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.nio.charset.StandardCharsets;
  28 import java.security.AccessController;
  29 import java.security.PrivilegedAction;
  30 import java.util.Locale;
  31 import java.util.Objects;
  32 import java.util.concurrent.Callable;
  33 import java.util.concurrent.CountDownLatch;
  34 import java.util.concurrent.Executor;
  35 import java.util.concurrent.Executors;
  36 import java.util.concurrent.ForkJoinPool;
  37 import java.util.concurrent.ForkJoinPool.ForkJoinWorkerThreadFactory;
  38 import java.util.concurrent.ForkJoinTask;
  39 import java.util.concurrent.ForkJoinWorkerThread;
  40 import java.util.concurrent.Future;
  41 import java.util.concurrent.RejectedExecutionException;
  42 import java.util.concurrent.ScheduledExecutorService;
  43 import java.util.concurrent.ScheduledThreadPoolExecutor;
  44 import jdk.internal.event.VirtualThreadEndEvent;
  45 import jdk.internal.event.VirtualThreadStartEvent;
  46 import jdk.internal.event.VirtualThreadSubmitFailedEvent;
  47 import jdk.internal.misc.CarrierThread;
  48 import jdk.internal.misc.InnocuousThread;
  49 import jdk.internal.misc.Unsafe;
  50 import jdk.internal.vm.Continuation;
  51 import jdk.internal.vm.ContinuationScope;
  52 import jdk.internal.vm.StackableScope;
  53 import jdk.internal.vm.ThreadContainer;
  54 import jdk.internal.vm.ThreadContainers;
  55 import jdk.internal.vm.annotation.ChangesCurrentThread;
  56 import jdk.internal.vm.annotation.Hidden;
  57 import jdk.internal.vm.annotation.IntrinsicCandidate;
  58 import jdk.internal.vm.annotation.JvmtiMountTransition;
  59 import jdk.internal.vm.annotation.ReservedStackAccess;
  60 import sun.nio.ch.Interruptible;
  61 import sun.security.action.GetPropertyAction;
  62 import static java.util.concurrent.TimeUnit.*;
  63 
  64 /**
  65  * A thread that is scheduled by the Java virtual machine rather than the operating
  66  * system.
  67  */
  68 final class VirtualThread extends BaseVirtualThread {
  69     private static final Unsafe U = Unsafe.getUnsafe();
  70     private static final ContinuationScope VTHREAD_SCOPE = new ContinuationScope("VirtualThreads");
  71     private static final ForkJoinPool DEFAULT_SCHEDULER = createDefaultScheduler();
  72     private static final ScheduledExecutorService[] DELAYED_TASK_SCHEDULERS = createDelayedTaskSchedulers();
  73 
  74     private static final long STATE = U.objectFieldOffset(VirtualThread.class, "state");
  75     private static final long PARK_PERMIT = U.objectFieldOffset(VirtualThread.class, "parkPermit");
  76     private static final long CARRIER_THREAD = U.objectFieldOffset(VirtualThread.class, "carrierThread");
  77     private static final long TERMINATION = U.objectFieldOffset(VirtualThread.class, "termination");
  78     private static final long ON_WAITING_LIST = U.objectFieldOffset(VirtualThread.class, "onWaitingList");
  79 
  80     // scheduler and continuation
  81     private final Executor scheduler;
  82     private final Continuation cont;
  83     private final Runnable runContinuation;
  84 
  85     // virtual thread state, accessed by VM
  86     private volatile int state;
  87 
  88     /*
  89      * Virtual thread state transitions:
  90      *
  91      *      NEW -> STARTED         // Thread.start, schedule to run
  92      *  STARTED -> TERMINATED      // failed to start
  93      *  STARTED -> RUNNING         // first run
  94      *  RUNNING -> TERMINATED      // done
  95      *
  96      *  RUNNING -> PARKING         // Thread parking with LockSupport.park
  97      *  PARKING -> PARKED          // cont.yield successful, parked indefinitely
  98      *  PARKING -> PINNED          // cont.yield failed, parked indefinitely on carrier
  99      *   PARKED -> UNPARKED        // unparked, may be scheduled to continue
 100      *   PINNED -> RUNNING         // unparked, continue execution on same carrier
 101      * UNPARKED -> RUNNING         // continue execution after park
 102      *
 103      *       RUNNING -> TIMED_PARKING   // Thread parking with LockSupport.parkNanos
 104      * TIMED_PARKING -> TIMED_PARKED    // cont.yield successful, timed-parked
 105      * TIMED_PARKING -> TIMED_PINNED    // cont.yield failed, timed-parked on carrier
 106      *  TIMED_PARKED -> UNPARKED        // unparked, may be scheduled to continue
 107      *  TIMED_PINNED -> RUNNING         // unparked, continue execution on same carrier
 108      *
 109      *  RUNNABLE -> BLOCKING       // blocking on monitor enter
 110      *  BLOCKING -> BLOCKED        // blocked on monitor enter
 111      *   BLOCKED -> UNBLOCKED      // unblocked, may be scheduled to continue
 112      * UNBLOCKED -> RUNNING        // continue execution after blocked on monitor enter
 113      *
 114      *  RUNNING -> YIELDING        // Thread.yield
 115      * YIELDING -> YIELDED         // cont.yield successful, may be scheduled to continue
 116      * YIELDING -> RUNNING         // cont.yield failed
 117      *  YIELDED -> RUNNING         // continue execution after Thread.yield
 118      */
 119     private static final int NEW      = 0;
 120     private static final int STARTED  = 1;
 121     private static final int RUNNING  = 2;     // runnable-mounted
 122 
 123     // untimed and timed parking
 124     private static final int PARKING       = 3;
 125     private static final int PARKED        = 4;     // unmounted
 126     private static final int PINNED        = 5;     // mounted
 127     private static final int TIMED_PARKING = 6;
 128     private static final int TIMED_PARKED  = 7;     // unmounted
 129     private static final int TIMED_PINNED  = 8;     // mounted
 130     private static final int UNPARKED      = 9;     // unmounted but runnable
 131 
 132     // Thread.yield
 133     private static final int YIELDING = 10;
 134     private static final int YIELDED  = 11;         // unmounted but runnable
 135 
 136     // monitor enter
 137     private static final int BLOCKING  = 12;
 138     private static final int BLOCKED   = 13;        // unmounted
 139     private static final int UNBLOCKED = 14;        // unmounted but runnable
 140 
 141     private static final int TERMINATED = 99;  // final state
 142 
 143     // can be suspended from scheduling when unmounted
 144     private static final int SUSPENDED = 1 << 8;
 145 
 146     // parking permit
 147     private volatile boolean parkPermit;
 148 
 149     // used to mark thread as ready to be unblocked while it is concurrently blocking
 150     private volatile boolean unblocked;
 151 
 152     // a positive value if "responsible thread" blocked on monitor enter, accessed by VM
 153     private volatile byte recheckInterval;
 154 
 155     // carrier thread when mounted, accessed by VM
 156     private volatile Thread carrierThread;
 157 
 158     // termination object when joining, created lazily if needed
 159     private volatile CountDownLatch termination;
 160 
 161     // has the value 1 when on the list of virtual threads waiting to be unblocked
 162     private volatile byte onWaitingList;
 163 
 164     // next virtual thread on the list of virtual threads waiting to be unblocked
 165     private volatile VirtualThread next;
 166 
 167     /**
 168      * Returns the continuation scope used for virtual threads.
 169      */
 170     static ContinuationScope continuationScope() {
 171         return VTHREAD_SCOPE;
 172     }
 173 
 174     /**
 175      * Creates a new {@code VirtualThread} to run the given task with the given
 176      * scheduler. If the given scheduler is {@code null} and the current thread
 177      * is a platform thread then the newly created virtual thread will use the
 178      * default scheduler. If given scheduler is {@code null} and the current
 179      * thread is a virtual thread then the current thread's scheduler is used.
 180      *
 181      * @param scheduler the scheduler or null
 182      * @param name thread name
 183      * @param characteristics characteristics
 184      * @param task the task to execute
 185      */
 186     VirtualThread(Executor scheduler, String name, int characteristics, Runnable task) {
 187         super(name, characteristics, /*bound*/ false);
 188         Objects.requireNonNull(task);
 189 
 190         // choose scheduler if not specified
 191         if (scheduler == null) {
 192             Thread parent = Thread.currentThread();
 193             if (parent instanceof VirtualThread vparent) {
 194                 scheduler = vparent.scheduler;
 195             } else {
 196                 scheduler = DEFAULT_SCHEDULER;
 197             }
 198         }
 199 
 200         this.scheduler = scheduler;
 201         this.cont = new VThreadContinuation(this, task);
 202         this.runContinuation = this::runContinuation;
 203     }
 204 
 205     /**
 206      * The continuation that a virtual thread executes.
 207      */
 208     private static class VThreadContinuation extends Continuation {
 209         VThreadContinuation(VirtualThread vthread, Runnable task) {
 210             super(VTHREAD_SCOPE, wrap(vthread, task));
 211         }
 212         @Override
 213         protected void onPinned(Continuation.Pinned reason) {
 214             // emit JFR event
 215             virtualThreadPinnedEvent(reason.value());
 216         }
 217         private static Runnable wrap(VirtualThread vthread, Runnable task) {
 218             return new Runnable() {
 219                 @Hidden
 220                 public void run() {
 221                     vthread.run(task);
 222                 }
 223             };
 224         }
 225     }
 226 
 227     /**
 228      * Runs or continues execution on the current thread. The virtual thread is mounted
 229      * on the current thread before the task runs or continues. It unmounts when the
 230      * task completes or yields.
 231      */
 232     @ChangesCurrentThread
 233     private void runContinuation() {
 234         // the carrier must be a platform thread
 235         if (Thread.currentThread().isVirtual()) {
 236             throw new WrongThreadException();
 237         }
 238 
 239         // set state to RUNNING
 240         int initialState = state();
 241         if (initialState == STARTED || initialState == UNPARKED
 242                 || initialState == UNBLOCKED || initialState == YIELDED) {
 243             // newly started or continue after parking/blocking/Thread.yield
 244             if (!compareAndSetState(initialState, RUNNING)) {
 245                 return;
 246             }
 247             // consume parking permit when continuing after parking
 248             if (initialState == UNPARKED) {
 249                 setParkPermit(false);
 250             }
 251         } else {
 252             // not runnable
 253             return;
 254         }
 255 
 256         mount();
 257         try {
 258             cont.run();
 259         } finally {
 260             unmount();
 261             if (cont.isDone()) {
 262                 afterDone();
 263             } else {
 264                 afterYield();
 265             }
 266         }
 267     }
 268 
 269     /**
 270      * Submits the runContinuation task to the scheduler. For the default scheduler,
 271      * and calling it on a worker thread, the task will be pushed to the local queue,
 272      * otherwise it will be pushed to an external submission queue.
 273      * @throws RejectedExecutionException
 274      */
 275     private void submitRunContinuation() {
 276         try {
 277             scheduler.execute(runContinuation);
 278         } catch (RejectedExecutionException ree) {
 279             submitFailed(ree);
 280             throw ree;
 281         }
 282     }
 283 
 284     /**
 285      * Submits the runContinuation task the scheduler. For the default scheduler, the task
 286      * will be pushed to an external submission queue.
 287      * @throws RejectedExecutionException
 288      */
 289     private void externalSubmitRunContinuation() {
 290         if (scheduler == DEFAULT_SCHEDULER
 291                 && currentCarrierThread() instanceof CarrierThread ct) {
 292             externalSubmitRunContinuation(ct.getPool());
 293         } else {
 294             submitRunContinuation();
 295         }
 296     }
 297 
 298     /**
 299      * Submits the runContinuation task to given scheduler with a lazy submit.
 300      * @throws RejectedExecutionException
 301      * @see ForkJoinPool#lazySubmit(ForkJoinTask)
 302      */
 303     private void lazySubmitRunContinuation(ForkJoinPool pool) {
 304         try {
 305             pool.lazySubmit(ForkJoinTask.adapt(runContinuation));
 306         } catch (RejectedExecutionException ree) {
 307             submitFailed(ree);
 308             throw ree;
 309         }
 310     }
 311 
 312     /**
 313      * Submits the runContinuation task to the given scheduler as an external submit.
 314      * @throws RejectedExecutionException
 315      * @see ForkJoinPool#externalSubmit(ForkJoinTask)
 316      */
 317     private void externalSubmitRunContinuation(ForkJoinPool pool) {
 318         try {
 319             pool.externalSubmit(ForkJoinTask.adapt(runContinuation));
 320         } catch (RejectedExecutionException ree) {
 321             submitFailed(ree);
 322             throw ree;
 323         }
 324     }
 325 
 326     /**
 327      * If enabled, emits a JFR VirtualThreadSubmitFailedEvent.
 328      */
 329     private void submitFailed(RejectedExecutionException ree) {
 330         var event = new VirtualThreadSubmitFailedEvent();
 331         if (event.isEnabled()) {
 332             event.javaThreadId = threadId();
 333             event.exceptionMessage = ree.getMessage();
 334             event.commit();
 335         }
 336     }
 337 
 338     /**
 339      * Runs a task in the context of this virtual thread.
 340      */
 341     private void run(Runnable task) {
 342         assert Thread.currentThread() == this && state == RUNNING;
 343 
 344         // notify JVMTI, may post VirtualThreadStart event
 345         notifyJvmtiStart();
 346 
 347         // emit JFR event if enabled
 348         if (VirtualThreadStartEvent.isTurnedOn()) {
 349             var event = new VirtualThreadStartEvent();
 350             event.javaThreadId = threadId();
 351             event.commit();
 352         }
 353 
 354         Object bindings = Thread.scopedValueBindings();
 355         try {
 356             runWith(bindings, task);
 357         } catch (Throwable exc) {
 358             dispatchUncaughtException(exc);
 359         } finally {
 360             try {
 361                 // pop any remaining scopes from the stack, this may block
 362                 StackableScope.popAll();
 363 
 364                 // emit JFR event if enabled
 365                 if (VirtualThreadEndEvent.isTurnedOn()) {
 366                     var event = new VirtualThreadEndEvent();
 367                     event.javaThreadId = threadId();
 368                     event.commit();
 369                 }
 370 
 371             } finally {
 372                 // notify JVMTI, may post VirtualThreadEnd event
 373                 notifyJvmtiEnd();
 374             }
 375         }
 376     }
 377 
 378     /**
 379      * Mounts this virtual thread onto the current platform thread. On
 380      * return, the current thread is the virtual thread.
 381      */
 382     @ChangesCurrentThread
 383     @ReservedStackAccess
 384     private void mount() {
 385         // notify JVMTI before mount
 386         notifyJvmtiMount(/*hide*/true);
 387 
 388         // sets the carrier thread
 389         Thread carrier = Thread.currentCarrierThread();
 390         setCarrierThread(carrier);
 391 
 392         // sync up carrier thread interrupt status if needed
 393         if (interrupted) {
 394             carrier.setInterrupt();
 395         } else if (carrier.isInterrupted()) {
 396             synchronized (interruptLock) {
 397                 // need to recheck interrupt status
 398                 if (!interrupted) {
 399                     carrier.clearInterrupt();
 400                 }
 401             }
 402         }
 403 
 404         // set Thread.currentThread() to return this virtual thread
 405         carrier.setCurrentThread(this);
 406     }
 407 
 408     /**
 409      * Unmounts this virtual thread from the carrier. On return, the
 410      * current thread is the current platform thread.
 411      */
 412     @ChangesCurrentThread
 413     @ReservedStackAccess
 414     private void unmount() {
 415         assert !Thread.holdsLock(interruptLock);
 416 
 417         // set Thread.currentThread() to return the platform thread
 418         Thread carrier = this.carrierThread;
 419         carrier.setCurrentThread(carrier);
 420 
 421         // break connection to carrier thread, synchronized with interrupt
 422         synchronized (interruptLock) {
 423             setCarrierThread(null);
 424         }
 425         carrier.clearInterrupt();
 426 
 427         // notify JVMTI after unmount
 428         notifyJvmtiUnmount(/*hide*/false);
 429     }
 430 
 431     /**
 432      * Sets the current thread to the current carrier thread.
 433      */
 434     @ChangesCurrentThread
 435     @JvmtiMountTransition
 436     private void switchToCarrierThread() {
 437         notifyJvmtiHideFrames(true);
 438         Thread carrier = this.carrierThread;
 439         assert Thread.currentThread() == this
 440                 && carrier == Thread.currentCarrierThread();
 441         carrier.setCurrentThread(carrier);
 442         setLockId(this.threadId()); // keep lockid of vthread
 443     }
 444 
 445     /**
 446      * Sets the current thread to the given virtual thread.
 447      */
 448     @ChangesCurrentThread
 449     @JvmtiMountTransition
 450     private void switchToVirtualThread(VirtualThread vthread) {
 451         Thread carrier = vthread.carrierThread;
 452         assert carrier == Thread.currentCarrierThread();
 453         carrier.setCurrentThread(vthread);
 454         notifyJvmtiHideFrames(false);
 455     }
 456 
 457     /**
 458      * Executes the given value returning task on the current carrier thread.
 459      */
 460     @ChangesCurrentThread
 461     <V> V executeOnCarrierThread(Callable<V> task) throws Exception {
 462         assert Thread.currentThread() == this;
 463         switchToCarrierThread();
 464         try {
 465             return task.call();
 466         } finally {
 467             switchToVirtualThread(this);
 468         }
 469      }
 470 
 471     /**
 472      * Invokes Continuation.yield, notifying JVMTI (if enabled) to hide frames until
 473      * the continuation continues.
 474      */
 475     @Hidden
 476     private boolean yieldContinuation() {
 477         notifyJvmtiUnmount(/*hide*/true);
 478         try {
 479             return Continuation.yield(VTHREAD_SCOPE);
 480         } finally {
 481             notifyJvmtiMount(/*hide*/false);
 482         }
 483     }
 484 
 485     /**
 486      * Invoked after the continuation yields. If parking then it sets the state
 487      * and also re-submits the task to continue if unparked while parking.
 488      * If yielding due to Thread.yield then it just submits the task to continue.
 489      */
 490     private void afterYield() {
 491         assert carrierThread == null;
 492 
 493         // re-adjust parallelism if the virtual thread yielded when compensating
 494         if (currentThread() instanceof CarrierThread ct) {
 495             ct.endBlocking();
 496         }
 497 
 498         int s = state();
 499 
 500         // LockSupport.park/parkNanos
 501         if (s == PARKING || s == TIMED_PARKING) {
 502             int newState = (s == PARKING) ? PARKED : TIMED_PARKED;
 503             setState(newState);
 504 
 505             // may have been unparked while parking
 506             if (parkPermit && compareAndSetState(newState, UNPARKED)) {
 507                 // lazy submit to continue on the current thread as carrier if possible
 508                 if (currentThread() instanceof CarrierThread ct) {
 509                     lazySubmitRunContinuation(ct.getPool());
 510                 } else {
 511                     submitRunContinuation();
 512                 }
 513 
 514             }
 515             return;
 516         }
 517 
 518         // Thread.yield
 519         if (s == YIELDING) {
 520             setState(YIELDED);
 521 
 522             // external submit if there are no tasks in the local task queue
 523             if (currentThread() instanceof CarrierThread ct && ct.getQueuedTaskCount() == 0) {
 524                 externalSubmitRunContinuation(ct.getPool());
 525             } else {
 526                 submitRunContinuation();
 527             }
 528             return;
 529         }
 530 
 531         // blocking on monitorenter
 532         if (s == BLOCKING) {
 533             setState(BLOCKED);
 534 
 535             // may have been unblocked while blocking
 536             if (unblocked && compareAndSetState(BLOCKED, UNBLOCKED)) {
 537                 unblocked = false;
 538                 submitRunContinuation();
 539                 return;
 540             }
 541 
 542             // if thread is the designated responsible thread for a monitor then schedule
 543             // it to wakeup so that it can check and recover. See objectMonitor.cpp.
 544             int recheckInterval = this.recheckInterval;
 545             if (recheckInterval > 0 && state() == BLOCKED) {
 546                 assert recheckInterval >= 1 && recheckInterval <= 6;
 547                 // 4 ^ (recheckInterval - 1) = 1, 4, 16, ... 1024
 548                 long delay = 1 << (recheckInterval - 1) << (recheckInterval - 1);
 549                 Future<?> unblocker = delayedTaskScheduler().schedule(this::unblock, delay, MILLISECONDS);
 550                 // cancel if unblocked while scheduling the unblock
 551                 if (state() != BLOCKED) {
 552                     unblocker.cancel(false);
 553                 }
 554             }
 555             return;
 556         }
 557 
 558         assert false;
 559     }
 560 
 561     /**
 562      * Invoked after the continuation completes.
 563      */
 564     private void afterDone() {
 565         afterDone(true);
 566     }
 567 
 568     /**
 569      * Invoked after the continuation completes (or start failed). Sets the thread
 570      * state to TERMINATED and notifies anyone waiting for the thread to terminate.
 571      *
 572      * @param notifyContainer true if its container should be notified
 573      */
 574     private void afterDone(boolean notifyContainer) {
 575         assert carrierThread == null;
 576         setState(TERMINATED);
 577 
 578         // notify anyone waiting for this virtual thread to terminate
 579         CountDownLatch termination = this.termination;
 580         if (termination != null) {
 581             assert termination.getCount() == 1;
 582             termination.countDown();
 583         }
 584 
 585         // notify container
 586         if (notifyContainer) {
 587             threadContainer().onExit(this);
 588         }
 589 
 590         // clear references to thread locals
 591         clearReferences();
 592     }
 593 
 594     /**
 595      * Schedules this {@code VirtualThread} to execute.
 596      *
 597      * @throws IllegalStateException if the container is shutdown or closed
 598      * @throws IllegalThreadStateException if the thread has already been started
 599      * @throws RejectedExecutionException if the scheduler cannot accept a task
 600      */
 601     @Override
 602     void start(ThreadContainer container) {
 603         if (!compareAndSetState(NEW, STARTED)) {
 604             throw new IllegalThreadStateException("Already started");
 605         }
 606 
 607         // bind thread to container
 608         assert threadContainer() == null;
 609         setThreadContainer(container);
 610 
 611         // start thread
 612         boolean addedToContainer = false;
 613         boolean started = false;
 614         try {
 615             container.onStart(this);  // may throw
 616             addedToContainer = true;
 617 
 618             // scoped values may be inherited
 619             inheritScopedValueBindings(container);
 620 
 621             // submit task to run thread, using externalSubmit if possible
 622             externalSubmitRunContinuation();
 623             started = true;
 624         } finally {
 625             if (!started) {
 626                 afterDone(addedToContainer);
 627             }
 628         }
 629     }
 630 
 631     @Override
 632     public void start() {
 633         start(ThreadContainers.root());
 634     }
 635 
 636     @Override
 637     public void run() {
 638         // do nothing
 639     }
 640 
 641     /**
 642      * Parks until unparked or interrupted. If already unparked then the parking
 643      * permit is consumed and this method completes immediately (meaning it doesn't
 644      * yield). It also completes immediately if the interrupt status is set.
 645      */
 646     @Override
 647     void park() {
 648         assert Thread.currentThread() == this;
 649 
 650         // complete immediately if parking permit available or interrupted
 651         if (getAndSetParkPermit(false) || interrupted)
 652             return;
 653 
 654         // park the thread
 655         boolean yielded = false;
 656         setState(PARKING);
 657         try {
 658             yielded = yieldContinuation();  // may throw
 659         } finally {
 660             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 661             if (!yielded) {
 662                 assert state() == PARKING;
 663                 setState(RUNNING);
 664             }
 665         }
 666 
 667         // park on the carrier thread when pinned
 668         if (!yielded) {
 669             parkOnCarrierThread(false, 0);
 670         }
 671     }
 672 
 673     /**
 674      * Parks up to the given waiting time or until unparked or interrupted.
 675      * If already unparked then the parking permit is consumed and this method
 676      * completes immediately (meaning it doesn't yield). It also completes immediately
 677      * if the interrupt status is set or the waiting time is {@code <= 0}.
 678      *
 679      * @param nanos the maximum number of nanoseconds to wait.
 680      */
 681     @Override
 682     void parkNanos(long nanos) {
 683         assert Thread.currentThread() == this;
 684 
 685         // complete immediately if parking permit available or interrupted
 686         if (getAndSetParkPermit(false) || interrupted)
 687             return;
 688 
 689         // park the thread for the waiting time
 690         if (nanos > 0) {
 691             long startTime = System.nanoTime();
 692 
 693             boolean yielded = false;
 694             Future<?> unparker = scheduleUnpark(nanos);  // may throw OOME
 695             setState(TIMED_PARKING);
 696             try {
 697                 yielded = yieldContinuation();  // may throw
 698             } finally {
 699                 assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 700                 if (!yielded) {
 701                     assert state() == TIMED_PARKING;
 702                     setState(RUNNING);
 703                 }
 704                 cancel(unparker);
 705             }
 706 
 707             // park on carrier thread for remaining time when pinned
 708             if (!yielded) {
 709                 long remainingNanos = nanos - (System.nanoTime() - startTime);
 710                 parkOnCarrierThread(true, remainingNanos);
 711             }
 712         }
 713     }
 714 
 715     /**
 716      * Parks the current carrier thread up to the given waiting time or until
 717      * unparked or interrupted. If the virtual thread is interrupted then the
 718      * interrupt status will be propagated to the carrier thread.
 719      * @param timed true for a timed park, false for untimed
 720      * @param nanos the waiting time in nanoseconds
 721      */
 722     private void parkOnCarrierThread(boolean timed, long nanos) {
 723         assert state() == RUNNING;
 724 
 725         setState(timed ? TIMED_PINNED : PINNED);
 726         try {
 727             if (!parkPermit) {
 728                 if (!timed) {
 729                     U.park(false, 0);
 730                 } else if (nanos > 0) {
 731                     U.park(false, nanos);
 732                 }
 733             }
 734         } finally {
 735             setState(RUNNING);
 736         }
 737 
 738         // consume parking permit
 739         setParkPermit(false);
 740     }
 741 
 742     /**
 743      * jdk.VirtualThreadPinned is emitted by HotSpot VM when pinned. Call into VM to
 744      * emit event to avoid having a JFR in Java with the same name (but different ID)
 745      * to events emitted by the VM.
 746      */
 747     private static native void virtualThreadPinnedEvent(int reason);
 748 
 749     /**
 750      * Schedule this virtual thread to be unparked after a given delay.
 751      */
 752     @ChangesCurrentThread
 753     private Future<?> scheduleUnpark(long nanos) {
 754         assert Thread.currentThread() == this;
 755         // need to switch to current carrier thread to avoid nested parking
 756         switchToCarrierThread();
 757         try {
 758             return delayedTaskScheduler().schedule(this::unpark, nanos, NANOSECONDS);
 759         } finally {
 760             switchToVirtualThread(this);
 761         }
 762     }
 763 
 764     /**
 765      * Cancels a task if it has not completed.
 766      */
 767     @ChangesCurrentThread
 768     private void cancel(Future<?> future) {
 769         assert Thread.currentThread() == this;
 770         if (!future.isDone()) {
 771             // need to switch to current carrier thread to avoid nested parking
 772             switchToCarrierThread();
 773             try {
 774                 future.cancel(false);
 775             } finally {
 776                 switchToVirtualThread(this);
 777             }
 778         }
 779     }
 780 
 781     /**
 782      * Re-enables this virtual thread for scheduling. If the virtual thread was
 783      * {@link #park() parked} then it will be unblocked, otherwise its next call
 784      * to {@code park} or {@linkplain #parkNanos(long) parkNanos} is guaranteed
 785      * not to block.
 786      * @throws RejectedExecutionException if the scheduler cannot accept a task
 787      */
 788     @Override
 789     @ChangesCurrentThread
 790     void unpark() {
 791         Thread currentThread = Thread.currentThread();
 792         if (!getAndSetParkPermit(true) && currentThread != this) {
 793             int s = state();
 794             boolean parked = (s == PARKED) || (s == TIMED_PARKED);
 795             if (parked && compareAndSetState(s, UNPARKED)) {
 796                 if (currentThread instanceof VirtualThread vthread) {
 797                     vthread.switchToCarrierThread();
 798                     try {
 799                         submitRunContinuation();
 800                     } finally {
 801                         switchToVirtualThread(vthread);
 802                     }
 803                 } else {
 804                     submitRunContinuation();
 805                 }
 806             } else if ((s == PINNED) || (s == TIMED_PINNED)) {
 807                 // unpark carrier thread when pinned
 808                 disableSuspendAndPreempt();
 809                 try {
 810                     synchronized (carrierThreadAccessLock()) {
 811                         Thread carrier = carrierThread;
 812                         if (carrier != null && ((s = state()) == PINNED || s == TIMED_PINNED)) {
 813                             U.unpark(carrier);
 814                         }
 815                     }
 816                 } finally {
 817                     enableSuspendAndPreempt();
 818                 }
 819             }
 820         }
 821     }
 822 
 823     /**
 824      * Re-enables this virtual thread for scheduling after blocking on monitor enter.
 825      * @throws RejectedExecutionException if the scheduler cannot accept a task
 826      */
 827     private void unblock() {
 828         assert !Thread.currentThread().isVirtual();
 829         unblocked = true;
 830         if (state() == BLOCKED && compareAndSetState(BLOCKED, UNBLOCKED)) {
 831             unblocked = false;
 832             submitRunContinuation();
 833         }
 834     }
 835 
 836     /**
 837      * Attempts to yield the current virtual thread (Thread.yield).
 838      */
 839     void tryYield() {
 840         assert Thread.currentThread() == this;
 841         setState(YIELDING);
 842         boolean yielded = false;
 843         try {
 844             yielded = yieldContinuation();  // may throw
 845         } finally {
 846             assert (Thread.currentThread() == this) && (yielded == (state() == RUNNING));
 847             if (!yielded) {
 848                 assert state() == YIELDING;
 849                 setState(RUNNING);
 850             }
 851         }
 852     }
 853 
 854     /**
 855      * Sleep the current thread for the given sleep time (in nanoseconds). If
 856      * nanos is 0 then the thread will attempt to yield.
 857      *
 858      * @implNote This implementation parks the thread for the given sleeping time
 859      * and will therefore be observed in PARKED state during the sleep. Parking
 860      * will consume the parking permit so this method makes available the parking
 861      * permit after the sleep. This may be observed as a spurious, but benign,
 862      * wakeup when the thread subsequently attempts to park.
 863      *
 864      * @param nanos the maximum number of nanoseconds to sleep
 865      * @throws InterruptedException if interrupted while sleeping
 866      */
 867     void sleepNanos(long nanos) throws InterruptedException {
 868         assert Thread.currentThread() == this && nanos >= 0;
 869         if (getAndClearInterrupt())
 870             throw new InterruptedException();
 871         if (nanos == 0) {
 872             tryYield();
 873         } else {
 874             // park for the sleep time
 875             try {
 876                 long remainingNanos = nanos;
 877                 long startNanos = System.nanoTime();
 878                 while (remainingNanos > 0) {
 879                     parkNanos(remainingNanos);
 880                     if (getAndClearInterrupt()) {
 881                         throw new InterruptedException();
 882                     }
 883                     remainingNanos = nanos - (System.nanoTime() - startNanos);
 884                 }
 885             } finally {
 886                 // may have been unparked while sleeping
 887                 setParkPermit(true);
 888             }
 889         }
 890     }
 891 
 892     /**
 893      * Waits up to {@code nanos} nanoseconds for this virtual thread to terminate.
 894      * A timeout of {@code 0} means to wait forever.
 895      *
 896      * @throws InterruptedException if interrupted while waiting
 897      * @return true if the thread has terminated
 898      */
 899     boolean joinNanos(long nanos) throws InterruptedException {
 900         if (state() == TERMINATED)
 901             return true;
 902 
 903         // ensure termination object exists, then re-check state
 904         CountDownLatch termination = getTermination();
 905         if (state() == TERMINATED)
 906             return true;
 907 
 908         // wait for virtual thread to terminate
 909         if (nanos == 0) {
 910             termination.await();
 911         } else {
 912             boolean terminated = termination.await(nanos, NANOSECONDS);
 913             if (!terminated) {
 914                 // waiting time elapsed
 915                 return false;
 916             }
 917         }
 918         assert state() == TERMINATED;
 919         return true;
 920     }
 921 
 922     @Override
 923     void blockedOn(Interruptible b) {
 924         disableSuspendAndPreempt();
 925         try {
 926             super.blockedOn(b);
 927         } finally {
 928             enableSuspendAndPreempt();
 929         }
 930     }
 931 
 932     @Override
 933     @SuppressWarnings("removal")
 934     public void interrupt() {
 935         if (Thread.currentThread() != this) {
 936             checkAccess();
 937 
 938             // if current thread is a virtual thread then prevent it from being
 939             // suspended or unmounted when entering or holding interruptLock
 940             Interruptible blocker;
 941             disableSuspendAndPreempt();
 942             try {
 943                 synchronized (interruptLock) {
 944                     interrupted = true;
 945                     blocker = nioBlocker();
 946                     if (blocker != null) {
 947                         blocker.interrupt(this);
 948                     }
 949 
 950                     // interrupt carrier thread if mounted
 951                     Thread carrier = carrierThread;
 952                     if (carrier != null) carrier.setInterrupt();
 953                 }
 954             } finally {
 955                 enableSuspendAndPreempt();
 956             }
 957 
 958             // notify blocker after releasing interruptLock
 959             if (blocker != null) {
 960                 blocker.postInterrupt();
 961             }
 962 
 963         } else {
 964             interrupted = true;
 965             carrierThread.setInterrupt();
 966         }
 967         unpark();
 968     }
 969 
 970     @Override
 971     public boolean isInterrupted() {
 972         return interrupted;
 973     }
 974 
 975     @Override
 976     boolean getAndClearInterrupt() {
 977         assert Thread.currentThread() == this;
 978         boolean oldValue = interrupted;
 979         if (oldValue) {
 980             disableSuspendAndPreempt();
 981             try {
 982                 synchronized (interruptLock) {
 983                     interrupted = false;
 984                     carrierThread.clearInterrupt();
 985                 }
 986             } finally {
 987                 enableSuspendAndPreempt();
 988             }
 989         }
 990         return oldValue;
 991     }
 992 
 993     @Override
 994     Thread.State threadState() {
 995         int s = state();
 996         switch (s & ~SUSPENDED) {
 997             case NEW:
 998                 return Thread.State.NEW;
 999             case STARTED:
1000                 // return NEW if thread container not yet set
1001                 if (threadContainer() == null) {
1002                     return Thread.State.NEW;
1003                 } else {
1004                     return Thread.State.RUNNABLE;
1005                 }
1006             case UNPARKED:
1007             case YIELDED:
1008                 // runnable, not mounted
1009                 return Thread.State.RUNNABLE;
1010             case UNBLOCKED:
1011                 // if designated responsible thread for monitor then thread is blocked
1012                 if (isResponsibleForMonitor()) {
1013                     return Thread.State.BLOCKED;
1014                 } else {
1015                     return Thread.State.RUNNABLE;
1016                 }
1017             case RUNNING:
1018                 // if designated responsible thread for monitor then thread is blocked
1019                 if (isResponsibleForMonitor()) {
1020                     return Thread.State.BLOCKED;
1021                 }
1022                 // if mounted then return state of carrier thread
1023                 if (Thread.currentThread() != this) {
1024                     disableSuspendAndPreempt();
1025                     try {
1026                         synchronized (carrierThreadAccessLock()) {
1027                             Thread carrierThread = this.carrierThread;
1028                             if (carrierThread != null) {
1029                                 return carrierThread.threadState();
1030                             }
1031                         }
1032                     } finally {
1033                         enableSuspendAndPreempt();
1034                     }
1035                 }
1036                 // runnable, mounted
1037                 return Thread.State.RUNNABLE;
1038             case PARKING:
1039             case TIMED_PARKING:
1040             case YIELDING:
1041                 // runnable, in transition
1042                 return Thread.State.RUNNABLE;
1043             case PARKED:
1044             case PINNED:
1045                 return State.WAITING;
1046             case TIMED_PARKED:
1047             case TIMED_PINNED:
1048                 return State.TIMED_WAITING;
1049             case BLOCKING:
1050             case BLOCKED:
1051                 return State.BLOCKED;
1052             case TERMINATED:
1053                 return Thread.State.TERMINATED;
1054             default:
1055                 throw new InternalError();
1056         }
1057     }
1058 
1059     /**
1060      * Returns true if thread is the designated responsible thread for a monitor.
1061      * See objectMonitor.cpp for details.
1062      */
1063     private boolean isResponsibleForMonitor() {
1064         return (recheckInterval > 0);
1065     }
1066 
1067     @Override
1068     boolean alive() {
1069         int s = state;
1070         return (s != NEW && s != TERMINATED);
1071     }
1072 
1073     @Override
1074     boolean isTerminated() {
1075         return (state == TERMINATED);
1076     }
1077 
1078     @Override
1079     StackTraceElement[] asyncGetStackTrace() {
1080         StackTraceElement[] stackTrace;
1081         do {
1082             stackTrace = (carrierThread != null)
1083                     ? super.asyncGetStackTrace()  // mounted
1084                     : tryGetStackTrace();         // unmounted
1085             if (stackTrace == null) {
1086                 Thread.yield();
1087             }
1088         } while (stackTrace == null);
1089         return stackTrace;
1090     }
1091 
1092     /**
1093      * Returns the stack trace for this virtual thread if it is unmounted.
1094      * Returns null if the thread is mounted or in transition.
1095      */
1096     private StackTraceElement[] tryGetStackTrace() {
1097         int initialState = state() & ~SUSPENDED;
1098         switch (initialState) {
1099             case NEW, STARTED, TERMINATED -> {
1100                 return new StackTraceElement[0];  // unmounted, empty stack
1101             }
1102             case RUNNING, PINNED, TIMED_PINNED -> {
1103                 return null;   // mounted
1104             }
1105             case PARKED, TIMED_PARKED, BLOCKED -> {
1106                 // unmounted, not runnable
1107             }
1108             case UNPARKED, UNBLOCKED, YIELDED -> {
1109                 // unmounted, runnable
1110             }
1111             case PARKING, TIMED_PARKING, BLOCKING, YIELDING -> {
1112                 return null;  // in transition
1113             }
1114             default -> throw new InternalError("" + initialState);
1115         }
1116 
1117         // thread is unmounted, prevent it from continuing
1118         int suspendedState = initialState | SUSPENDED;
1119         if (!compareAndSetState(initialState, suspendedState)) {
1120             return null;
1121         }
1122 
1123         // get stack trace and restore state
1124         StackTraceElement[] stack;
1125         try {
1126             stack = cont.getStackTrace();
1127         } finally {
1128             assert state == suspendedState;
1129             setState(initialState);
1130         }
1131         boolean resubmit = switch (initialState) {
1132             case UNPARKED, UNBLOCKED, YIELDED -> {
1133                 // resubmit as task may have run while suspended
1134                 yield true;
1135             }
1136             case PARKED, TIMED_PARKED -> {
1137                 // resubmit if unparked while suspended
1138                 yield parkPermit && compareAndSetState(initialState, UNPARKED);
1139             }
1140             case BLOCKED -> {
1141                 // resubmit if unblocked while suspended
1142                 yield unblocked && compareAndSetState(BLOCKED, UNBLOCKED);
1143             }
1144             default -> throw new InternalError();
1145         };
1146         if (resubmit) {
1147             submitRunContinuation();
1148         }
1149         return stack;
1150     }
1151 
1152     @Override
1153     public String toString() {
1154         StringBuilder sb = new StringBuilder("VirtualThread[#");
1155         sb.append(threadId());
1156         String name = getName();
1157         if (!name.isEmpty()) {
1158             sb.append(",");
1159             sb.append(name);
1160         }
1161         sb.append("]/");
1162 
1163         // add the carrier state and thread name when mounted
1164         boolean mounted;
1165         if (Thread.currentThread() == this) {
1166             mounted = appendCarrierInfo(sb);
1167         } else {
1168             disableSuspendAndPreempt();
1169             try {
1170                 synchronized (carrierThreadAccessLock()) {
1171                     mounted = appendCarrierInfo(sb);
1172                 }
1173             } finally {
1174                 enableSuspendAndPreempt();
1175             }
1176         }
1177 
1178         // add virtual thread state when not mounted
1179         if (!mounted) {
1180             String stateAsString = threadState().toString();
1181             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1182         }
1183 
1184         return sb.toString();
1185     }
1186 
1187     /**
1188      * Appends the carrier state and thread name to the string buffer if mounted.
1189      * @return true if mounted, false if not mounted
1190      */
1191     private boolean appendCarrierInfo(StringBuilder sb) {
1192         assert Thread.currentThread() == this || Thread.holdsLock(carrierThreadAccessLock());
1193         Thread carrier = carrierThread;
1194         if (carrier != null) {
1195             String stateAsString = carrier.threadState().toString();
1196             sb.append(stateAsString.toLowerCase(Locale.ROOT));
1197             sb.append('@');
1198             sb.append(carrier.getName());
1199             return true;
1200         } else {
1201             return false;
1202         }
1203     }
1204 
1205     @Override
1206     public int hashCode() {
1207         return (int) threadId();
1208     }
1209 
1210     @Override
1211     public boolean equals(Object obj) {
1212         return obj == this;
1213     }
1214 
1215     /**
1216      * Returns a ScheduledExecutorService to execute a delayed task.
1217      */
1218     private ScheduledExecutorService delayedTaskScheduler() {
1219         long tid = Thread.currentThread().threadId();
1220         int index = (int) tid & (DELAYED_TASK_SCHEDULERS.length - 1);
1221         return DELAYED_TASK_SCHEDULERS[index];
1222     }
1223 
1224     /**
1225      * Returns the termination object, creating it if needed.
1226      */
1227     private CountDownLatch getTermination() {
1228         CountDownLatch termination = this.termination;
1229         if (termination == null) {
1230             termination = new CountDownLatch(1);
1231             if (!U.compareAndSetReference(this, TERMINATION, null, termination)) {
1232                 termination = this.termination;
1233             }
1234         }
1235         return termination;
1236     }
1237 
1238     /**
1239      * Returns the lock object to synchronize on when accessing carrierThread.
1240      * The lock prevents carrierThread from being reset to null during unmount.
1241      */
1242     private Object carrierThreadAccessLock() {
1243         // return interruptLock as unmount has to coordinate with interrupt
1244         return interruptLock;
1245     }
1246 
1247     /**
1248      * Disallow the current thread be suspended or preempted.
1249      */
1250     private void disableSuspendAndPreempt() {
1251         notifyJvmtiDisableSuspend(true);
1252         Continuation.pin();
1253     }
1254 
1255     /**
1256      * Allow the current thread be suspended or preempted.
1257      */
1258     private void enableSuspendAndPreempt() {
1259         Continuation.unpin();
1260         notifyJvmtiDisableSuspend(false);
1261     }
1262 
1263     // -- wrappers for get/set of state, parking permit, and carrier thread --
1264 
1265     private int state() {
1266         return state;  // volatile read
1267     }
1268 
1269     private void setState(int newValue) {
1270         state = newValue;  // volatile write
1271     }
1272 
1273     private boolean compareAndSetState(int expectedValue, int newValue) {
1274         return U.compareAndSetInt(this, STATE, expectedValue, newValue);
1275     }
1276 
1277     private boolean compareAndSetOnWaitingList(byte expectedValue, byte newValue) {
1278         return U.compareAndSetByte(this, ON_WAITING_LIST, expectedValue, newValue);
1279     }
1280 
1281     private void setParkPermit(boolean newValue) {
1282         if (parkPermit != newValue) {
1283             parkPermit = newValue;
1284         }
1285     }
1286 
1287     private boolean getAndSetParkPermit(boolean newValue) {
1288         if (parkPermit != newValue) {
1289             return U.getAndSetBoolean(this, PARK_PERMIT, newValue);
1290         } else {
1291             return newValue;
1292         }
1293     }
1294 
1295     private void setCarrierThread(Thread carrier) {
1296         // U.putReferenceRelease(this, CARRIER_THREAD, carrier);
1297         this.carrierThread = carrier;
1298     }
1299 
1300     @IntrinsicCandidate
1301     private static native void setLockId(long tid);
1302 
1303     // -- JVM TI support --
1304 
1305     @IntrinsicCandidate
1306     @JvmtiMountTransition
1307     private native void notifyJvmtiStart();
1308 
1309     @IntrinsicCandidate
1310     @JvmtiMountTransition
1311     private native void notifyJvmtiEnd();
1312 
1313     @IntrinsicCandidate
1314     @JvmtiMountTransition
1315     private native void notifyJvmtiMount(boolean hide);
1316 
1317     @IntrinsicCandidate
1318     @JvmtiMountTransition
1319     private native void notifyJvmtiUnmount(boolean hide);
1320 
1321     @IntrinsicCandidate
1322     @JvmtiMountTransition
1323     private static native void notifyJvmtiHideFrames(boolean hide);
1324 
1325     @IntrinsicCandidate
1326     private static native void notifyJvmtiDisableSuspend(boolean enter);
1327 
1328     private static native void registerNatives();
1329     static {
1330         registerNatives();
1331     }
1332 
1333     /**
1334      * Creates the default scheduler.
1335      */
1336     @SuppressWarnings("removal")
1337     private static ForkJoinPool createDefaultScheduler() {
1338         ForkJoinWorkerThreadFactory factory = pool -> {
1339             PrivilegedAction<ForkJoinWorkerThread> pa = () -> new CarrierThread(pool);
1340             return AccessController.doPrivileged(pa);
1341         };
1342         PrivilegedAction<ForkJoinPool> pa = () -> {
1343             int parallelism, maxPoolSize, minRunnable;
1344             String parallelismValue = System.getProperty("jdk.virtualThreadScheduler.parallelism");
1345             String maxPoolSizeValue = System.getProperty("jdk.virtualThreadScheduler.maxPoolSize");
1346             String minRunnableValue = System.getProperty("jdk.virtualThreadScheduler.minRunnable");
1347             if (parallelismValue != null) {
1348                 parallelism = Integer.parseInt(parallelismValue);
1349             } else {
1350                 parallelism = Runtime.getRuntime().availableProcessors();
1351             }
1352             if (maxPoolSizeValue != null) {
1353                 maxPoolSize = Integer.parseInt(maxPoolSizeValue);
1354                 parallelism = Integer.min(parallelism, maxPoolSize);
1355             } else {
1356                 maxPoolSize = Integer.max(parallelism, 256);
1357             }
1358             if (minRunnableValue != null) {
1359                 minRunnable = Integer.parseInt(minRunnableValue);
1360             } else {
1361                 minRunnable = Integer.max(parallelism / 2, 1);
1362             }
1363             Thread.UncaughtExceptionHandler handler = (t, e) -> { };
1364             boolean asyncMode = true; // FIFO
1365             return new ForkJoinPool(parallelism, factory, handler, asyncMode,
1366                          0, maxPoolSize, minRunnable, pool -> true, 30, SECONDS);
1367         };
1368         return AccessController.doPrivileged(pa);
1369     }
1370 
1371     /**
1372      * Invoked by the VM for the Thread.vthread_scheduler diagnostic command.
1373      */
1374     private static byte[] printDefaultScheduler() {
1375         return String.format("%s%n", DEFAULT_SCHEDULER.toString())
1376                 .getBytes(StandardCharsets.UTF_8);
1377     }
1378 
1379     /**
1380      * Creates the ScheduledThreadPoolExecutors used to execute delayed tasks.
1381      */
1382     private static ScheduledExecutorService[] createDelayedTaskSchedulers() {
1383         String propName = "jdk.virtualThreadScheduler.timerQueues";
1384         String propValue = GetPropertyAction.privilegedGetProperty(propName);
1385         int queueCount;
1386         if (propValue != null) {
1387             queueCount = Integer.parseInt(propValue);
1388             if (queueCount != Integer.highestOneBit(queueCount)) {
1389                 throw new RuntimeException("Value of " + propName + " must be power of 2");
1390             }
1391         } else {
1392             int ncpus = Runtime.getRuntime().availableProcessors();
1393             queueCount = Math.max(Integer.highestOneBit(ncpus / 4), 1);
1394         }
1395         var schedulers = new ScheduledExecutorService[queueCount];
1396         for (int i = 0; i < queueCount; i++) {
1397             ScheduledThreadPoolExecutor stpe = (ScheduledThreadPoolExecutor)
1398                 Executors.newScheduledThreadPool(1, task -> {
1399                     Thread t = InnocuousThread.newThread("VirtualThread-unparker", task);
1400                     t.setDaemon(true);
1401                     return t;
1402                 });
1403             stpe.setRemoveOnCancelPolicy(true);
1404             schedulers[i] = stpe;
1405         }
1406         return schedulers;
1407     }
1408 
1409     /**
1410      * Schedule virtual threads that are ready to be scheduled after they blocked on
1411      * monitor enter.
1412      */
1413     private static void unblockVirtualThreads() {
1414         while (true) {
1415             VirtualThread vthread = takeVirtualThreadListToUnblock();
1416             while (vthread != null) {
1417                 assert vthread.onWaitingList == 1;
1418                 VirtualThread nextThread = vthread.next;
1419 
1420                 // remove from list and unblock
1421                 vthread.next = null;
1422                 boolean changed = vthread.compareAndSetOnWaitingList((byte) 1, (byte) 0);
1423                 assert changed;
1424                 vthread.unblock();
1425 
1426                 vthread = nextThread;
1427             }
1428         }
1429     }
1430 
1431     /**
1432      * Retrieves the list of virtual threads that are waiting to be unblocked, waiting
1433      * if necessary until a list of one or more threads becomes available.
1434      */
1435     private static native VirtualThread takeVirtualThreadListToUnblock();
1436 
1437     static {
1438         var unblocker = InnocuousThread.newThread("VirtualThread-unblocker",
1439                 VirtualThread::unblockVirtualThreads);
1440         unblocker.setDaemon(true);
1441         unblocker.start();
1442     }
1443 }