1 /*
2 * Copyright (c) 2018, 2024, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25 package java.lang;
26
27 import java.util.Locale;
28 import java.util.Objects;
29 import java.util.concurrent.CountDownLatch;
30 import java.util.concurrent.Executor;
31 import java.util.concurrent.Executors;
32 import java.util.concurrent.ForkJoinPool;
33 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
34 import java.util.concurrent.ForkJoinTask;
35 import java.util.concurrent.ForkJoinWorkerThread;
36 import java.util.concurrent.Future;
37 import java.util.concurrent.RejectedExecutionException;
38 import java.util.concurrent.ScheduledExecutorService;
39 import java.util.concurrent.ScheduledThreadPoolExecutor;
40 import java.util.concurrent.TimeUnit;
41 import jdk.internal.event.VirtualThreadEndEvent;
42 import jdk.internal.event.VirtualThreadStartEvent;
43 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
44 import jdk.internal.misc.CarrierThread;
45 import jdk.internal.misc.InnocuousThread;
46 import jdk.internal.misc.Unsafe;
47 import jdk.internal.vm.Continuation;
48 import jdk.internal.vm.ContinuationScope;
49 import jdk.internal.vm.StackableScope;
50 import jdk.internal.vm.ThreadContainer;
51 import jdk.internal.vm.ThreadContainers;
52 import jdk.internal.vm.annotation.ChangesCurrentThread;
53 import jdk.internal.vm.annotation.Hidden;
54 import jdk.internal.vm.annotation.IntrinsicCandidate;
55 import jdk.internal.vm.annotation.JvmtiHideEvents;
56 import jdk.internal.vm.annotation.JvmtiMountTransition;
57 import jdk.internal.vm.annotation.ReservedStackAccess;
58 import sun.nio.ch.Interruptible;
59 import static java.util.concurrent.TimeUnit.*;
60
61 /**
62 * A thread that is scheduled by the Java virtual machine rather than the operating system.
63 */
64 final class VirtualThread extends BaseVirtualThread {
65 private static final Unsafe U = Unsafe.getUnsafe();
66 private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
67 private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
68 private static final ScheduledExecutorService[] DELAYED_TASK_SCHEDULERS = createDelayedTaskSchedulers();
69
70 private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
71 private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
72 private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
73 private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
74 private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
75
76 // scheduler and continuation
77 private final Executor scheduler;
78 private final Continuation cont;
79 private final Runnable runContinuation;
80
81 // virtual thread state, accessed by VM
82 private volatile int state;
83
84 /*
85 * Virtual thread state transitions:
86 *
87 * NEW -> STARTED // Thread.start, schedule to run
88 * STARTED -> TERMINATED // failed to start
89 * STARTED -> RUNNING // first run
90 * RUNNING -> TERMINATED // done
91 *
92 * RUNNING -> PARKING // Thread parking with LockSupport.park
93 * PARKING -> PARKED // cont.yield successful, parked indefinitely
94 * PARKING -> PINNED // cont.yield failed, parked indefinitely on carrier
95 * PARKED -> UNPARKED // unparked, may be scheduled to continue
96 * PINNED -> RUNNING // unparked, continue execution on same carrier
97 * UNPARKED -> RUNNING // continue execution after park
98 *
99 * RUNNING -> TIMED_PARKING // Thread parking with LockSupport.parkNanos
100 * TIMED_PARKING -> TIMED_PARKED // cont.yield successful, timed-parked
101 * TIMED_PARKING -> TIMED_PINNED // cont.yield failed, timed-parked on carrier
102 * TIMED_PARKED -> UNPARKED // unparked, may be scheduled to continue
103 * TIMED_PINNED -> RUNNING // unparked, continue execution on same carrier
104 *
105 * RUNNING -> BLOCKING // blocking on monitor enter
106 * BLOCKING -> BLOCKED // blocked on monitor enter
107 * BLOCKED -> UNBLOCKED // unblocked, may be scheduled to continue
108 * UNBLOCKED -> RUNNING // continue execution after blocked on monitor enter
109 *
110 * RUNNING -> WAITING // transitional state during wait on monitor
111 * WAITING -> WAITED // waiting on monitor
112 * WAITED -> BLOCKED // notified, waiting to be unblocked by monitor owner
113 * WAITED -> UNBLOCKED // timed-out/interrupted
114 *
115 * RUNNING -> TIMED_WAITING // transition state during timed-waiting on monitor
116 * TIMED_WAITING -> TIMED_WAITED // timed-waiting on monitor
117 * TIMED_WAITED -> BLOCKED // notified, waiting to be unblocked by monitor owner
118 * TIMED_WAITED -> UNBLOCKED // timed-out/interrupted
119 *
120 * RUNNING -> YIELDING // Thread.yield
121 * YIELDING -> YIELDED // cont.yield successful, may be scheduled to continue
122 * YIELDING -> RUNNING // cont.yield failed
123 * YIELDED -> RUNNING // continue execution after Thread.yield
124 */
125 private static final int NEW = 0;
126 private static final int STARTED = 1;
127 private static final int RUNNING = 2; // runnable-mounted
128
129 // untimed and timed parking
130 private static final int PARKING = 3;
131 private static final int PARKED = 4; // unmounted
132 private static final int PINNED = 5; // mounted
133 private static final int TIMED_PARKING = 6;
134 private static final int TIMED_PARKED = 7; // unmounted
135 private static final int TIMED_PINNED = 8; // mounted
136 private static final int UNPARKED = 9; // unmounted but runnable
137
138 // Thread.yield
139 private static final int YIELDING = 10;
140 private static final int YIELDED = 11; // unmounted but runnable
141
142 // monitor enter
143 private static final int BLOCKING = 12;
144 private static final int BLOCKED = 13; // unmounted
145 private static final int UNBLOCKED = 14; // unmounted but runnable
146
147 // monitor wait/timed-wait
148 private static final int WAITING = 15;
149 private static final int WAIT = 16; // waiting in Object.wait
150 private static final int TIMED_WAITING = 17;
151 private static final int TIMED_WAIT = 18; // waiting in timed-Object.wait
152
153 private static final int TERMINATED = 99; // final state
154
155 // can be suspended from scheduling when unmounted
156 private static final int SUSPENDED = 1 << 8;
157
158 // parking permit made available by LockSupport.unpark
159 private volatile boolean parkPermit;
160
161 // blocking permit made available by unblocker thread when another thread exits monitor
162 private volatile boolean blockPermit;
163
164 // true when on the list of virtual threads waiting to be unblocked
165 private volatile boolean onWaitingList;
166
167 // next virtual thread on the list of virtual threads waiting to be unblocked
168 private volatile VirtualThread next;
169
170 // notified by Object.notify/notifyAll while waiting in Object.wait
171 private volatile boolean notified;
172
173 // timed-wait support
174 private byte timedWaitSeqNo;
175
176 // timeout for timed-park and timed-wait, only accessed on current/carrier thread
177 private long timeout;
178
179 // timer task for timed-park and timed-wait, only accessed on current/carrier thread
180 private Future<?> timeoutTask;
181
182 // carrier thread when mounted, accessed by VM
183 private volatile Thread carrierThread;
184
185 // termination object when joining, created lazily if needed
186 private volatile CountDownLatch termination;
187
188 /**
189 * Returns the default scheduler.
190 */
191 static Executor defaultScheduler() {
192 return DEFAULT_SCHEDULER;
193 }
194
195 /**
196 * Returns the continuation scope used for virtual threads.
197 */
198 static ContinuationScope continuationScope() {
199 return VTHREAD_SCOPE;
200 }
201
202 /**
203 * Creates a new {@code VirtualThread} to run the given task with the given
204 * scheduler. If the given scheduler is {@code null} and the current thread
205 * is a platform thread then the newly created virtual thread will use the
206 * default scheduler. If given scheduler is {@code null} and the current
207 * thread is a virtual thread then the current thread's scheduler is used.
208 *
209 * @param scheduler the scheduler or null
210 * @param name thread name
211 * @param characteristics characteristics
212 * @param task the task to execute
213 */
214 VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
215 super(name, characteristics, /*bound*/ false);
216 Objects.requireNonNull(task);
217
218 // choose scheduler if not specified
219 if (scheduler == null) {
220 Thread parent = Thread.currentThread();
221 if (parent instanceof VirtualThread vparent) {
222 scheduler = vparent.scheduler;
223 } else {
224 scheduler = DEFAULT_SCHEDULER;
225 }
226 }
227
228 this.scheduler = scheduler;
229 this.cont = new VThreadContinuation(this, task);
230 this.runContinuation = this::runContinuation;
231 }
232
233 /**
234 * The continuation that a virtual thread executes.
235 */
236 private static class VThreadContinuation extends Continuation {
237 VThreadContinuation(VirtualThread vthread, Runnable task) {
238 super(VTHREAD_SCOPE, wrap(vthread, task));
239 }
240 @Override
241 protected void onPinned(Continuation.Pinned reason) {
242 }
243 private static Runnable wrap(VirtualThread vthread, Runnable task) {
244 return new Runnable() {
245 @Hidden
246 @JvmtiHideEvents
247 public void run() {
248 vthread.notifyJvmtiStart(); // notify JVMTI
249 try {
250 vthread.run(task);
251 } finally {
252 vthread.notifyJvmtiEnd(); // notify JVMTI
253 }
254 }
255 };
256 }
257 }
258
259 /**
260 * Runs or continues execution on the current thread. The virtual thread is mounted
261 * on the current thread before the task runs or continues. It unmounts when the
262 * task completes or yields.
263 */
264 @ChangesCurrentThread // allow mount/unmount to be inlined
265 private void runContinuation() {
266 // the carrier must be a platform thread
267 if (Thread.currentThread().isVirtual()) {
268 throw new WrongThreadException();
269 }
270
271 // set state to RUNNING
272 int initialState = state();
273 if (initialState == STARTED || initialState == UNPARKED
274 || initialState == UNBLOCKED || initialState == YIELDED) {
275 // newly started or continue after parking/blocking/Thread.yield
276 if (!compareAndSetState(initialState, RUNNING)) {
277 return;
278 }
279 // consume permit when continuing after parking or blocking. If continue
280 // after a timed-park or timed-wait then the timeout task is cancelled.
281 if (initialState == UNPARKED) {
282 cancelTimeoutTask();
283 setParkPermit(false);
284 } else if (initialState == UNBLOCKED) {
285 cancelTimeoutTask();
286 blockPermit = false;
287 }
288 } else {
289 // not runnable
290 return;
291 }
292
293 mount();
294 try {
295 cont.run();
296 } finally {
297 unmount();
298 if (cont.isDone()) {
299 afterDone();
300 } else {
301 afterYield();
302 }
303 }
304 }
305
306 /**
307 * Cancel timeout task when continuing after timed-park or timed-wait.
308 * The timeout task may be executing, or may have already completed.
309 */
310 private void cancelTimeoutTask() {
311 if (timeoutTask != null) {
312 timeoutTask.cancel(false);
313 timeoutTask = null;
314 }
315 }
316
317 /**
318 * Submits the runContinuation task to the scheduler. For the default scheduler,
319 * and calling it on a worker thread, the task will be pushed to the local queue,
320 * otherwise it will be pushed to an external submission queue.
321 * @param scheduler the scheduler
322 * @param retryOnOOME true to retry indefinitely if OutOfMemoryError is thrown
323 * @throws RejectedExecutionException
324 */
325 private void submitRunContinuation(Executor scheduler, boolean retryOnOOME) {
326 boolean done = false;
327 while (!done) {
328 try {
329 // Pin the continuation to prevent the virtual thread from unmounting
330 // when submitting a task. For the default scheduler this ensures that
331 // the carrier doesn't change when pushing a task. For other schedulers
332 // it avoids deadlock that could arise due to carriers and virtual
333 // threads contending for a lock.
334 if (currentThread().isVirtual()) {
335 Continuation.pin();
336 try {
337 scheduler.execute(runContinuation);
338 } finally {
339 Continuation.unpin();
340 }
341 } else {
342 scheduler.execute(runContinuation);
343 }
344 done = true;
345 } catch (RejectedExecutionException ree) {
346 submitFailed(ree);
347 throw ree;
348 } catch (OutOfMemoryError e) {
349 if (retryOnOOME) {
350 U.park(false, 100_000_000); // 100ms
351 } else {
352 throw e;
353 }
354 }
355 }
356 }
357
358 /**
359 * Submits the runContinuation task to the given scheduler as an external submit.
360 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
361 * @throws RejectedExecutionException
362 * @see ForkJoinPool#externalSubmit(ForkJoinTask)
363 */
364 private void externalSubmitRunContinuation(ForkJoinPool pool) {
365 assert Thread.currentThread() instanceof CarrierThread;
366 try {
367 pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
368 } catch (RejectedExecutionException ree) {
369 submitFailed(ree);
370 throw ree;
371 } catch (OutOfMemoryError e) {
372 submitRunContinuation(pool, true);
373 }
374 }
375
376 /**
377 * Submits the runContinuation task to the scheduler. For the default scheduler,
378 * and calling it on a worker thread, the task will be pushed to the local queue,
379 * otherwise it will be pushed to an external submission queue.
380 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
381 * @throws RejectedExecutionException
382 */
383 private void submitRunContinuation() {
384 submitRunContinuation(scheduler, true);
385 }
386
387 /**
388 * Lazy submit the runContinuation task if invoked on a carrier thread and its local
389 * queue is empty. If not empty, or invoked by another thread, then this method works
390 * like submitRunContinuation and just submits the task to the scheduler.
391 * If OutOfMemoryError is thrown then the submit will be retried until it succeeds.
392 * @throws RejectedExecutionException
393 * @see ForkJoinPool#lazySubmit(ForkJoinTask)
394 */
395 private void lazySubmitRunContinuation() {
396 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
397 ForkJoinPool pool = ct.getPool();
398 try {
399 pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
400 } catch (RejectedExecutionException ree) {
401 submitFailed(ree);
402 throw ree;
403 } catch (OutOfMemoryError e) {
404 submitRunContinuation();
405 }
406 } else {
407 submitRunContinuation();
408 }
409 }
410
411 /**
412 * Submits the runContinuation task to the scheduler. For the default scheduler, and
413 * calling it a virtual thread that uses the default scheduler, the task will be
414 * pushed to an external submission queue. This method may throw OutOfMemoryError.
415 * @throws RejectedExecutionException
416 * @throws OutOfMemoryError
417 */
418 private void externalSubmitRunContinuationOrThrow() {
419 if (scheduler == DEFAULT_SCHEDULER && currentCarrierThread() instanceof CarrierThread ct) {
420 try {
421 ct.getPool().externalSubmit(ForkJoinTask.adapt(runContinuation));
422 } catch (RejectedExecutionException ree) {
423 submitFailed(ree);
424 throw ree;
425 }
426 } else {
427 submitRunContinuation(scheduler, false);
428 }
429 }
430
431 /**
432 * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
433 */
434 private void submitFailed(RejectedExecutionException ree) {
435 var event = new VirtualThreadSubmitFailedEvent();
436 if (event.isEnabled()) {
437 event.javaThreadId = threadId();
438 event.exceptionMessage = ree.getMessage();
439 event.commit();
440 }
441 }
442
443 /**
444 * Runs a task in the context of this virtual thread.
445 */
446 private void run(Runnable task) {
447 assert Thread.currentThread() == this && state == RUNNING;
448
449 // emit JFR event if enabled
450 if (VirtualThreadStartEvent.isTurnedOn()) {
451 var event = new VirtualThreadStartEvent();
452 event.javaThreadId = threadId();
453 event.commit();
454 }
455
456 Object bindings = Thread.scopedValueBindings();
457 try {
458 runWith(bindings, task);
459 } catch (Throwable exc) {
460 dispatchUncaughtException(exc);
461 } finally {
462 // pop any remaining scopes from the stack, this may block
463 StackableScope.popAll();
464
465 // emit JFR event if enabled
466 if (VirtualThreadEndEvent.isTurnedOn()) {
467 var event = new VirtualThreadEndEvent();
468 event.javaThreadId = threadId();
469 event.commit();
470 }
471 }
472 }
473
474 /**
475 * Mounts this virtual thread onto the current platform thread. On
476 * return, the current thread is the virtual thread.
477 */
478 @ChangesCurrentThread
479 @ReservedStackAccess
480 private void mount() {
481 // notify JVMTI before mount
482 notifyJvmtiMount(/*hide*/true);
483
484 // sets the carrier thread
485 Thread carrier = Thread.currentCarrierThread();
486 setCarrierThread(carrier);
487
488 // sync up carrier thread interrupt status if needed
489 if (interrupted) {
490 carrier.setInterrupt();
491 } else if (carrier.isInterrupted()) {
492 synchronized (interruptLock) {
493 // need to recheck interrupt status
494 if (!interrupted) {
495 carrier.clearInterrupt();
496 }
497 }
498 }
499
500 // set Thread.currentThread() to return this virtual thread
501 carrier.setCurrentThread(this);
502 }
503
504 /**
505 * Unmounts this virtual thread from the carrier. On return, the
506 * current thread is the current platform thread.
507 */
508 @ChangesCurrentThread
509 @ReservedStackAccess
510 private void unmount() {
511 assert !Thread.holdsLock(interruptLock);
512
513 // set Thread.currentThread() to return the platform thread
514 Thread carrier = this.carrierThread;
515 carrier.setCurrentThread(carrier);
516
517 // break connection to carrier thread, synchronized with interrupt
518 synchronized (interruptLock) {
519 setCarrierThread(null);
520 }
521 carrier.clearInterrupt();
522
523 // notify JVMTI after unmount
524 notifyJvmtiUnmount(/*hide*/false);
525 }
526
527 /**
528 * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
529 * the continuation continues.
530 */
531 @Hidden
532 private boolean yieldContinuation() {
533 notifyJvmtiUnmount(/*hide*/true);
534 try {
535 return Continuation.yield(VTHREAD_SCOPE);
536 } finally {
537 notifyJvmtiMount(/*hide*/false);
538 }
539 }
540
541 /**
542 * Invoked in the context of the carrier thread after the Continuation yields when
543 * parking, blocking on monitor enter, Object.wait, or Thread.yield.
544 */
545 private void afterYield() {
546 assert carrierThread == null;
547
548 // re-adjust parallelism if the virtual thread yielded when compensating
549 if (currentThread() instanceof CarrierThread ct) {
550 ct.endBlocking();
551 }
552
553 int s = state();
554
555 // LockSupport.park/parkNanos
556 if (s == PARKING || s == TIMED_PARKING) {
557 int newState;
558 if (s == PARKING) {
559 setState(newState = PARKED);
560 } else {
561 // schedule unpark
562 assert timeout > 0;
563 timeoutTask = schedule(this::unpark, timeout, NANOSECONDS);
564 setState(newState = TIMED_PARKED);
565 }
566
567 // may have been unparked while parking
568 if (parkPermit && compareAndSetState(newState, UNPARKED)) {
569 // lazy submit if local queue is empty
570 lazySubmitRunContinuation();
571 }
572 return;
573 }
574
575 // Thread.yield
576 if (s == YIELDING) {
577 setState(YIELDED);
578
579 // external submit if there are no tasks in the local task queue
580 if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
581 externalSubmitRunContinuation(ct.getPool());
582 } else {
583 submitRunContinuation();
584 }
585 return;
586 }
587
588 // blocking on monitorenter
589 if (s == BLOCKING) {
590 setState(BLOCKED);
591
592 // may have been unblocked while blocking
593 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
594 // lazy submit if local queue is empty
595 lazySubmitRunContinuation();
596 }
597 return;
598 }
599
600 // Object.wait
601 if (s == WAITING || s == TIMED_WAITING) {
602 int newState;
603 if (s == WAITING) {
604 setState(newState = WAIT);
605 } else {
606 // For timed-wait, a timeout task is scheduled to execute. The timeout
607 // task will change the thread state to UNBLOCKED and submit the thread
608 // to the scheduler. A sequence number is used to ensure that the timeout
609 // task only unblocks the thread for this timed-wait. We synchronize with
610 // the timeout task to coordinate access to the sequence number and to
611 // ensure the timeout task doesn't execute until the thread has got to
612 // the TIMED_WAIT state.
613 assert timeout > 0;
614 synchronized (timedWaitLock()) {
615 byte seqNo = ++timedWaitSeqNo;
616 timeoutTask = schedule(() -> waitTimeoutExpired(seqNo), timeout, MILLISECONDS);
617 setState(newState = TIMED_WAIT);
618 }
619 }
620
621 // may have been notified while in transition to wait state
622 if (notified && compareAndSetState(newState, BLOCKED)) {
623 // may have even been unblocked already
624 if (blockPermit && compareAndSetState(BLOCKED, UNBLOCKED)) {
625 submitRunContinuation();
626 }
627 return;
628 }
629
630 // may have been interrupted while in transition to wait state
631 if (interrupted && compareAndSetState(newState, UNBLOCKED)) {
632 submitRunContinuation();
633 return;
634 }
635 return;
636 }
637
638 assert false;
639 }
640
641 /**
642 * Invoked after the continuation completes.
643 */
644 private void afterDone() {
645 afterDone(true);
646 }
647
648 /**
649 * Invoked after the continuation completes (or start failed). Sets the thread
650 * state to TERMINATED and notifies anyone waiting for the thread to terminate.
651 *
652 * @param notifyContainer true if its container should be notified
653 */
654 private void afterDone(boolean notifyContainer) {
655 assert carrierThread == null;
656 setState(TERMINATED);
657
658 // notify anyone waiting for this virtual thread to terminate
659 CountDownLatch termination = this.termination;
660 if (termination != null) {
661 assert termination.getCount() == 1;
662 termination.countDown();
663 }
664
665 // notify container
666 if (notifyContainer) {
667 threadContainer().onExit(this);
668 }
669
670 // clear references to thread locals
671 clearReferences();
672 }
673
674 /**
675 * Schedules this {@code VirtualThread} to execute.
676 *
677 * @throws IllegalStateException if the container is shutdown or closed
678 * @throws IllegalThreadStateException if the thread has already been started
679 * @throws RejectedExecutionException if the scheduler cannot accept a task
680 */
681 @Override
682 void start(ThreadContainer container) {
683 if (!compareAndSetState(NEW, STARTED)) {
684 throw new IllegalThreadStateException("Already started");
685 }
686
687 // bind thread to container
688 assert threadContainer() == null;
689 setThreadContainer(container);
690
691 // start thread
692 boolean addedToContainer = false;
693 boolean started = false;
694 try {
695 container.onStart(this); // may throw
696 addedToContainer = true;
697
698 // scoped values may be inherited
699 inheritScopedValueBindings(container);
700
701 // submit task to run thread, using externalSubmit if possible
702 externalSubmitRunContinuationOrThrow();
703 started = true;
704 } finally {
705 if (!started) {
706 afterDone(addedToContainer);
707 }
708 }
709 }
710
711 @Override
712 public void start() {
713 start(ThreadContainers.root());
714 }
715
716 @Override
717 public void run() {
718 // do nothing
719 }
720
721 /**
722 * Parks until unparked or interrupted. If already unparked then the parking
723 * permit is consumed and this method completes immediately (meaning it doesn't
724 * yield). It also completes immediately if the interrupt status is set.
725 */
726 @Override
727 void park() {
728 assert Thread.currentThread() == this;
729
730 // complete immediately if parking permit available or interrupted
731 if (getAndSetParkPermit(false) || interrupted)
732 return;
733
734 // park the thread
735 boolean yielded = false;
736 setState(PARKING);
737 try {
738 yielded = yieldContinuation();
739 } catch (OutOfMemoryError e) {
740 // park on carrier
741 } finally {
742 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
743 if (!yielded) {
744 assert state() == PARKING;
745 setState(RUNNING);
746 }
747 }
748
749 // park on the carrier thread when pinned
750 if (!yielded) {
751 parkOnCarrierThread(false, 0);
752 }
753 }
754
755 /**
756 * Parks up to the given waiting time or until unparked or interrupted.
757 * If already unparked then the parking permit is consumed and this method
758 * completes immediately (meaning it doesn't yield). It also completes immediately
759 * if the interrupt status is set or the waiting time is {@code <= 0}.
760 *
761 * @param nanos the maximum number of nanoseconds to wait.
762 */
763 @Override
764 void parkNanos(long nanos) {
765 assert Thread.currentThread() == this;
766
767 // complete immediately if parking permit available or interrupted
768 if (getAndSetParkPermit(false) || interrupted)
769 return;
770
771 // park the thread for the waiting time
772 if (nanos > 0) {
773 long startTime = System.nanoTime();
774
775 // park the thread, afterYield will schedule the thread to unpark
776 boolean yielded = false;
777 timeout = nanos;
778 setState(TIMED_PARKING);
779 try {
780 yielded = yieldContinuation();
781 } catch (OutOfMemoryError e) {
782 // park on carrier
783 } finally {
784 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
785 if (!yielded) {
786 assert state() == TIMED_PARKING;
787 setState(RUNNING);
788 }
789 }
790
791 // park on carrier thread for remaining time when pinned (or OOME)
792 if (!yielded) {
793 long remainingNanos = nanos - (System.nanoTime() - startTime);
794 parkOnCarrierThread(true, remainingNanos);
795 }
796 }
797 }
798
799 /**
800 * Parks the current carrier thread up to the given waiting time or until
801 * unparked or interrupted. If the virtual thread is interrupted then the
802 * interrupt status will be propagated to the carrier thread.
803 * @param timed true for a timed park, false for untimed
804 * @param nanos the waiting time in nanoseconds
805 */
806 private void parkOnCarrierThread(boolean timed, long nanos) {
807 assert state() == RUNNING;
808
809 setState(timed ? TIMED_PINNED : PINNED);
810 try {
811 if (!parkPermit) {
812 if (!timed) {
813 U.park(false, 0);
814 } else if (nanos > 0) {
815 U.park(false, nanos);
816 }
817 }
818 } finally {
819 setState(RUNNING);
820 }
821
822 // consume parking permit
823 setParkPermit(false);
824
825 // JFR jdk.VirtualThreadPinned event
826 postPinnedEvent("LockSupport.park");
827 }
828
829 /**
830 * Call into VM when pinned to record a JFR jdk.VirtualThreadPinned event.
831 * Recording the event in the VM avoids having JFR event recorded in Java
832 * with the same name, but different ID, to events recorded by the VM.
833 */
834 @Hidden
835 private static native void postPinnedEvent(String op);
836
837 /**
838 * Re-enables this virtual thread for scheduling. If this virtual thread is parked
839 * then its task is scheduled to continue, otherwise its next call to {@code park} or
840 * {@linkplain #parkNanos(long) parkNanos} is guaranteed not to block.
841 * @throws RejectedExecutionException if the scheduler cannot accept a task
842 */
843 @Override
844 void unpark() {
845 if (!getAndSetParkPermit(true) && currentThread() != this) {
846 int s = state();
847
848 // unparked while parked
849 if ((s == PARKED || s == TIMED_PARKED) && compareAndSetState(s, UNPARKED)) {
850 submitRunContinuation();
851 return;
852 }
853
854 // unparked while parked when pinned
855 if (s == PINNED || s == TIMED_PINNED) {
856 // unpark carrier thread when pinned
857 disableSuspendAndPreempt();
858 try {
859 synchronized (carrierThreadAccessLock()) {
860 Thread carrier = carrierThread;
861 if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
862 U.unpark(carrier);
863 }
864 }
865 } finally {
866 enableSuspendAndPreempt();
867 }
868 return;
869 }
870 }
871 }
872
873 /**
874 * Invoked by unblocker thread to unblock this virtual thread.
875 */
876 private void unblock() {
877 assert !Thread.currentThread().isVirtual();
878 blockPermit = true;
879 if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
880 submitRunContinuation();
881 }
882 }
883
884 /**
885 * Invoked by timer thread when wait timeout for virtual thread has expired.
886 * If the virtual thread is in timed-wait then this method will unblock the thread
887 * and submit its task so that it continues and attempts to reenter the monitor.
888 * This method does nothing if the thread has been woken by notify or interrupt.
889 */
890 private void waitTimeoutExpired(byte seqNo) {
891 assert !Thread.currentThread().isVirtual();
892 for (;;) {
893 boolean unblocked = false;
894 synchronized (timedWaitLock()) {
895 if (seqNo != timedWaitSeqNo) {
896 // this timeout task is for a past timed-wait
897 return;
898 }
899 int s = state();
900 if (s == TIMED_WAIT) {
901 unblocked = compareAndSetState(TIMED_WAIT, UNBLOCKED);
902 } else if (s != (TIMED_WAIT | SUSPENDED)) {
903 // notified or interrupted, no longer waiting
904 return;
905 }
906 }
907 if (unblocked) {
908 submitRunContinuation();
909 return;
910 }
911 // need to retry when thread is suspended in time-wait
912 Thread.yield();
913 }
914 }
915
916 /**
917 * Attempts to yield the current virtual thread (Thread.yield).
918 */
919 void tryYield() {
920 assert Thread.currentThread() == this;
921 setState(YIELDING);
922 boolean yielded = false;
923 try {
924 yielded = yieldContinuation(); // may throw
925 } finally {
926 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
927 if (!yielded) {
928 assert state() == YIELDING;
929 setState(RUNNING);
930 }
931 }
932 }
933
934 /**
935 * Sleep the current thread for the given sleep time (in nanoseconds). If
936 * nanos is 0 then the thread will attempt to yield.
937 *
938 * @implNote This implementation parks the thread for the given sleeping time
939 * and will therefore be observed in PARKED state during the sleep. Parking
940 * will consume the parking permit so this method makes available the parking
941 * permit after the sleep. This may be observed as a spurious, but benign,
942 * wakeup when the thread subsequently attempts to park.
943 *
944 * @param nanos the maximum number of nanoseconds to sleep
945 * @throws InterruptedException if interrupted while sleeping
946 */
947 void sleepNanos(long nanos) throws InterruptedException {
948 assert Thread.currentThread() == this && nanos >= 0;
949 if (getAndClearInterrupt())
950 throw new InterruptedException();
951 if (nanos == 0) {
952 tryYield();
953 } else {
954 // park for the sleep time
955 try {
956 long remainingNanos = nanos;
957 long startNanos = System.nanoTime();
958 while (remainingNanos > 0) {
959 parkNanos(remainingNanos);
960 if (getAndClearInterrupt()) {
961 throw new InterruptedException();
962 }
963 remainingNanos = nanos - (System.nanoTime() - startNanos);
964 }
965 } finally {
966 // may have been unparked while sleeping
967 setParkPermit(true);
968 }
969 }
970 }
971
972 /**
973 * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
974 * A timeout of {@code 0} means to wait forever.
975 *
976 * @throws InterruptedException if interrupted while waiting
977 * @return true if the thread has terminated
978 */
979 boolean joinNanos(long nanos) throws InterruptedException {
980 if (state() == TERMINATED)
981 return true;
982
983 // ensure termination object exists, then re-check state
984 CountDownLatch termination = getTermination();
985 if (state() == TERMINATED)
986 return true;
987
988 // wait for virtual thread to terminate
989 if (nanos == 0) {
990 termination.await();
991 } else {
992 boolean terminated = termination.await(nanos, NANOSECONDS);
993 if (!terminated) {
994 // waiting time elapsed
995 return false;
996 }
997 }
998 assert state() == TERMINATED;
999 return true;
1000 }
1001
1002 @Override
1003 void blockedOn(Interruptible b) {
1004 disableSuspendAndPreempt();
1005 try {
1006 super.blockedOn(b);
1007 } finally {
1008 enableSuspendAndPreempt();
1009 }
1010 }
1011
1012 @Override
1013 public void interrupt() {
1014 if (Thread.currentThread() != this) {
1015 // if current thread is a virtual thread then prevent it from being
1016 // suspended or unmounted when entering or holding interruptLock
1017 Interruptible blocker;
1018 disableSuspendAndPreempt();
1019 try {
1020 synchronized (interruptLock) {
1021 interrupted = true;
1022 blocker = nioBlocker();
1023 if (blocker != null) {
1024 blocker.interrupt(this);
1025 }
1026
1027 // interrupt carrier thread if mounted
1028 Thread carrier = carrierThread;
1029 if (carrier != null) carrier.setInterrupt();
1030 }
1031 } finally {
1032 enableSuspendAndPreempt();
1033 }
1034
1035 // notify blocker after releasing interruptLock
1036 if (blocker != null) {
1037 blocker.postInterrupt();
1038 }
1039
1040 // make available parking permit, unpark thread if parked
1041 unpark();
1042
1043 // if thread is waiting in Object.wait then schedule to try to reenter
1044 int s = state();
1045 if ((s == WAIT || s == TIMED_WAIT) && compareAndSetState(s, UNBLOCKED)) {
1046 submitRunContinuation();
1047 }
1048
1049 } else {
1050 interrupted = true;
1051 carrierThread.setInterrupt();
1052 setParkPermit(true);
1053 }
1054 }
1055
1056 @Override
1057 public boolean isInterrupted() {
1058 return interrupted;
1059 }
1060
1061 @Override
1062 boolean getAndClearInterrupt() {
1063 assert Thread.currentThread() == this;
1064 boolean oldValue = interrupted;
1065 if (oldValue) {
1066 disableSuspendAndPreempt();
1067 try {
1068 synchronized (interruptLock) {
1069 interrupted = false;
1070 carrierThread.clearInterrupt();
1071 }
1072 } finally {
1073 enableSuspendAndPreempt();
1074 }
1075 }
1076 return oldValue;
1077 }
1078
1079 @Override
1080 Thread.State threadState() {
1081 int s = state();
1082 switch (s & ~SUSPENDED) {
1083 case NEW:
1084 return Thread.State.NEW;
1085 case STARTED:
1086 // return NEW if thread container not yet set
1087 if (threadContainer() == null) {
1088 return Thread.State.NEW;
1089 } else {
1090 return Thread.State.RUNNABLE;
1091 }
1092 case UNPARKED:
1093 case UNBLOCKED:
1094 case YIELDED:
1095 // runnable, not mounted
1096 return Thread.State.RUNNABLE;
1097 case RUNNING:
1098 // if mounted then return state of carrier thread
1099 if (Thread.currentThread() != this) {
1100 disableSuspendAndPreempt();
1101 try {
1102 synchronized (carrierThreadAccessLock()) {
1103 Thread carrierThread = this.carrierThread;
1104 if (carrierThread != null) {
1105 return carrierThread.threadState();
1106 }
1107 }
1108 } finally {
1109 enableSuspendAndPreempt();
1110 }
1111 }
1112 // runnable, mounted
1113 return Thread.State.RUNNABLE;
1114 case PARKING:
1115 case TIMED_PARKING:
1116 case WAITING:
1117 case TIMED_WAITING:
1118 case YIELDING:
1119 // runnable, in transition
1120 return Thread.State.RUNNABLE;
1121 case PARKED:
1122 case PINNED:
1123 case WAIT:
1124 return Thread.State.WAITING;
1125 case TIMED_PARKED:
1126 case TIMED_PINNED:
1127 case TIMED_WAIT:
1128 return Thread.State.TIMED_WAITING;
1129 case BLOCKING:
1130 case BLOCKED:
1131 return Thread.State.BLOCKED;
1132 case TERMINATED:
1133 return Thread.State.TERMINATED;
1134 default:
1135 throw new InternalError();
1136 }
1137 }
1138
1139 @Override
1140 boolean alive() {
1141 int s = state;
1142 return (s != NEW && s != TERMINATED);
1143 }
1144
1145 @Override
1146 boolean isTerminated() {
1147 return (state == TERMINATED);
1148 }
1149
1150 @Override
1151 StackTraceElement[] asyncGetStackTrace() {
1152 StackTraceElement[] stackTrace;
1153 do {
1154 stackTrace = (carrierThread != null)
1155 ? super.asyncGetStackTrace() // mounted
1156 : tryGetStackTrace(); // unmounted
1157 if (stackTrace == null) {
1158 Thread.yield();
1159 }
1160 } while (stackTrace == null);
1161 return stackTrace;
1162 }
1163
1164 /**
1165 * Returns the stack trace for this virtual thread if it is unmounted.
1166 * Returns null if the thread is mounted or in transition.
1167 */
1168 private StackTraceElement[] tryGetStackTrace() {
1169 int initialState = state() & ~SUSPENDED;
1170 switch (initialState) {
1171 case NEW, STARTED, TERMINATED -> {
1172 return new StackTraceElement[0]; // unmounted, empty stack
1173 }
1174 case RUNNING, PINNED, TIMED_PINNED -> {
1175 return null; // mounted
1176 }
1177 case PARKED, TIMED_PARKED, BLOCKED, WAIT, TIMED_WAIT -> {
1178 // unmounted, not runnable
1179 }
1180 case UNPARKED, UNBLOCKED, YIELDED -> {
1181 // unmounted, runnable
1182 }
1183 case PARKING, TIMED_PARKING, BLOCKING, YIELDING, WAITING, TIMED_WAITING -> {
1184 return null; // in transition
1185 }
1186 default -> throw new InternalError("" + initialState);
1187 }
1188
1189 // thread is unmounted, prevent it from continuing
1190 int suspendedState = initialState | SUSPENDED;
1191 if (!compareAndSetState(initialState, suspendedState)) {
1192 return null;
1193 }
1194
1195 // get stack trace and restore state
1196 StackTraceElement[] stack;
1197 try {
1198 stack = cont.getStackTrace();
1199 } finally {
1200 assert state == suspendedState;
1201 setState(initialState);
1202 }
1203 boolean resubmit = switch (initialState) {
1204 case UNPARKED, UNBLOCKED, YIELDED -> {
1205 // resubmit as task may have run while suspended
1206 yield true;
1207 }
1208 case PARKED, TIMED_PARKED -> {
1209 // resubmit if unparked while suspended
1210 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1211 }
1212 case BLOCKED -> {
1213 // resubmit if unblocked while suspended
1214 yield blockPermit && compareAndSetState(BLOCKED, UNBLOCKED);
1215 }
1216 case WAIT, TIMED_WAIT -> {
1217 // resubmit if notified or interrupted while waiting (Object.wait)
1218 // waitTimeoutExpired will retry if the timed expired when suspended
1219 yield (notified || interrupted) && compareAndSetState(initialState, UNBLOCKED);
1220 }
1221 default -> throw new InternalError();
1222 };
1223 if (resubmit) {
1224 submitRunContinuation();
1225 }
1226 return stack;
1227 }
1228
1229 @Override
1230 public String toString() {
1231 StringBuilder sb = new StringBuilder("VirtualThread[#");
1232 sb.append(threadId());
1233 String name = getName();
1234 if (!name.isEmpty()) {
1235 sb.append(",");
1236 sb.append(name);
1237 }
1238 sb.append("]/");
1239
1240 // add the carrier state and thread name when mounted
1241 boolean mounted;
1242 if (Thread.currentThread() == this) {
1243 mounted = appendCarrierInfo(sb);
1244 } else {
1245 disableSuspendAndPreempt();
1246 try {
1247 synchronized (carrierThreadAccessLock()) {
1248 mounted = appendCarrierInfo(sb);
1249 }
1250 } finally {
1251 enableSuspendAndPreempt();
1252 }
1253 }
1254
1255 // add virtual thread state when not mounted
1256 if (!mounted) {
1257 String stateAsString = threadState().toString();
1258 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1259 }
1260
1261 return sb.toString();
1262 }
1263
1264 /**
1265 * Appends the carrier state and thread name to the string buffer if mounted.
1266 * @return true if mounted, false if not mounted
1267 */
1268 private boolean appendCarrierInfo(StringBuilder sb) {
1269 assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1270 Thread carrier = carrierThread;
1271 if (carrier != null) {
1272 String stateAsString = carrier.threadState().toString();
1273 sb.append(stateAsString.toLowerCase(Locale.ROOT));
1274 sb.append('@');
1275 sb.append(carrier.getName());
1276 return true;
1277 } else {
1278 return false;
1279 }
1280 }
1281
1282 @Override
1283 public int hashCode() {
1284 return (int) threadId();
1285 }
1286
1287 @Override
1288 public boolean equals(Object obj) {
1289 return obj == this;
1290 }
1291
1292 /**
1293 * Returns the termination object, creating it if needed.
1294 */
1295 private CountDownLatch getTermination() {
1296 CountDownLatch termination = this.termination;
1297 if (termination == null) {
1298 termination = new CountDownLatch(1);
1299 if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1300 termination = this.termination;
1301 }
1302 }
1303 return termination;
1304 }
1305
1306 /**
1307 * Returns the lock object to synchronize on when accessing carrierThread.
1308 * The lock prevents carrierThread from being reset to null during unmount.
1309 */
1310 private Object carrierThreadAccessLock() {
1311 // return interruptLock as unmount has to coordinate with interrupt
1312 return interruptLock;
1313 }
1314
1315 /**
1316 * Returns a lock object for coordinating timed-wait setup and timeout handling.
1317 */
1318 private Object timedWaitLock() {
1319 // use this object for now to avoid the overhead of introducing another lock
1320 return runContinuation;
1321 }
1322
1323 /**
1324 * Disallow the current thread be suspended or preempted.
1325 */
1326 private void disableSuspendAndPreempt() {
1327 notifyJvmtiDisableSuspend(true);
1328 Continuation.pin();
1329 }
1330
1331 /**
1332 * Allow the current thread be suspended or preempted.
1333 */
1334 private void enableSuspendAndPreempt() {
1335 Continuation.unpin();
1336 notifyJvmtiDisableSuspend(false);
1337 }
1338
1339 // -- wrappers for get/set of state, parking permit, and carrier thread --
1340
1341 private int state() {
1342 return state; // volatile read
1343 }
1344
1345 private void setState(int newValue) {
1346 state = newValue; // volatile write
1347 }
1348
1349 private boolean compareAndSetState(int expectedValue, int newValue) {
1350 return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1351 }
1352
1353 private boolean compareAndSetOnWaitingList(boolean expectedValue, boolean newValue) {
1354 return U.compareAndSetBoolean(this, ON_WAITING_LIST, expectedValue, newValue);
1355 }
1356
1357 private void setParkPermit(boolean newValue) {
1358 if (parkPermit != newValue) {
1359 parkPermit = newValue;
1360 }
1361 }
1362
1363 private boolean getAndSetParkPermit(boolean newValue) {
1364 if (parkPermit != newValue) {
1365 return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1366 } else {
1367 return newValue;
1368 }
1369 }
1370
1371 private void setCarrierThread(Thread carrier) {
1372 // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1373 this.carrierThread = carrier;
1374 }
1375
1376 // -- JVM TI support --
1377
1378 @IntrinsicCandidate
1379 @JvmtiMountTransition
1380 private native void notifyJvmtiStart();
1381
1382 @IntrinsicCandidate
1383 @JvmtiMountTransition
1384 private native void notifyJvmtiEnd();
1385
1386 @IntrinsicCandidate
1387 @JvmtiMountTransition
1388 private native void notifyJvmtiMount(boolean hide);
1389
1390 @IntrinsicCandidate
1391 @JvmtiMountTransition
1392 private native void notifyJvmtiUnmount(boolean hide);
1393
1394 @IntrinsicCandidate
1395 private static native void notifyJvmtiDisableSuspend(boolean enter);
1396
1397 private static native void registerNatives();
1398 static {
1399 registerNatives();
1400
1401 // ensure VTHREAD_GROUP is created, may be accessed by JVMTI
1402 var group = Thread.virtualThreadGroup();
1403 }
1404
1405 /**
1406 * Creates the default ForkJoinPool scheduler.
1407 */
1408 private static ForkJoinPool createDefaultScheduler() {
1409 ForkJoinWorkerThreadFactory factory = pool -> new CarrierThread(pool);
1410 int parallelism, maxPoolSize, minRunnable;
1411 String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1412 String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1413 String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1414 if (parallelismValue != null) {
1415 parallelism = Integer.parseInt(parallelismValue);
1416 } else {
1417 parallelism = Runtime.getRuntime().availableProcessors();
1418 }
1419 if (maxPoolSizeValue != null) {
1420 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1421 parallelism = Integer.min(parallelism, maxPoolSize);
1422 } else {
1423 maxPoolSize = Integer.max(parallelism, 256);
1424 }
1425 if (minRunnableValue != null) {
1426 minRunnable = Integer.parseInt(minRunnableValue);
1427 } else {
1428 minRunnable = Integer.max(parallelism / 2, 1);
1429 }
1430 Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1431 boolean asyncMode = true; // FIFO
1432 return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1433 0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1434 }
1435
1436 /**
1437 * Schedule a runnable task to run after a delay.
1438 */
1439 private static Future<?> schedule(Runnable command, long delay, TimeUnit unit) {
1440 long tid = Thread.currentThread().threadId();
1441 int index = (int) tid & (DELAYED_TASK_SCHEDULERS.length - 1);
1442 return DELAYED_TASK_SCHEDULERS[index].schedule(command, delay, unit);
1443 }
1444
1445 /**
1446 * Creates the ScheduledThreadPoolExecutors used to execute delayed tasks.
1447 */
1448 private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1449 String propName = "jdk.virtualThreadScheduler.timerQueues";
1450 String propValue = System.getProperty(propName);
1451 int queueCount;
1452 if (propValue != null) {
1453 queueCount = Integer.parseInt(propValue);
1454 if (queueCount != Integer.highestOneBit(queueCount)) {
1455 throw new RuntimeException("Value of " + propName + " must be power of 2");
1456 }
1457 } else {
1458 int ncpus = Runtime.getRuntime().availableProcessors();
1459 queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1460 }
1461 var schedulers = new ScheduledExecutorService[queueCount];
1462 for (int i = 0; i < queueCount; i++) {
1463 ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1464 Executors.newScheduledThreadPool(1, task -> {
1465 Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1466 t.setDaemon(true);
1467 return t;
1468 });
1469 stpe.setRemoveOnCancelPolicy(true);
1470 schedulers[i] = stpe;
1471 }
1472 return schedulers;
1473 }
1474
1475 /**
1476 * Schedule virtual threads that are ready to be scheduled after they blocked on
1477 * monitor enter.
1478 */
1479 private static void unblockVirtualThreads() {
1480 while (true) {
1481 VirtualThread vthread = takeVirtualThreadListToUnblock();
1482 while (vthread != null) {
1483 assert vthread.onWaitingList;
1484 VirtualThread nextThread = vthread.next;
1485
1486 // remove from list and unblock
1487 vthread.next = null;
1488 boolean changed = vthread.compareAndSetOnWaitingList(true, false);
1489 assert changed;
1490 vthread.unblock();
1491
1492 vthread = nextThread;
1493 }
1494 }
1495 }
1496
1497 /**
1498 * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1499 * if necessary until a list of one or more threads becomes available.
1500 */
1501 private static native VirtualThread takeVirtualThreadListToUnblock();
1502
1503 static {
1504 var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1505 VirtualThread::unblockVirtualThreads);
1506 unblocker.setDaemon(true);
1507 unblocker.start();
1508 }
1509 }