1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.lang.Thread.UncaughtExceptionHandler;
39 import java.lang.reflect.Field;
40 import java.util.ArrayList;
41 import java.util.Collection;
42 import java.util.Collections;
43 import java.util.List;
44 import java.util.Objects;
45 import java.util.function.Consumer;
46 import java.util.function.Predicate;
47 import java.util.concurrent.CountDownLatch;
48 import java.util.concurrent.locks.LockSupport;
49 import jdk.internal.access.JavaLangAccess;
50 import jdk.internal.access.JavaUtilConcurrentFJPAccess;
51 import jdk.internal.access.SharedSecrets;
52 import jdk.internal.misc.Unsafe;
53 import jdk.internal.vm.SharedThreadContainer;
54 import static java.util.concurrent.DelayScheduler.ScheduledForkJoinTask;
55
56 /**
57 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
58 * A {@code ForkJoinPool} provides the entry point for submissions
59 * from non-{@code ForkJoinTask} clients, as well as management and
60 * monitoring operations.
61 *
62 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
63 * ExecutorService} mainly by virtue of employing
64 * <em>work-stealing</em>: all threads in the pool attempt to find and
65 * execute tasks submitted to the pool and/or created by other active
66 * tasks (eventually blocking waiting for work if none exist). This
67 * enables efficient processing when most tasks spawn other subtasks
68 * (as do most {@code ForkJoinTask}s), as well as when many small
69 * tasks are submitted to the pool from external clients. Especially
70 * when setting <em>asyncMode</em> to true in constructors, {@code
71 * ForkJoinPool}s may also be appropriate for use with event-style
72 * tasks that are never joined. All worker threads are initialized
73 * with {@link Thread#isDaemon} set {@code true}.
74 *
75 * <p>A static {@link #commonPool()} is available and appropriate for
76 * most applications. The common pool is used by any ForkJoinTask that
77 * is not explicitly submitted to a specified pool. Using the common
78 * pool normally reduces resource usage (its threads are slowly
79 * reclaimed during periods of non-use, and reinstated upon subsequent
80 * use).
81 *
82 * <p>For applications that require separate or custom pools, a {@code
83 * ForkJoinPool} may be constructed with a given target parallelism
84 * level; by default, equal to the number of available processors.
85 * The pool attempts to maintain enough active (or available) threads
86 * by dynamically adding, suspending, or resuming internal worker
87 * threads, even if some tasks are stalled waiting to join others.
88 * However, no such adjustments are guaranteed in the face of blocked
89 * I/O or other unmanaged synchronization. The nested {@link
90 * ManagedBlocker} interface enables extension of the kinds of
91 * synchronization accommodated. The default policies may be
92 * overridden using a constructor with parameters corresponding to
93 * those documented in class {@link ThreadPoolExecutor}.
94 *
95 * <p>In addition to execution and lifecycle control methods, this
96 * class provides status check methods (for example
97 * {@link #getStealCount}) that are intended to aid in developing,
98 * tuning, and monitoring fork/join applications. Also, method
99 * {@link #toString} returns indications of pool state in a
100 * convenient form for informal monitoring.
101 *
102 * <p>As is the case with other ExecutorServices, there are three
103 * main task execution methods summarized in the following table.
104 * These are designed to be used primarily by clients not already
105 * engaged in fork/join computations in the current pool. The main
106 * forms of these methods accept instances of {@code ForkJoinTask},
107 * but overloaded forms also allow mixed execution of plain {@code
108 * Runnable}- or {@code Callable}- based activities as well. However,
109 * tasks that are already executing in a pool should normally instead
110 * use the within-computation forms listed in the table unless using
111 * async event-style tasks that are not usually joined, in which case
112 * there is little difference among choice of methods.
113 *
114 * <table class="plain">
115 * <caption>Summary of task execution methods</caption>
116 * <tr>
117 * <td></td>
118 * <th scope="col"> Call from non-fork/join clients</th>
119 * <th scope="col"> Call from within fork/join computations</th>
120 * </tr>
121 * <tr>
122 * <th scope="row" style="text-align:left"> Arrange async execution</th>
123 * <td> {@link #execute(ForkJoinTask)}</td>
124 * <td> {@link ForkJoinTask#fork}</td>
125 * </tr>
126 * <tr>
127 * <th scope="row" style="text-align:left"> Await and obtain result</th>
128 * <td> {@link #invoke(ForkJoinTask)}</td>
129 * <td> {@link ForkJoinTask#invoke}</td>
130 * </tr>
131 * <tr>
132 * <th scope="row" style="text-align:left"> Arrange exec and obtain Future</th>
133 * <td> {@link #submit(ForkJoinTask)}</td>
134 * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
135 * </tr>
136 * </table>
137 *
138 * <p>Additionally, this class supports {@link
139 * ScheduledExecutorService} methods to delay or periodically execute
140 * tasks, as well as method {@link #submitWithTimeout} to cancel tasks
141 * that take too long. The scheduled functions or actions may create
142 * and invoke other {@linkplain ForkJoinTask ForkJoinTasks}. Delayed
143 * actions become enabled for execution and behave as ordinary submitted
144 * tasks when their delays elapse. Scheduling methods return
145 * {@linkplain ForkJoinTask ForkJoinTasks} that implement the {@link
146 * ScheduledFuture} interface. Resource exhaustion encountered after
147 * initial submission results in task cancellation. When time-based
148 * methods are used, shutdown policies match the default policies of
149 * class {@link ScheduledThreadPoolExecutor}: upon {@link #shutdown},
150 * existing periodic tasks will not re-execute, and the pool
151 * terminates when quiescent and existing delayed tasks
152 * complete. Method {@link #cancelDelayedTasksOnShutdown} may be used
153 * to disable all delayed tasks upon shutdown, and method {@link
154 * #shutdownNow} may be used to instead unconditionally initiate pool
155 * termination. Monitoring methods such as {@link #getQueuedTaskCount}
156 * do not include scheduled tasks that are not yet enabled for execution,
157 * which are reported separately by method {@link
158 * #getDelayedTaskCount}.
159 *
160 * <p>The parameters used to construct the common pool may be controlled by
161 * setting the following {@linkplain System#getProperty system properties}:
162 * <ul>
163 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.parallelism}
164 * - the parallelism level, a non-negative integer. Usage is discouraged.
165 * Use {@link #setParallelism} instead.
166 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.threadFactory}
167 * - the class name of a {@link ForkJoinWorkerThreadFactory}.
168 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
169 * is used to load this class.
170 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.exceptionHandler}
171 * - the class name of a {@link UncaughtExceptionHandler}.
172 * The {@linkplain ClassLoader#getSystemClassLoader() system class loader}
173 * is used to load this class.
174 * <li>{@systemProperty java.util.concurrent.ForkJoinPool.common.maximumSpares}
175 * - the maximum number of allowed extra threads to maintain target
176 * parallelism (default 256).
177 * </ul>
178 * If no thread factory is supplied via a system property, then the
179 * common pool uses a factory that uses the system class loader as the
180 * {@linkplain Thread#getContextClassLoader() thread context class loader}.
181 *
182 * Upon any error in establishing these settings, default parameters
183 * are used. It is possible to disable use of threads by using a
184 * factory that may return {@code null}, in which case some tasks may
185 * never execute. While possible, it is strongly discouraged to set
186 * the parallelism property to zero, which may be internally
187 * overridden in the presence of intrinsically async tasks.
188 *
189 * @implNote This implementation restricts the maximum number of
190 * running threads to 32767. Attempts to create pools with greater
191 * than the maximum number result in {@code
192 * IllegalArgumentException}. Also, this implementation rejects
193 * submitted tasks (that is, by throwing {@link
194 * RejectedExecutionException}) only when the pool is shut down or
195 * internal resources have been exhausted.
196 *
197 * @since 1.7
198 * @author Doug Lea
199 */
200 public class ForkJoinPool extends AbstractExecutorService
201 implements ScheduledExecutorService {
202
203 /*
204 * Implementation Overview
205 *
206 * This class and its nested classes provide the main
207 * functionality and control for a set of worker threads. Because
208 * most internal methods and nested classes are interrelated,
209 * their main rationale and descriptions are presented here;
210 * individual methods and nested classes contain only brief
211 * comments about details. Broadly: submissions from non-FJ
212 * threads enter into submission queues. Workers take these tasks
213 * and typically split them into subtasks that may be stolen by
214 * other workers. Work-stealing based on randomized scans
215 * generally leads to better throughput than "work dealing" in
216 * which producers assign tasks to idle threads, in part because
217 * threads that have finished other tasks before the signalled
218 * thread wakes up (which can be a long time) can take the task
219 * instead. Preference rules give first priority to processing
220 * tasks from their own queues (LIFO or FIFO, depending on mode),
221 * then to randomized FIFO steals of tasks in other queues.
222 *
223 * This framework began as vehicle for supporting structured
224 * parallelism using work-stealing, designed to work best when
225 * tasks are dag-structured (wrt completion dependencies), nested
226 * (generated using recursion or completions), of reasonable
227 * granularity, independent (wrt memory and resources) and where
228 * callers participate in task execution. These are properties
229 * that anyone aiming for efficient parallel multicore execution
230 * should design for. Over time, the scalability advantages of
231 * this framework led to extensions to better support more diverse
232 * usage contexts, amounting to weakenings or violations of each
233 * of these properties. Accommodating them may compromise
234 * performance, but mechanics discussed below include tradeoffs
235 * attempting to arrange that no single performance issue dominates.
236 *
237 * Here's a brief history of major revisions, each also with other
238 * minor features and changes.
239 *
240 * 1. Only handle recursively structured computational tasks
241 * 2. Async (FIFO) mode and striped submission queues
242 * 3. Completion-based tasks (mainly CountedCompleters)
243 * 4. CommonPool and parallelStream support
244 * 5. InterruptibleTasks for externally submitted tasks
245 * 6. Support ScheduledExecutorService methods
246 *
247 * Most changes involve adaptions of base algorithms using
248 * combinations of static and dynamic bitwise mode settings (both
249 * here and in ForkJoinTask), and subclassing of ForkJoinTask.
250 * There are a fair number of odd code constructions and design
251 * decisions for components that reside at the edge of Java vs JVM
252 * functionality.
253 *
254 * WorkQueues
255 * ==========
256 *
257 * Most operations occur within work-stealing queues (in nested
258 * class WorkQueue). These are special forms of Deques that
259 * support only three of the four possible end-operations -- push,
260 * pop, and poll (aka steal), under the further constraints that
261 * push and pop are called only from the owning thread (or, as
262 * extended here, under a lock), while poll may be called from
263 * other threads. (If you are unfamiliar with them, you probably
264 * want to read Herlihy and Shavit's book "The Art of
265 * Multiprocessor programming", chapter 16 describing these in
266 * more detail before proceeding.) The main work-stealing queue
267 * design is roughly similar to those in the papers "Dynamic
268 * Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
269 * (http://research.sun.com/scalable/pubs/index.html) and
270 * "Idempotent work stealing" by Michael, Saraswat, and Vechev,
271 * PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
272 * The main differences ultimately stem from GC requirements that
273 * we null out taken slots as soon as we can, to maintain as small
274 * a footprint as possible even in programs generating huge
275 * numbers of tasks. To accomplish this, we shift the CAS
276 * arbitrating pop vs poll (steal) from being on the indices
277 * ("base" and "top") to the slots themselves. These provide the
278 * primary required memory ordering -- see "Correct and Efficient
279 * Work-Stealing for Weak Memory Models" by Le, Pop, Cohen, and
280 * Nardelli, PPoPP 2013
281 * (http://www.di.ens.fr/~zappa/readings/ppopp13.pdf) for an
282 * analysis of memory ordering requirements in work-stealing
283 * algorithms similar to the one used here. We use per-operation
284 * ordered writes of various kinds for accesses when required.
285 *
286 * We also support a user mode in which local task processing is
287 * in FIFO, not LIFO order, simply by using a local version of
288 * poll rather than pop. This can be useful in message-passing
289 * frameworks in which tasks are never joined, although with
290 * increased contention among task producers and consumers. Also,
291 * the same data structure (and class) is used for "submission
292 * queues" (described below) holding externally submitted tasks,
293 * that differ only in that a lock (using field "phase"; see below) is
294 * required by external callers to push and pop tasks.
295 *
296 * Adding tasks then takes the form of a classic array push(task)
297 * in a circular buffer:
298 * q.array[q.top++ % length] = task;
299 *
300 * The actual code needs to null-check and size-check the array,
301 * uses masking, not mod, for indexing a power-of-two-sized array,
302 * enforces memory ordering, supports resizing, and possibly
303 * signals waiting workers to start scanning (described below),
304 * which requires stronger forms of order accesses.
305 *
306 * The pop operation (always performed by owner) is of the form:
307 * if ((task = getAndSet(q.array, (q.top-1) % length, null)) != null)
308 * decrement top and return task;
309 * If this fails, the queue is empty. This operation is one part
310 * of the nextLocalTask method, that instead does a local-poll
311 * in FIFO mode.
312 *
313 * The poll operation is, basically:
314 * if (CAS nonnull task t = q.array[k = q.base % length] to null)
315 * increment base and return task;
316 *
317 * However, there are several more cases that must be dealt with.
318 * Some of them are just due to asynchrony; others reflect
319 * contention and stealing policies. Stepping through them
320 * illustrates some of the implementation decisions in this class.
321 *
322 * * Slot k must be read with an acquiring read, which it must
323 * anyway to dereference and run the task if the (acquiring)
324 * CAS succeeds.
325 *
326 * * q.base may change between reading and using its value to
327 * index the slot. To avoid trying to use the wrong t, the
328 * index and slot must be reread (not necessarily immediately)
329 * until consistent, unless this is a local poll by owner, in
330 * which case this form of inconsistency can only appear as t
331 * being null, below.
332 *
333 * * Similarly, q.array may change (due to a resize), unless this
334 * is a local poll by owner. Otherwise, when t is present, this
335 * only needs consideration on CAS failure (since a CAS
336 * confirms the non-resized case.)
337 *
338 * * t may appear null because a previous poll operation has not
339 * yet incremented q.base, so the read is from an already-taken
340 * index. This form of stall reflects the non-lock-freedom of
341 * the poll operation. Stalls can be detected by observing that
342 * q.base doesn't change on repeated reads of null t and when
343 * no other alternatives apply, spin-wait for it to settle. To
344 * reduce producing these kinds of stalls by other stealers, we
345 * encourage timely writes to indices using otherwise
346 * unnecessarily strong writes.
347 *
348 * * The CAS may fail, in which case we may want to retry unless
349 * there is too much contention. One goal is to balance and
350 * spread out the many forms of contention that may be
351 * encountered across polling and other operations to avoid
352 * sustained performance degradations. Across all cases where
353 * alternatives exist, a bounded number of CAS misses or stalls
354 * are tolerated (for slots, ctl, and elsewhere described
355 * below) before taking alternative action. These may move
356 * contention or retries elsewhere, which is still preferable
357 * to single-point bottlenecks.
358 *
359 * * Even though the check "top == base" is quiescently accurate
360 * to determine whether a queue is empty, it is not of much use
361 * when deciding whether to try to poll or repoll after a
362 * failure. Both top and base may move independently, and both
363 * lag updates to the underlying array. To reduce memory
364 * contention, non-owners avoid reading the "top" when
365 * possible, by using one-ahead reads to check whether to
366 * repoll, relying on the fact that a non-empty queue does not
367 * have two null slots in a row, except in cases (resizes and
368 * shifts) that can be detected with a secondary recheck that
369 * is less likely to conflict with owner writes.
370 *
371 * The poll operations in q.poll(), runWorker(), helpJoin(), and
372 * elsewhere differ with respect to whether other queues are
373 * available to try, and the presence or nature of screening steps
374 * when only some kinds of tasks can be taken. When alternatives
375 * (or failing) is an option, they uniformly give up after
376 * bounded numbers of stalls and/or CAS failures, which reduces
377 * contention when too many workers are polling too few tasks.
378 * Overall, in the aggregate, we ensure probabilistic
379 * non-blockingness of work-stealing at least until checking
380 * quiescence (which is intrinsically blocking): If an attempted
381 * steal fails in these ways, a scanning thief chooses a different
382 * target to try next. In contexts where alternatives aren't
383 * available, and when progress conditions can be isolated to
384 * values of a single variable, simple spinloops (using
385 * Thread.onSpinWait) are used to reduce memory traffic.
386 *
387 * WorkQueues are also used in a similar way for tasks submitted
388 * to the pool. We cannot mix these tasks in the same queues used
389 * by workers. Instead, we randomly associate submission queues
390 * with submitting threads (or carriers when using VirtualThreads)
391 * using a form of hashing. The ThreadLocalRandom probe value
392 * serves as a hash code for choosing existing queues, and may be
393 * randomly repositioned upon contention with other submitters.
394 * In essence, submitters act like workers except that they are
395 * restricted to executing local tasks that they submitted (or
396 * when known, subtasks thereof). Insertion of tasks in shared
397 * mode requires a lock. We use only a simple spinlock (as one
398 * role of field "phase") because submitters encountering a busy
399 * queue move to a different position to use or create other
400 * queues. They (spin) block when registering new queues, or
401 * indirectly elsewhere, by revisiting later.
402 *
403 * Management
404 * ==========
405 *
406 * The main throughput advantages of work-stealing stem from
407 * decentralized control -- workers mostly take tasks from
408 * themselves or each other, at rates that can exceed a billion
409 * per second. Most non-atomic control is performed by some form
410 * of scanning across or within queues. The pool itself creates,
411 * activates (enables scanning for and running tasks),
412 * deactivates, blocks, and terminates threads, all with minimal
413 * central information. There are only a few properties that we
414 * can globally track or maintain, so we pack them into a small
415 * number of variables, often maintaining atomicity without
416 * blocking or locking. Nearly all essentially atomic control
417 * state is held in a few variables that are by far most often
418 * read (not written) as status and consistency checks. We pack as
419 * much information into them as we can.
420 *
421 * Field "ctl" contains 64 bits holding information needed to
422 * atomically decide to add, enqueue (on an event queue), and
423 * dequeue and release workers. To enable this packing, we
424 * restrict maximum parallelism to (1<<15)-1 (which is far in
425 * excess of normal operating range) to allow ids, counts, and
426 * their negations (used for thresholding) to fit into 16bit
427 * subfields.
428 *
429 * Field "runState" and per-WorkQueue field "phase" play similar
430 * roles, as lockable, versioned counters. Field runState also
431 * includes monotonic event bits:
432 * * SHUTDOWN: no more external tasks accepted; STOP when quiescent
433 * * STOP: no more tasks run, and deregister all workers
434 * * CLEANED: all unexecuted tasks have been cancelled
435 * * TERMINATED: all workers deregistered and all queues cleaned
436 * The version tags enable detection of state changes (by
437 * comparing two reads) modulo bit wraparound. The bit range in
438 * each case suffices for purposes of determining quiescence,
439 * termination, avoiding ABA-like errors, and signal control, most
440 * of which are ultimately based on at most 15bit ranges (due to
441 * 32767 max total workers). RunState updates do not need to be
442 * atomic with respect to ctl updates, but because they are not,
443 * some care is required to avoid stalls. The seqLock properties
444 * detect changes and conditionally upgrade to coordinate with
445 * updates. It is typically held for less than a dozen
446 * instructions unless the queue array is being resized, during
447 * which contention is rare. To be conservative, lockRunState is
448 * implemented as a spin/sleep loop. Here and elsewhere spin
449 * constants are short enough to apply even on systems with few
450 * available processors. In addition to checking pool status,
451 * reads of runState sometimes serve as acquire fences before
452 * reading other fields.
453 *
454 * Field "parallelism" holds the target parallelism (normally
455 * corresponding to pool size). Users can dynamically reset target
456 * parallelism, but is only accessed when signalling or awaiting
457 * work, so only slowly has an effect in creating threads or
458 * letting them time out and terminate when idle.
459 *
460 * Array "queues" holds references to WorkQueues. It is updated
461 * (only during worker creation and termination) under the
462 * runState lock. It is otherwise concurrently readable but reads
463 * for use in scans (see below) are always prefaced by a volatile
464 * read of runState (or equivalent constructions), ensuring that
465 * its state is current at the point it is used (which is all we
466 * require). To simplify index-based operations, the array size is
467 * always a power of two, and all readers must tolerate null
468 * slots. Worker queues are at odd indices. Worker phase ids
469 * masked with SMASK match their index. Shared (submission) queues
470 * are at even indices. Grouping them together in this way aids in
471 * task scanning: At top-level, both kinds of queues should be
472 * sampled with approximately the same probability, which is
473 * simpler if they are all in the same array. But we also need to
474 * identify what kind they are without looking at them, leading to
475 * this odd/even scheme. One disadvantage is that there are
476 * usually many fewer submission queues, so there can be many
477 * wasted probes (null slots). But this is still cheaper than
478 * alternatives. Other loops over the queues array vary in origin
479 * and stride depending on whether they cover only submission
480 * (even) or worker (odd) queues or both, and whether they require
481 * randomness (in which case cyclically exhaustive strides may be
482 * used).
483 *
484 * All worker thread creation is on-demand, triggered by task
485 * submissions, replacement of terminated workers, and/or
486 * compensation for blocked workers. However, all other support
487 * code is set up to work with other policies. To ensure that we
488 * do not hold on to worker or task references that would prevent
489 * GC, all accesses to workQueues in waiting, signalling, and
490 * control methods are via indices into the queues array (which is
491 * one source of some of the messy code constructions here). In
492 * essence, the queues array serves as a weak reference
493 * mechanism. In particular, the stack top subfield of ctl stores
494 * indices, not references. Operations on queues obtained from
495 * these indices remain valid (with at most some unnecessary extra
496 * work) even if an underlying worker failed and was replaced by
497 * another at the same index. During termination, worker queue
498 * array updates are disabled.
499 *
500 * Queuing Idle Workers. Unlike HPC work-stealing frameworks, we
501 * cannot let workers spin indefinitely scanning for tasks when
502 * none can be found immediately, and we cannot start/resume
503 * workers unless there appear to be tasks available. On the
504 * other hand, we must quickly prod them into action when new
505 * tasks are submitted or generated. These latencies are mainly a
506 * function of JVM park/unpark (and underlying OS) performance,
507 * which can be slow and variable (even though usages are
508 * streamlined as much as possible). In many usages, ramp-up time
509 * is the main limiting factor in overall performance, which is
510 * compounded at program start-up by JIT compilation and
511 * allocation. On the other hand, throughput degrades when too
512 * many threads poll for too few tasks. (See below.)
513 *
514 * The "ctl" field atomically maintains total and "released"
515 * worker counts, plus the head of the available worker queue
516 * (actually stack, represented by the lower 32bit subfield of
517 * ctl). Released workers are those known to be scanning for
518 * and/or running tasks (we cannot accurately determine
519 * which). Unreleased ("available") workers are recorded in the
520 * ctl stack. These workers are made eligible for signalling by
521 * enqueuing in ctl (see method deactivate). This "queue" is a
522 * form of Treiber stack. This is ideal for activating threads in
523 * most-recently used order, and improves performance and
524 * locality, outweighing the disadvantages of being prone to
525 * contention and inability to release a worker unless it is
526 * topmost on stack. The top stack state holds the value of the
527 * "phase" field of the worker: its index and status, plus a
528 * version counter that, in addition to the count subfields (also
529 * serving as version stamps) provide protection against Treiber
530 * stack ABA effects.
531 *
532 * Creating workers. To create a worker, we pre-increment counts
533 * (serving as a reservation), and attempt to construct a
534 * ForkJoinWorkerThread via its factory. On starting, the new
535 * thread first invokes registerWorker, where it is assigned an
536 * index in the queues array (expanding the array if necessary).
537 * Upon any exception across these steps, or null return from
538 * factory, deregisterWorker adjusts counts and records
539 * accordingly. If a null return, the pool continues running with
540 * fewer than the target number workers. If exceptional, the
541 * exception is propagated, generally to some external caller.
542 *
543 * WorkQueue field "phase" encodes the queue array id in lower
544 * bits, and otherwise acts similarly to the pool runState field:
545 * The "IDLE" bit is clear while active (either a released worker
546 * or a locked external queue), with other bits serving as a
547 * version counter to distinguish changes across multiple reads.
548 * Note that phase field updates lag queue CAS releases; seeing a
549 * non-idle phase does not guarantee that the worker is available
550 * (and so is never checked in this way).
551 *
552 * The ctl field also serves as the basis for memory
553 * synchronization surrounding activation. This uses a more
554 * efficient version of a Dekker-like rule that task producers and
555 * consumers sync with each other by both writing/CASing ctl (even
556 * if to its current value). However, rather than CASing ctl to
557 * its current value in the common case where no action is
558 * required, we reduce write contention by ensuring that
559 * signalWork invocations are prefaced with a fully fenced memory
560 * access (which is usually needed anyway).
561 *
562 * Signalling. Signals (in signalWork) cause new or reactivated
563 * workers to scan for tasks. SignalWork is invoked in two cases:
564 * (1) When a task is pushed onto an empty queue, and (2) When a
565 * worker takes a top-level task from a queue that has additional
566 * tasks. Together, these suffice in O(log(#threads)) steps to
567 * fully activate with at least enough workers, and ideally no
568 * more than required. This ideal is unobtainable: Callers do not
569 * know whether another worker will finish its current task and
570 * poll for others without need of a signal (which is otherwise an
571 * advantage of work-stealing vs other schemes), and also must
572 * conservatively estimate the triggering conditions of emptiness
573 * or non-emptiness; all of which usually cause more activations
574 * than necessary (see below). (Method signalWork is also used as
575 * failsafe in case of Thread failures in deregisterWorker.)
576 *
577 * Top-Level scheduling
578 * ====================
579 *
580 * Scanning. Method runWorker performs top-level scanning for (and
581 * execution of) tasks by polling a pseudo-random permutation of
582 * the array (by starting at a given index, and using a constant
583 * cyclically exhaustive stride.) It uses the same basic polling
584 * method as WorkQueue.poll(), but restarts with a different
585 * permutation on each rescan. The pseudorandom generator need
586 * not have high-quality statistical properties in the long
587 * term. We use Marsaglia XorShifts, seeded with the Weyl sequence
588 * from ThreadLocalRandom probes, which are cheap and suffice.
589 *
590 * Deactivation. When no tasks are found by a worker in runWorker,
591 * it invokes awaitWork, that first deactivates (to an IDLE
592 * phase). Avoiding missed signals during deactivation requires a
593 * (conservative) rescan, reactivating if there may be tasks to
594 * poll. Because idle workers are often not yet blocked (parked),
595 * we use a WorkQueue field to advertise that a waiter actually
596 * needs unparking upon signal.
597 *
598 * When tasks are constructed as (recursive) dags, top-level
599 * scanning is usually infrequent, and doesn't encounter most
600 * of the following problems addressed by runWorker and awaitWork:
601 *
602 * Locality. Polls are organized into "runs", continuing until
603 * empty or contended, while also minimizing interference by
604 * postponing bookeeping to ends of runs. This may reduce
605 * fairness, which is partially counteracted by the following.
606 *
607 * Contention. When many workers try to poll few queues, they
608 * often collide, generating CAS failures and disrupting locality
609 * of workers already running their tasks. This also leads to
610 * stalls when tasks cannot be taken because other workers have
611 * not finished poll operations, which is detected by reading
612 * ahead in queue arrays. In both caes, workers restart scans in a
613 * way that approximates randomized backoff.
614 *
615 * Oversignalling. When many short top-level tasks are present in
616 * a small number of queues, the above signalling strategy may
617 * activate many more workers than needed, worsening locality and
618 * contention problems, while also generating more global
619 * contention (field is CASed on every activation and
620 * deactivation). We filter out (both in runWorker and
621 * signalWork) attempted signals that are surely not needed
622 * because the signalled tasks are already taken.
623 *
624 * Shutdown and Quiescence
625 * =======================
626 *
627 * Quiescence. Workers scan looking for work, giving up when they
628 * don't find any, without being sure that none are available.
629 * However, some required functionality relies on consensus about
630 * quiescence (also termination, discussed below). The count
631 * fields in ctl allow accurate discovery of states in which all
632 * workers are idle. However, because external (asynchronous)
633 * submitters are not part of this vote, these mechanisms
634 * themselves do not guarantee that the pool is in a quiescent
635 * state with respect to methods isQuiescent, shutdown (which
636 * begins termination when quiescent), helpQuiesce, and indirectly
637 * others including tryCompensate. Method quiescent() is used in
638 * all of these contexts. It provides checks that all workers are
639 * idle and there are no submissions that they could poll if they
640 * were not idle, retrying on inconsistent reads of queues and
641 * using the runState seqLock to retry on queue array updates.
642 * (It also reports quiescence if the pool is terminating.) A true
643 * report means only that there was a moment at which quiescence
644 * held. False negatives are inevitable (for example when queues
645 * indices lag updates, as described above), which is accommodated
646 * when (tentatively) idle by scanning for work etc, and then
647 * re-invoking. This includes cases in which the final unparked
648 * thread (in deactivate()) uses quiescent() to check for tasks
649 * that could have been added during a race window that would not
650 * be accompanied by a signal, in which case re-activating itself
651 * (or any other worker) to rescan. Method helpQuiesce acts
652 * similarly but cannot rely on ctl counts to determine that all
653 * workers are inactive because the caller and any others
654 * executing helpQuiesce are not included in counts.
655 *
656 * Termination. Termination is initiated by setting STOP in one of
657 * three ways (via methods tryTerminate and quiescent):
658 * * A call to shutdownNow, in which case all workers are
659 * interrupted, first ensuring that the queues array is stable,
660 * to avoid missing any workers.
661 * * A call to shutdown when quiescent, in which case method
662 * releaseWaiters is used to dequeue them, at which point they notice
663 * STOP state and return from runWorker to deregister();
664 * * The pool becomes quiescent() sometime after shutdown has
665 * been called, in which case releaseWaiters is also used to
666 * propagate as they deregister.
667 * Upon STOP, each worker, as well as external callers to
668 * tryTerminate (via close() etc) race to set CLEANED, indicating
669 * that all tasks have been cancelled. The implementation (method
670 * cleanQueues) balances cases in which there may be many tasks to
671 * cancel (benefitting from parallelism) versus contention and
672 * interference when many threads try to poll remaining queues,
673 * while also avoiding unnecessary rechecks, by using
674 * pseudorandom scans and giving up upon interference. This may be
675 * retried by the same caller only when there are no more
676 * registered workers, using the same criteria as method
677 * quiescent. When CLEANED and all workers have deregistered,
678 * TERMINATED is set, also signalling any caller of
679 * awaitTermination or close. Because shutdownNow-based
680 * termination relies on interrupts, there is no guarantee that
681 * workers will stop if their tasks ignore interrupts. Class
682 * InterruptibleTask (see below) further arranges runState checks
683 * before executing task bodies, and ensures interrupts while
684 * terminating. Even so, there are no guarantees because tasks may
685 * internally enter unbounded loops.
686 *
687 * Trimming workers. To release resources after periods of lack of
688 * use, a worker starting to wait when the pool is quiescent will
689 * time out and terminate if the pool has remained quiescent for
690 * period given by field keepAlive (default 60sec), which applies
691 * to the first timeout of a quiescent pool. Subsequent cases use
692 * minimal delays such that, if still quiescent, all will be
693 * released soon thereafter. This is checked by setting the
694 * "source" field of signallee to an invalid value, that will
695 * remain invalid only if it did not process any tasks.
696 *
697 * Joining Tasks
698 * =============
699 *
700 * The "Join" part of ForkJoinPools consists of a set of
701 * mechanisms that sometimes or always (depending on the kind of
702 * task) avoid context switching or adding worker threads when one
703 * task would otherwise be blocked waiting for completion of
704 * another, basically, just by running that task or one of its
705 * subtasks if not already taken. These mechanics are disabled for
706 * InterruptibleTasks, that guarantee that callers do not execute
707 * submitted tasks.
708 *
709 * The basic structure of joining is an extended spin/block scheme
710 * in which workers check for task completions status between
711 * steps to find other work, until relevant pool state stabilizes
712 * enough to believe that no such tasks are available, at which
713 * point blocking. This is usually a good choice of when to block
714 * that would otherwise be harder to approximate.
715 *
716 * These forms of helping may increase stack space usage, but that
717 * space is bounded in tree/dag structured procedurally parallel
718 * designs to be no more than that if a task were executed only by
719 * the joining thread. This is arranged by associated task
720 * subclasses that also help detect and control the ways in which
721 * this may occur.
722 *
723 * Normally, the first option when joining a task that is not done
724 * is to try to take it from the local queue and run it. Method
725 * tryRemoveAndExec tries to do so. For tasks with any form of
726 * subtasks that must be completed first, we try to locate these
727 * subtasks and run them as well. This is easy when local, but
728 * when stolen, steal-backs are restricted to the same rules as
729 * stealing (polling), which requires additional bookkeeping and
730 * scanning. This cost is still very much worthwhile because of
731 * its impact on task scheduling and resource control.
732 *
733 * The two methods for finding and executing subtasks vary in
734 * details. The algorithm in helpJoin entails a form of "linear
735 * helping". Each worker records (in field "source") the index of
736 * the internal queue from which it last stole a task. (Note:
737 * because chains cannot include even-numbered external queues,
738 * they are ignored, and 0 is an OK default. However, the source
739 * field is set anyway, or eventually to DROPPED, to ensure
740 * volatile memory synchronization effects.) The scan in method
741 * helpJoin uses these markers to try to find a worker to help
742 * (i.e., steal back a task from and execute it) that could make
743 * progress toward completion of the actively joined task. Thus,
744 * the joiner executes a task that would be on its own local deque
745 * if the to-be-joined task had not been stolen. This is a
746 * conservative variant of the approach described in Wagner &
747 * Calder "Leapfrogging: a portable technique for implementing
748 * efficient futures" SIGPLAN Notices, 1993
749 * (http://portal.acm.org/citation.cfm?id=155354). It differs
750 * mainly in that we only record queues, not full dependency
751 * links. This requires a linear scan of the queues to locate
752 * stealers, but isolates cost to when it is needed, rather than
753 * adding to per-task overhead. For CountedCompleters, the
754 * analogous method helpComplete doesn't need stealer-tracking,
755 * but requires a similar (but simpler) check of completion
756 * chains.
757 *
758 * In either case, searches can fail to locate stealers when
759 * stalls delay recording sources or issuing subtasks. We avoid
760 * some of these cases by using snapshotted values of ctl as a
761 * check that the numbers of workers are not changing, along with
762 * rescans to deal with contention and stalls. But even when
763 * accurately identified, stealers might not ever produce a task
764 * that the joiner can in turn help with.
765 *
766 * Related method helpAsyncBlocker does not directly rely on
767 * subtask structure, but instead avoids or postpones blocking of
768 * tagged tasks (CompletableFuture.AsynchronousCompletionTask) by
769 * executing other asyncs that can be processed in any order.
770 * This is currently invoked only in non-join-based blocking
771 * contexts from classes CompletableFuture and
772 * SubmissionPublisher, that could be further generalized.
773 *
774 * When any of the above fail to avoid blocking, we rely on
775 * "compensation" -- an indirect form of context switching that
776 * either activates an existing worker to take the place of the
777 * blocked one, or expands the number of workers.
778 *
779 * Compensation does not by default aim to keep exactly the target
780 * parallelism number of unblocked threads running at any given
781 * time. Some previous versions of this class employed immediate
782 * compensations for any blocked join. However, in practice, the
783 * vast majority of blockages are transient byproducts of GC and
784 * other JVM or OS activities that are made worse by replacement
785 * by causing longer-term oversubscription. These are inevitable
786 * without (unobtainably) perfect information about whether worker
787 * creation is actually necessary. False alarms are common enough
788 * to negatively impact performance, so compensation is by default
789 * attempted only when it appears possible that the pool could
790 * stall due to lack of any unblocked workers. However, we allow
791 * users to override defaults using the long form of the
792 * ForkJoinPool constructor. The compensation mechanism may also
793 * be bounded. Bounds for the commonPool better enable JVMs to
794 * cope with programming errors and abuse before running out of
795 * resources to do so.
796 *
797 * The ManagedBlocker extension API can't use helping so relies
798 * only on compensation in method awaitBlocker. This API was
799 * designed to highlight the uncertainty of compensation decisions
800 * by requiring implementation of method isReleasable to abort
801 * compensation during attempts to obtain a stable snapshot. But
802 * users now rely upon the fact that if isReleasable always
803 * returns false, the API can be used to obtain precautionary
804 * compensation, which is sometimes the only reasonable option
805 * when running unknown code in tasks; which is now supported more
806 * simply (see method beginCompensatedBlock).
807 *
808 * Common Pool
809 * ===========
810 *
811 * The static common pool always exists after static
812 * initialization. Since it (or any other created pool) need
813 * never be used, we minimize initial construction overhead and
814 * footprint to the setup of about a dozen fields, although with
815 * some System property parsing properties are set. The common pool is
816 * distinguished by having a null workerNamePrefix (which is an
817 * odd convention, but avoids the need to decode status in factory
818 * classes). It also has PRESET_SIZE config set if parallelism
819 * was configured by system property.
820 *
821 * When external threads use the common pool, they can perform
822 * subtask processing (see helpComplete and related methods) upon
823 * joins, unless they are submitted using ExecutorService
824 * submission methods, which implicitly disallow this. This
825 * caller-helps policy makes it sensible to set common pool
826 * parallelism level to one (or more) less than the total number
827 * of available cores, or even zero for pure caller-runs. External
828 * threads waiting for joins first check the common pool for their
829 * task, which fails quickly if the caller did not fork to common
830 * pool.
831 *
832 * Guarantees for common pool parallelism zero are limited to
833 * tasks that are joined by their callers in a tree-structured
834 * fashion or use CountedCompleters (as is true for jdk
835 * parallelStreams). Support infiltrates several methods,
836 * including those that retry helping steps until we are sure that
837 * none apply if there are no workers. To deal with conflicting
838 * requirements, uses of the commonPool that require async because
839 * caller-runs need not apply, ensure threads are enabled (by
840 * setting parallelism) via method asyncCommonPool before
841 * proceeding. (In principle, overriding zero parallelism needs to
842 * ensure at least one worker, but due to other backward
843 * compatibility contraints, ensures two.)
844 *
845 * As a more appropriate default in managed environments, unless
846 * overridden by system properties, we use workers of subclass
847 * InnocuousForkJoinWorkerThread for the commonPool. These
848 * workers do not belong to any user-defined ThreadGroup, and
849 * clear all ThreadLocals and reset the ContextClassLoader before
850 * (re)activating to execute top-level tasks. The associated
851 * mechanics may be JVM-dependent and must access particular
852 * Thread class fields to achieve this effect.
853 *
854 * InterruptibleTasks
855 * ====================
856 *
857 * Regular ForkJoinTasks manage task cancellation (method cancel)
858 * independently from the interrupt status of threads running
859 * tasks. Interrupts are issued internally only while
860 * terminating, to wake up workers and cancel queued tasks. By
861 * default, interrupts are cleared only when necessary to ensure
862 * that calls to LockSupport.park do not loop indefinitely (park
863 * returns immediately if the current thread is interrupted).
864 *
865 * To comply with ExecutorService specs, we use subclasses of
866 * abstract class InterruptibleTask for tasks that require
867 * stronger interruption and cancellation guarantees. External
868 * submitters never run these tasks, even if in the common pool
869 * (as indicated by ForkJoinTask.noUserHelp status bit).
870 * InterruptibleTasks include a "runner" field (implemented
871 * similarly to FutureTask) to support cancel(true). Upon pool
872 * shutdown, runners are interrupted so they can cancel. Since
873 * external joining callers never run these tasks, they must await
874 * cancellation by others, which can occur along several different
875 * paths.
876 *
877 * Across these APIs, rules for reporting exceptions for tasks
878 * with results accessed via join() differ from those via get(),
879 * which differ from those invoked using pool submit methods by
880 * non-workers (which comply with Future.get() specs). Internal
881 * usages of ForkJoinTasks ignore interrupt status when executing
882 * or awaiting completion. Otherwise, reporting task results or
883 * exceptions is preferred to throwing InterruptedExceptions,
884 * which are in turn preferred to timeouts. Similarly, completion
885 * status is preferred to reporting cancellation. Cancellation is
886 * reported as an unchecked exception by join(), and by worker
887 * calls to get(), but is otherwise wrapped in a (checked)
888 * ExecutionException.
889 *
890 * Worker Threads cannot be VirtualThreads, as enforced by
891 * requiring ForkJoinWorkerThreads in factories. There are
892 * several constructions relying on this. However as of this
893 * writing, virtual thread bodies are by default run as some form
894 * of InterruptibleTask.
895 *
896 * DelayScheduler
897 * ================
898 *
899 * This class supports ScheduledExecutorService methods by
900 * creating and starting a DelayScheduler on first use of these
901 * methods (via startDelayScheduler). The scheduler operates
902 * independently in its own thread, relaying tasks to the pool to
903 * execute when their delays elapse (see method
904 * executeEnabledScheduledTask). The only other interactions with
905 * the delayScheduler are to control shutdown and maintain
906 * shutdown-related policies in methods quiescent() and
907 * tryTerminate(). In particular, processing must deal with cases
908 * in which tasks are submitted before shutdown, but not enabled
909 * until afterwards, in which case they must bypass some screening
910 * to be allowed to run. Conversely, the DelayScheduler checks
911 * runState status and when enabled, completes termination, using
912 * only methods shutdownStatus and tryStopIfShutdown. All of these
913 * methods are final and have signatures referencing
914 * DelaySchedulers, so cannot conflict with those of any existing
915 * FJP subclasses.
916 *
917 * Memory placement
918 * ================
919 *
920 * Performance is very sensitive to placement of instances of
921 * ForkJoinPool and WorkQueues and their queue arrays, as well as
922 * the placement of their fields. Caches misses and contention due
923 * to false-sharing have been observed to slow down some programs
924 * by more than a factor of four. Effects may vary across initial
925 * memory configuarations, applications, and different garbage
926 * collectors and GC settings, so there is no perfect solution.
927 * Too much isolation may generate more cache misses in common
928 * cases (because some fields snd slots are usually read at the
929 * same time). The @Contended annotation provides only rough
930 * control (for good reason). Similarly for relying on fields
931 * being placed in size-sorted declaration order.
932 *
933 * We isolate the ForkJoinPool.ctl field that otherwise causes the
934 * most false-sharing misses with respect to other fields. Also,
935 * ForkJoinPool fields are ordered such that fields less prone to
936 * contention effects are first, offsetting those that otherwise
937 * would be, while also reducing total footprint vs using
938 * multiple @Contended regions, which tends to slow down
939 * less-contended applications. To help arrange this, some
940 * non-reference fields are declared as "long" even when ints or
941 * shorts would suffice. For class WorkQueue, an
942 * embedded @Contended region segregates fields most heavily
943 * updated by owners from those most commonly read by stealers or
944 * other management.
945 *
946 * Initial sizing and resizing of WorkQueue arrays is an even more
947 * delicate tradeoff because the best strategy systematically
948 * varies across garbage collectors. Small arrays are better for
949 * locality and reduce GC scan time, but large arrays reduce both
950 * direct false-sharing and indirect cases due to GC bookkeeping
951 * (cardmarks etc), and reduce the number of resizes, which are
952 * not especially fast because they require atomic transfers.
953 * Currently, arrays are initialized to be just large enough to
954 * avoid resizing in most tree-structured tasks, but grow rapidly
955 * until large. (Maintenance note: any changes in fields, queues,
956 * or their uses, or JVM layout policies, must be accompanied by
957 * re-evaluation of these placement and sizing decisions.)
958 *
959 * Style notes
960 * ===========
961 *
962 * Memory ordering relies mainly on atomic operations (CAS,
963 * getAndSet, getAndAdd) along with moded accesses. These use
964 * jdk-internal Unsafe for atomics and special memory modes,
965 * rather than VarHandles, to avoid initialization dependencies in
966 * other jdk components that require early parallelism. This can
967 * be awkward and ugly, but also reflects the need to control
968 * outcomes across the unusual cases that arise in very racy code
969 * with very few invariants. All atomic task slot updates use
970 * Unsafe operations requiring offset positions, not indices, as
971 * computed by method slotOffset. All fields are read into locals
972 * before use, and null-checked if they are references, even if
973 * they can never be null under current usages. Usually,
974 * computations (held in local variables) are defined as soon as
975 * logically enabled, sometimes to convince compilers that they
976 * may be performed despite memory ordering constraints. Array
977 * accesses using masked indices include checks (that are always
978 * true) that the array length is non-zero to avoid compilers
979 * inserting more expensive traps. This is usually done in a
980 * "C"-like style of listing declarations at the heads of methods
981 * or blocks, and using inline assignments on first encounter.
982 * Nearly all explicit checks lead to bypass/return, not exception
983 * throws, because they may legitimately arise during shutdown. A
984 * few unusual loop constructions encourage (with varying
985 * effectiveness) JVMs about where (not) to place safepoints. All
986 * public methods screen arguments (mainly null checks) before
987 * creating or executing tasks.
988 *
989 * There is a lot of representation-level coupling among classes
990 * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The
991 * fields of WorkQueue maintain data structures managed by
992 * ForkJoinPool, so are directly accessed. There is little point
993 * trying to reduce this, since any associated future changes in
994 * representations will need to be accompanied by algorithmic
995 * changes anyway. Several methods intrinsically sprawl because
996 * they must accumulate sets of consistent reads of fields held in
997 * local variables. Some others are artificially broken up to
998 * reduce producer/consumer imbalances due to dynamic compilation.
999 * There are also other coding oddities (including several
1000 * unnecessary-looking hoisted null checks) that help some methods
1001 * perform reasonably even when interpreted (not compiled).
1002 *
1003 * The order of declarations in this file is (with a few exceptions):
1004 * (1) Static configuration constants
1005 * (2) Static utility functions
1006 * (3) Nested (static) classes
1007 * (4) Fields, along with constants used when unpacking some of them
1008 * (5) Internal control methods
1009 * (6) Callbacks and other support for ForkJoinTask methods
1010 * (7) Exported methods
1011 * (8) Static block initializing statics in minimally dependent order
1012 *
1013 */
1014
1015 // static configuration constants
1016
1017 /**
1018 * Default idle timeout value (in milliseconds) for idle threads
1019 * to park waiting for new work before terminating.
1020 */
1021 static final long DEFAULT_KEEPALIVE = 60_000L;
1022
1023 /**
1024 * Undershoot tolerance for idle timeouts, also serving as the
1025 * minimum allowed timeout value.
1026 */
1027 static final long TIMEOUT_SLOP = 20L;
1028
1029 /**
1030 * The default value for common pool maxSpares. Overridable using
1031 * the "java.util.concurrent.ForkJoinPool.common.maximumSpares"
1032 * system property. The default value is far in excess of normal
1033 * requirements, but also far short of maximum capacity and typical OS
1034 * thread limits, so allows JVMs to catch misuse/abuse before
1035 * running out of resources needed to do so.
1036 */
1037 static final int DEFAULT_COMMON_MAX_SPARES = 256;
1038
1039 /**
1040 * Initial capacity of work-stealing queue array.
1041 * Must be a power of two, at least 2. See above.
1042 */
1043 static final int INITIAL_QUEUE_CAPACITY = 1 << 6;
1044
1045 // conversions among short, int, long
1046 static final int SMASK = 0xffff; // (unsigned) short bits
1047 static final long LMASK = 0xffffffffL; // lower 32 bits of long
1048 static final long UMASK = ~LMASK; // upper 32 bits
1049
1050 // masks and sentinels for queue indices
1051 static final int MAX_CAP = 0x7fff; // max # workers
1052 static final int EXTERNAL_ID_MASK = 0x3ffe; // max external queue id
1053 static final int INVALID_ID = 0x4000; // unused external queue id
1054
1055 // pool.runState bits
1056 static final long STOP = 1L << 0; // terminating
1057 static final long SHUTDOWN = 1L << 1; // terminate when quiescent
1058 static final long CLEANED = 1L << 2; // stopped and queues cleared
1059 static final long TERMINATED = 1L << 3; // only set if STOP also set
1060 static final long RS_LOCK = 1L << 4; // lowest seqlock bit
1061
1062 // spin/sleep limits for runState locking and elsewhere
1063 static final int SPIN_WAITS = 1 << 7; // max calls to onSpinWait
1064 static final int MIN_SLEEP = 1 << 10; // approx 1 usec as nanos
1065 static final int MAX_SLEEP = 1 << 20; // approx 1 sec as nanos
1066
1067 // {pool, workQueue} config bits
1068 static final int FIFO = 1 << 0; // fifo queue or access mode
1069 static final int CLEAR_TLS = 1 << 1; // set for Innocuous workers
1070 static final int PRESET_SIZE = 1 << 2; // size was set by property
1071
1072 // others
1073 static final int DROPPED = 1 << 16; // removed from ctl counts
1074 static final int UNCOMPENSATE = 1 << 16; // tryCompensate return
1075 static final int IDLE = 1 << 16; // phase seqlock/version count
1076 static final int MIN_QUEUES_SIZE = 1 << 4; // ensure external slots
1077
1078 /*
1079 * Bits and masks for ctl and bounds are packed with 4 16 bit subfields:
1080 * RC: Number of released (unqueued) workers
1081 * TC: Number of total workers
1082 * SS: version count and status of top waiting thread
1083 * ID: poolIndex of top of Treiber stack of waiters
1084 *
1085 * When convenient, we can extract the lower 32 stack top bits
1086 * (including version bits) as sp=(int)ctl. When sp is non-zero,
1087 * there are waiting workers. Count fields may be transiently
1088 * negative during termination because of out-of-order updates.
1089 * To deal with this, we use casts in and out of "short" and/or
1090 * signed shifts to maintain signedness. Because it occupies
1091 * uppermost bits, we can add one release count using getAndAdd of
1092 * RC_UNIT, rather than CAS, when returning from a blocked join.
1093 * Other updates of multiple subfields require CAS.
1094 */
1095
1096 // Release counts
1097 static final int RC_SHIFT = 48;
1098 static final long RC_UNIT = 0x0001L << RC_SHIFT;
1099 static final long RC_MASK = 0xffffL << RC_SHIFT;
1100 // Total counts
1101 static final int TC_SHIFT = 32;
1102 static final long TC_UNIT = 0x0001L << TC_SHIFT;
1103 static final long TC_MASK = 0xffffL << TC_SHIFT;
1104
1105 /*
1106 * All atomic operations on task arrays (queues) use Unsafe
1107 * operations that take array offsets versus indices, based on
1108 * array base and shift constants established during static
1109 * initialization.
1110 */
1111 static final long ABASE;
1112 static final int ASHIFT;
1113
1114 // Static utilities
1115
1116 /**
1117 * Returns the array offset corresponding to the given index for
1118 * Unsafe task queue operations
1119 */
1120 static long slotOffset(int index) {
1121 return ((long)index << ASHIFT) + ABASE;
1122 }
1123
1124 // Nested classes
1125
1126 /**
1127 * Factory for creating new {@link ForkJoinWorkerThread}s.
1128 * A {@code ForkJoinWorkerThreadFactory} must be defined and used
1129 * for {@code ForkJoinWorkerThread} subclasses that extend base
1130 * functionality or initialize threads with different contexts.
1131 */
1132 public static interface ForkJoinWorkerThreadFactory {
1133 /**
1134 * Returns a new worker thread operating in the given pool.
1135 * Returning null or throwing an exception may result in tasks
1136 * never being executed. If this method throws an exception,
1137 * it is relayed to the caller of the method (for example
1138 * {@code execute}) causing attempted thread creation. If this
1139 * method returns null or throws an exception, it is not
1140 * retried until the next attempted creation (for example
1141 * another call to {@code execute}).
1142 *
1143 * @param pool the pool this thread works in
1144 * @return the new worker thread, or {@code null} if the request
1145 * to create a thread is rejected
1146 * @throws NullPointerException if the pool is null
1147 */
1148 public ForkJoinWorkerThread newThread(ForkJoinPool pool);
1149 }
1150
1151 /**
1152 * Default ForkJoinWorkerThreadFactory implementation; creates a
1153 * new ForkJoinWorkerThread using the system class loader as the
1154 * thread context class loader.
1155 */
1156 static final class DefaultForkJoinWorkerThreadFactory
1157 implements ForkJoinWorkerThreadFactory {
1158 public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
1159 return ((pool.workerNamePrefix == null) ? // is commonPool
1160 new ForkJoinWorkerThread.InnocuousForkJoinWorkerThread(pool) :
1161 new ForkJoinWorkerThread(null, pool, true, false));
1162 }
1163 }
1164
1165 /**
1166 * Queues supporting work-stealing as well as external task
1167 * submission. See above for descriptions and algorithms.
1168 */
1169 static final class WorkQueue {
1170 // fields declared in order of their likely layout on most VMs
1171 final ForkJoinWorkerThread owner; // null if shared
1172 ForkJoinTask<?>[] array; // the queued tasks; power of 2 size
1173 int base; // index of next slot for poll
1174 final int config; // mode bits
1175
1176 // fields otherwise causing more unnecessary false-sharing cache misses
1177 @jdk.internal.vm.annotation.Contended("w")
1178 int top; // index of next slot for push
1179 @jdk.internal.vm.annotation.Contended("w")
1180 volatile int phase; // versioned active status
1181 @jdk.internal.vm.annotation.Contended("w")
1182 int stackPred; // pool stack (ctl) predecessor link
1183 @jdk.internal.vm.annotation.Contended("w")
1184 volatile int source; // source queue id (or DROPPED)
1185 @jdk.internal.vm.annotation.Contended("w")
1186 int nsteals; // number of steals from other queues
1187 @jdk.internal.vm.annotation.Contended("w")
1188 volatile int parking; // nonzero if parked in awaitWork
1189
1190 // Support for atomic operations
1191 private static final Unsafe U;
1192 private static final long PHASE;
1193 private static final long BASE;
1194 private static final long TOP;
1195 private static final long ARRAY;
1196
1197 final void updateBase(int v) {
1198 U.putIntVolatile(this, BASE, v);
1199 }
1200 final void updateTop(int v) {
1201 U.putIntOpaque(this, TOP, v);
1202 }
1203 final void updateArray(ForkJoinTask<?>[] a) {
1204 U.getAndSetReference(this, ARRAY, a);
1205 }
1206 final void unlockPhase() {
1207 U.getAndAddInt(this, PHASE, IDLE);
1208 }
1209 final boolean tryLockPhase() { // seqlock acquire
1210 int p;
1211 return (((p = phase) & IDLE) != 0 &&
1212 U.compareAndSetInt(this, PHASE, p, p + IDLE));
1213 }
1214
1215 /**
1216 * Constructor. For internal queues, most fields are initialized
1217 * upon thread start in pool.registerWorker.
1218 */
1219 WorkQueue(ForkJoinWorkerThread owner, int id, int cfg,
1220 boolean clearThreadLocals) {
1221 this.config = (clearThreadLocals) ? cfg | CLEAR_TLS : cfg;
1222 if ((this.owner = owner) == null) {
1223 phase = id | IDLE;
1224 array = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
1225 }
1226 }
1227
1228 /**
1229 * Returns an exportable index (used by ForkJoinWorkerThread).
1230 */
1231 final int getPoolIndex() {
1232 return (phase & 0xffff) >>> 1; // ignore odd/even tag bit
1233 }
1234
1235 /**
1236 * Returns the approximate number of tasks in the queue.
1237 */
1238 final int queueSize() {
1239 int unused = phase; // for ordering effect
1240 return Math.max(top - base, 0); // ignore transient negative
1241 }
1242
1243 /**
1244 * Pushes a task. Called only by owner or if already locked
1245 *
1246 * @param task the task; no-op if null
1247 * @param pool the pool to signal if was previously empty, else null
1248 * @param internal if caller owns this queue
1249 * @throws RejectedExecutionException if array could not be resized
1250 */
1251 final void push(ForkJoinTask<?> task, ForkJoinPool pool, boolean internal) {
1252 int s = top, b = base, m, cap, room; ForkJoinTask<?>[] a;
1253 if ((a = array) != null && (cap = a.length) > 0) { // else disabled
1254 if ((room = (m = cap - 1) - (s - b)) >= 0) {
1255 top = s + 1;
1256 long pos = slotOffset(m & s);
1257 if (!internal)
1258 U.putReference(a, pos, task); // inside lock
1259 else
1260 U.getAndSetReference(a, pos, task); // fully fenced
1261 if (room == 0 && (a = growArray(a, cap, s)) != null)
1262 m = a.length - 1; // resize
1263 }
1264 if (!internal)
1265 unlockPhase();
1266 if (room < 0)
1267 throw new RejectedExecutionException("Queue capacity exceeded");
1268 if (pool != null && a != null &&
1269 U.getReferenceAcquire(a, slotOffset(m & (s - 1))) == null)
1270 pool.signalWork(a, m & s); // may have appeared empty
1271 }
1272 }
1273
1274 /**
1275 * Resizes the queue array unless out of memory.
1276 * @param a old array
1277 * @param cap old array capacity
1278 * @param s current top
1279 * @return new array, or null on failure
1280 */
1281 private ForkJoinTask<?>[] growArray(ForkJoinTask<?>[] a, int cap, int s) {
1282 int newCap = (cap >= 1 << 16) ? cap << 1 : cap << 2;
1283 ForkJoinTask<?>[] newArray = null;
1284 if (a != null && a.length == cap && cap > 0 && newCap > 0) {
1285 try {
1286 newArray = new ForkJoinTask<?>[newCap];
1287 } catch (OutOfMemoryError ex) {
1288 }
1289 if (newArray != null) { // else throw on next push
1290 int mask = cap - 1, newMask = newCap - 1;
1291 for (int k = s, j = cap; j > 0; --j, --k) {
1292 ForkJoinTask<?> u; // poll old, push to new
1293 if ((u = (ForkJoinTask<?>)U.getAndSetReference(
1294 a, slotOffset(k & mask), null)) == null)
1295 break; // lost to pollers
1296 newArray[k & newMask] = u;
1297 }
1298 updateArray(newArray); // fully fenced
1299 }
1300 }
1301 return newArray;
1302 }
1303
1304 /**
1305 * Takes next task, if one exists, in order specified by mode,
1306 * so acts as either local-pop or local-poll. Called only by owner.
1307 * @param fifo nonzero if FIFO mode
1308 */
1309 private ForkJoinTask<?> nextLocalTask(int fifo) {
1310 ForkJoinTask<?> t = null;
1311 ForkJoinTask<?>[] a = array;
1312 int b = base, p = top, cap;
1313 if (p - b > 0 && a != null && (cap = a.length) > 0) {
1314 for (int m = cap - 1, s, nb;;) {
1315 if (fifo == 0 || (nb = b + 1) == p) {
1316 if ((t = (ForkJoinTask<?>)U.getAndSetReference(
1317 a, slotOffset(m & (s = p - 1)), null)) != null)
1318 updateTop(s); // else lost race for only task
1319 break;
1320 }
1321 if ((t = (ForkJoinTask<?>)U.getAndSetReference(
1322 a, slotOffset(m & b), null)) != null) {
1323 updateBase(nb);
1324 break;
1325 }
1326 while (b == (b = U.getIntAcquire(this, BASE)))
1327 Thread.onSpinWait(); // spin to reduce memory traffic
1328 if (p - b <= 0)
1329 break;
1330 }
1331 }
1332 return t;
1333 }
1334
1335 /**
1336 * Takes next task, if one exists, using configured mode.
1337 * (Always internal, never called for Common pool.)
1338 */
1339 final ForkJoinTask<?> nextLocalTask() {
1340 return nextLocalTask(config & FIFO);
1341 }
1342
1343 /**
1344 * Pops the given task only if it is at the current top.
1345 * @param task the task. Caller must ensure non-null.
1346 * @param internal if caller owns this queue
1347 */
1348 final boolean tryUnpush(ForkJoinTask<?> task, boolean internal) {
1349 boolean taken = false;
1350 ForkJoinTask<?>[] a = array;
1351 int p = top, s = p - 1, cap; long k;
1352 if (a != null && (cap = a.length) > 0 &&
1353 U.getReference(a, k = slotOffset((cap - 1) & s)) == task &&
1354 (internal || tryLockPhase())) {
1355 if (top == p && U.compareAndSetReference(a, k, task, null)) {
1356 taken = true;
1357 updateTop(s);
1358 }
1359 if (!internal)
1360 unlockPhase();
1361 }
1362 return taken;
1363 }
1364
1365 /**
1366 * Returns next task, if one exists, in order specified by mode.
1367 */
1368 final ForkJoinTask<?> peek() {
1369 ForkJoinTask<?>[] a = array;
1370 int b = base, cfg = config, p = top, cap;
1371 if (p != b && a != null && (cap = a.length) > 0) {
1372 if ((cfg & FIFO) == 0)
1373 return a[(cap - 1) & (p - 1)];
1374 else { // skip over in-progress removals
1375 ForkJoinTask<?> t;
1376 for ( ; p - b > 0; ++b) {
1377 if ((t = a[(cap - 1) & b]) != null)
1378 return t;
1379 }
1380 }
1381 }
1382 return null;
1383 }
1384
1385 /**
1386 * Polls for a task. Used only by non-owners.
1387 */
1388 final ForkJoinTask<?> poll() {
1389 for (int pb = -1, b; ; pb = b) { // track progress
1390 ForkJoinTask<?> t; int cap, nb; long k; ForkJoinTask<?>[] a;
1391 if ((a = array) == null || (cap = a.length) <= 0)
1392 break;
1393 t = (ForkJoinTask<?>)U.getReferenceAcquire(
1394 a, k = slotOffset((cap - 1) & (b = base)));
1395 Object u = U.getReference( // next slot
1396 a, slotOffset((cap - 1) & (nb = b + 1)));
1397 if (base != b) // inconsistent
1398 ;
1399 else if (t == null) {
1400 if (u == null && top - b <= 0)
1401 break; // empty
1402 if (pb == b)
1403 Thread.onSpinWait(); // stalled
1404 }
1405 else if (U.compareAndSetReference(a, k, t, null)) {
1406 updateBase(nb);
1407 return t;
1408 }
1409 }
1410 return null;
1411 }
1412
1413 // specialized execution methods
1414
1415 /**
1416 * Runs the given task, as well as remaining local tasks.
1417 */
1418 final void topLevelExec(ForkJoinTask<?> task, int fifo) {
1419 while (task != null) {
1420 task.doExec();
1421 task = nextLocalTask(fifo);
1422 }
1423 }
1424
1425 /**
1426 * Deep form of tryUnpush: Traverses from top and removes and
1427 * runs task if present.
1428 */
1429 final void tryRemoveAndExec(ForkJoinTask<?> task, boolean internal) {
1430 ForkJoinTask<?>[] a = array;
1431 int b = base, p = top, s = p - 1, d = p - b, cap;
1432 if (a != null && (cap = a.length) > 0) {
1433 for (int m = cap - 1, i = s; d > 0; --i, --d) {
1434 long k; boolean taken;
1435 ForkJoinTask<?> t = (ForkJoinTask<?>)U.getReference(
1436 a, k = slotOffset(i & m));
1437 if (t == null)
1438 break;
1439 if (t == task) {
1440 if (!internal && !tryLockPhase())
1441 break; // fail if locked
1442 if (taken =
1443 (top == p &&
1444 U.compareAndSetReference(a, k, task, null))) {
1445 if (i == s) // act as pop
1446 updateTop(s);
1447 else if (i == base) // act as poll
1448 updateBase(i + 1);
1449 else { // swap with top
1450 U.putReferenceVolatile(
1451 a, k, (ForkJoinTask<?>)
1452 U.getAndSetReference(
1453 a, slotOffset(s & m), null));
1454 updateTop(s);
1455 }
1456 }
1457 if (!internal)
1458 unlockPhase();
1459 if (taken)
1460 task.doExec();
1461 break;
1462 }
1463 }
1464 }
1465 }
1466
1467 /**
1468 * Tries to pop and run tasks within the target's computation
1469 * until done, not found, or limit exceeded.
1470 *
1471 * @param task root of computation
1472 * @param limit max runs, or zero for no limit
1473 * @return task status if known to be done
1474 */
1475 final int helpComplete(ForkJoinTask<?> task, boolean internal, int limit) {
1476 int status = 0;
1477 if (task != null) {
1478 outer: for (;;) {
1479 ForkJoinTask<?>[] a; boolean taken; Object o;
1480 int stat, p, s, cap;
1481 if ((stat = task.status) < 0) {
1482 status = stat;
1483 break;
1484 }
1485 if ((a = array) == null || (cap = a.length) <= 0)
1486 break;
1487 long k = slotOffset((cap - 1) & (s = (p = top) - 1));
1488 if (!((o = U.getReference(a, k)) instanceof CountedCompleter))
1489 break;
1490 CountedCompleter<?> t = (CountedCompleter<?>)o, f = t;
1491 for (int steps = cap;;) { // bound path
1492 if (f == task)
1493 break;
1494 if ((f = f.completer) == null || --steps == 0)
1495 break outer;
1496 }
1497 if (!internal && !tryLockPhase())
1498 break;
1499 if (taken =
1500 (top == p &&
1501 U.compareAndSetReference(a, k, t, null)))
1502 updateTop(s);
1503 if (!internal)
1504 unlockPhase();
1505 if (!taken)
1506 break;
1507 t.doExec();
1508 if (limit != 0 && --limit == 0)
1509 break;
1510 }
1511 }
1512 return status;
1513 }
1514
1515 /**
1516 * Tries to poll and run AsynchronousCompletionTasks until
1517 * none found or blocker is released
1518 *
1519 * @param blocker the blocker
1520 */
1521 final void helpAsyncBlocker(ManagedBlocker blocker) {
1522 for (;;) {
1523 ForkJoinTask<?> t; ForkJoinTask<?>[] a; int b, cap; long k;
1524 if ((a = array) == null || (cap = a.length) <= 0)
1525 break;
1526 t = (ForkJoinTask<?>)U.getReferenceAcquire(
1527 a, k = slotOffset((cap - 1) & (b = base)));
1528 if (t == null) {
1529 if (top - b <= 0)
1530 break;
1531 }
1532 else if (!(t instanceof CompletableFuture
1533 .AsynchronousCompletionTask))
1534 break;
1535 if (blocker != null && blocker.isReleasable())
1536 break;
1537 if (base == b && t != null &&
1538 U.compareAndSetReference(a, k, t, null)) {
1539 updateBase(b + 1);
1540 t.doExec();
1541 }
1542 }
1543 }
1544
1545 // misc
1546
1547 /**
1548 * Cancels all local tasks. Called only by owner.
1549 */
1550 final void cancelTasks() {
1551 for (ForkJoinTask<?> t; (t = nextLocalTask(0)) != null; ) {
1552 try {
1553 t.cancel(false);
1554 } catch (Throwable ignore) {
1555 }
1556 }
1557 }
1558
1559 /**
1560 * Returns true if internal and not known to be blocked.
1561 */
1562 final boolean isApparentlyUnblocked() {
1563 Thread wt; Thread.State s;
1564 return ((wt = owner) != null && (phase & IDLE) != 0 &&
1565 (s = wt.getState()) != Thread.State.BLOCKED &&
1566 s != Thread.State.WAITING &&
1567 s != Thread.State.TIMED_WAITING);
1568 }
1569
1570 static {
1571 U = Unsafe.getUnsafe();
1572 Class<WorkQueue> klass = WorkQueue.class;
1573 PHASE = U.objectFieldOffset(klass, "phase");
1574 BASE = U.objectFieldOffset(klass, "base");
1575 TOP = U.objectFieldOffset(klass, "top");
1576 ARRAY = U.objectFieldOffset(klass, "array");
1577 }
1578 }
1579
1580 // static fields (initialized in static initializer below)
1581
1582 /**
1583 * Creates a new ForkJoinWorkerThread. This factory is used unless
1584 * overridden in ForkJoinPool constructors.
1585 */
1586 public static final ForkJoinWorkerThreadFactory
1587 defaultForkJoinWorkerThreadFactory;
1588
1589 /**
1590 * Common (static) pool. Non-null for public use unless a static
1591 * construction exception, but internal usages null-check on use
1592 * to paranoically avoid potential initialization circularities
1593 * as well as to simplify generated code.
1594 */
1595 static final ForkJoinPool common;
1596
1597 /**
1598 * Sequence number for creating worker names
1599 */
1600 private static volatile int poolIds;
1601
1602 /**
1603 * For VirtualThread intrinsics
1604 */
1605 private static final JavaLangAccess JLA;
1606
1607 // fields declared in order of their likely layout on most VMs
1608 volatile CountDownLatch termination; // lazily constructed
1609 final Predicate<? super ForkJoinPool> saturate;
1610 final ForkJoinWorkerThreadFactory factory;
1611 final UncaughtExceptionHandler ueh; // per-worker UEH
1612 final SharedThreadContainer container;
1613 final String workerNamePrefix; // null for common pool
1614 final String poolName;
1615 volatile DelayScheduler delayScheduler; // lazily constructed
1616 WorkQueue[] queues; // main registry
1617 volatile long runState; // versioned, lockable
1618 final long keepAlive; // milliseconds before dropping if idle
1619 final long config; // static configuration bits
1620 volatile long stealCount; // collects worker nsteals
1621 volatile long threadIds; // for worker thread names
1622
1623 @jdk.internal.vm.annotation.Contended("fjpctl") // segregate
1624 volatile long ctl; // main pool control
1625 @jdk.internal.vm.annotation.Contended("fjpctl") // colocate
1626 int parallelism; // target number of workers
1627
1628 // Support for atomic operations
1629 private static final Unsafe U;
1630 private static final long CTL;
1631 private static final long RUNSTATE;
1632 private static final long PARALLELISM;
1633 private static final long THREADIDS;
1634 private static final long TERMINATION;
1635 private static final Object POOLIDS_BASE;
1636 private static final long POOLIDS;
1637
1638 private boolean compareAndSetCtl(long c, long v) {
1639 return U.compareAndSetLong(this, CTL, c, v);
1640 }
1641 private long compareAndExchangeCtl(long c, long v) {
1642 return U.compareAndExchangeLong(this, CTL, c, v);
1643 }
1644 private long getAndAddCtl(long v) {
1645 return U.getAndAddLong(this, CTL, v);
1646 }
1647 private long incrementThreadIds() {
1648 return U.getAndAddLong(this, THREADIDS, 1L);
1649 }
1650 private static int getAndAddPoolIds(int x) {
1651 return U.getAndAddInt(POOLIDS_BASE, POOLIDS, x);
1652 }
1653 private int getAndSetParallelism(int v) {
1654 return U.getAndSetInt(this, PARALLELISM, v);
1655 }
1656 private int getParallelismOpaque() {
1657 return U.getIntOpaque(this, PARALLELISM);
1658 }
1659 private CountDownLatch cmpExTerminationSignal(CountDownLatch x) {
1660 return (CountDownLatch)
1661 U.compareAndExchangeReference(this, TERMINATION, null, x);
1662 }
1663
1664 // runState operations
1665
1666 private long getAndBitwiseOrRunState(long v) { // for status bits
1667 return U.getAndBitwiseOrLong(this, RUNSTATE, v);
1668 }
1669 private boolean casRunState(long c, long v) {
1670 return U.compareAndSetLong(this, RUNSTATE, c, v);
1671 }
1672 private void unlockRunState() { // increment lock bit
1673 U.getAndAddLong(this, RUNSTATE, RS_LOCK);
1674 }
1675 private long lockRunState() { // lock and return current state
1676 long s, u; // locked when RS_LOCK set
1677 if (((s = runState) & RS_LOCK) == 0L && casRunState(s, u = s + RS_LOCK))
1678 return u;
1679 else
1680 return spinLockRunState();
1681 }
1682 private long spinLockRunState() { // spin/sleep
1683 for (int waits = 0;;) {
1684 long s, u;
1685 if (((s = runState) & RS_LOCK) == 0L) {
1686 if (casRunState(s, u = s + RS_LOCK))
1687 return u;
1688 waits = 0;
1689 } else if (waits < SPIN_WAITS) {
1690 ++waits;
1691 Thread.onSpinWait();
1692 } else {
1693 if (waits < MIN_SLEEP)
1694 waits = MIN_SLEEP;
1695 LockSupport.parkNanos(this, (long)waits);
1696 if (waits < MAX_SLEEP)
1697 waits <<= 1;
1698 }
1699 }
1700 }
1701
1702 static boolean poolIsStopping(ForkJoinPool p) { // Used by ForkJoinTask
1703 return p != null && (p.runState & STOP) != 0L;
1704 }
1705
1706 // Creating, registering, and deregistering workers
1707
1708 /**
1709 * Tries to construct and start one worker. Assumes that total
1710 * count has already been incremented as a reservation. Invokes
1711 * deregisterWorker on any failure.
1712 *
1713 * @return true if successful
1714 */
1715 private boolean createWorker() {
1716 ForkJoinWorkerThreadFactory fac = factory;
1717 SharedThreadContainer ctr = container;
1718 Throwable ex = null;
1719 ForkJoinWorkerThread wt = null;
1720 try {
1721 if ((runState & STOP) == 0L && // avoid construction if terminating
1722 fac != null && (wt = fac.newThread(this)) != null) {
1723 if (ctr != null)
1724 ctr.start(wt);
1725 else
1726 wt.start();
1727 return true;
1728 }
1729 } catch (Throwable rex) {
1730 ex = rex;
1731 }
1732 deregisterWorker(wt, ex);
1733 return false;
1734 }
1735
1736 /**
1737 * Provides a name for ForkJoinWorkerThread constructor.
1738 */
1739 final String nextWorkerThreadName() {
1740 String prefix = workerNamePrefix;
1741 long tid = incrementThreadIds() + 1L;
1742 if (prefix == null) // commonPool has no prefix
1743 prefix = "ForkJoinPool.commonPool-worker-";
1744 return prefix.concat(Long.toString(tid));
1745 }
1746
1747 /**
1748 * Finishes initializing and records internal queue.
1749 *
1750 * @param w caller's WorkQueue
1751 */
1752 final void registerWorker(WorkQueue w) {
1753 if (w != null) {
1754 w.array = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
1755 ThreadLocalRandom.localInit();
1756 int seed = w.stackPred = ThreadLocalRandom.getProbe();
1757 int phaseSeq = seed & ~((IDLE << 1) - 1); // initial phase tag
1758 int id = ((seed << 1) | 1) & SMASK; // base of linear-probe-like scan
1759 long stop = lockRunState() & STOP;
1760 try {
1761 WorkQueue[] qs; int n;
1762 if (stop == 0L && (qs = queues) != null && (n = qs.length) > 0) {
1763 for (int k = n, m = n - 1; ; id += 2) {
1764 if (qs[id &= m] == null)
1765 break;
1766 if ((k -= 2) <= 0) {
1767 id |= n;
1768 break;
1769 }
1770 }
1771 w.phase = id | phaseSeq; // now publishable
1772 if (id < n)
1773 qs[id] = w;
1774 else { // expand
1775 int an = n << 1, am = an - 1;
1776 WorkQueue[] as = new WorkQueue[an];
1777 as[id & am] = w;
1778 for (int j = 1; j < n; j += 2)
1779 as[j] = qs[j];
1780 for (int j = 0; j < n; j += 2) {
1781 WorkQueue q; // shared queues may move
1782 if ((q = qs[j]) != null)
1783 as[q.phase & EXTERNAL_ID_MASK & am] = q;
1784 }
1785 U.storeFence(); // fill before publish
1786 queues = as;
1787 }
1788 }
1789 } finally {
1790 unlockRunState();
1791 }
1792 }
1793 }
1794
1795 /**
1796 * Final callback from terminating worker, as well as upon failure
1797 * to construct or start a worker. Removes record of worker from
1798 * array, and adjusts counts. If pool is shutting down, tries to
1799 * complete termination.
1800 *
1801 * @param wt the worker thread, or null if construction failed
1802 * @param ex the exception causing failure, or null if none
1803 */
1804 final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
1805 WorkQueue w = null; // null if not created
1806 int phase = 0; // 0 if not registered
1807 if (wt != null && (w = wt.workQueue) != null &&
1808 (phase = w.phase) != 0 && (phase & IDLE) != 0)
1809 releaseWaiters(); // ensure released
1810 if (w == null || w.source != DROPPED) {
1811 long c = ctl; // decrement counts
1812 do {} while (c != (c = compareAndExchangeCtl(
1813 c, ((RC_MASK & (c - RC_UNIT)) |
1814 (TC_MASK & (c - TC_UNIT)) |
1815 (LMASK & c)))));
1816 }
1817 if (phase != 0 && w != null) { // remove index unless terminating
1818 long ns = w.nsteals & 0xffffffffL;
1819 if ((runState & STOP) == 0L) {
1820 WorkQueue[] qs; int n, i;
1821 if ((lockRunState() & STOP) == 0L &&
1822 (qs = queues) != null && (n = qs.length) > 0 &&
1823 qs[i = phase & SMASK & (n - 1)] == w) {
1824 qs[i] = null;
1825 stealCount += ns; // accumulate steals
1826 }
1827 unlockRunState();
1828 }
1829 }
1830 if ((tryTerminate(false, false) & STOP) == 0L &&
1831 phase != 0 && w != null && w.source != DROPPED) {
1832 w.cancelTasks(); // clean queue
1833 signalWork(null, 0); // possibly replace
1834 }
1835 if (ex != null)
1836 ForkJoinTask.rethrow(ex);
1837 }
1838
1839 /**
1840 * Releases an idle worker, or creates one if not enough exist,
1841 * giving up if array a is nonnull and task at a[k] already taken.
1842 */
1843 final void signalWork(ForkJoinTask<?>[] a, int k) {
1844 int pc = parallelism;
1845 for (long c = ctl;;) {
1846 WorkQueue[] qs = queues;
1847 long ac = (c + RC_UNIT) & RC_MASK, nc;
1848 int sp = (int)c, i = sp & SMASK;
1849 if ((short)(c >>> RC_SHIFT) >= pc)
1850 break;
1851 if (qs == null)
1852 break;
1853 if (qs.length <= i)
1854 break;
1855 WorkQueue w = qs[i], v = null;
1856 if (sp == 0) {
1857 if ((short)(c >>> TC_SHIFT) >= pc)
1858 break;
1859 nc = ((c + TC_UNIT) & TC_MASK) | ac;
1860 }
1861 else if ((v = w) == null)
1862 break;
1863 else
1864 nc = (v.stackPred & LMASK) | (c & TC_MASK) | ac;
1865 if (a != null && k < a.length && k >= 0 && a[k] == null)
1866 break;
1867 if (c == (c = ctl) && c == (c = compareAndExchangeCtl(c, nc))) {
1868 if (v == null)
1869 createWorker();
1870 else {
1871 v.phase = sp;
1872 if (v.parking != 0)
1873 U.unpark(v.owner);
1874 }
1875 break;
1876 }
1877 }
1878 }
1879
1880 /**
1881 * Releases all waiting workers. Called only during shutdown.
1882 */
1883 private void releaseWaiters() {
1884 for (long c = ctl;;) {
1885 WorkQueue[] qs; WorkQueue v; int sp, i;
1886 if ((sp = (int)c) == 0 || (qs = queues) == null ||
1887 qs.length <= (i = sp & SMASK) || (v = qs[i]) == null)
1888 break;
1889 if (c == (c = compareAndExchangeCtl(
1890 c, ((UMASK & (c + RC_UNIT)) | (c & TC_MASK) |
1891 (v.stackPred & LMASK))))) {
1892 v.phase = sp;
1893 if (v.parking != 0)
1894 U.unpark(v.owner);
1895 }
1896 }
1897 }
1898
1899 /**
1900 * Internal version of isQuiescent and related functionality.
1901 * @return positive if stopping, nonnegative if terminating or all
1902 * workers are inactive and submission queues are empty and
1903 * unlocked; if so, setting STOP if shutdown is enabled
1904 */
1905 private int quiescent() {
1906 for (;;) {
1907 long phaseSum = 0L;
1908 boolean swept = false;
1909 for (long e, prevRunState = 0L; ; prevRunState = e) {
1910 DelayScheduler ds;
1911 long c = ctl;
1912 if (((e = runState) & STOP) != 0L)
1913 return 1; // terminating
1914 else if ((c & RC_MASK) > 0L)
1915 return -1; // at least one active
1916 else if (!swept || e != prevRunState || (e & RS_LOCK) != 0) {
1917 long sum = c;
1918 WorkQueue[] qs = queues;
1919 int n = (qs == null) ? 0 : qs.length;
1920 for (int i = 0; i < n; ++i) { // scan queues
1921 WorkQueue q;
1922 if ((q = qs[i]) != null) {
1923 int p = q.phase, s = q.top, b = q.base;
1924 sum += (p & 0xffffffffL) | ((long)b << 32);
1925 if ((p & IDLE) == 0 || s - b > 0)
1926 return -1;
1927 }
1928 }
1929 swept = (phaseSum == (phaseSum = sum));
1930 }
1931 else if ((e & SHUTDOWN) == 0)
1932 return 0;
1933 else if ((ds = delayScheduler) != null && !ds.canShutDown())
1934 return 0;
1935 else if (compareAndSetCtl(c, c) && casRunState(e, e | STOP))
1936 return 1; // enable termination
1937 else
1938 break; // restart
1939 }
1940 }
1941 }
1942
1943 /**
1944 * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
1945 * See above for explanation.
1946 *
1947 * @param w caller's WorkQueue (may be null on failed initialization)
1948 */
1949 final void runWorker(WorkQueue w) {
1950 if (w != null) {
1951 int phase = w.phase;
1952 int r = w.stackPred, origin = r; // seed from registerWorker
1953 int cfg = w.config, fifo = cfg & FIFO, clearLocals = cfg & CLEAR_TLS;
1954 int src = -1; // current source queue
1955 int taken = 0, ptaken = 0, staken = 0; // takes per phase and scan
1956 rescan: while ((runState & STOP) == 0L) {
1957 WorkQueue[] qs = queues;
1958 int n = (qs == null) ? 0 : qs.length;
1959 int i = origin, step = (r >>> 16) | 1;
1960 r ^= r << 13; r ^= r >>> 17; origin = r ^= r << 5; // xorshift
1961 for (int l = n; l > 0; --l, i += step) { // scan queues
1962 WorkQueue q; int j;
1963 if ((q = qs[j = i & (n - 1)]) != null) {
1964 for (;;) { // poll q
1965 ForkJoinTask<?>[] a; int cap, b, m, k;
1966 if ((a = q.array) == null || (cap = a.length) <= 0)
1967 break;
1968 long bp = slotOffset(k = (b = q.base) & (m = cap - 1));
1969 int nb = b + 1, nk = nb & m;
1970 ForkJoinTask<?> t = (ForkJoinTask<?>)
1971 U.getReferenceAcquire(a, bp);
1972 if (q.array != a || q.base != b || a[k] != t)
1973 continue; // inconsistent
1974 if (t == null) {
1975 if (taken != staken) {
1976 staken = taken;
1977 continue rescan; // sweep until clean
1978 }
1979 if (a[nk] != null || a[(b + 2) & m] != null)
1980 continue rescan; // stalled; reorder scan
1981 break; // probably empty
1982 }
1983 if (U.compareAndSetReference(a, bp, t, null)) {
1984 q.base = nb;
1985 Object nt = U.getReferenceAcquire
1986 (a, slotOffset(nk)); // confirm below
1987 ++taken;
1988 if (src != j)
1989 w.source = src = j;
1990 if (nt != null && nt == a[nk])
1991 signalWork(a, nk); // propagate
1992 w.topLevelExec(t, fifo); // run t & its subtasks
1993 }
1994 }
1995 }
1996 }
1997 if (taken != ptaken) { // end run
1998 ptaken = taken;
1999 origin = src; // hint for next run
2000 if (clearLocals != 0 &&
2001 Thread.currentThread() instanceof ForkJoinWorkerThread wt)
2002 wt.resetThreadLocals();
2003 w.nsteals = taken;
2004 }
2005 w.phase = phase += IDLE; // deactivate
2006 if ((phase = awaitWork(w, phase)) == IDLE)
2007 break;
2008 }
2009 }
2010 }
2011
2012 /**
2013 * Awaits signal or termination.
2014 *
2015 * @param w the work queue
2016 * @param p current phase (known to be idle
2017 * @return current phase or IDLE if worker should exit
2018 */
2019 private int awaitWork(WorkQueue w, int p) {
2020 if (w == null) // never true; hoist checks
2021 return IDLE;
2022 int activePhase = p + IDLE;
2023 long ap = activePhase & LMASK, pc = ctl, qc;
2024 do { // enqueue
2025 qc = ap | ((pc - RC_UNIT) & UMASK);
2026 w.stackPred = (int)pc; // set ctl stack link
2027 } while (pc != (pc = compareAndExchangeCtl(pc, qc)));
2028 long psp = pc & LMASK; // reactivation stack prefix
2029 WorkQueue[] qs; int n; // missed signal check
2030 if ((runState & STOP) != 0 || (qs = queues) == null || (n = qs.length) <= 0)
2031 return IDLE; // already terminating
2032 for (int m = n - 1, origin = p + 1, i = 0; i < m; ++i) {
2033 WorkQueue q; long cc; // stagger origins
2034 if ((q = qs[(origin + i) & m]) != null && q.top - q.base > 0) {
2035 if ((p = w.phase) == activePhase)
2036 break;
2037 if ((int)(cc = ctl) == activePhase &&
2038 compareAndSetCtl(cc, psp | ((cc + RC_UNIT) & UMASK))) {
2039 p = w.phase = activePhase;
2040 break; // reactivated
2041 }
2042 }
2043 }
2044 if (p != activePhase && (p = w.phase) != activePhase) {
2045 long deadline = 0L, c, e; // quiescence checks
2046 if (((e = runState) & STOP) != 0)
2047 return IDLE;
2048 else if ((int)(c = ctl) != activePhase || (c & RC_MASK) > 0L) {
2049 for (int spins = n; (p = w.phase) != activePhase && --spins > 0;)
2050 Thread.onSpinWait(); // spin unless possibly quiescent
2051 }
2052 else if ((e & SHUTDOWN) != 0L && quiescent() > 0)
2053 return IDLE; // quiescent termination
2054 else { // use trim timeout
2055 long d = ((w.source != INVALID_ID) ? keepAlive :
2056 TIMEOUT_SLOP) + System.currentTimeMillis();
2057 deadline = (d == 0L)? 1L : d; // avoid zero
2058 p = w.phase;
2059 }
2060 if (p != activePhase) { // block
2061 LockSupport.setCurrentBlocker(this);
2062 w.parking = 1; // enable unpark
2063 while ((p = w.phase) != activePhase) {
2064 boolean trimmable = false; int trim;
2065 Thread.interrupted(); // clear status
2066 if ((runState & STOP) != 0L)
2067 break;
2068 if (deadline != 0L) {
2069 if ((trim = tryTrim(w, p, deadline)) > 0)
2070 break;
2071 else if (trim < 0)
2072 deadline = 0L;
2073 else
2074 trimmable = true;
2075 }
2076 U.park(trimmable, deadline);
2077 }
2078 w.parking = 0;
2079 LockSupport.setCurrentBlocker(null);
2080 if (p != activePhase)
2081 return IDLE;
2082 }
2083 }
2084 return activePhase;
2085 }
2086
2087 /**
2088 * Tries to remove and deregister worker after timeout, and release
2089 * another to do the same.
2090 * @return > 0: trimmed, < 0 : not trimmable, else 0
2091 */
2092 private int tryTrim(WorkQueue w, int phase, long deadline) {
2093 long c, nc; int stat, activePhase, vp, i; WorkQueue[] vs; WorkQueue v;
2094 if ((activePhase = phase + IDLE) != (int)(c = ctl) || w == null)
2095 stat = -1; // no longer ctl top
2096 else if (deadline - System.currentTimeMillis() >= TIMEOUT_SLOP)
2097 stat = 0; // spurious wakeup
2098 else if (!compareAndSetCtl(
2099 c, nc = ((w.stackPred & LMASK) | (RC_MASK & c) |
2100 (TC_MASK & (c - TC_UNIT)))))
2101 stat = -1; // lost race to signaller
2102 else {
2103 stat = 1;
2104 w.source = DROPPED;
2105 w.phase = activePhase;
2106 if ((vp = (int)nc) != 0 && (vs = queues) != null &&
2107 vs.length > (i = vp & SMASK) && (v = vs[i]) != null &&
2108 compareAndSetCtl( // try to wake up next waiter
2109 nc, ((UMASK & (nc + RC_UNIT)) |
2110 (nc & TC_MASK) | (v.stackPred & LMASK)))) {
2111 v.source = INVALID_ID; // enable cascaded timeouts
2112 v.phase = vp;
2113 U.unpark(v.owner);
2114 }
2115 }
2116 return stat;
2117 }
2118
2119 /**
2120 * Scans for and returns a polled task, if available. Used only
2121 * for untracked polls. Begins scan at a random index to avoid
2122 * systematic unfairness.
2123 *
2124 * @param submissionsOnly if true, only scan submission queues
2125 */
2126 private ForkJoinTask<?> pollScan(boolean submissionsOnly) {
2127 if ((runState & STOP) == 0L) {
2128 WorkQueue[] qs; int n; WorkQueue q; ForkJoinTask<?> t;
2129 int r = ThreadLocalRandom.nextSecondarySeed();
2130 if (submissionsOnly) // even indices only
2131 r &= ~1;
2132 int step = (submissionsOnly) ? 2 : 1;
2133 if ((qs = queues) != null && (n = qs.length) > 0) {
2134 for (int i = n; i > 0; i -= step, r += step) {
2135 if ((q = qs[r & (n - 1)]) != null &&
2136 (t = q.poll()) != null)
2137 return t;
2138 }
2139 }
2140 }
2141 return null;
2142 }
2143
2144 /**
2145 * Tries to decrement counts (sometimes implicitly) and possibly
2146 * arrange for a compensating worker in preparation for
2147 * blocking. May fail due to interference, in which case -1 is
2148 * returned so caller may retry. A zero return value indicates
2149 * that the caller doesn't need to re-adjust counts when later
2150 * unblocked.
2151 *
2152 * @param c incoming ctl value
2153 * @return UNCOMPENSATE: block then adjust, 0: block, -1 : retry
2154 */
2155 private int tryCompensate(long c) {
2156 Predicate<? super ForkJoinPool> sat;
2157 long b = config;
2158 int pc = parallelism, // unpack fields
2159 minActive = (short)(b >>> RC_SHIFT),
2160 maxTotal = (short)(b >>> TC_SHIFT) + pc,
2161 active = (short)(c >>> RC_SHIFT),
2162 total = (short)(c >>> TC_SHIFT),
2163 sp = (int)c,
2164 stat = -1; // default retry return
2165 if (sp != 0 && active <= pc) { // activate idle worker
2166 WorkQueue[] qs; WorkQueue v; int i;
2167 if ((qs = queues) != null && qs.length > (i = sp & SMASK) &&
2168 (v = qs[i]) != null &&
2169 compareAndSetCtl(c, (c & UMASK) | (v.stackPred & LMASK))) {
2170 v.phase = sp;
2171 if (v.parking != 0)
2172 U.unpark(v.owner);
2173 stat = UNCOMPENSATE;
2174 }
2175 }
2176 else if (active > minActive && total >= pc) { // reduce active workers
2177 if (compareAndSetCtl(c, ((c - RC_UNIT) & RC_MASK) | (c & ~RC_MASK)))
2178 stat = UNCOMPENSATE;
2179 }
2180 else if (total < maxTotal && total < MAX_CAP) { // try to expand pool
2181 long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
2182 if ((runState & STOP) != 0L) // terminating
2183 stat = 0;
2184 else if (compareAndSetCtl(c, nc))
2185 stat = createWorker() ? UNCOMPENSATE : 0;
2186 }
2187 else if (!compareAndSetCtl(c, c)) // validate
2188 ;
2189 else if ((sat = saturate) != null && sat.test(this))
2190 stat = 0;
2191 else
2192 throw new RejectedExecutionException(
2193 "Thread limit exceeded replacing blocked worker");
2194 return stat;
2195 }
2196
2197 /**
2198 * Readjusts RC count; called from ForkJoinTask after blocking.
2199 */
2200 final void uncompensate() {
2201 getAndAddCtl(RC_UNIT);
2202 }
2203
2204 /**
2205 * Helps if possible until the given task is done. Processes
2206 * compatible local tasks and scans other queues for task produced
2207 * by w's stealers; returning compensated blocking sentinel if
2208 * none are found.
2209 *
2210 * @param task the task
2211 * @param w caller's WorkQueue
2212 * @param internal true if w is owned by a ForkJoinWorkerThread
2213 * @return task status on exit, or UNCOMPENSATE for compensated blocking
2214 */
2215 final int helpJoin(ForkJoinTask<?> task, WorkQueue w, boolean internal) {
2216 if (w != null)
2217 w.tryRemoveAndExec(task, internal);
2218 int s = 0;
2219 if (task != null && (s = task.status) >= 0 && internal && w != null) {
2220 int wid = w.phase & SMASK, r = wid + 2, wsrc = w.source;
2221 long sctl = 0L; // track stability
2222 outer: for (boolean rescan = true;;) {
2223 if ((s = task.status) < 0)
2224 break;
2225 if (!rescan) {
2226 if ((runState & STOP) != 0L)
2227 break;
2228 if (sctl == (sctl = ctl) && (s = tryCompensate(sctl)) >= 0)
2229 break;
2230 }
2231 rescan = false;
2232 WorkQueue[] qs = queues;
2233 int n = (qs == null) ? 0 : qs.length;
2234 scan: for (int l = n >>> 1; l > 0; --l, r += 2) {
2235 int j; WorkQueue q;
2236 if ((q = qs[j = r & SMASK & (n - 1)]) != null) {
2237 for (;;) {
2238 ForkJoinTask<?> t; ForkJoinTask<?>[] a;
2239 boolean eligible = false;
2240 int sq = q.source, b, cap; long k;
2241 if ((a = q.array) == null || (cap = a.length) <= 0)
2242 break;
2243 t = (ForkJoinTask<?>)U.getReferenceAcquire(
2244 a, k = slotOffset((cap - 1) & (b = q.base)));
2245 if (t == task)
2246 eligible = true;
2247 else if (t != null) { // check steal chain
2248 for (int v = sq, d = cap;;) {
2249 WorkQueue p;
2250 if (v == wid) {
2251 eligible = true;
2252 break;
2253 }
2254 if ((v & 1) == 0 || // external or none
2255 --d < 0 || // bound depth
2256 (p = qs[v & (n - 1)]) == null)
2257 break;
2258 v = p.source;
2259 }
2260 }
2261 if ((s = task.status) < 0)
2262 break outer; // validate
2263 if (q.source == sq && q.base == b &&
2264 U.getReference(a, k) == t) {
2265 if (!eligible) { // revisit if nonempty
2266 if (!rescan && t == null && q.top - b > 0)
2267 rescan = true;
2268 break;
2269 }
2270 if (U.compareAndSetReference(a, k, t, null)) {
2271 q.base = b + 1;
2272 w.source = j; // volatile write
2273 t.doExec();
2274 w.source = wsrc;
2275 rescan = true; // restart at index r
2276 break scan;
2277 }
2278 }
2279 }
2280 }
2281 }
2282 }
2283 }
2284 return s;
2285 }
2286
2287 /**
2288 * Version of helpJoin for CountedCompleters.
2289 *
2290 * @param task root of computation (only called when a CountedCompleter)
2291 * @param w caller's WorkQueue
2292 * @param internal true if w is owned by a ForkJoinWorkerThread
2293 * @return task status on exit, or UNCOMPENSATE for compensated blocking
2294 */
2295 final int helpComplete(ForkJoinTask<?> task, WorkQueue w, boolean internal) {
2296 int s = 0;
2297 if (task != null && (s = task.status) >= 0 && w != null) {
2298 int r = w.phase + 1; // for indexing
2299 long sctl = 0L; // track stability
2300 outer: for (boolean rescan = true, locals = true;;) {
2301 if (locals && (s = w.helpComplete(task, internal, 0)) < 0)
2302 break;
2303 if ((s = task.status) < 0)
2304 break;
2305 if (!rescan) {
2306 if ((runState & STOP) != 0L)
2307 break;
2308 if (sctl == (sctl = ctl) &&
2309 (!internal || (s = tryCompensate(sctl)) >= 0))
2310 break;
2311 }
2312 rescan = locals = false;
2313 WorkQueue[] qs = queues;
2314 int n = (qs == null) ? 0 : qs.length;
2315 scan: for (int l = n; l > 0; --l, ++r) {
2316 int j; WorkQueue q;
2317 if ((q = qs[j = r & SMASK & (n - 1)]) != null) {
2318 for (;;) {
2319 ForkJoinTask<?> t; ForkJoinTask<?>[] a;
2320 int b, cap, nb; long k;
2321 boolean eligible = false;
2322 if ((a = q.array) == null || (cap = a.length) <= 0)
2323 break;
2324 t = (ForkJoinTask<?>)U.getReferenceAcquire(
2325 a, k = slotOffset((cap - 1) & (b = q.base)));
2326 if (t instanceof CountedCompleter) {
2327 CountedCompleter<?> f = (CountedCompleter<?>)t;
2328 for (int steps = cap; steps > 0; --steps) {
2329 if (f == task) {
2330 eligible = true;
2331 break;
2332 }
2333 if ((f = f.completer) == null)
2334 break;
2335 }
2336 }
2337 if ((s = task.status) < 0) // validate
2338 break outer;
2339 if (q.base == b) {
2340 if (eligible) {
2341 if (U.compareAndSetReference(
2342 a, k, t, null)) {
2343 q.updateBase(b + 1);
2344 t.doExec();
2345 locals = rescan = true;
2346 break scan;
2347 }
2348 }
2349 else if (U.getReference(a, k) == t) {
2350 if (!rescan && t == null && q.top - b > 0)
2351 rescan = true; // revisit
2352 break;
2353 }
2354 }
2355 }
2356 }
2357 }
2358 }
2359 }
2360 return s;
2361 }
2362
2363 /**
2364 * Runs tasks until all workers are inactive and no tasks are
2365 * found. Rather than blocking when tasks cannot be found, rescans
2366 * until all others cannot find tasks either.
2367 *
2368 * @param nanos max wait time (Long.MAX_VALUE if effectively untimed)
2369 * @param interruptible true if return on interrupt
2370 * @return positive if quiescent, negative if interrupted, else 0
2371 */
2372 private int helpQuiesce(WorkQueue w, long nanos, boolean interruptible) {
2373 int phase; // w.phase inactive bit set when temporarily quiescent
2374 if (w == null || ((phase = w.phase) & IDLE) != 0)
2375 return 0;
2376 int wsrc = w.source;
2377 long startTime = System.nanoTime();
2378 long maxSleep = Math.min(nanos >>> 8, MAX_SLEEP); // approx 1% nanos
2379 long prevSum = 0L;
2380 int activePhase = phase, inactivePhase = phase + IDLE;
2381 int r = phase + 1, waits = 0, returnStatus = 1;
2382 boolean locals = true;
2383 for (long e = runState;;) {
2384 if ((e & STOP) != 0L)
2385 break; // terminating
2386 if (interruptible && Thread.interrupted()) {
2387 returnStatus = -1;
2388 break;
2389 }
2390 if (locals) { // run local tasks before (re)polling
2391 locals = false;
2392 for (ForkJoinTask<?> u; (u = w.nextLocalTask()) != null;)
2393 u.doExec();
2394 }
2395 WorkQueue[] qs = queues;
2396 int n = (qs == null) ? 0 : qs.length;
2397 long phaseSum = 0L;
2398 boolean rescan = false, busy = false;
2399 scan: for (int l = n; l > 0; --l, ++r) {
2400 int j; WorkQueue q;
2401 if ((q = qs[j = r & SMASK & (n - 1)]) != null && q != w) {
2402 for (;;) {
2403 ForkJoinTask<?> t; ForkJoinTask<?>[] a;
2404 int b, cap; long k;
2405 if ((a = q.array) == null || (cap = a.length) <= 0)
2406 break;
2407 t = (ForkJoinTask<?>)U.getReferenceAcquire(
2408 a, k = slotOffset((cap - 1) & (b = q.base)));
2409 if (t != null && phase == inactivePhase) // reactivate
2410 w.phase = phase = activePhase;
2411 if (q.base == b && U.getReference(a, k) == t) {
2412 int nb = b + 1;
2413 if (t == null) {
2414 if (!rescan) {
2415 int qp = q.phase, mq = qp & (IDLE | 1);
2416 phaseSum += qp;
2417 if (mq == 0 || q.top - b > 0)
2418 rescan = true;
2419 else if (mq == 1)
2420 busy = true;
2421 }
2422 break;
2423 }
2424 if (U.compareAndSetReference(a, k, t, null)) {
2425 q.base = nb;
2426 w.source = j; // volatile write
2427 t.doExec();
2428 w.source = wsrc;
2429 rescan = locals = true;
2430 break scan;
2431 }
2432 }
2433 }
2434 }
2435 }
2436 if (e != (e = runState) || prevSum != (prevSum = phaseSum) ||
2437 rescan || (e & RS_LOCK) != 0L)
2438 ; // inconsistent
2439 else if (!busy)
2440 break;
2441 else if (phase == activePhase) {
2442 waits = 0; // recheck, then sleep
2443 w.phase = phase = inactivePhase;
2444 }
2445 else if (System.nanoTime() - startTime > nanos) {
2446 returnStatus = 0; // timed out
2447 break;
2448 }
2449 else if (waits == 0) // same as spinLockRunState except
2450 waits = MIN_SLEEP; // with rescan instead of onSpinWait
2451 else {
2452 LockSupport.parkNanos(this, (long)waits);
2453 if (waits < maxSleep)
2454 waits <<= 1;
2455 }
2456 }
2457 w.phase = activePhase;
2458 return returnStatus;
2459 }
2460
2461 /**
2462 * Helps quiesce from external caller until done, interrupted, or timeout
2463 *
2464 * @param nanos max wait time (Long.MAX_VALUE if effectively untimed)
2465 * @param interruptible true if return on interrupt
2466 * @return positive if quiescent, negative if interrupted, else 0
2467 */
2468 private int externalHelpQuiesce(long nanos, boolean interruptible) {
2469 if (quiescent() < 0) {
2470 long startTime = System.nanoTime();
2471 long maxSleep = Math.min(nanos >>> 8, MAX_SLEEP);
2472 for (int waits = 0;;) {
2473 ForkJoinTask<?> t;
2474 if (interruptible && Thread.interrupted())
2475 return -1;
2476 else if ((t = pollScan(false)) != null) {
2477 waits = 0;
2478 t.doExec();
2479 }
2480 else if (quiescent() >= 0)
2481 break;
2482 else if (System.nanoTime() - startTime > nanos)
2483 return 0;
2484 else if (waits == 0)
2485 waits = MIN_SLEEP;
2486 else {
2487 LockSupport.parkNanos(this, (long)waits);
2488 if (waits < maxSleep)
2489 waits <<= 1;
2490 }
2491 }
2492 }
2493 return 1;
2494 }
2495
2496 /**
2497 * Helps quiesce from either internal or external caller
2498 *
2499 * @param pool the pool to use, or null if any
2500 * @param nanos max wait time (Long.MAX_VALUE if effectively untimed)
2501 * @param interruptible true if return on interrupt
2502 * @return positive if quiescent, negative if interrupted, else 0
2503 */
2504 static final int helpQuiescePool(ForkJoinPool pool, long nanos,
2505 boolean interruptible) {
2506 Thread t; ForkJoinPool p; ForkJoinWorkerThread wt;
2507 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
2508 (p = (wt = (ForkJoinWorkerThread)t).pool) != null &&
2509 (p == pool || pool == null))
2510 return p.helpQuiesce(wt.workQueue, nanos, interruptible);
2511 else if ((p = pool) != null || (p = common) != null)
2512 return p.externalHelpQuiesce(nanos, interruptible);
2513 else
2514 return 0;
2515 }
2516
2517 /**
2518 * Gets and removes a local or stolen task for the given worker.
2519 *
2520 * @return a task, if available
2521 */
2522 final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
2523 ForkJoinTask<?> t;
2524 if (w == null || (t = w.nextLocalTask()) == null)
2525 t = pollScan(false);
2526 return t;
2527 }
2528
2529 // External operations
2530
2531 /**
2532 * Finds and locks a WorkQueue for an external submitter, or
2533 * throws RejectedExecutionException if shutdown
2534 * @param rejectOnShutdown true if RejectedExecutionException
2535 * should be thrown when shutdown
2536 */
2537 final WorkQueue externalSubmissionQueue(boolean rejectOnShutdown) {
2538 int r;
2539 if ((r = ThreadLocalRandom.getProbe()) == 0) {
2540 ThreadLocalRandom.localInit(); // initialize caller's probe
2541 r = ThreadLocalRandom.getProbe();
2542 }
2543 for (;;) {
2544 WorkQueue q; WorkQueue[] qs; int n, id, i;
2545 if ((qs = queues) == null || (n = qs.length) <= 0)
2546 break;
2547 if ((q = qs[i = (id = r & EXTERNAL_ID_MASK) & (n - 1)]) == null) {
2548 WorkQueue newq = new WorkQueue(null, id, 0, false);
2549 lockRunState();
2550 if (qs[i] == null && queues == qs)
2551 q = qs[i] = newq; // else lost race to install
2552 unlockRunState();
2553 }
2554 if (q != null && q.tryLockPhase()) {
2555 if (rejectOnShutdown && (runState & SHUTDOWN) != 0L) {
2556 q.unlockPhase(); // check while q lock held
2557 break;
2558 }
2559 return q;
2560 }
2561 r = ThreadLocalRandom.advanceProbe(r); // move
2562 }
2563 throw new RejectedExecutionException();
2564 }
2565
2566 private <T> ForkJoinTask<T> poolSubmit(boolean signalIfEmpty, ForkJoinTask<T> task) {
2567 Thread t; ForkJoinWorkerThread wt; WorkQueue q; boolean internal;
2568 if (((t = JLA.currentCarrierThread()) instanceof ForkJoinWorkerThread) &&
2569 (wt = (ForkJoinWorkerThread)t).pool == this) {
2570 internal = true;
2571 q = wt.workQueue;
2572 }
2573 else { // find and lock queue
2574 internal = false;
2575 q = externalSubmissionQueue(true);
2576 }
2577 q.push(task, signalIfEmpty ? this : null, internal);
2578 return task;
2579 }
2580
2581 /**
2582 * Returns queue for an external thread, if one exists that has
2583 * possibly ever submitted to the given pool (nonzero probe), or
2584 * null if none.
2585 */
2586 static WorkQueue externalQueue(ForkJoinPool p) {
2587 WorkQueue[] qs; int n;
2588 int r = ThreadLocalRandom.getProbe();
2589 return (p != null && (qs = p.queues) != null &&
2590 (n = qs.length) > 0 && r != 0) ?
2591 qs[r & EXTERNAL_ID_MASK & (n - 1)] : null;
2592 }
2593
2594 /**
2595 * Returns external queue for common pool.
2596 */
2597 static WorkQueue commonQueue() {
2598 return externalQueue(common);
2599 }
2600
2601 /**
2602 * If the given executor is a ForkJoinPool, poll and execute
2603 * AsynchronousCompletionTasks from worker's queue until none are
2604 * available or blocker is released.
2605 */
2606 static void helpAsyncBlocker(Executor e, ManagedBlocker blocker) {
2607 WorkQueue w = null; Thread t; ForkJoinWorkerThread wt;
2608 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2609 (wt = (ForkJoinWorkerThread)t).pool == e)
2610 w = wt.workQueue;
2611 else if (e instanceof ForkJoinPool)
2612 w = externalQueue((ForkJoinPool)e);
2613 if (w != null)
2614 w.helpAsyncBlocker(blocker);
2615 }
2616
2617 /**
2618 * Returns a cheap heuristic guide for task partitioning when
2619 * programmers, frameworks, tools, or languages have little or no
2620 * idea about task granularity. In essence, by offering this
2621 * method, we ask users only about tradeoffs in overhead vs
2622 * expected throughput and its variance, rather than how finely to
2623 * partition tasks.
2624 *
2625 * In a steady state strict (tree-structured) computation, each
2626 * thread makes available for stealing enough tasks for other
2627 * threads to remain active. Inductively, if all threads play by
2628 * the same rules, each thread should make available only a
2629 * constant number of tasks.
2630 *
2631 * The minimum useful constant is just 1. But using a value of 1
2632 * would require immediate replenishment upon each steal to
2633 * maintain enough tasks, which is infeasible. Further,
2634 * partitionings/granularities of offered tasks should minimize
2635 * steal rates, which in general means that threads nearer the top
2636 * of computation tree should generate more than those nearer the
2637 * bottom. In perfect steady state, each thread is at
2638 * approximately the same level of computation tree. However,
2639 * producing extra tasks amortizes the uncertainty of progress and
2640 * diffusion assumptions.
2641 *
2642 * So, users will want to use values larger (but not much larger)
2643 * than 1 to both smooth over transient shortages and hedge
2644 * against uneven progress; as traded off against the cost of
2645 * extra task overhead. We leave the user to pick a threshold
2646 * value to compare with the results of this call to guide
2647 * decisions, but recommend values such as 3.
2648 *
2649 * When all threads are active, it is on average OK to estimate
2650 * surplus strictly locally. In steady-state, if one thread is
2651 * maintaining say 2 surplus tasks, then so are others. So we can
2652 * just use estimated queue length. However, this strategy alone
2653 * leads to serious mis-estimates in some non-steady-state
2654 * conditions (ramp-up, ramp-down, other stalls). We can detect
2655 * many of these by further considering the number of "idle"
2656 * threads, that are known to have zero queued tasks, so
2657 * compensate by a factor of (#idle/#active) threads.
2658 */
2659 static int getSurplusQueuedTaskCount() {
2660 Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
2661 if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) &&
2662 (pool = (wt = (ForkJoinWorkerThread)t).pool) != null &&
2663 (q = wt.workQueue) != null) {
2664 int n = q.top - q.base;
2665 int p = pool.parallelism;
2666 int a = (short)(pool.ctl >>> RC_SHIFT);
2667 return n - (a > (p >>>= 1) ? 0 :
2668 a > (p >>>= 1) ? 1 :
2669 a > (p >>>= 1) ? 2 :
2670 a > (p >>>= 1) ? 4 :
2671 8);
2672 }
2673 return 0;
2674 }
2675
2676 // Termination
2677
2678 /**
2679 * Possibly initiates and/or completes pool termination.
2680 *
2681 * @param now if true, unconditionally terminate, else only
2682 * if no work and no active workers
2683 * @param enable if true, terminate when next possible
2684 * @return runState on exit
2685 */
2686 private long tryTerminate(boolean now, boolean enable) {
2687 long e, isShutdown, ps;
2688 if (((e = runState) & TERMINATED) != 0L)
2689 now = false;
2690 else if ((e & STOP) != 0L)
2691 now = true;
2692 else if (now) {
2693 if (((ps = getAndBitwiseOrRunState(SHUTDOWN|STOP) & STOP)) == 0L) {
2694 if ((ps & RS_LOCK) != 0L) {
2695 spinLockRunState(); // ensure queues array stable after stop
2696 unlockRunState();
2697 }
2698 interruptAll();
2699 }
2700 }
2701 else if ((isShutdown = (e & SHUTDOWN)) != 0L || enable) {
2702 long quiet; DelayScheduler ds;
2703 if (isShutdown == 0L)
2704 getAndBitwiseOrRunState(SHUTDOWN);
2705 if ((quiet = quiescent()) > 0)
2706 now = true;
2707 else if (quiet == 0 && (ds = delayScheduler) != null)
2708 ds.signal();
2709 }
2710
2711 if (now) {
2712 DelayScheduler ds;
2713 releaseWaiters();
2714 if ((ds = delayScheduler) != null)
2715 ds.signal();
2716 for (;;) {
2717 if (((e = runState) & CLEANED) == 0L) {
2718 boolean clean = cleanQueues();
2719 if (((e = runState) & CLEANED) == 0L && clean)
2720 e = getAndBitwiseOrRunState(CLEANED) | CLEANED;
2721 }
2722 if ((e & TERMINATED) != 0L)
2723 break;
2724 if (ctl != 0L) // else loop if didn't finish cleaning
2725 break;
2726 if ((ds = delayScheduler) != null && ds.signal() >= 0)
2727 break;
2728 if ((e & CLEANED) != 0L) {
2729 e |= TERMINATED;
2730 if ((getAndBitwiseOrRunState(TERMINATED) & TERMINATED) == 0L) {
2731 CountDownLatch done; SharedThreadContainer ctr;
2732 if ((done = termination) != null)
2733 done.countDown();
2734 if ((ctr = container) != null)
2735 ctr.close();
2736 }
2737 break;
2738 }
2739 }
2740 }
2741 return e;
2742 }
2743
2744 /**
2745 * Scans queues in a psuedorandom order based on thread id,
2746 * cancelling tasks until empty, or returning early upon
2747 * interference or still-active external queues, in which case
2748 * other calls will finish cancellation.
2749 *
2750 * @return true if all queues empty
2751 */
2752 private boolean cleanQueues() {
2753 int r = (int)Thread.currentThread().threadId();
2754 r ^= r << 13; r ^= r >>> 17; r ^= r << 5; // xorshift
2755 int step = (r >>> 16) | 1; // randomize traversals
2756 WorkQueue[] qs = queues;
2757 int n = (qs == null) ? 0 : qs.length;
2758 for (int l = n; l > 0; --l, r += step) {
2759 WorkQueue q; ForkJoinTask<?>[] a; int cap;
2760 if ((q = qs[r & (n - 1)]) != null &&
2761 (a = q.array) != null && (cap = a.length) > 0) {
2762 for (;;) {
2763 ForkJoinTask<?> t; int b; long k;
2764 t = (ForkJoinTask<?>)U.getReferenceAcquire(
2765 a, k = slotOffset((cap - 1) & (b = q.base)));
2766 if (q.base == b && t != null &&
2767 U.compareAndSetReference(a, k, t, null)) {
2768 q.updateBase(b + 1);
2769 try {
2770 t.cancel(false);
2771 } catch (Throwable ignore) {
2772 }
2773 }
2774 else if ((q.phase & (IDLE|1)) == 0 || // externally locked
2775 q.top - q.base > 0)
2776 return false; // incomplete
2777 else
2778 break;
2779 }
2780 }
2781 }
2782 return true;
2783 }
2784
2785 /**
2786 * Interrupts all workers
2787 */
2788 private void interruptAll() {
2789 Thread current = Thread.currentThread();
2790 WorkQueue[] qs = queues;
2791 int n = (qs == null) ? 0 : qs.length;
2792 for (int i = 1; i < n; i += 2) {
2793 WorkQueue q; Thread o;
2794 if ((q = qs[i]) != null && (o = q.owner) != null && o != current) {
2795 try {
2796 o.interrupt();
2797 } catch (Throwable ignore) {
2798 }
2799 }
2800 }
2801 }
2802
2803 /**
2804 * Returns termination signal, constructing if necessary
2805 */
2806 private CountDownLatch terminationSignal() {
2807 CountDownLatch signal, s, u;
2808 if ((signal = termination) == null)
2809 signal = ((u = cmpExTerminationSignal(
2810 s = new CountDownLatch(1))) == null) ? s : u;
2811 return signal;
2812 }
2813
2814 // Exported methods
2815
2816 // Constructors
2817
2818 /**
2819 * Creates a {@code ForkJoinPool} with parallelism equal to {@link
2820 * java.lang.Runtime#availableProcessors}, using defaults for all
2821 * other parameters (see {@link #ForkJoinPool(int,
2822 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2823 * int, int, int, Predicate, long, TimeUnit)}).
2824 */
2825 public ForkJoinPool() {
2826 this(Math.min(MAX_CAP, Runtime.getRuntime().availableProcessors()),
2827 defaultForkJoinWorkerThreadFactory, null, false,
2828 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2829 }
2830
2831 /**
2832 * Creates a {@code ForkJoinPool} with the indicated parallelism
2833 * level, using defaults for all other parameters (see {@link
2834 * #ForkJoinPool(int, ForkJoinWorkerThreadFactory,
2835 * UncaughtExceptionHandler, boolean, int, int, int, Predicate,
2836 * long, TimeUnit)}).
2837 *
2838 * @param parallelism the parallelism level
2839 * @throws IllegalArgumentException if parallelism less than or
2840 * equal to zero, or greater than implementation limit
2841 */
2842 public ForkJoinPool(int parallelism) {
2843 this(parallelism, defaultForkJoinWorkerThreadFactory, null, false,
2844 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2845 }
2846
2847 /**
2848 * Creates a {@code ForkJoinPool} with the given parameters (using
2849 * defaults for others -- see {@link #ForkJoinPool(int,
2850 * ForkJoinWorkerThreadFactory, UncaughtExceptionHandler, boolean,
2851 * int, int, int, Predicate, long, TimeUnit)}).
2852 *
2853 * @param parallelism the parallelism level. For default value,
2854 * use {@link java.lang.Runtime#availableProcessors}.
2855 * @param factory the factory for creating new threads. For default value,
2856 * use {@link #defaultForkJoinWorkerThreadFactory}.
2857 * @param handler the handler for internal worker threads that
2858 * terminate due to unrecoverable errors encountered while executing
2859 * tasks. For default value, use {@code null}.
2860 * @param asyncMode if true,
2861 * establishes local first-in-first-out scheduling mode for forked
2862 * tasks that are never joined. This mode may be more appropriate
2863 * than default locally stack-based mode in applications in which
2864 * worker threads only process event-style asynchronous tasks.
2865 * For default value, use {@code false}.
2866 * @throws IllegalArgumentException if parallelism less than or
2867 * equal to zero, or greater than implementation limit
2868 * @throws NullPointerException if the factory is null
2869 */
2870 public ForkJoinPool(int parallelism,
2871 ForkJoinWorkerThreadFactory factory,
2872 UncaughtExceptionHandler handler,
2873 boolean asyncMode) {
2874 this(parallelism, factory, handler, asyncMode,
2875 0, MAX_CAP, 1, null, DEFAULT_KEEPALIVE, TimeUnit.MILLISECONDS);
2876 }
2877
2878 /**
2879 * Creates a {@code ForkJoinPool} with the given parameters.
2880 *
2881 * @param parallelism the parallelism level. For default value,
2882 * use {@link java.lang.Runtime#availableProcessors}.
2883 *
2884 * @param factory the factory for creating new threads. For
2885 * default value, use {@link #defaultForkJoinWorkerThreadFactory}.
2886 *
2887 * @param handler the handler for internal worker threads that
2888 * terminate due to unrecoverable errors encountered while
2889 * executing tasks. For default value, use {@code null}.
2890 *
2891 * @param asyncMode if true, establishes local first-in-first-out
2892 * scheduling mode for forked tasks that are never joined. This
2893 * mode may be more appropriate than default locally stack-based
2894 * mode in applications in which worker threads only process
2895 * event-style asynchronous tasks. For default value, use {@code
2896 * false}.
2897 *
2898 * @param corePoolSize ignored: used in previous releases of this
2899 * class but no longer applicable. Using {@code 0} maintains
2900 * compatibility across releases.
2901 *
2902 * @param maximumPoolSize the maximum number of threads allowed.
2903 * When the maximum is reached, attempts to replace blocked
2904 * threads fail. (However, because creation and termination of
2905 * different threads may overlap, and may be managed by the given
2906 * thread factory, this value may be transiently exceeded.) To
2907 * arrange the same value as is used by default for the common
2908 * pool, use {@code 256} plus the {@code parallelism} level. (By
2909 * default, the common pool allows a maximum of 256 spare
2910 * threads.) Using a value (for example {@code
2911 * Integer.MAX_VALUE}) larger than the implementation's total
2912 * thread limit has the same effect as using this limit (which is
2913 * the default).
2914 *
2915 * @param minimumRunnable the minimum allowed number of core
2916 * threads not blocked by a join or {@link ManagedBlocker}. To
2917 * ensure progress, when too few unblocked threads exist and
2918 * unexecuted tasks may exist, new threads are constructed, up to
2919 * the given maximumPoolSize. For the default value, use {@code
2920 * 1}, that ensures liveness. A larger value might improve
2921 * throughput in the presence of blocked activities, but might
2922 * not, due to increased overhead. A value of zero may be
2923 * acceptable when submitted tasks cannot have dependencies
2924 * requiring additional threads.
2925 *
2926 * @param saturate if non-null, a predicate invoked upon attempts
2927 * to create more than the maximum total allowed threads. By
2928 * default, when a thread is about to block on a join or {@link
2929 * ManagedBlocker}, but cannot be replaced because the
2930 * maximumPoolSize would be exceeded, a {@link
2931 * RejectedExecutionException} is thrown. But if this predicate
2932 * returns {@code true}, then no exception is thrown, so the pool
2933 * continues to operate with fewer than the target number of
2934 * runnable threads, which might not ensure progress.
2935 *
2936 * @param keepAliveTime the elapsed time since last use before
2937 * a thread is terminated (and then later replaced if needed).
2938 * For the default value, use {@code 60, TimeUnit.SECONDS}.
2939 *
2940 * @param unit the time unit for the {@code keepAliveTime} argument
2941 *
2942 * @throws IllegalArgumentException if parallelism is less than or
2943 * equal to zero, or is greater than implementation limit,
2944 * or if maximumPoolSize is less than parallelism,
2945 * of if the keepAliveTime is less than or equal to zero.
2946 * @throws NullPointerException if the factory is null
2947 * @since 9
2948 */
2949 public ForkJoinPool(int parallelism,
2950 ForkJoinWorkerThreadFactory factory,
2951 UncaughtExceptionHandler handler,
2952 boolean asyncMode,
2953 int corePoolSize,
2954 int maximumPoolSize,
2955 int minimumRunnable,
2956 Predicate<? super ForkJoinPool> saturate,
2957 long keepAliveTime,
2958 TimeUnit unit) {
2959 int p = parallelism;
2960 if (p <= 0 || p > MAX_CAP || p > maximumPoolSize || keepAliveTime <= 0L)
2961 throw new IllegalArgumentException();
2962 if (factory == null || unit == null)
2963 throw new NullPointerException();
2964 int size = Math.max(MIN_QUEUES_SIZE,
2965 1 << (33 - Integer.numberOfLeadingZeros(p - 1)));
2966 this.parallelism = p;
2967 this.factory = factory;
2968 this.ueh = handler;
2969 this.saturate = saturate;
2970 this.keepAlive = Math.max(unit.toMillis(keepAliveTime), TIMEOUT_SLOP);
2971 int maxSpares = Math.clamp(maximumPoolSize - p, 0, MAX_CAP);
2972 int minAvail = Math.clamp(minimumRunnable, 0, MAX_CAP);
2973 this.config = (((asyncMode ? FIFO : 0) & LMASK) |
2974 (((long)maxSpares) << TC_SHIFT) |
2975 (((long)minAvail) << RC_SHIFT));
2976 this.queues = new WorkQueue[size];
2977 String pid = Integer.toString(getAndAddPoolIds(1) + 1);
2978 String name = "ForkJoinPool-" + pid;
2979 this.poolName = name;
2980 this.workerNamePrefix = name + "-worker-";
2981 this.container = SharedThreadContainer.create(name);
2982 }
2983
2984 /**
2985 * Constructor for common pool using parameters possibly
2986 * overridden by system properties
2987 */
2988 private ForkJoinPool(byte forCommonPoolOnly) {
2989 String name = "ForkJoinPool.commonPool";
2990 ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory;
2991 UncaughtExceptionHandler handler = null;
2992 int maxSpares = DEFAULT_COMMON_MAX_SPARES;
2993 int pc = 0, preset = 0; // nonzero if size set as property
2994 try { // ignore exceptions in accessing/parsing properties
2995 String pp = System.getProperty
2996 ("java.util.concurrent.ForkJoinPool.common.parallelism");
2997 if (pp != null) {
2998 pc = Math.max(0, Integer.parseInt(pp));
2999 preset = PRESET_SIZE;
3000 }
3001 String ms = System.getProperty
3002 ("java.util.concurrent.ForkJoinPool.common.maximumSpares");
3003 if (ms != null)
3004 maxSpares = Math.clamp(Integer.parseInt(ms), 0, MAX_CAP);
3005 String sf = System.getProperty
3006 ("java.util.concurrent.ForkJoinPool.common.threadFactory");
3007 String sh = System.getProperty
3008 ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
3009 if (sf != null || sh != null) {
3010 ClassLoader ldr = ClassLoader.getSystemClassLoader();
3011 if (sf != null)
3012 fac = (ForkJoinWorkerThreadFactory)
3013 ldr.loadClass(sf).getConstructor().newInstance();
3014 if (sh != null)
3015 handler = (UncaughtExceptionHandler)
3016 ldr.loadClass(sh).getConstructor().newInstance();
3017 }
3018 } catch (Exception ignore) {
3019 }
3020 if (preset == 0)
3021 pc = Math.max(1, Runtime.getRuntime().availableProcessors() - 1);
3022 int p = Math.min(pc, MAX_CAP);
3023 int size = Math.max(MIN_QUEUES_SIZE,
3024 (p == 0) ? 1 :
3025 1 << (33 - Integer.numberOfLeadingZeros(p-1)));
3026 this.parallelism = p;
3027 this.config = ((preset & LMASK) | (((long)maxSpares) << TC_SHIFT) |
3028 (1L << RC_SHIFT));
3029 this.factory = fac;
3030 this.ueh = handler;
3031 this.keepAlive = DEFAULT_KEEPALIVE;
3032 this.saturate = null;
3033 this.workerNamePrefix = null;
3034 this.poolName = name;
3035 this.queues = new WorkQueue[size];
3036 this.container = SharedThreadContainer.create(name);
3037 }
3038
3039 /**
3040 * Returns the common pool instance. This pool is statically
3041 * constructed; its run state is unaffected by attempts to {@link
3042 * #shutdown} or {@link #shutdownNow}. However this pool and any
3043 * ongoing processing are automatically terminated upon program
3044 * {@link System#exit}. Any program that relies on asynchronous
3045 * task processing to complete before program termination should
3046 * invoke {@code commonPool().}{@link #awaitQuiescence awaitQuiescence},
3047 * before exit.
3048 *
3049 * @return the common pool instance
3050 * @since 1.8
3051 */
3052 public static ForkJoinPool commonPool() {
3053 // assert common != null : "static init error";
3054 return common;
3055 }
3056
3057 /**
3058 * Package-private access to commonPool overriding zero parallelism
3059 */
3060 static ForkJoinPool asyncCommonPool() {
3061 ForkJoinPool cp; int p;
3062 if ((p = (cp = common).parallelism) == 0)
3063 U.compareAndSetInt(cp, PARALLELISM, 0, 2);
3064 return cp;
3065 }
3066
3067 // Execution methods
3068
3069 /**
3070 * Performs the given task, returning its result upon completion.
3071 * If the computation encounters an unchecked Exception or Error,
3072 * it is rethrown as the outcome of this invocation. Rethrown
3073 * exceptions behave in the same way as regular exceptions, but,
3074 * when possible, contain stack traces (as displayed for example
3075 * using {@code ex.printStackTrace()}) of both the current thread
3076 * as well as the thread actually encountering the exception;
3077 * minimally only the latter.
3078 *
3079 * @param task the task
3080 * @param <T> the type of the task's result
3081 * @return the task's result
3082 * @throws NullPointerException if the task is null
3083 * @throws RejectedExecutionException if the task cannot be
3084 * scheduled for execution
3085 */
3086 public <T> T invoke(ForkJoinTask<T> task) {
3087 poolSubmit(true, Objects.requireNonNull(task));
3088 try {
3089 return task.join();
3090 } catch (RuntimeException | Error unchecked) {
3091 throw unchecked;
3092 } catch (Exception checked) {
3093 throw new RuntimeException(checked);
3094 }
3095 }
3096
3097 /**
3098 * Arranges for (asynchronous) execution of the given task.
3099 *
3100 * @param task the task
3101 * @throws NullPointerException if the task is null
3102 * @throws RejectedExecutionException if the task cannot be
3103 * scheduled for execution
3104 */
3105 public void execute(ForkJoinTask<?> task) {
3106 poolSubmit(true, Objects.requireNonNull(task));
3107 }
3108
3109 // AbstractExecutorService methods
3110
3111 /**
3112 * @throws NullPointerException if the task is null
3113 * @throws RejectedExecutionException if the task cannot be
3114 * scheduled for execution
3115 */
3116 @Override
3117 @SuppressWarnings("unchecked")
3118 public void execute(Runnable task) {
3119 poolSubmit(true, (Objects.requireNonNull(task) instanceof ForkJoinTask<?>)
3120 ? (ForkJoinTask<Void>) task // avoid re-wrap
3121 : new ForkJoinTask.RunnableExecuteAction(task));
3122 }
3123
3124 /**
3125 * Submits a ForkJoinTask for execution.
3126 *
3127 * @implSpec
3128 * This method is equivalent to {@link #externalSubmit(ForkJoinTask)}
3129 * when called from a thread that is not in this pool.
3130 *
3131 * @param task the task to submit
3132 * @param <T> the type of the task's result
3133 * @return the task
3134 * @throws NullPointerException if the task is null
3135 * @throws RejectedExecutionException if the task cannot be
3136 * scheduled for execution
3137 */
3138 public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
3139 return poolSubmit(true, Objects.requireNonNull(task));
3140 }
3141
3142 /**
3143 * @throws NullPointerException if the task is null
3144 * @throws RejectedExecutionException if the task cannot be
3145 * scheduled for execution
3146 */
3147 @Override
3148 public <T> ForkJoinTask<T> submit(Callable<T> task) {
3149 Objects.requireNonNull(task);
3150 return poolSubmit(
3151 true,
3152 (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
3153 new ForkJoinTask.AdaptedCallable<T>(task) :
3154 new ForkJoinTask.AdaptedInterruptibleCallable<T>(task));
3155 }
3156
3157 /**
3158 * @throws NullPointerException if the task is null
3159 * @throws RejectedExecutionException if the task cannot be
3160 * scheduled for execution
3161 */
3162 @Override
3163 public <T> ForkJoinTask<T> submit(Runnable task, T result) {
3164 Objects.requireNonNull(task);
3165 return poolSubmit(
3166 true,
3167 (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
3168 new ForkJoinTask.AdaptedRunnable<T>(task, result) :
3169 new ForkJoinTask.AdaptedInterruptibleRunnable<T>(task, result));
3170 }
3171
3172 /**
3173 * @throws NullPointerException if the task is null
3174 * @throws RejectedExecutionException if the task cannot be
3175 * scheduled for execution
3176 */
3177 @Override
3178 @SuppressWarnings("unchecked")
3179 public ForkJoinTask<?> submit(Runnable task) {
3180 Objects.requireNonNull(task);
3181 return poolSubmit(
3182 true,
3183 (task instanceof ForkJoinTask<?>) ?
3184 (ForkJoinTask<Void>) task : // avoid re-wrap
3185 ((Thread.currentThread() instanceof ForkJoinWorkerThread) ?
3186 new ForkJoinTask.AdaptedRunnable<Void>(task, null) :
3187 new ForkJoinTask.AdaptedInterruptibleRunnable<Void>(task, null)));
3188 }
3189
3190 /**
3191 * Submits the given task as if submitted from a non-{@code ForkJoinTask}
3192 * client. The task is added to a scheduling queue for submissions to the
3193 * pool even when called from a thread in the pool.
3194 *
3195 * @implSpec
3196 * This method is equivalent to {@link #submit(ForkJoinTask)} when called
3197 * from a thread that is not in this pool.
3198 *
3199 * @return the task
3200 * @param task the task to submit
3201 * @param <T> the type of the task's result
3202 * @throws NullPointerException if the task is null
3203 * @throws RejectedExecutionException if the task cannot be
3204 * scheduled for execution
3205 * @since 20
3206 */
3207 public <T> ForkJoinTask<T> externalSubmit(ForkJoinTask<T> task) {
3208 Objects.requireNonNull(task);
3209 externalSubmissionQueue(true).push(task, this, false);
3210 return task;
3211 }
3212
3213 /**
3214 * Submits the given task without guaranteeing that it will
3215 * eventually execute in the absence of available active threads.
3216 * In some contexts, this method may reduce contention and
3217 * overhead by relying on context-specific knowledge that existing
3218 * threads (possibly including the calling thread if operating in
3219 * this pool) will eventually be available to execute the task.
3220 *
3221 * @param task the task
3222 * @param <T> the type of the task's result
3223 * @return the task
3224 * @throws NullPointerException if the task is null
3225 * @throws RejectedExecutionException if the task cannot be
3226 * scheduled for execution
3227 * @since 19
3228 */
3229 public <T> ForkJoinTask<T> lazySubmit(ForkJoinTask<T> task) {
3230 return poolSubmit(false, Objects.requireNonNull(task));
3231 }
3232
3233 /**
3234 * Changes the target parallelism of this pool, controlling the
3235 * future creation, use, and termination of worker threads.
3236 * Applications include contexts in which the number of available
3237 * processors changes over time.
3238 *
3239 * @implNote This implementation restricts the maximum number of
3240 * running threads to 32767
3241 *
3242 * @param size the target parallelism level
3243 * @return the previous parallelism level.
3244 * @throws IllegalArgumentException if size is less than 1 or
3245 * greater than the maximum supported by this pool.
3246 * @throws UnsupportedOperationException this is the{@link
3247 * #commonPool()} and parallelism level was set by System
3248 * property {@systemProperty
3249 * java.util.concurrent.ForkJoinPool.common.parallelism}.
3250 * @since 19
3251 */
3252 public int setParallelism(int size) {
3253 if (size < 1 || size > MAX_CAP)
3254 throw new IllegalArgumentException();
3255 if ((config & PRESET_SIZE) != 0)
3256 throw new UnsupportedOperationException("Cannot override System property");
3257 return getAndSetParallelism(size);
3258 }
3259
3260 /**
3261 * Uninterrupible version of {@code invokeAll}. Executes the given
3262 * tasks, returning a list of Futures holding their status and
3263 * results when all complete, ignoring interrupts. {@link
3264 * Future#isDone} is {@code true} for each element of the returned
3265 * list. Note that a <em>completed</em> task could have
3266 * terminated either normally or by throwing an exception. The
3267 * results of this method are undefined if the given collection is
3268 * modified while this operation is in progress.
3269 *
3270 * @apiNote This method supports usages that previously relied on an
3271 * incompatible override of
3272 * {@link ExecutorService#invokeAll(java.util.Collection)}.
3273 *
3274 * @param tasks the collection of tasks
3275 * @param <T> the type of the values returned from the tasks
3276 * @return a list of Futures representing the tasks, in the same
3277 * sequential order as produced by the iterator for the
3278 * given task list, each of which has completed
3279 * @throws NullPointerException if tasks or any of its elements are {@code null}
3280 * @throws RejectedExecutionException if any task cannot be
3281 * scheduled for execution
3282 * @since 22
3283 */
3284 public <T> List<Future<T>> invokeAllUninterruptibly(Collection<? extends Callable<T>> tasks) {
3285 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
3286 try {
3287 for (Callable<T> t : tasks) {
3288 ForkJoinTask<T> f = ForkJoinTask.adapt(t);
3289 futures.add(f);
3290 poolSubmit(true, f);
3291 }
3292 for (int i = futures.size() - 1; i >= 0; --i)
3293 ((ForkJoinTask<?>)futures.get(i)).quietlyJoin();
3294 return futures;
3295 } catch (Throwable t) {
3296 for (Future<T> e : futures)
3297 e.cancel(true);
3298 throw t;
3299 }
3300 }
3301
3302 /**
3303 * Common support for timed and untimed invokeAll
3304 */
3305 private <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
3306 long deadline)
3307 throws InterruptedException {
3308 ArrayList<Future<T>> futures = new ArrayList<>(tasks.size());
3309 try {
3310 for (Callable<T> t : tasks) {
3311 ForkJoinTask<T> f = ForkJoinTask.adaptInterruptible(t);
3312 futures.add(f);
3313 poolSubmit(true, f);
3314 }
3315 for (int i = futures.size() - 1; i >= 0; --i)
3316 ((ForkJoinTask<?>)futures.get(i))
3317 .quietlyJoinPoolInvokeAllTask(deadline);
3318 return futures;
3319 } catch (Throwable t) {
3320 for (Future<T> e : futures)
3321 e.cancel(true);
3322 throw t;
3323 }
3324 }
3325
3326 @Override
3327 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
3328 throws InterruptedException {
3329 return invokeAll(tasks, 0L);
3330 }
3331 // for jdk version < 22, replace with
3332 // /**
3333 // * @throws NullPointerException {@inheritDoc}
3334 // * @throws RejectedExecutionException {@inheritDoc}
3335 // */
3336 // @Override
3337 // public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
3338 // return invokeAllUninterruptibly(tasks);
3339 // }
3340
3341 @Override
3342 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
3343 long timeout, TimeUnit unit)
3344 throws InterruptedException {
3345 return invokeAll(tasks, (System.nanoTime() + unit.toNanos(timeout)) | 1L);
3346 }
3347
3348 @Override
3349 public <T> T invokeAny(Collection<? extends Callable<T>> tasks)
3350 throws InterruptedException, ExecutionException {
3351 try {
3352 return new ForkJoinTask.InvokeAnyRoot<T>()
3353 .invokeAny(tasks, this, false, 0L);
3354 } catch (TimeoutException cannotHappen) {
3355 assert false;
3356 return null;
3357 }
3358 }
3359
3360 @Override
3361 public <T> T invokeAny(Collection<? extends Callable<T>> tasks,
3362 long timeout, TimeUnit unit)
3363 throws InterruptedException, ExecutionException, TimeoutException {
3364 return new ForkJoinTask.InvokeAnyRoot<T>()
3365 .invokeAny(tasks, this, true, unit.toNanos(timeout));
3366 }
3367
3368 // Support for delayed tasks
3369
3370 /**
3371 * Returns STOP and SHUTDOWN status (zero if neither), masking or
3372 * truncating out other bits.
3373 */
3374 final int shutdownStatus(DelayScheduler ds) {
3375 return (int)(runState & (SHUTDOWN | STOP));
3376 }
3377
3378 /**
3379 * Tries to stop and possibly terminate if already enabled, return success.
3380 */
3381 final boolean tryStopIfShutdown(DelayScheduler ds) {
3382 return (tryTerminate(false, false) & STOP) != 0L;
3383 }
3384
3385 /**
3386 * Creates and starts DelayScheduler
3387 */
3388 private DelayScheduler startDelayScheduler() {
3389 DelayScheduler ds;
3390 if ((ds = delayScheduler) == null) {
3391 boolean start = false;
3392 String name = poolName + "-delayScheduler";
3393 if (workerNamePrefix == null)
3394 asyncCommonPool(); // override common parallelism zero
3395 long isShutdown = lockRunState() & SHUTDOWN;
3396 try {
3397 if (isShutdown == 0L && (ds = delayScheduler) == null) {
3398 ds = delayScheduler = new DelayScheduler(this, name);
3399 start = true;
3400 }
3401 } finally {
3402 unlockRunState();
3403 }
3404 if (start) { // start outside of lock
3405 SharedThreadContainer ctr;
3406 try {
3407 if ((ctr = container) != null)
3408 ctr.start(ds);
3409 else
3410 ds.start();
3411 } catch (RuntimeException | Error ex) { // back out
3412 lockRunState();
3413 ds = delayScheduler = null;
3414 unlockRunState();
3415 tryTerminate(false, false);
3416 if (ex instanceof Error)
3417 throw ex;
3418 }
3419 }
3420 }
3421 return ds;
3422 }
3423
3424 /**
3425 * Arranges execution of a ScheduledForkJoinTask whose delay has
3426 * elapsed
3427 */
3428 final void executeEnabledScheduledTask(ScheduledForkJoinTask<?> task) {
3429 externalSubmissionQueue(false).push(task, this, false);
3430 }
3431
3432 /**
3433 * Arranges delayed execution of a ScheduledForkJoinTask via the
3434 * DelayScheduler, creating and starting it if necessary.
3435 * @return the task
3436 */
3437 final <T> ScheduledForkJoinTask<T> scheduleDelayedTask(ScheduledForkJoinTask<T> task) {
3438 DelayScheduler ds;
3439 if (((ds = delayScheduler) == null &&
3440 (ds = startDelayScheduler()) == null) ||
3441 (runState & SHUTDOWN) != 0L)
3442 throw new RejectedExecutionException();
3443 ds.pend(task);
3444 return task;
3445 }
3446
3447 /**
3448 * Submits a one-shot task that becomes enabled for execution after the given
3449 * delay. At that point it will execute unless explicitly
3450 * cancelled, or fail to execute (eventually reporting
3451 * cancellation) when encountering resource exhaustion, or the
3452 * pool is {@link #shutdownNow}, or is {@link #shutdown} when
3453 * otherwise quiescent and {@link #cancelDelayedTasksOnShutdown}
3454 * is in effect.
3455 *
3456 * @param command the task to execute
3457 * @param delay the time from now to delay execution
3458 * @param unit the time unit of the delay parameter
3459 * @return a ForkJoinTask implementing the ScheduledFuture
3460 * interface, whose {@code get()} method will return
3461 * {@code null} upon normal completion.
3462 * @throws RejectedExecutionException if the pool is shutdown or
3463 * submission encounters resource exhaustion.
3464 * @throws NullPointerException if command or unit is null
3465 * @since 25
3466 */
3467 public ScheduledFuture<?> schedule(Runnable command,
3468 long delay, TimeUnit unit) {
3469 return scheduleDelayedTask(
3470 new ScheduledForkJoinTask<Void>(
3471 unit.toNanos(delay), 0L, false, // implicit null check of unit
3472 Objects.requireNonNull(command), null, this));
3473 }
3474
3475 /**
3476 * Submits a value-returning one-shot task that becomes enabled for execution
3477 * after the given delay. At that point it will execute unless
3478 * explicitly cancelled, or fail to execute (eventually reporting
3479 * cancellation) when encountering resource exhaustion, or the
3480 * pool is {@link #shutdownNow}, or is {@link #shutdown} when
3481 * otherwise quiescent and {@link #cancelDelayedTasksOnShutdown}
3482 * is in effect.
3483 *
3484 * @param callable the function to execute
3485 * @param delay the time from now to delay execution
3486 * @param unit the time unit of the delay parameter
3487 * @param <V> the type of the callable's result
3488 * @return a ForkJoinTask implementing the ScheduledFuture
3489 * interface, whose {@code get()} method will return the
3490 * value from the callable upon normal completion.
3491 * @throws RejectedExecutionException if the pool is shutdown or
3492 * submission encounters resource exhaustion.
3493 * @throws NullPointerException if command or unit is null
3494 * @since 25
3495 */
3496 public <V> ScheduledFuture<V> schedule(Callable<V> callable,
3497 long delay, TimeUnit unit) {
3498 return scheduleDelayedTask(
3499 new ScheduledForkJoinTask<V>(
3500 unit.toNanos(delay), 0L, false, null, // implicit null check of unit
3501 Objects.requireNonNull(callable), this));
3502 }
3503
3504 /**
3505 * Submits a periodic action that becomes enabled for execution first after the
3506 * given initial delay, and subsequently with the given period;
3507 * that is, executions will commence after
3508 * {@code initialDelay}, then {@code initialDelay + period}, then
3509 * {@code initialDelay + 2 * period}, and so on.
3510 *
3511 * <p>The sequence of task executions continues indefinitely until
3512 * one of the following exceptional completions occur:
3513 * <ul>
3514 * <li>The task is {@linkplain Future#cancel explicitly cancelled}
3515 * <li>Method {@link #shutdownNow} is called
3516 * <li>Method {@link #shutdown} is called and the pool is
3517 * otherwise quiescent, in which case existing executions continue
3518 * but subsequent executions do not.
3519 * <li>An execution or the task encounters resource exhaustion.
3520 * <li>An execution of the task throws an exception. In this case
3521 * calling {@link Future#get() get} on the returned future will throw
3522 * {@link ExecutionException}, holding the exception as its cause.
3523 * </ul>
3524 * Subsequent executions are suppressed. Subsequent calls to
3525 * {@link Future#isDone isDone()} on the returned future will
3526 * return {@code true}.
3527 *
3528 * <p>If any execution of this task takes longer than its period, then
3529 * subsequent executions may start late, but will not concurrently
3530 * execute.
3531 * @param command the task to execute
3532 * @param initialDelay the time to delay first execution
3533 * @param period the period between successive executions
3534 * @param unit the time unit of the initialDelay and period parameters
3535 * @return a ForkJoinTask implementing the ScheduledFuture
3536 * interface. The future's {@link Future#get() get()}
3537 * method will never return normally, and will throw an
3538 * exception upon task cancellation or abnormal
3539 * termination of a task execution.
3540 * @throws RejectedExecutionException if the pool is shutdown or
3541 * submission encounters resource exhaustion.
3542 * @throws NullPointerException if command or unit is null
3543 * @throws IllegalArgumentException if period less than or equal to zero
3544 * @since 25
3545 */
3546 public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
3547 long initialDelay,
3548 long period, TimeUnit unit) {
3549 if (period <= 0L)
3550 throw new IllegalArgumentException();
3551 return scheduleDelayedTask(
3552 new ScheduledForkJoinTask<Void>(
3553 unit.toNanos(initialDelay), // implicit null check of unit
3554 unit.toNanos(period), false,
3555 Objects.requireNonNull(command), null, this));
3556 }
3557
3558 /**
3559 * Submits a periodic action that becomes enabled for execution first after the
3560 * given initial delay, and subsequently with the given delay
3561 * between the termination of one execution and the commencement of
3562 * the next.
3563 * <p>The sequence of task executions continues indefinitely until
3564 * one of the following exceptional completions occur:
3565 * <ul>
3566 * <li>The task is {@linkplain Future#cancel explicitly cancelled}
3567 * <li>Method {@link #shutdownNow} is called
3568 * <li>Method {@link #shutdown} is called and the pool is
3569 * otherwise quiescent, in which case existing executions continue
3570 * but subsequent executions do not.
3571 * <li>An execution or the task encounters resource exhaustion.
3572 * <li>An execution of the task throws an exception. In this case
3573 * calling {@link Future#get() get} on the returned future will throw
3574 * {@link ExecutionException}, holding the exception as its cause.
3575 * </ul>
3576 * Subsequent executions are suppressed. Subsequent calls to
3577 * {@link Future#isDone isDone()} on the returned future will
3578 * return {@code true}.
3579 * @param command the task to execute
3580 * @param initialDelay the time to delay first execution
3581 * @param delay the delay between the termination of one
3582 * execution and the commencement of the next
3583 * @param unit the time unit of the initialDelay and delay parameters
3584 * @return a ForkJoinTask implementing the ScheduledFuture
3585 * interface. The future's {@link Future#get() get()}
3586 * method will never return normally, and will throw an
3587 * exception upon task cancellation or abnormal
3588 * termination of a task execution.
3589 * @throws RejectedExecutionException if the pool is shutdown or
3590 * submission encounters resource exhaustion.
3591 * @throws NullPointerException if command or unit is null
3592 * @throws IllegalArgumentException if delay less than or equal to zero
3593 * @since 25
3594 */
3595 public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
3596 long initialDelay,
3597 long delay, TimeUnit unit) {
3598 if (delay <= 0L)
3599 throw new IllegalArgumentException();
3600 return scheduleDelayedTask(
3601 new ScheduledForkJoinTask<Void>(
3602 unit.toNanos(initialDelay), // implicit null check of unit
3603 -unit.toNanos(delay), false, // negative for fixed delay
3604 Objects.requireNonNull(command), null, this));
3605 }
3606
3607 /**
3608 * Body of a task performed on timeout of another task
3609 */
3610 static final class TimeoutAction<V> implements Runnable {
3611 // set after construction, nulled after use
3612 ForkJoinTask.CallableWithTimeout<V> task;
3613 Consumer<? super ForkJoinTask<V>> action;
3614 TimeoutAction(Consumer<? super ForkJoinTask<V>> action) {
3615 this.action = action;
3616 }
3617 public void run() {
3618 ForkJoinTask.CallableWithTimeout<V> t = task;
3619 Consumer<? super ForkJoinTask<V>> a = action;
3620 task = null;
3621 action = null;
3622 if (t != null && t.status >= 0) {
3623 if (a == null)
3624 t.cancel(true);
3625 else {
3626 a.accept(t);
3627 t.interruptIfRunning(true);
3628 }
3629 }
3630 }
3631 }
3632
3633 /**
3634 * Submits a task executing the given function, cancelling the
3635 * task or performing a given timeoutAction if not completed
3636 * within the given timeout period. If the optional {@code
3637 * timeoutAction} is null, the task is cancelled (via {@code
3638 * cancel(true)}. Otherwise, the action is applied and the task
3639 * may be interrupted if running. Actions may include {@link
3640 * ForkJoinTask#complete} to set a replacement value or {@link
3641 * ForkJoinTask#completeExceptionally} to throw an appropriate
3642 * exception. Note that these can succeed only if the task has
3643 * not already completed when the timeoutAction executes.
3644 *
3645 * @param callable the function to execute
3646 * @param <V> the type of the callable's result
3647 * @param timeout the time to wait before cancelling if not completed
3648 * @param timeoutAction if nonnull, an action to perform on
3649 * timeout, otherwise the default action is to cancel using
3650 * {@code cancel(true)}.
3651 * @param unit the time unit of the timeout parameter
3652 * @return a Future that can be used to extract result or cancel
3653 * @throws RejectedExecutionException if the task cannot be
3654 * scheduled for execution
3655 * @throws NullPointerException if callable or unit is null
3656 * @since 25
3657 */
3658 public <V> ForkJoinTask<V> submitWithTimeout(Callable<V> callable,
3659 long timeout, TimeUnit unit,
3660 Consumer<? super ForkJoinTask<V>> timeoutAction) {
3661 ForkJoinTask.CallableWithTimeout<V> task; TimeoutAction<V> onTimeout;
3662 Objects.requireNonNull(callable);
3663 ScheduledForkJoinTask<Void> timeoutTask =
3664 new ScheduledForkJoinTask<Void>(
3665 unit.toNanos(timeout), 0L, true,
3666 onTimeout = new TimeoutAction<V>(timeoutAction), null, this);
3667 onTimeout.task = task =
3668 new ForkJoinTask.CallableWithTimeout<V>(callable, timeoutTask);
3669 scheduleDelayedTask(timeoutTask);
3670 return poolSubmit(true, task);
3671 }
3672
3673 /**
3674 * Arranges that scheduled tasks that are not executing and have
3675 * not already been enabled for execution will not be executed and
3676 * will be cancelled upon {@link #shutdown} (unless this pool is
3677 * the {@link #commonPool()} which never shuts down). This method
3678 * may be invoked either before {@link #shutdown} to take effect
3679 * upon the next call, or afterwards to cancel such tasks, which
3680 * may then allow termination. Note that subsequent executions of
3681 * periodic tasks are always disabled upon shutdown, so this
3682 * method applies meaningfully only to non-periodic tasks.
3683 * @since 25
3684 */
3685 public void cancelDelayedTasksOnShutdown() {
3686 DelayScheduler ds;
3687 if ((ds = delayScheduler) != null ||
3688 (ds = startDelayScheduler()) != null)
3689 ds.cancelDelayedTasksOnShutdown();
3690 }
3691
3692 /**
3693 * Returns the factory used for constructing new workers.
3694 *
3695 * @return the factory used for constructing new workers
3696 */
3697 public ForkJoinWorkerThreadFactory getFactory() {
3698 return factory;
3699 }
3700
3701 /**
3702 * Returns the handler for internal worker threads that terminate
3703 * due to unrecoverable errors encountered while executing tasks.
3704 *
3705 * @return the handler, or {@code null} if none
3706 */
3707 public UncaughtExceptionHandler getUncaughtExceptionHandler() {
3708 return ueh;
3709 }
3710
3711 /**
3712 * Returns the targeted parallelism level of this pool.
3713 *
3714 * @return the targeted parallelism level of this pool
3715 */
3716 public int getParallelism() {
3717 return Math.max(getParallelismOpaque(), 1);
3718 }
3719
3720 /**
3721 * Returns the targeted parallelism level of the common pool.
3722 *
3723 * @return the targeted parallelism level of the common pool
3724 * @since 1.8
3725 */
3726 public static int getCommonPoolParallelism() {
3727 return common.getParallelism();
3728 }
3729
3730 /**
3731 * Returns the number of worker threads that have started but not
3732 * yet terminated. The result returned by this method may differ
3733 * from {@link #getParallelism} when threads are created to
3734 * maintain parallelism when others are cooperatively blocked.
3735 *
3736 * @return the number of worker threads
3737 */
3738 public int getPoolSize() {
3739 return (short)(ctl >>> TC_SHIFT);
3740 }
3741
3742 /**
3743 * Returns {@code true} if this pool uses local first-in-first-out
3744 * scheduling mode for forked tasks that are never joined.
3745 *
3746 * @return {@code true} if this pool uses async mode
3747 */
3748 public boolean getAsyncMode() {
3749 return (config & FIFO) != 0;
3750 }
3751
3752 /**
3753 * Returns an estimate of the number of worker threads that are
3754 * not blocked waiting to join tasks or for other managed
3755 * synchronization. This method may overestimate the
3756 * number of running threads.
3757 *
3758 * @return the number of worker threads
3759 */
3760 public int getRunningThreadCount() {
3761 WorkQueue[] qs; WorkQueue q;
3762 int rc = 0;
3763 if ((runState & TERMINATED) == 0L && (qs = queues) != null) {
3764 for (int i = 1; i < qs.length; i += 2) {
3765 if ((q = qs[i]) != null && q.isApparentlyUnblocked())
3766 ++rc;
3767 }
3768 }
3769 return rc;
3770 }
3771
3772 /**
3773 * Returns an estimate of the number of threads that are currently
3774 * stealing or executing tasks. This method may overestimate the
3775 * number of active threads.
3776 *
3777 * @return the number of active threads
3778 */
3779 public int getActiveThreadCount() {
3780 return Math.max((short)(ctl >>> RC_SHIFT), 0);
3781 }
3782
3783 /**
3784 * Returns {@code true} if all worker threads are currently idle.
3785 * An idle worker is one that cannot obtain a task to execute
3786 * because none are available to steal from other threads, and
3787 * there are no pending submissions to the pool. This method is
3788 * conservative; it might not return {@code true} immediately upon
3789 * idleness of all threads, but will eventually become true if
3790 * threads remain inactive.
3791 *
3792 * @return {@code true} if all threads are currently idle
3793 */
3794 public boolean isQuiescent() {
3795 return quiescent() >= 0;
3796 }
3797
3798 /**
3799 * Returns an estimate of the total number of completed tasks that
3800 * were executed by a thread other than their submitter. The
3801 * reported value underestimates the actual total number of steals
3802 * when the pool is not quiescent. This value may be useful for
3803 * monitoring and tuning fork/join programs: in general, steal
3804 * counts should be high enough to keep threads busy, but low
3805 * enough to avoid overhead and contention across threads.
3806 *
3807 * @return the number of steals
3808 */
3809 public long getStealCount() {
3810 long count = stealCount;
3811 WorkQueue[] qs; WorkQueue q;
3812 if ((qs = queues) != null) {
3813 for (int i = 1; i < qs.length; i += 2) {
3814 if ((q = qs[i]) != null)
3815 count += (long)q.nsteals & 0xffffffffL;
3816 }
3817 }
3818 return count;
3819 }
3820
3821 /**
3822 * Returns an estimate of the total number of tasks currently held
3823 * in queues by worker threads (but not including tasks submitted
3824 * to the pool that have not begun executing). This value is only
3825 * an approximation, obtained by iterating across all threads in
3826 * the pool. This method may be useful for tuning task
3827 * granularities.The returned count does not include scheduled
3828 * tasks that are not yet ready to execute, which are reported
3829 * separately by method {@link getDelayedTaskCount}.
3830 *
3831 * @return the number of queued tasks
3832 * @see ForkJoinWorkerThread#getQueuedTaskCount()
3833 */
3834 public long getQueuedTaskCount() {
3835 WorkQueue[] qs; WorkQueue q;
3836 long count = 0;
3837 if ((runState & TERMINATED) == 0L && (qs = queues) != null) {
3838 for (int i = 1; i < qs.length; i += 2) {
3839 if ((q = qs[i]) != null)
3840 count += q.queueSize();
3841 }
3842 }
3843 return count;
3844 }
3845
3846 /**
3847 * Returns an estimate of the number of tasks submitted to this
3848 * pool that have not yet begun executing. This method may take
3849 * time proportional to the number of submissions.
3850 *
3851 * @return the number of queued submissions
3852 */
3853 public int getQueuedSubmissionCount() {
3854 WorkQueue[] qs; WorkQueue q;
3855 int count = 0;
3856 if ((runState & TERMINATED) == 0L && (qs = queues) != null) {
3857 for (int i = 0; i < qs.length; i += 2) {
3858 if ((q = qs[i]) != null)
3859 count += q.queueSize();
3860 }
3861 }
3862 return count;
3863 }
3864
3865 /**
3866 * Returns an estimate of the number of delayed (including
3867 * periodic) tasks scheduled in this pool that are not yet ready
3868 * to submit for execution. The returned value is inaccurate while
3869 * delayed tasks are being processed.
3870 *
3871 * @return an estimate of the number of delayed tasks
3872 * @since 25
3873 */
3874 public long getDelayedTaskCount() {
3875 DelayScheduler ds;
3876 return ((ds = delayScheduler) == null ? 0 : ds.lastStableSize());
3877 }
3878
3879 /**
3880 * Returns {@code true} if there are any tasks submitted to this
3881 * pool that have not yet begun executing.
3882 *
3883 * @return {@code true} if there are any queued submissions
3884 */
3885 public boolean hasQueuedSubmissions() {
3886 WorkQueue[] qs; WorkQueue q;
3887 if ((runState & STOP) == 0L && (qs = queues) != null) {
3888 for (int i = 0; i < qs.length; i += 2) {
3889 if ((q = qs[i]) != null && q.queueSize() > 0)
3890 return true;
3891 }
3892 }
3893 return false;
3894 }
3895
3896 /**
3897 * Removes and returns the next unexecuted submission if one is
3898 * available. This method may be useful in extensions to this
3899 * class that re-assign work in systems with multiple pools.
3900 *
3901 * @return the next submission, or {@code null} if none
3902 */
3903 protected ForkJoinTask<?> pollSubmission() {
3904 return pollScan(true);
3905 }
3906
3907 /**
3908 * Removes all available unexecuted submitted and forked tasks
3909 * from scheduling queues and adds them to the given collection,
3910 * without altering their execution status. These may include
3911 * artificially generated or wrapped tasks. This method is
3912 * designed to be invoked only when the pool is known to be
3913 * quiescent. Invocations at other times may not remove all
3914 * tasks. A failure encountered while attempting to add elements
3915 * to collection {@code c} may result in elements being in
3916 * neither, either or both collections when the associated
3917 * exception is thrown. The behavior of this operation is
3918 * undefined if the specified collection is modified while the
3919 * operation is in progress.
3920 *
3921 * @param c the collection to transfer elements into
3922 * @return the number of elements transferred
3923 */
3924 protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
3925 int count = 0;
3926 for (ForkJoinTask<?> t; (t = pollScan(false)) != null; ) {
3927 c.add(t);
3928 ++count;
3929 }
3930 return count;
3931 }
3932
3933 /**
3934 * Returns a string identifying this pool, as well as its state,
3935 * including indications of run state, parallelism level, and
3936 * worker and task counts.
3937 *
3938 * @return a string identifying this pool, as well as its state
3939 */
3940 public String toString() {
3941 // Use a single pass through queues to collect counts
3942 DelayScheduler ds;
3943 long e = runState;
3944 long st = stealCount;
3945 long qt = 0L, ss = 0L; int rc = 0;
3946 WorkQueue[] qs; WorkQueue q;
3947 if ((qs = queues) != null) {
3948 for (int i = 0; i < qs.length; ++i) {
3949 if ((q = qs[i]) != null) {
3950 int size = q.queueSize();
3951 if ((i & 1) == 0)
3952 ss += size;
3953 else {
3954 qt += size;
3955 st += (long)q.nsteals & 0xffffffffL;
3956 if (q.isApparentlyUnblocked())
3957 ++rc;
3958 }
3959 }
3960 }
3961 }
3962 String delayed = ((ds = delayScheduler) == null ? "" :
3963 ", delayed = " + ds.lastStableSize());
3964 int pc = parallelism;
3965 long c = ctl;
3966 int tc = (short)(c >>> TC_SHIFT);
3967 int ac = (short)(c >>> RC_SHIFT);
3968 if (ac < 0) // ignore transient negative
3969 ac = 0;
3970 String level = ((e & TERMINATED) != 0L ? "Terminated" :
3971 (e & STOP) != 0L ? "Terminating" :
3972 (e & SHUTDOWN) != 0L ? "Shutting down" :
3973 "Running");
3974 return super.toString() +
3975 "[" + level +
3976 ", parallelism = " + pc +
3977 ", size = " + tc +
3978 ", active = " + ac +
3979 ", running = " + rc +
3980 ", steals = " + st +
3981 ", tasks = " + qt +
3982 ", submissions = " + ss +
3983 delayed +
3984 "]";
3985 }
3986
3987 /**
3988 * Possibly initiates an orderly shutdown in which previously
3989 * submitted tasks are executed, but no new tasks will be
3990 * accepted. Invocation has no effect on execution state if this
3991 * is the {@link #commonPool()}, and no additional effect if
3992 * already shut down. Tasks that are in the process of being
3993 * submitted concurrently during the course of this method may or
3994 * may not be rejected.
3995 */
3996 public void shutdown() {
3997 if (workerNamePrefix != null) // not common pool
3998 tryTerminate(false, true);
3999 }
4000
4001 /**
4002 * Possibly attempts to cancel and/or stop all tasks, and reject
4003 * all subsequently submitted tasks. Invocation has no effect on
4004 * execution state if this is the {@link #commonPool()}, and no
4005 * additional effect if already shut down. Otherwise, tasks that
4006 * are in the process of being submitted or executed concurrently
4007 * during the course of this method may or may not be
4008 * rejected. This method cancels both existing and unexecuted
4009 * tasks, in order to permit termination in the presence of task
4010 * dependencies. So the method always returns an empty list
4011 * (unlike the case for some other Executors).
4012 *
4013 * @return an empty list
4014 */
4015 public List<Runnable> shutdownNow() {
4016 if (workerNamePrefix != null) // not common pool
4017 tryTerminate(true, true);
4018 return Collections.emptyList();
4019 }
4020
4021 /**
4022 * Returns {@code true} if all tasks have completed following shut down.
4023 *
4024 * @return {@code true} if all tasks have completed following shut down
4025 */
4026 public boolean isTerminated() {
4027 return (tryTerminate(false, false) & TERMINATED) != 0;
4028 }
4029
4030 /**
4031 * Returns {@code true} if the process of termination has
4032 * commenced but not yet completed. This method may be useful for
4033 * debugging. A return of {@code true} reported a sufficient
4034 * period after shutdown may indicate that submitted tasks have
4035 * ignored or suppressed interruption, or are waiting for I/O,
4036 * causing this executor not to properly terminate. (See the
4037 * advisory notes for class {@link ForkJoinTask} stating that
4038 * tasks should not normally entail blocking operations. But if
4039 * they do, they must abort them on interrupt.)
4040 *
4041 * @return {@code true} if terminating but not yet terminated
4042 */
4043 public boolean isTerminating() {
4044 return (tryTerminate(false, false) & (STOP | TERMINATED)) == STOP;
4045 }
4046
4047 /**
4048 * Returns {@code true} if this pool has been shut down.
4049 *
4050 * @return {@code true} if this pool has been shut down
4051 */
4052 public boolean isShutdown() {
4053 return (runState & SHUTDOWN) != 0L;
4054 }
4055
4056 /**
4057 * Blocks until all tasks have completed execution after a
4058 * shutdown request, or the timeout occurs, or the current thread
4059 * is interrupted, whichever happens first. Because the {@link
4060 * #commonPool()} never terminates until program shutdown, when
4061 * applied to the common pool, this method is equivalent to {@link
4062 * #awaitQuiescence(long, TimeUnit)} but always returns {@code false}.
4063 *
4064 * @param timeout the maximum time to wait
4065 * @param unit the time unit of the timeout argument
4066 * @return {@code true} if this executor terminated and
4067 * {@code false} if the timeout elapsed before termination
4068 * @throws InterruptedException if interrupted while waiting
4069 */
4070 public boolean awaitTermination(long timeout, TimeUnit unit)
4071 throws InterruptedException {
4072 long nanos = unit.toNanos(timeout);
4073 CountDownLatch done;
4074 if (workerNamePrefix == null) { // is common pool
4075 if (helpQuiescePool(this, nanos, true) < 0)
4076 throw new InterruptedException();
4077 return false;
4078 }
4079 else if ((tryTerminate(false, false) & TERMINATED) != 0 ||
4080 (done = terminationSignal()) == null ||
4081 (runState & TERMINATED) != 0L)
4082 return true;
4083 else
4084 return done.await(nanos, TimeUnit.NANOSECONDS);
4085 }
4086
4087 /**
4088 * If called by a ForkJoinTask operating in this pool, equivalent
4089 * in effect to {@link ForkJoinTask#helpQuiesce}. Otherwise,
4090 * waits and/or attempts to assist performing tasks until this
4091 * pool {@link #isQuiescent} or the indicated timeout elapses.
4092 *
4093 * @param timeout the maximum time to wait
4094 * @param unit the time unit of the timeout argument
4095 * @return {@code true} if quiescent; {@code false} if the
4096 * timeout elapsed.
4097 */
4098 public boolean awaitQuiescence(long timeout, TimeUnit unit) {
4099 return (helpQuiescePool(this, unit.toNanos(timeout), false) > 0);
4100 }
4101
4102 /**
4103 * Unless this is the {@link #commonPool()}, initiates an orderly
4104 * shutdown in which previously submitted tasks are executed, but
4105 * no new tasks will be accepted, and waits until all tasks have
4106 * completed execution and the executor has terminated.
4107 *
4108 * <p> If already terminated, or this is the {@link
4109 * #commonPool()}, this method has no effect on execution, and
4110 * does not wait. Otherwise, if interrupted while waiting, this
4111 * method stops all executing tasks as if by invoking {@link
4112 * #shutdownNow()}. It then continues to wait until all actively
4113 * executing tasks have completed. Tasks that were awaiting
4114 * execution are not executed. The interrupt status will be
4115 * re-asserted before this method returns.
4116 *
4117 * @since 19
4118 */
4119 @Override
4120 public void close() {
4121 if (workerNamePrefix != null) {
4122 CountDownLatch done = null;
4123 boolean interrupted = false;
4124 while ((tryTerminate(interrupted, true) & TERMINATED) == 0) {
4125 if (done == null)
4126 done = terminationSignal();
4127 else {
4128 try {
4129 done.await();
4130 break;
4131 } catch (InterruptedException ex) {
4132 interrupted = true;
4133 }
4134 }
4135 }
4136 if (interrupted)
4137 Thread.currentThread().interrupt();
4138 }
4139 }
4140
4141 /**
4142 * Interface for extending managed parallelism for tasks running
4143 * in {@link ForkJoinPool}s.
4144 *
4145 * <p>A {@code ManagedBlocker} provides two methods. Method
4146 * {@link #isReleasable} must return {@code true} if blocking is
4147 * not necessary. Method {@link #block} blocks the current thread
4148 * if necessary (perhaps internally invoking {@code isReleasable}
4149 * before actually blocking). These actions are performed by any
4150 * thread invoking {@link
4151 * ForkJoinPool#managedBlock(ManagedBlocker)}. The unusual
4152 * methods in this API accommodate synchronizers that may, but
4153 * don't usually, block for long periods. Similarly, they allow
4154 * more efficient internal handling of cases in which additional
4155 * workers may be, but usually are not, needed to ensure
4156 * sufficient parallelism. Toward this end, implementations of
4157 * method {@code isReleasable} must be amenable to repeated
4158 * invocation. Neither method is invoked after a prior invocation
4159 * of {@code isReleasable} or {@code block} returns {@code true}.
4160 *
4161 * <p>For example, here is a ManagedBlocker based on a
4162 * ReentrantLock:
4163 * <pre> {@code
4164 * class ManagedLocker implements ManagedBlocker {
4165 * final ReentrantLock lock;
4166 * boolean hasLock = false;
4167 * ManagedLocker(ReentrantLock lock) { this.lock = lock; }
4168 * public boolean block() {
4169 * if (!hasLock)
4170 * lock.lock();
4171 * return true;
4172 * }
4173 * public boolean isReleasable() {
4174 * return hasLock || (hasLock = lock.tryLock());
4175 * }
4176 * }}</pre>
4177 *
4178 * <p>Here is a class that possibly blocks waiting for an
4179 * item on a given queue:
4180 * <pre> {@code
4181 * class QueueTaker<E> implements ManagedBlocker {
4182 * final BlockingQueue<E> queue;
4183 * volatile E item = null;
4184 * QueueTaker(BlockingQueue<E> q) { this.queue = q; }
4185 * public boolean block() throws InterruptedException {
4186 * if (item == null)
4187 * item = queue.take();
4188 * return true;
4189 * }
4190 * public boolean isReleasable() {
4191 * return item != null || (item = queue.poll()) != null;
4192 * }
4193 * public E getItem() { // call after pool.managedBlock completes
4194 * return item;
4195 * }
4196 * }}</pre>
4197 */
4198 public static interface ManagedBlocker {
4199 /**
4200 * Possibly blocks the current thread, for example waiting for
4201 * a lock or condition.
4202 *
4203 * @return {@code true} if no additional blocking is necessary
4204 * (i.e., if isReleasable would return true)
4205 * @throws InterruptedException if interrupted while waiting
4206 * (the method is not required to do so, but is allowed to)
4207 */
4208 boolean block() throws InterruptedException;
4209
4210 /**
4211 * Returns {@code true} if blocking is unnecessary.
4212 * @return {@code true} if blocking is unnecessary
4213 */
4214 boolean isReleasable();
4215 }
4216
4217 /**
4218 * Runs the given possibly blocking task. When {@linkplain
4219 * ForkJoinTask#inForkJoinPool() running in a ForkJoinPool}, this
4220 * method possibly arranges for a spare thread to be activated if
4221 * necessary to ensure sufficient parallelism while the current
4222 * thread is blocked in {@link ManagedBlocker#block blocker.block()}.
4223 *
4224 * <p>This method repeatedly calls {@code blocker.isReleasable()} and
4225 * {@code blocker.block()} until either method returns {@code true}.
4226 * Every call to {@code blocker.block()} is preceded by a call to
4227 * {@code blocker.isReleasable()} that returned {@code false}.
4228 *
4229 * <p>If not running in a ForkJoinPool, this method is
4230 * behaviorally equivalent to
4231 * <pre> {@code
4232 * while (!blocker.isReleasable())
4233 * if (blocker.block())
4234 * break;}</pre>
4235 *
4236 * If running in a ForkJoinPool, the pool may first be expanded to
4237 * ensure sufficient parallelism available during the call to
4238 * {@code blocker.block()}.
4239 *
4240 * @param blocker the blocker task
4241 * @throws InterruptedException if {@code blocker.block()} did so
4242 */
4243 public static void managedBlock(ManagedBlocker blocker)
4244 throws InterruptedException {
4245 Thread t; ForkJoinPool p;
4246 if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread &&
4247 (p = ((ForkJoinWorkerThread)t).pool) != null)
4248 p.compensatedBlock(blocker);
4249 else
4250 unmanagedBlock(blocker);
4251 }
4252
4253 /** ManagedBlock for ForkJoinWorkerThreads */
4254 private void compensatedBlock(ManagedBlocker blocker)
4255 throws InterruptedException {
4256 Objects.requireNonNull(blocker);
4257 for (;;) {
4258 int comp; boolean done;
4259 long c = ctl;
4260 if (blocker.isReleasable())
4261 break;
4262 if ((runState & STOP) != 0L)
4263 throw new InterruptedException();
4264 if ((comp = tryCompensate(c)) >= 0) {
4265 try {
4266 done = blocker.block();
4267 } finally {
4268 if (comp > 0)
4269 getAndAddCtl(RC_UNIT);
4270 }
4271 if (done)
4272 break;
4273 }
4274 }
4275 }
4276
4277 /**
4278 * Invokes tryCompensate to create or re-activate a spare thread to
4279 * compensate for a thread that performs a blocking operation. When the
4280 * blocking operation is done then endCompensatedBlock must be invoked
4281 * with the value returned by this method to re-adjust the parallelism.
4282 * @return value to use in endCompensatedBlock
4283 */
4284 final long beginCompensatedBlock() {
4285 int c;
4286 do {} while ((c = tryCompensate(ctl)) < 0);
4287 return (c == 0) ? 0L : RC_UNIT;
4288 }
4289
4290 /**
4291 * Re-adjusts parallelism after a blocking operation completes.
4292 * @param post value from beginCompensatedBlock
4293 */
4294 void endCompensatedBlock(long post) {
4295 if (post > 0L) {
4296 getAndAddCtl(post);
4297 }
4298 }
4299
4300 /** ManagedBlock for external threads */
4301 private static void unmanagedBlock(ManagedBlocker blocker)
4302 throws InterruptedException {
4303 Objects.requireNonNull(blocker);
4304 do {} while (!blocker.isReleasable() && !blocker.block());
4305 }
4306
4307 @Override
4308 protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
4309 Objects.requireNonNull(runnable);
4310 return (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
4311 new ForkJoinTask.AdaptedRunnable<T>(runnable, value) :
4312 new ForkJoinTask.AdaptedInterruptibleRunnable<T>(runnable, value);
4313 }
4314
4315 @Override
4316 protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
4317 Objects.requireNonNull(callable);
4318 return (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
4319 new ForkJoinTask.AdaptedCallable<T>(callable) :
4320 new ForkJoinTask.AdaptedInterruptibleCallable<T>(callable);
4321 }
4322
4323 static {
4324 U = Unsafe.getUnsafe();
4325 Class<ForkJoinPool> klass = ForkJoinPool.class;
4326 try {
4327 Field poolIdsField = klass.getDeclaredField("poolIds");
4328 POOLIDS_BASE = U.staticFieldBase(poolIdsField);
4329 POOLIDS = U.staticFieldOffset(poolIdsField);
4330 } catch (NoSuchFieldException e) {
4331 throw new ExceptionInInitializerError(e);
4332 }
4333 CTL = U.objectFieldOffset(klass, "ctl");
4334 RUNSTATE = U.objectFieldOffset(klass, "runState");
4335 PARALLELISM = U.objectFieldOffset(klass, "parallelism");
4336 THREADIDS = U.objectFieldOffset(klass, "threadIds");
4337 TERMINATION = U.objectFieldOffset(klass, "termination");
4338 Class<ForkJoinTask[]> aklass = ForkJoinTask[].class;
4339 ABASE = U.arrayBaseOffset(aklass);
4340 int scale = U.arrayIndexScale(aklass);
4341 ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
4342 if ((scale & (scale - 1)) != 0)
4343 throw new Error("array index scale not a power of two");
4344
4345 Class<?> dep = LockSupport.class; // ensure loaded
4346 // allow access to non-public methods
4347 JLA = SharedSecrets.getJavaLangAccess();
4348 SharedSecrets.setJavaUtilConcurrentFJPAccess(
4349 new JavaUtilConcurrentFJPAccess() {
4350 @Override
4351 public long beginCompensatedBlock(ForkJoinPool pool) {
4352 return pool.beginCompensatedBlock();
4353 }
4354 public void endCompensatedBlock(ForkJoinPool pool, long post) {
4355 pool.endCompensatedBlock(post);
4356 }
4357 });
4358 defaultForkJoinWorkerThreadFactory =
4359 new DefaultForkJoinWorkerThreadFactory();
4360 common = new ForkJoinPool((byte)0);
4361 }
4362 }