1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent;
37
38 import java.util.ArrayList;
39 import java.util.Arrays;
40 import java.util.Collection;
41 import java.util.Collections;
42 import java.util.List;
43 import java.util.Random;
44 import java.util.concurrent.AbstractExecutorService;
45 import java.util.concurrent.Callable;
46 import java.util.concurrent.ExecutorService;
47 import java.util.concurrent.Future;
48 import java.util.concurrent.RejectedExecutionException;
49 import java.util.concurrent.RunnableFuture;
50 import java.util.concurrent.TimeUnit;
51 import java.util.concurrent.TimeoutException;
52 import java.util.concurrent.atomic.AtomicInteger;
53 import java.util.concurrent.locks.LockSupport;
54 import java.util.concurrent.locks.ReentrantLock;
55 import java.util.concurrent.locks.Condition;
56
57 /**
58 * An {@link ExecutorService} for running {@link ForkJoinTask}s.
59 * A {@code ForkJoinPool} provides the entry point for submissions
60 * from non-{@code ForkJoinTask} clients, as well as management and
61 * monitoring operations.
62 *
63 * <p>A {@code ForkJoinPool} differs from other kinds of {@link
64 * ExecutorService} mainly by virtue of employing
65 * <em>work-stealing</em>: all threads in the pool attempt to find and
66 * execute subtasks created by other active tasks (eventually blocking
67 * waiting for work if none exist). This enables efficient processing
68 * when most tasks spawn other subtasks (as do most {@code
69 * ForkJoinTask}s). When setting <em>asyncMode</em> to true in
70 * constructors, {@code ForkJoinPool}s may also be appropriate for use
71 * with event-style tasks that are never joined.
72 *
73 * <p>A {@code ForkJoinPool} is constructed with a given target
74 * parallelism level; by default, equal to the number of available
75 * processors. The pool attempts to maintain enough active (or
76 * available) threads by dynamically adding, suspending, or resuming
77 * internal worker threads, even if some tasks are stalled waiting to
78 * join others. However, no such adjustments are guaranteed in the
79 * face of blocked IO or other unmanaged synchronization. The nested
80 * {@link ManagedBlocker} interface enables extension of the kinds of
81 * synchronization accommodated.
82 *
83 * <p>In addition to execution and lifecycle control methods, this
84 * class provides status check methods (for example
85 * {@link #getStealCount}) that are intended to aid in developing,
86 * tuning, and monitoring fork/join applications. Also, method
87 * {@link #toString} returns indications of pool state in a
88 * convenient form for informal monitoring.
89 *
90 * <p> As is the case with other ExecutorServices, there are three
91 * main task execution methods summarized in the following
92 * table. These are designed to be used by clients not already engaged
93 * in fork/join computations in the current pool. The main forms of
94 * these methods accept instances of {@code ForkJoinTask}, but
95 * overloaded forms also allow mixed execution of plain {@code
96 * Runnable}- or {@code Callable}- based activities as well. However,
97 * tasks that are already executing in a pool should normally
98 * <em>NOT</em> use these pool execution methods, but instead use the
99 * within-computation forms listed in the table.
100 *
101 * <table BORDER CELLPADDING=3 CELLSPACING=1>
102 * <tr>
103 * <td></td>
104 * <td ALIGN=CENTER> <b>Call from non-fork/join clients</b></td>
105 * <td ALIGN=CENTER> <b>Call from within fork/join computations</b></td>
106 * </tr>
107 * <tr>
108 * <td> <b>Arrange async execution</td>
109 * <td> {@link #execute(ForkJoinTask)}</td>
110 * <td> {@link ForkJoinTask#fork}</td>
111 * </tr>
112 * <tr>
113 * <td> <b>Await and obtain result</td>
114 * <td> {@link #invoke(ForkJoinTask)}</td>
115 * <td> {@link ForkJoinTask#invoke}</td>
116 * </tr>
117 * <tr>
118 * <td> <b>Arrange exec and obtain Future</td>
119 * <td> {@link #submit(ForkJoinTask)}</td>
120 * <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
121 * </tr>
122 * </table>
123 *
124 * <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
125 * used for all parallel task execution in a program or subsystem.
126 * Otherwise, use would not usually outweigh the construction and
127 * bookkeeping overhead of creating a large set of threads. For
128 * example, a common pool could be used for the {@code SortTasks}
129 * illustrated in {@link RecursiveAction}. Because {@code
130 * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
131 * daemon} mode, there is typically no need to explicitly {@link
132 * #shutdown} such a pool upon program exit.
133 *
134 * <pre>
135 * static final ForkJoinPool mainPool = new ForkJoinPool();
136 * ...
137 * public void sort(long[] array) {
138 * mainPool.invoke(new SortTask(array, 0, array.length));
139 * }
140 * </pre>
141 *
142 * <p><b>Implementation notes</b>: This implementation restricts the
143 * maximum number of running threads to 32767. Attempts to create
144 * pools with greater than the maximum number result in
145 * {@code IllegalArgumentException}.
146 *
147 * <p>This implementation rejects submitted tasks (that is, by throwing
148 * {@link RejectedExecutionException}) only when the pool is shut down
149 * or internal resources have been exhausted.
150 *
151 * @since 1.7
152 * @author Doug Lea
153 */
154 public class ForkJoinPool extends AbstractExecutorService {
155
156 /*
157 * Implementation Overview
158 *
159 * This class provides the central bookkeeping and control for a
160 * set of worker threads: Submissions from non-FJ threads enter
161 * into a submission queue. Workers take these tasks and typically
162 * split them into subtasks that may be stolen by other workers.
163 * Preference rules give first priority to processing tasks from
164 * their own queues (LIFO or FIFO, depending on mode), then to
165 * randomized FIFO steals of tasks in other worker queues, and
166 * lastly to new submissions.
167 *
168 * The main throughput advantages of work-stealing stem from
169 * decentralized control -- workers mostly take tasks from
170 * themselves or each other. We cannot negate this in the
171 * implementation of other management responsibilities. The main
172 * tactic for avoiding bottlenecks is packing nearly all
173 * essentially atomic control state into a single 64bit volatile
174 * variable ("ctl"). This variable is read on the order of 10-100
175 * times as often as it is modified (always via CAS). (There is
176 * some additional control state, for example variable "shutdown"
177 * for which we can cope with uncoordinated updates.) This
178 * streamlines synchronization and control at the expense of messy
179 * constructions needed to repack status bits upon updates.
180 * Updates tend not to contend with each other except during
181 * bursts while submitted tasks begin or end. In some cases when
182 * they do contend, threads can instead do something else
183 * (usually, scan for tasks) until contention subsides.
184 *
185 * To enable packing, we restrict maximum parallelism to (1<<15)-1
186 * (which is far in excess of normal operating range) to allow
187 * ids, counts, and their negations (used for thresholding) to fit
188 * into 16bit fields.
189 *
190 * Recording Workers. Workers are recorded in the "workers" array
191 * that is created upon pool construction and expanded if (rarely)
192 * necessary. This is an array as opposed to some other data
193 * structure to support index-based random steals by workers.
194 * Updates to the array recording new workers and unrecording
195 * terminated ones are protected from each other by a seqLock
196 * (scanGuard) but the array is otherwise concurrently readable,
197 * and accessed directly by workers. To simplify index-based
198 * operations, the array size is always a power of two, and all
199 * readers must tolerate null slots. To avoid flailing during
200 * start-up, the array is presized to hold twice #parallelism
201 * workers (which is unlikely to need further resizing during
202 * execution). But to avoid dealing with so many null slots,
203 * variable scanGuard includes a mask for the nearest power of two
204 * that contains all current workers. All worker thread creation
205 * is on-demand, triggered by task submissions, replacement of
206 * terminated workers, and/or compensation for blocked
207 * workers. However, all other support code is set up to work with
208 * other policies. To ensure that we do not hold on to worker
209 * references that would prevent GC, ALL accesses to workers are
210 * via indices into the workers array (which is one source of some
211 * of the messy code constructions here). In essence, the workers
212 * array serves as a weak reference mechanism. Thus for example
213 * the wait queue field of ctl stores worker indices, not worker
214 * references. Access to the workers in associated methods (for
215 * example signalWork) must both index-check and null-check the
216 * IDs. All such accesses ignore bad IDs by returning out early
217 * from what they are doing, since this can only be associated
218 * with termination, in which case it is OK to give up.
219 *
220 * All uses of the workers array, as well as queue arrays, check
221 * that the array is non-null (even if previously non-null). This
222 * allows nulling during termination, which is currently not
223 * necessary, but remains an option for resource-revocation-based
224 * shutdown schemes.
225 *
226 * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
227 * let workers spin indefinitely scanning for tasks when none can
228 * be found immediately, and we cannot start/resume workers unless
229 * there appear to be tasks available. On the other hand, we must
230 * quickly prod them into action when new tasks are submitted or
231 * generated. We park/unpark workers after placing in an event
232 * wait queue when they cannot find work. This "queue" is actually
233 * a simple Treiber stack, headed by the "id" field of ctl, plus a
234 * 15bit counter value to both wake up waiters (by advancing their
235 * count) and avoid ABA effects. Successors are held in worker
236 * field "nextWait". Queuing deals with several intrinsic races,
237 * mainly that a task-producing thread can miss seeing (and
238 * signalling) another thread that gave up looking for work but
239 * has not yet entered the wait queue. We solve this by requiring
240 * a full sweep of all workers both before (in scan()) and after
241 * (in tryAwaitWork()) a newly waiting worker is added to the wait
242 * queue. During a rescan, the worker might release some other
243 * queued worker rather than itself, which has the same net
244 * effect. Because enqueued workers may actually be rescanning
245 * rather than waiting, we set and clear the "parked" field of
246 * ForkJoinWorkerThread to reduce unnecessary calls to unpark.
247 * (Use of the parked field requires a secondary recheck to avoid
248 * missed signals.)
249 *
250 * Signalling. We create or wake up workers only when there
251 * appears to be at least one task they might be able to find and
252 * execute. When a submission is added or another worker adds a
253 * task to a queue that previously had two or fewer tasks, they
254 * signal waiting workers (or trigger creation of new ones if
255 * fewer than the given parallelism level -- see signalWork).
256 * These primary signals are buttressed by signals during rescans
257 * as well as those performed when a worker steals a task and
258 * notices that there are more tasks too; together these cover the
259 * signals needed in cases when more than two tasks are pushed
260 * but untaken.
261 *
262 * Trimming workers. To release resources after periods of lack of
263 * use, a worker starting to wait when the pool is quiescent will
264 * time out and terminate if the pool has remained quiescent for
265 * SHRINK_RATE nanosecs. This will slowly propagate, eventually
266 * terminating all workers after long periods of non-use.
267 *
268 * Submissions. External submissions are maintained in an
269 * array-based queue that is structured identically to
270 * ForkJoinWorkerThread queues except for the use of
271 * submissionLock in method addSubmission. Unlike the case for
272 * worker queues, multiple external threads can add new
273 * submissions, so adding requires a lock.
274 *
275 * Compensation. Beyond work-stealing support and lifecycle
276 * control, the main responsibility of this framework is to take
277 * actions when one worker is waiting to join a task stolen (or
278 * always held by) another. Because we are multiplexing many
279 * tasks on to a pool of workers, we can't just let them block (as
280 * in Thread.join). We also cannot just reassign the joiner's
281 * run-time stack with another and replace it later, which would
282 * be a form of "continuation", that even if possible is not
283 * necessarily a good idea since we sometimes need both an
284 * unblocked task and its continuation to progress. Instead we
285 * combine two tactics:
286 *
287 * Helping: Arranging for the joiner to execute some task that it
288 * would be running if the steal had not occurred. Method
289 * ForkJoinWorkerThread.joinTask tracks joining->stealing
290 * links to try to find such a task.
291 *
292 * Compensating: Unless there are already enough live threads,
293 * method tryPreBlock() may create or re-activate a spare
294 * thread to compensate for blocked joiners until they
295 * unblock.
296 *
297 * The ManagedBlocker extension API can't use helping so relies
298 * only on compensation in method awaitBlocker.
299 *
300 * It is impossible to keep exactly the target parallelism number
301 * of threads running at any given time. Determining the
302 * existence of conservatively safe helping targets, the
303 * availability of already-created spares, and the apparent need
304 * to create new spares are all racy and require heuristic
305 * guidance, so we rely on multiple retries of each. Currently,
306 * in keeping with on-demand signalling policy, we compensate only
307 * if blocking would leave less than one active (non-waiting,
308 * non-blocked) worker. Additionally, to avoid some false alarms
309 * due to GC, lagging counters, system activity, etc, compensated
310 * blocking for joins is only attempted after rechecks stabilize
311 * (retries are interspersed with Thread.yield, for good
312 * citizenship). The variable blockedCount, incremented before
313 * blocking and decremented after, is sometimes needed to
314 * distinguish cases of waiting for work vs blocking on joins or
315 * other managed sync. Both cases are equivalent for most pool
316 * control, so we can update non-atomically. (Additionally,
317 * contention on blockedCount alleviates some contention on ctl).
318 *
319 * Shutdown and Termination. A call to shutdownNow atomically sets
320 * the ctl stop bit and then (non-atomically) sets each workers
321 * "terminate" status, cancels all unprocessed tasks, and wakes up
322 * all waiting workers. Detecting whether termination should
323 * commence after a non-abrupt shutdown() call requires more work
324 * and bookkeeping. We need consensus about quiesence (i.e., that
325 * there is no more work) which is reflected in active counts so
326 * long as there are no current blockers, as well as possible
327 * re-evaluations during independent changes in blocking or
328 * quiescing workers.
329 *
330 * Style notes: There is a lot of representation-level coupling
331 * among classes ForkJoinPool, ForkJoinWorkerThread, and
332 * ForkJoinTask. Most fields of ForkJoinWorkerThread maintain
333 * data structures managed by ForkJoinPool, so are directly
334 * accessed. Conversely we allow access to "workers" array by
335 * workers, and direct access to ForkJoinTask.status by both
336 * ForkJoinPool and ForkJoinWorkerThread. There is little point
337 * trying to reduce this, since any associated future changes in
338 * representations will need to be accompanied by algorithmic
339 * changes anyway. All together, these low-level implementation
340 * choices produce as much as a factor of 4 performance
341 * improvement compared to naive implementations, and enable the
342 * processing of billions of tasks per second, at the expense of
343 * some ugliness.
344 *
345 * Methods signalWork() and scan() are the main bottlenecks so are
346 * especially heavily micro-optimized/mangled. There are lots of
347 * inline assignments (of form "while ((local = field) != 0)")
348 * which are usually the simplest way to ensure the required read
349 * orderings (which are sometimes critical). This leads to a
350 * "C"-like style of listing declarations of these locals at the
351 * heads of methods or blocks. There are several occurrences of
352 * the unusual "do {} while (!cas...)" which is the simplest way
353 * to force an update of a CAS'ed variable. There are also other
354 * coding oddities that help some methods perform reasonably even
355 * when interpreted (not compiled).
356 *
357 * The order of declarations in this file is: (1) declarations of
358 * statics (2) fields (along with constants used when unpacking
359 * some of them), listed in an order that tends to reduce
360 * contention among them a bit under most JVMs. (3) internal
361 * control methods (4) callbacks and other support for
362 * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
363 * methods (plus a few little helpers). (6) static block
364 * initializing all statics in a minimally dependent order.
365 */
366
367 /**
368 * Factory for creating new {@link ForkJoinWorkerThread}s.
369 * A {@code ForkJoinWorkerThreadFactory} must be defined and used
370 * for {@code ForkJoinWorkerThread} subclasses that extend base
371 * functionality or initialize threads with different contexts.
372 */
373 public static interface ForkJoinWorkerThreadFactory {
374 /**
375 * Returns a new worker thread operating in the given pool.
376 *
377 * @param pool the pool this thread works in
378 * @throws NullPointerException if the pool is null
379 */
380 public ForkJoinWorkerThread newThread(ForkJoinPool pool);
381 }
382
383 /**
384 * Default ForkJoinWorkerThreadFactory implementation; creates a
385 * new ForkJoinWorkerThread.
386 */
387 static class DefaultForkJoinWorkerThreadFactory
388 implements ForkJoinWorkerThreadFactory {
389 public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
390 return new ForkJoinWorkerThread(pool);
391 }
392 }
393
394 /**
395 * Creates a new ForkJoinWorkerThread. This factory is used unless
396 * overridden in ForkJoinPool constructors.
397 */
398 public static final ForkJoinWorkerThreadFactory
399 defaultForkJoinWorkerThreadFactory;
400
401 /**
402 * Permission required for callers of methods that may start or
403 * kill threads.
404 */
405 private static final RuntimePermission modifyThreadPermission;
406
407 /**
408 * If there is a security manager, makes sure caller has
409 * permission to modify threads.
410 */
411 private static void checkPermission() {
412 SecurityManager security = System.getSecurityManager();
413 if (security != null)
414 security.checkPermission(modifyThreadPermission);
415 }
416
417 /**
418 * Generator for assigning sequence numbers as pool names.
419 */
420 private static final AtomicInteger poolNumberGenerator;
421
422 /**
423 * Generator for initial random seeds for worker victim
424 * selection. This is used only to create initial seeds. Random
425 * steals use a cheaper xorshift generator per steal attempt. We
426 * don't expect much contention on seedGenerator, so just use a
427 * plain Random.
428 */
429 static final Random workerSeedGenerator;
430
431 /**
432 * Array holding all worker threads in the pool. Initialized upon
433 * construction. Array size must be a power of two. Updates and
434 * replacements are protected by scanGuard, but the array is
435 * always kept in a consistent enough state to be randomly
436 * accessed without locking by workers performing work-stealing,
437 * as well as other traversal-based methods in this class, so long
438 * as reads memory-acquire by first reading ctl. All readers must
439 * tolerate that some array slots may be null.
440 */
441 ForkJoinWorkerThread[] workers;
442
443 /**
444 * Initial size for submission queue array. Must be a power of
445 * two. In many applications, these always stay small so we use a
446 * small initial cap.
447 */
448 private static final int INITIAL_QUEUE_CAPACITY = 8;
449
450 /**
451 * Maximum size for submission queue array. Must be a power of two
452 * less than or equal to 1 << (31 - width of array entry) to
453 * ensure lack of index wraparound, but is capped at a lower
454 * value to help users trap runaway computations.
455 */
456 private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
457
458 /**
459 * Array serving as submission queue. Initialized upon construction.
460 */
461 private ForkJoinTask<?>[] submissionQueue;
462
463 /**
464 * Lock protecting submissions array for addSubmission
465 */
466 private final ReentrantLock submissionLock;
467
468 /**
469 * Condition for awaitTermination, using submissionLock for
470 * convenience.
471 */
472 private final Condition termination;
473
474 /**
475 * Creation factory for worker threads.
476 */
477 private final ForkJoinWorkerThreadFactory factory;
478
479 /**
480 * The uncaught exception handler used when any worker abruptly
481 * terminates.
482 */
483 final Thread.UncaughtExceptionHandler ueh;
484
485 /**
486 * Prefix for assigning names to worker threads
487 */
488 private final String workerNamePrefix;
489
490 /**
491 * Sum of per-thread steal counts, updated only when threads are
492 * idle or terminating.
493 */
494 private volatile long stealCount;
495
496 /**
497 * Main pool control -- a long packed with:
498 * AC: Number of active running workers minus target parallelism (16 bits)
499 * TC: Number of total workers minus target parallelism (16bits)
500 * ST: true if pool is terminating (1 bit)
501 * EC: the wait count of top waiting thread (15 bits)
502 * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
503 *
504 * When convenient, we can extract the upper 32 bits of counts and
505 * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
506 * (int)ctl. The ec field is never accessed alone, but always
507 * together with id and st. The offsets of counts by the target
508 * parallelism and the positionings of fields makes it possible to
509 * perform the most common checks via sign tests of fields: When
510 * ac is negative, there are not enough active workers, when tc is
511 * negative, there are not enough total workers, when id is
512 * negative, there is at least one waiting worker, and when e is
513 * negative, the pool is terminating. To deal with these possibly
514 * negative fields, we use casts in and out of "short" and/or
515 * signed shifts to maintain signedness.
516 */
517 volatile long ctl;
518
519 // bit positions/shifts for fields
520 private static final int AC_SHIFT = 48;
521 private static final int TC_SHIFT = 32;
522 private static final int ST_SHIFT = 31;
523 private static final int EC_SHIFT = 16;
524
525 // bounds
526 private static final int MAX_ID = 0x7fff; // max poolIndex
527 private static final int SMASK = 0xffff; // mask short bits
528 private static final int SHORT_SIGN = 1 << 15;
529 private static final int INT_SIGN = 1 << 31;
530
531 // masks
532 private static final long STOP_BIT = 0x0001L << ST_SHIFT;
533 private static final long AC_MASK = ((long)SMASK) << AC_SHIFT;
534 private static final long TC_MASK = ((long)SMASK) << TC_SHIFT;
535
536 // units for incrementing and decrementing
537 private static final long TC_UNIT = 1L << TC_SHIFT;
538 private static final long AC_UNIT = 1L << AC_SHIFT;
539
540 // masks and units for dealing with u = (int)(ctl >>> 32)
541 private static final int UAC_SHIFT = AC_SHIFT - 32;
542 private static final int UTC_SHIFT = TC_SHIFT - 32;
543 private static final int UAC_MASK = SMASK << UAC_SHIFT;
544 private static final int UTC_MASK = SMASK << UTC_SHIFT;
545 private static final int UAC_UNIT = 1 << UAC_SHIFT;
546 private static final int UTC_UNIT = 1 << UTC_SHIFT;
547
548 // masks and units for dealing with e = (int)ctl
549 private static final int E_MASK = 0x7fffffff; // no STOP_BIT
550 private static final int EC_UNIT = 1 << EC_SHIFT;
551
552 /**
553 * The target parallelism level.
554 */
555 final int parallelism;
556
557 /**
558 * Index (mod submission queue length) of next element to take
559 * from submission queue. Usage is identical to that for
560 * per-worker queues -- see ForkJoinWorkerThread internal
561 * documentation.
562 */
563 volatile int queueBase;
564
565 /**
566 * Index (mod submission queue length) of next element to add
567 * in submission queue. Usage is identical to that for
568 * per-worker queues -- see ForkJoinWorkerThread internal
569 * documentation.
570 */
571 int queueTop;
572
573 /**
574 * True when shutdown() has been called.
575 */
576 volatile boolean shutdown;
577
578 /**
579 * True if use local fifo, not default lifo, for local polling
580 * Read by, and replicated by ForkJoinWorkerThreads
581 */
582 final boolean locallyFifo;
583
584 /**
585 * The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
586 * When non-zero, suppresses automatic shutdown when active
587 * counts become zero.
588 */
589 volatile int quiescerCount;
590
591 /**
592 * The number of threads blocked in join.
593 */
594 volatile int blockedCount;
595
596 /**
597 * Counter for worker Thread names (unrelated to their poolIndex)
598 */
599 private volatile int nextWorkerNumber;
600
601 /**
602 * The index for the next created worker. Accessed under scanGuard.
603 */
604 private int nextWorkerIndex;
605
606 /**
607 * SeqLock and index masking for updates to workers array. Locked
608 * when SG_UNIT is set. Unlocking clears bit by adding
609 * SG_UNIT. Staleness of read-only operations can be checked by
610 * comparing scanGuard to value before the reads. The low 16 bits
611 * (i.e, anding with SMASK) hold (the smallest power of two
612 * covering all worker indices, minus one, and is used to avoid
613 * dealing with large numbers of null slots when the workers array
614 * is overallocated.
615 */
616 volatile int scanGuard;
617
618 private static final int SG_UNIT = 1 << 16;
619
620 /**
621 * The wakeup interval (in nanoseconds) for a worker waiting for a
622 * task when the pool is quiescent to instead try to shrink the
623 * number of workers. The exact value does not matter too
624 * much. It must be short enough to release resources during
625 * sustained periods of idleness, but not so short that threads
626 * are continually re-created.
627 */
628 private static final long SHRINK_RATE =
629 4L * 1000L * 1000L * 1000L; // 4 seconds
630
631 /**
632 * Top-level loop for worker threads: On each step: if the
633 * previous step swept through all queues and found no tasks, or
634 * there are excess threads, then possibly blocks. Otherwise,
635 * scans for and, if found, executes a task. Returns when pool
636 * and/or worker terminate.
637 *
638 * @param w the worker
639 */
640 final void work(ForkJoinWorkerThread w) {
641 boolean swept = false; // true on empty scans
642 long c;
643 while (!w.terminate && (int)(c = ctl) >= 0) {
644 int a; // active count
645 if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
646 swept = scan(w, a);
647 else if (tryAwaitWork(w, c))
648 swept = false;
649 }
650 }
651
652 // Signalling
653
654 /**
655 * Wakes up or creates a worker.
656 */
657 final void signalWork() {
658 /*
659 * The while condition is true if: (there is are too few total
660 * workers OR there is at least one waiter) AND (there are too
661 * few active workers OR the pool is terminating). The value
662 * of e distinguishes the remaining cases: zero (no waiters)
663 * for create, negative if terminating (in which case do
664 * nothing), else release a waiter. The secondary checks for
665 * release (non-null array etc) can fail if the pool begins
666 * terminating after the test, and don't impose any added cost
667 * because JVMs must perform null and bounds checks anyway.
668 */
669 long c; int e, u;
670 while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
671 (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
672 if (e > 0) { // release a waiting worker
673 int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
674 if ((ws = workers) == null ||
675 (i = ~e & SMASK) >= ws.length ||
676 (w = ws[i]) == null)
677 break;
678 long nc = (((long)(w.nextWait & E_MASK)) |
679 ((long)(u + UAC_UNIT) << 32));
680 if (w.eventCount == e &&
681 UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
682 w.eventCount = (e + EC_UNIT) & E_MASK;
683 if (w.parked)
684 UNSAFE.unpark(w);
685 break;
686 }
687 }
688 else if (UNSAFE.compareAndSwapLong
689 (this, ctlOffset, c,
690 (long)(((u + UTC_UNIT) & UTC_MASK) |
691 ((u + UAC_UNIT) & UAC_MASK)) << 32)) {
692 addWorker();
693 break;
694 }
695 }
696 }
697
698 /**
699 * Variant of signalWork to help release waiters on rescans.
700 * Tries once to release a waiter if active count < 0.
701 *
702 * @return false if failed due to contention, else true
703 */
704 private boolean tryReleaseWaiter() {
705 long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
706 if ((e = (int)(c = ctl)) > 0 &&
707 (int)(c >> AC_SHIFT) < 0 &&
708 (ws = workers) != null &&
709 (i = ~e & SMASK) < ws.length &&
710 (w = ws[i]) != null) {
711 long nc = ((long)(w.nextWait & E_MASK) |
712 ((c + AC_UNIT) & (AC_MASK|TC_MASK)));
713 if (w.eventCount != e ||
714 !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
715 return false;
716 w.eventCount = (e + EC_UNIT) & E_MASK;
717 if (w.parked)
718 UNSAFE.unpark(w);
719 }
720 return true;
721 }
722
723 // Scanning for tasks
724
725 /**
726 * Scans for and, if found, executes one task. Scans start at a
727 * random index of workers array, and randomly select the first
728 * (2*#workers)-1 probes, and then, if all empty, resort to 2
729 * circular sweeps, which is necessary to check quiescence. and
730 * taking a submission only if no stealable tasks were found. The
731 * steal code inside the loop is a specialized form of
732 * ForkJoinWorkerThread.deqTask, followed bookkeeping to support
733 * helpJoinTask and signal propagation. The code for submission
734 * queues is almost identical. On each steal, the worker completes
735 * not only the task, but also all local tasks that this task may
736 * have generated. On detecting staleness or contention when
737 * trying to take a task, this method returns without finishing
738 * sweep, which allows global state rechecks before retry.
739 *
740 * @param w the worker
741 * @param a the number of active workers
742 * @return true if swept all queues without finding a task
743 */
744 private boolean scan(ForkJoinWorkerThread w, int a) {
745 int g = scanGuard; // mask 0 avoids useless scans if only one active
746 int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
747 ForkJoinWorkerThread[] ws = workers;
748 if (ws == null || ws.length <= m) // staleness check
749 return false;
750 for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
751 ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
752 ForkJoinWorkerThread v = ws[k & m];
753 if (v != null && (b = v.queueBase) != v.queueTop &&
754 (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
755 long u = (i << ASHIFT) + ABASE;
756 if ((t = q[i]) != null && v.queueBase == b &&
757 UNSAFE.compareAndSwapObject(q, u, t, null)) {
758 int d = (v.queueBase = b + 1) - v.queueTop;
759 v.stealHint = w.poolIndex;
760 if (d != 0)
761 signalWork(); // propagate if nonempty
762 w.execTask(t);
763 }
764 r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
765 return false; // store next seed
766 }
767 else if (j < 0) { // xorshift
768 r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
769 }
770 else
771 ++k;
772 }
773 if (scanGuard != g) // staleness check
774 return false;
775 else { // try to take submission
776 ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
777 if ((b = queueBase) != queueTop &&
778 (q = submissionQueue) != null &&
779 (i = (q.length - 1) & b) >= 0) {
780 long u = (i << ASHIFT) + ABASE;
781 if ((t = q[i]) != null && queueBase == b &&
782 UNSAFE.compareAndSwapObject(q, u, t, null)) {
783 queueBase = b + 1;
784 w.execTask(t);
785 }
786 return false;
787 }
788 return true; // all queues empty
789 }
790 }
791
792 /**
793 * Tries to enqueue worker w in wait queue and await change in
794 * worker's eventCount. If the pool is quiescent and there is
795 * more than one worker, possibly terminates worker upon exit.
796 * Otherwise, before blocking, rescans queues to avoid missed
797 * signals. Upon finding work, releases at least one worker
798 * (which may be the current worker). Rescans restart upon
799 * detected staleness or failure to release due to
800 * contention. Note the unusual conventions about Thread.interrupt
801 * here and elsewhere: Because interrupts are used solely to alert
802 * threads to check termination, which is checked here anyway, we
803 * clear status (using Thread.interrupted) before any call to
804 * park, so that park does not immediately return due to status
805 * being set via some other unrelated call to interrupt in user
806 * code.
807 *
808 * @param w the calling worker
809 * @param c the ctl value on entry
810 * @return true if waited or another thread was released upon enq
811 */
812 private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
813 int v = w.eventCount;
814 w.nextWait = (int)c; // w's successor record
815 long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
816 if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
817 long d = ctl; // return true if lost to a deq, to force scan
818 return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
819 }
820 for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount
821 long s = stealCount;
822 if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
823 sc = w.stealCount = 0;
824 else if (w.eventCount != v)
825 return true; // update next time
826 }
827 if ((!shutdown || !tryTerminate(false)) &&
828 (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 &&
829 blockedCount == 0 && quiescerCount == 0)
830 idleAwaitWork(w, nc, c, v); // quiescent
831 for (boolean rescanned = false;;) {
832 if (w.eventCount != v)
833 return true;
834 if (!rescanned) {
835 int g = scanGuard, m = g & SMASK;
836 ForkJoinWorkerThread[] ws = workers;
837 if (ws != null && m < ws.length) {
838 rescanned = true;
839 for (int i = 0; i <= m; ++i) {
840 ForkJoinWorkerThread u = ws[i];
841 if (u != null) {
842 if (u.queueBase != u.queueTop &&
843 !tryReleaseWaiter())
844 rescanned = false; // contended
845 if (w.eventCount != v)
846 return true;
847 }
848 }
849 }
850 if (scanGuard != g || // stale
851 (queueBase != queueTop && !tryReleaseWaiter()))
852 rescanned = false;
853 if (!rescanned)
854 Thread.yield(); // reduce contention
855 else
856 Thread.interrupted(); // clear before park
857 }
858 else {
859 w.parked = true; // must recheck
860 if (w.eventCount != v) {
861 w.parked = false;
862 return true;
863 }
864 LockSupport.park(this);
865 rescanned = w.parked = false;
866 }
867 }
868 }
869
870 /**
871 * If inactivating worker w has caused pool to become
872 * quiescent, check for pool termination, and wait for event
873 * for up to SHRINK_RATE nanosecs (rescans are unnecessary in
874 * this case because quiescence reflects consensus about lack
875 * of work). On timeout, if ctl has not changed, terminate the
876 * worker. Upon its termination (see deregisterWorker), it may
877 * wake up another worker to possibly repeat this process.
878 *
879 * @param w the calling worker
880 * @param currentCtl the ctl value after enqueuing w
881 * @param prevCtl the ctl value if w terminated
882 * @param v the eventCount w awaits change
883 */
884 private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
885 long prevCtl, int v) {
886 if (w.eventCount == v) {
887 if (shutdown)
888 tryTerminate(false);
889 ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
890 while (ctl == currentCtl) {
891 long startTime = System.nanoTime();
892 w.parked = true;
893 if (w.eventCount == v) // must recheck
894 LockSupport.parkNanos(this, SHRINK_RATE);
895 w.parked = false;
896 if (w.eventCount != v)
897 break;
898 else if (System.nanoTime() - startTime <
899 SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop
900 Thread.interrupted(); // spurious wakeup
901 else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
902 currentCtl, prevCtl)) {
903 w.terminate = true; // restore previous
904 w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
905 break;
906 }
907 }
908 }
909 }
910
911 // Submissions
912
913 /**
914 * Enqueues the given task in the submissionQueue. Same idea as
915 * ForkJoinWorkerThread.pushTask except for use of submissionLock.
916 *
917 * @param t the task
918 */
919 private void addSubmission(ForkJoinTask<?> t) {
920 final ReentrantLock lock = this.submissionLock;
921 lock.lock();
922 try {
923 ForkJoinTask<?>[] q; int s, m;
924 if ((q = submissionQueue) != null) { // ignore if queue removed
925 long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
926 UNSAFE.putOrderedObject(q, u, t);
927 queueTop = s + 1;
928 if (s - queueBase == m)
929 growSubmissionQueue();
930 }
931 } finally {
932 lock.unlock();
933 }
934 signalWork();
935 }
936
937 // (pollSubmission is defined below with exported methods)
938
939 /**
940 * Creates or doubles submissionQueue array.
941 * Basically identical to ForkJoinWorkerThread version.
942 */
943 private void growSubmissionQueue() {
944 ForkJoinTask<?>[] oldQ = submissionQueue;
945 int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
946 if (size > MAXIMUM_QUEUE_CAPACITY)
947 throw new RejectedExecutionException("Queue capacity exceeded");
948 if (size < INITIAL_QUEUE_CAPACITY)
949 size = INITIAL_QUEUE_CAPACITY;
950 ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size];
951 int mask = size - 1;
952 int top = queueTop;
953 int oldMask;
954 if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
955 for (int b = queueBase; b != top; ++b) {
956 long u = ((b & oldMask) << ASHIFT) + ABASE;
957 Object x = UNSAFE.getObjectVolatile(oldQ, u);
958 if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
959 UNSAFE.putObjectVolatile
960 (q, ((b & mask) << ASHIFT) + ABASE, x);
961 }
962 }
963 }
964
965 // Blocking support
966
967 /**
968 * Tries to increment blockedCount, decrement active count
969 * (sometimes implicitly) and possibly release or create a
970 * compensating worker in preparation for blocking. Fails
971 * on contention or termination.
972 *
973 * @return true if the caller can block, else should recheck and retry
974 */
975 private boolean tryPreBlock() {
976 int b = blockedCount;
977 if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
978 int pc = parallelism;
979 do {
980 ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
981 int e, ac, tc, rc, i;
982 long c = ctl;
983 int u = (int)(c >>> 32);
984 if ((e = (int)c) < 0) {
985 // skip -- terminating
986 }
987 else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
988 (ws = workers) != null &&
989 (i = ~e & SMASK) < ws.length &&
990 (w = ws[i]) != null) {
991 long nc = ((long)(w.nextWait & E_MASK) |
992 (c & (AC_MASK|TC_MASK)));
993 if (w.eventCount == e &&
994 UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
995 w.eventCount = (e + EC_UNIT) & E_MASK;
996 if (w.parked)
997 UNSAFE.unpark(w);
998 return true; // release an idle worker
999 }
1000 }
1001 else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
1002 long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
1003 if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
1004 return true; // no compensation needed
1005 }
1006 else if (tc + pc < MAX_ID) {
1007 long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
1008 if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
1009 addWorker();
1010 return true; // create a replacement
1011 }
1012 }
1013 // try to back out on any failure and let caller retry
1014 } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
1015 b = blockedCount, b - 1));
1016 }
1017 return false;
1018 }
1019
1020 /**
1021 * Decrements blockedCount and increments active count
1022 */
1023 private void postBlock() {
1024 long c;
1025 do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask
1026 c = ctl, c + AC_UNIT));
1027 int b;
1028 do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
1029 b = blockedCount, b - 1));
1030 }
1031
1032 /**
1033 * Possibly blocks waiting for the given task to complete, or
1034 * cancels the task if terminating. Fails to wait if contended.
1035 *
1036 * @param joinMe the task
1037 */
1038 final void tryAwaitJoin(ForkJoinTask<?> joinMe) {
1039 int s;
1040 Thread.interrupted(); // clear interrupts before checking termination
1041 if (joinMe.status >= 0) {
1042 if (tryPreBlock()) {
1043 joinMe.tryAwaitDone(0L);
1044 postBlock();
1045 }
1046 else if ((ctl & STOP_BIT) != 0L)
1047 joinMe.cancelIgnoringExceptions();
1048 }
1049 }
1050
1051 /**
1052 * Possibly blocks the given worker waiting for joinMe to
1053 * complete or timeout
1054 *
1055 * @param joinMe the task
1056 * @param millis the wait time for underlying Object.wait
1057 */
1058 final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) {
1059 while (joinMe.status >= 0) {
1060 Thread.interrupted();
1061 if ((ctl & STOP_BIT) != 0L) {
1062 joinMe.cancelIgnoringExceptions();
1063 break;
1064 }
1065 if (tryPreBlock()) {
1066 long last = System.nanoTime();
1067 while (joinMe.status >= 0) {
1068 long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
1069 if (millis <= 0)
1070 break;
1071 joinMe.tryAwaitDone(millis);
1072 if (joinMe.status < 0)
1073 break;
1074 if ((ctl & STOP_BIT) != 0L) {
1075 joinMe.cancelIgnoringExceptions();
1076 break;
1077 }
1078 long now = System.nanoTime();
1079 nanos -= now - last;
1080 last = now;
1081 }
1082 postBlock();
1083 break;
1084 }
1085 }
1086 }
1087
1088 /**
1089 * If necessary, compensates for blocker, and blocks
1090 */
1091 private void awaitBlocker(ManagedBlocker blocker)
1092 throws InterruptedException {
1093 while (!blocker.isReleasable()) {
1094 if (tryPreBlock()) {
1095 try {
1096 do {} while (!blocker.isReleasable() && !blocker.block());
1097 } finally {
1098 postBlock();
1099 }
1100 break;
1101 }
1102 }
1103 }
1104
1105 // Creating, registering and deregistring workers
1106
1107 /**
1108 * Tries to create and start a worker; minimally rolls back counts
1109 * on failure.
1110 */
1111 private void addWorker() {
1112 Throwable ex = null;
1113 ForkJoinWorkerThread t = null;
1114 try {
1115 t = factory.newThread(this);
1116 } catch (Throwable e) {
1117 ex = e;
1118 }
1119 if (t == null) { // null or exceptional factory return
1120 long c; // adjust counts
1121 do {} while (!UNSAFE.compareAndSwapLong
1122 (this, ctlOffset, c = ctl,
1123 (((c - AC_UNIT) & AC_MASK) |
1124 ((c - TC_UNIT) & TC_MASK) |
1125 (c & ~(AC_MASK|TC_MASK)))));
1126 // Propagate exception if originating from an external caller
1127 if (!tryTerminate(false) && ex != null &&
1128 !(Thread.currentThread() instanceof ForkJoinWorkerThread))
1129 UNSAFE.throwException(ex);
1130 }
1131 else
1132 t.start();
1133 }
1134
1135 /**
1136 * Callback from ForkJoinWorkerThread constructor to assign a
1137 * public name
1138 */
1139 final String nextWorkerName() {
1140 for (int n;;) {
1141 if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
1142 n = nextWorkerNumber, ++n))
1143 return workerNamePrefix + n;
1144 }
1145 }
1146
1147 /**
1148 * Callback from ForkJoinWorkerThread constructor to
1149 * determine its poolIndex and record in workers array.
1150 *
1151 * @param w the worker
1152 * @return the worker's pool index
1153 */
1154 final int registerWorker(ForkJoinWorkerThread w) {
1155 /*
1156 * In the typical case, a new worker acquires the lock, uses
1157 * next available index and returns quickly. Since we should
1158 * not block callers (ultimately from signalWork or
1159 * tryPreBlock) waiting for the lock needed to do this, we
1160 * instead help release other workers while waiting for the
1161 * lock.
1162 */
1163 for (int g;;) {
1164 ForkJoinWorkerThread[] ws;
1165 if (((g = scanGuard) & SG_UNIT) == 0 &&
1166 UNSAFE.compareAndSwapInt(this, scanGuardOffset,
1167 g, g | SG_UNIT)) {
1168 int k = nextWorkerIndex;
1169 try {
1170 if ((ws = workers) != null) { // ignore on shutdown
1171 int n = ws.length;
1172 if (k < 0 || k >= n || ws[k] != null) {
1173 for (k = 0; k < n && ws[k] != null; ++k)
1174 ;
1175 if (k == n)
1176 ws = workers = Arrays.copyOf(ws, n << 1);
1177 }
1178 ws[k] = w;
1179 nextWorkerIndex = k + 1;
1180 int m = g & SMASK;
1181 g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
1182 }
1183 } finally {
1184 scanGuard = g;
1185 }
1186 return k;
1187 }
1188 else if ((ws = workers) != null) { // help release others
1189 for (ForkJoinWorkerThread u : ws) {
1190 if (u != null && u.queueBase != u.queueTop) {
1191 if (tryReleaseWaiter())
1192 break;
1193 }
1194 }
1195 }
1196 }
1197 }
1198
1199 /**
1200 * Final callback from terminating worker. Removes record of
1201 * worker from array, and adjusts counts. If pool is shutting
1202 * down, tries to complete termination.
1203 *
1204 * @param w the worker
1205 */
1206 final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
1207 int idx = w.poolIndex;
1208 int sc = w.stealCount;
1209 int steps = 0;
1210 // Remove from array, adjust worker counts and collect steal count.
1211 // We can intermix failed removes or adjusts with steal updates
1212 do {
1213 long s, c;
1214 int g;
1215 if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
1216 UNSAFE.compareAndSwapInt(this, scanGuardOffset,
1217 g, g |= SG_UNIT)) {
1218 ForkJoinWorkerThread[] ws = workers;
1219 if (ws != null && idx >= 0 &&
1220 idx < ws.length && ws[idx] == w)
1221 ws[idx] = null; // verify
1222 nextWorkerIndex = idx;
1223 scanGuard = g + SG_UNIT;
1224 steps = 1;
1225 }
1226 if (steps == 1 &&
1227 UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
1228 (((c - AC_UNIT) & AC_MASK) |
1229 ((c - TC_UNIT) & TC_MASK) |
1230 (c & ~(AC_MASK|TC_MASK)))))
1231 steps = 2;
1232 if (sc != 0 &&
1233 UNSAFE.compareAndSwapLong(this, stealCountOffset,
1234 s = stealCount, s + sc))
1235 sc = 0;
1236 } while (steps != 2 || sc != 0);
1237 if (!tryTerminate(false)) {
1238 if (ex != null) // possibly replace if died abnormally
1239 signalWork();
1240 else
1241 tryReleaseWaiter();
1242 }
1243 }
1244
1245 // Shutdown and termination
1246
1247 /**
1248 * Possibly initiates and/or completes termination.
1249 *
1250 * @param now if true, unconditionally terminate, else only
1251 * if shutdown and empty queue and no active workers
1252 * @return true if now terminating or terminated
1253 */
1254 private boolean tryTerminate(boolean now) {
1255 long c;
1256 while (((c = ctl) & STOP_BIT) == 0) {
1257 if (!now) {
1258 if ((int)(c >> AC_SHIFT) != -parallelism)
1259 return false;
1260 if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
1261 queueBase != queueTop) {
1262 if (ctl == c) // staleness check
1263 return false;
1264 continue;
1265 }
1266 }
1267 if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
1268 startTerminating();
1269 }
1270 if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
1271 final ReentrantLock lock = this.submissionLock;
1272 lock.lock();
1273 try {
1274 termination.signalAll();
1275 } finally {
1276 lock.unlock();
1277 }
1278 }
1279 return true;
1280 }
1281
1282 /**
1283 * Runs up to three passes through workers: (0) Setting
1284 * termination status for each worker, followed by wakeups up to
1285 * queued workers; (1) helping cancel tasks; (2) interrupting
1286 * lagging threads (likely in external tasks, but possibly also
1287 * blocked in joins). Each pass repeats previous steps because of
1288 * potential lagging thread creation.
1289 */
1290 private void startTerminating() {
1291 cancelSubmissions();
1292 for (int pass = 0; pass < 3; ++pass) {
1293 ForkJoinWorkerThread[] ws = workers;
1294 if (ws != null) {
1295 for (ForkJoinWorkerThread w : ws) {
1296 if (w != null) {
1297 w.terminate = true;
1298 if (pass > 0) {
1299 w.cancelTasks();
1300 if (pass > 1 && !w.isInterrupted()) {
1301 try {
1302 w.interrupt();
1303 } catch (SecurityException ignore) {
1304 }
1305 }
1306 }
1307 }
1308 }
1309 terminateWaiters();
1310 }
1311 }
1312 }
1313
1314 /**
1315 * Polls and cancels all submissions. Called only during termination.
1316 */
1317 private void cancelSubmissions() {
1318 while (queueBase != queueTop) {
1319 ForkJoinTask<?> task = pollSubmission();
1320 if (task != null) {
1321 try {
1322 task.cancel(false);
1323 } catch (Throwable ignore) {
1324 }
1325 }
1326 }
1327 }
1328
1329 /**
1330 * Tries to set the termination status of waiting workers, and
1331 * then wakes them up (after which they will terminate).
1332 */
1333 private void terminateWaiters() {
1334 ForkJoinWorkerThread[] ws = workers;
1335 if (ws != null) {
1336 ForkJoinWorkerThread w; long c; int i, e;
1337 int n = ws.length;
1338 while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
1339 (w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
1340 if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
1341 (long)(w.nextWait & E_MASK) |
1342 ((c + AC_UNIT) & AC_MASK) |
1343 (c & (TC_MASK|STOP_BIT)))) {
1344 w.terminate = true;
1345 w.eventCount = e + EC_UNIT;
1346 if (w.parked)
1347 UNSAFE.unpark(w);
1348 }
1349 }
1350 }
1351 }
1352
1353 // misc ForkJoinWorkerThread support
1354
1355 /**
1356 * Increment or decrement quiescerCount. Needed only to prevent
1357 * triggering shutdown if a worker is transiently inactive while
1358 * checking quiescence.
1359 *
1360 * @param delta 1 for increment, -1 for decrement
1361 */
1362 final void addQuiescerCount(int delta) {
1363 int c;
1364 do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
1365 c = quiescerCount, c + delta));
1366 }
1367
1368 /**
1369 * Directly increment or decrement active count without
1370 * queuing. This method is used to transiently assert inactivation
1371 * while checking quiescence.
1372 *
1373 * @param delta 1 for increment, -1 for decrement
1374 */
1375 final void addActiveCount(int delta) {
1376 long d = delta < 0 ? -AC_UNIT : AC_UNIT;
1377 long c;
1378 do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
1379 ((c + d) & AC_MASK) |
1380 (c & ~AC_MASK)));
1381 }
1382
1383 /**
1384 * Returns the approximate (non-atomic) number of idle threads per
1385 * active thread.
1386 */
1387 final int idlePerActive() {
1388 // Approximate at powers of two for small values, saturate past 4
1389 int p = parallelism;
1390 int a = p + (int)(ctl >> AC_SHIFT);
1391 return (a > (p >>>= 1) ? 0 :
1392 a > (p >>>= 1) ? 1 :
1393 a > (p >>>= 1) ? 2 :
1394 a > (p >>>= 1) ? 4 :
1395 8);
1396 }
1397
1398 // Exported methods
1399
1400 // Constructors
1401
1402 /**
1403 * Creates a {@code ForkJoinPool} with parallelism equal to {@link
1404 * java.lang.Runtime#availableProcessors}, using the {@linkplain
1405 * #defaultForkJoinWorkerThreadFactory default thread factory},
1406 * no UncaughtExceptionHandler, and non-async LIFO processing mode.
1407 *
1408 * @throws SecurityException if a security manager exists and
1409 * the caller is not permitted to modify threads
1410 * because it does not hold {@link
1411 * java.lang.RuntimePermission}{@code ("modifyThread")}
1412 */
1413 public ForkJoinPool() {
1414 this(Runtime.getRuntime().availableProcessors(),
1415 defaultForkJoinWorkerThreadFactory, null, false);
1416 }
1417
1418 /**
1419 * Creates a {@code ForkJoinPool} with the indicated parallelism
1420 * level, the {@linkplain
1421 * #defaultForkJoinWorkerThreadFactory default thread factory},
1422 * no UncaughtExceptionHandler, and non-async LIFO processing mode.
1423 *
1424 * @param parallelism the parallelism level
1425 * @throws IllegalArgumentException if parallelism less than or
1426 * equal to zero, or greater than implementation limit
1427 * @throws SecurityException if a security manager exists and
1428 * the caller is not permitted to modify threads
1429 * because it does not hold {@link
1430 * java.lang.RuntimePermission}{@code ("modifyThread")}
1431 */
1432 public ForkJoinPool(int parallelism) {
1433 this(parallelism, defaultForkJoinWorkerThreadFactory, null, false);
1434 }
1435
1436 /**
1437 * Creates a {@code ForkJoinPool} with the given parameters.
1438 *
1439 * @param parallelism the parallelism level. For default value,
1440 * use {@link java.lang.Runtime#availableProcessors}.
1441 * @param factory the factory for creating new threads. For default value,
1442 * use {@link #defaultForkJoinWorkerThreadFactory}.
1443 * @param handler the handler for internal worker threads that
1444 * terminate due to unrecoverable errors encountered while executing
1445 * tasks. For default value, use {@code null}.
1446 * @param asyncMode if true,
1447 * establishes local first-in-first-out scheduling mode for forked
1448 * tasks that are never joined. This mode may be more appropriate
1449 * than default locally stack-based mode in applications in which
1450 * worker threads only process event-style asynchronous tasks.
1451 * For default value, use {@code false}.
1452 * @throws IllegalArgumentException if parallelism less than or
1453 * equal to zero, or greater than implementation limit
1454 * @throws NullPointerException if the factory is null
1455 * @throws SecurityException if a security manager exists and
1456 * the caller is not permitted to modify threads
1457 * because it does not hold {@link
1458 * java.lang.RuntimePermission}{@code ("modifyThread")}
1459 */
1460 public ForkJoinPool(int parallelism,
1461 ForkJoinWorkerThreadFactory factory,
1462 Thread.UncaughtExceptionHandler handler,
1463 boolean asyncMode) {
1464 checkPermission();
1465 if (factory == null)
1466 throw new NullPointerException();
1467 if (parallelism <= 0 || parallelism > MAX_ID)
1468 throw new IllegalArgumentException();
1469 this.parallelism = parallelism;
1470 this.factory = factory;
1471 this.ueh = handler;
1472 this.locallyFifo = asyncMode;
1473 long np = (long)(-parallelism); // offset ctl counts
1474 this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
1475 this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
1476 // initialize workers array with room for 2*parallelism if possible
1477 int n = parallelism << 1;
1478 if (n >= MAX_ID)
1479 n = MAX_ID;
1480 else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
1481 n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
1482 }
1483 workers = new ForkJoinWorkerThread[n + 1];
1484 this.submissionLock = new ReentrantLock();
1485 this.termination = submissionLock.newCondition();
1486 StringBuilder sb = new StringBuilder("ForkJoinPool-");
1487 sb.append(poolNumberGenerator.incrementAndGet());
1488 sb.append("-worker-");
1489 this.workerNamePrefix = sb.toString();
1490 }
1491
1492 // Execution methods
1493
1494 /**
1495 * Performs the given task, returning its result upon completion.
1496 * If the computation encounters an unchecked Exception or Error,
1497 * it is rethrown as the outcome of this invocation. Rethrown
1498 * exceptions behave in the same way as regular exceptions, but,
1499 * when possible, contain stack traces (as displayed for example
1500 * using {@code ex.printStackTrace()}) of both the current thread
1501 * as well as the thread actually encountering the exception;
1502 * minimally only the latter.
1503 *
1504 * @param task the task
1505 * @return the task's result
1506 * @throws NullPointerException if the task is null
1507 * @throws RejectedExecutionException if the task cannot be
1508 * scheduled for execution
1509 */
1510 public <T> T invoke(ForkJoinTask<T> task) {
1511 Thread t = Thread.currentThread();
1512 if (task == null)
1513 throw new NullPointerException();
1514 if (shutdown)
1515 throw new RejectedExecutionException();
1516 if ((t instanceof ForkJoinWorkerThread) &&
1517 ((ForkJoinWorkerThread)t).pool == this)
1518 return task.invoke(); // bypass submit if in same pool
1519 else {
1520 addSubmission(task);
1521 return task.join();
1522 }
1523 }
1524
1525 /**
1526 * Unless terminating, forks task if within an ongoing FJ
1527 * computation in the current pool, else submits as external task.
1528 */
1529 private <T> void forkOrSubmit(ForkJoinTask<T> task) {
1530 ForkJoinWorkerThread w;
1531 Thread t = Thread.currentThread();
1532 if (shutdown)
1533 throw new RejectedExecutionException();
1534 if ((t instanceof ForkJoinWorkerThread) &&
1535 (w = (ForkJoinWorkerThread)t).pool == this)
1536 w.pushTask(task);
1537 else
1538 addSubmission(task);
1539 }
1540
1541 /**
1542 * Arranges for (asynchronous) execution of the given task.
1543 *
1544 * @param task the task
1545 * @throws NullPointerException if the task is null
1546 * @throws RejectedExecutionException if the task cannot be
1547 * scheduled for execution
1548 */
1549 public void execute(ForkJoinTask<?> task) {
1550 if (task == null)
1551 throw new NullPointerException();
1552 forkOrSubmit(task);
1553 }
1554
1555 // AbstractExecutorService methods
1556
1557 /**
1558 * @throws NullPointerException if the task is null
1559 * @throws RejectedExecutionException if the task cannot be
1560 * scheduled for execution
1561 */
1562 public void execute(Runnable task) {
1563 if (task == null)
1564 throw new NullPointerException();
1565 ForkJoinTask<?> job;
1566 if (task instanceof ForkJoinTask<?>) // avoid re-wrap
1567 job = (ForkJoinTask<?>) task;
1568 else
1569 job = ForkJoinTask.adapt(task, null);
1570 forkOrSubmit(job);
1571 }
1572
1573 /**
1574 * Submits a ForkJoinTask for execution.
1575 *
1576 * @param task the task to submit
1577 * @return the task
1578 * @throws NullPointerException if the task is null
1579 * @throws RejectedExecutionException if the task cannot be
1580 * scheduled for execution
1581 */
1582 public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
1583 if (task == null)
1584 throw new NullPointerException();
1585 forkOrSubmit(task);
1586 return task;
1587 }
1588
1589 /**
1590 * @throws NullPointerException if the task is null
1591 * @throws RejectedExecutionException if the task cannot be
1592 * scheduled for execution
1593 */
1594 public <T> ForkJoinTask<T> submit(Callable<T> task) {
1595 if (task == null)
1596 throw new NullPointerException();
1597 ForkJoinTask<T> job = ForkJoinTask.adapt(task);
1598 forkOrSubmit(job);
1599 return job;
1600 }
1601
1602 /**
1603 * @throws NullPointerException if the task is null
1604 * @throws RejectedExecutionException if the task cannot be
1605 * scheduled for execution
1606 */
1607 public <T> ForkJoinTask<T> submit(Runnable task, T result) {
1608 if (task == null)
1609 throw new NullPointerException();
1610 ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
1611 forkOrSubmit(job);
1612 return job;
1613 }
1614
1615 /**
1616 * @throws NullPointerException if the task is null
1617 * @throws RejectedExecutionException if the task cannot be
1618 * scheduled for execution
1619 */
1620 public ForkJoinTask<?> submit(Runnable task) {
1621 if (task == null)
1622 throw new NullPointerException();
1623 ForkJoinTask<?> job;
1624 if (task instanceof ForkJoinTask<?>) // avoid re-wrap
1625 job = (ForkJoinTask<?>) task;
1626 else
1627 job = ForkJoinTask.adapt(task, null);
1628 forkOrSubmit(job);
1629 return job;
1630 }
1631
1632 /**
1633 * @throws NullPointerException {@inheritDoc}
1634 * @throws RejectedExecutionException {@inheritDoc}
1635 */
1636 public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
1637 ArrayList<ForkJoinTask<T>> forkJoinTasks =
1638 new ArrayList<ForkJoinTask<T>>(tasks.size());
1639 for (Callable<T> task : tasks)
1640 forkJoinTasks.add(ForkJoinTask.adapt(task));
1641 invoke(new InvokeAll<T>(forkJoinTasks));
1642
1643 @SuppressWarnings({"unchecked", "rawtypes"})
1644 List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
1645 return futures;
1646 }
1647
1648 static final class InvokeAll<T> extends RecursiveAction {
1649 final ArrayList<ForkJoinTask<T>> tasks;
1650 InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
1651 public void compute() {
1652 try { invokeAll(tasks); }
1653 catch (Exception ignore) {}
1654 }
1655 private static final long serialVersionUID = -7914297376763021607L;
1656 }
1657
1658 /**
1659 * Returns the factory used for constructing new workers.
1660 *
1661 * @return the factory used for constructing new workers
1662 */
1663 public ForkJoinWorkerThreadFactory getFactory() {
1664 return factory;
1665 }
1666
1667 /**
1668 * Returns the handler for internal worker threads that terminate
1669 * due to unrecoverable errors encountered while executing tasks.
1670 *
1671 * @return the handler, or {@code null} if none
1672 */
1673 public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() {
1674 return ueh;
1675 }
1676
1677 /**
1678 * Returns the targeted parallelism level of this pool.
1679 *
1680 * @return the targeted parallelism level of this pool
1681 */
1682 public int getParallelism() {
1683 return parallelism;
1684 }
1685
1686 /**
1687 * Returns the number of worker threads that have started but not
1688 * yet terminated. The result returned by this method may differ
1689 * from {@link #getParallelism} when threads are created to
1690 * maintain parallelism when others are cooperatively blocked.
1691 *
1692 * @return the number of worker threads
1693 */
1694 public int getPoolSize() {
1695 return parallelism + (short)(ctl >>> TC_SHIFT);
1696 }
1697
1698 /**
1699 * Returns {@code true} if this pool uses local first-in-first-out
1700 * scheduling mode for forked tasks that are never joined.
1701 *
1702 * @return {@code true} if this pool uses async mode
1703 */
1704 public boolean getAsyncMode() {
1705 return locallyFifo;
1706 }
1707
1708 /**
1709 * Returns an estimate of the number of worker threads that are
1710 * not blocked waiting to join tasks or for other managed
1711 * synchronization. This method may overestimate the
1712 * number of running threads.
1713 *
1714 * @return the number of worker threads
1715 */
1716 public int getRunningThreadCount() {
1717 int r = parallelism + (int)(ctl >> AC_SHIFT);
1718 return (r <= 0) ? 0 : r; // suppress momentarily negative values
1719 }
1720
1721 /**
1722 * Returns an estimate of the number of threads that are currently
1723 * stealing or executing tasks. This method may overestimate the
1724 * number of active threads.
1725 *
1726 * @return the number of active threads
1727 */
1728 public int getActiveThreadCount() {
1729 int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount;
1730 return (r <= 0) ? 0 : r; // suppress momentarily negative values
1731 }
1732
1733 /**
1734 * Returns {@code true} if all worker threads are currently idle.
1735 * An idle worker is one that cannot obtain a task to execute
1736 * because none are available to steal from other threads, and
1737 * there are no pending submissions to the pool. This method is
1738 * conservative; it might not return {@code true} immediately upon
1739 * idleness of all threads, but will eventually become true if
1740 * threads remain inactive.
1741 *
1742 * @return {@code true} if all threads are currently idle
1743 */
1744 public boolean isQuiescent() {
1745 return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0;
1746 }
1747
1748 /**
1749 * Returns an estimate of the total number of tasks stolen from
1750 * one thread's work queue by another. The reported value
1751 * underestimates the actual total number of steals when the pool
1752 * is not quiescent. This value may be useful for monitoring and
1753 * tuning fork/join programs: in general, steal counts should be
1754 * high enough to keep threads busy, but low enough to avoid
1755 * overhead and contention across threads.
1756 *
1757 * @return the number of steals
1758 */
1759 public long getStealCount() {
1760 return stealCount;
1761 }
1762
1763 /**
1764 * Returns an estimate of the total number of tasks currently held
1765 * in queues by worker threads (but not including tasks submitted
1766 * to the pool that have not begun executing). This value is only
1767 * an approximation, obtained by iterating across all threads in
1768 * the pool. This method may be useful for tuning task
1769 * granularities.
1770 *
1771 * @return the number of queued tasks
1772 */
1773 public long getQueuedTaskCount() {
1774 long count = 0;
1775 ForkJoinWorkerThread[] ws;
1776 if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
1777 (ws = workers) != null) {
1778 for (ForkJoinWorkerThread w : ws)
1779 if (w != null)
1780 count -= w.queueBase - w.queueTop; // must read base first
1781 }
1782 return count;
1783 }
1784
1785 /**
1786 * Returns an estimate of the number of tasks submitted to this
1787 * pool that have not yet begun executing. This method may take
1788 * time proportional to the number of submissions.
1789 *
1790 * @return the number of queued submissions
1791 */
1792 public int getQueuedSubmissionCount() {
1793 return -queueBase + queueTop;
1794 }
1795
1796 /**
1797 * Returns {@code true} if there are any tasks submitted to this
1798 * pool that have not yet begun executing.
1799 *
1800 * @return {@code true} if there are any queued submissions
1801 */
1802 public boolean hasQueuedSubmissions() {
1803 return queueBase != queueTop;
1804 }
1805
1806 /**
1807 * Removes and returns the next unexecuted submission if one is
1808 * available. This method may be useful in extensions to this
1809 * class that re-assign work in systems with multiple pools.
1810 *
1811 * @return the next submission, or {@code null} if none
1812 */
1813 protected ForkJoinTask<?> pollSubmission() {
1814 ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
1815 while ((b = queueBase) != queueTop &&
1816 (q = submissionQueue) != null &&
1817 (i = (q.length - 1) & b) >= 0) {
1818 long u = (i << ASHIFT) + ABASE;
1819 if ((t = q[i]) != null &&
1820 queueBase == b &&
1821 UNSAFE.compareAndSwapObject(q, u, t, null)) {
1822 queueBase = b + 1;
1823 return t;
1824 }
1825 }
1826 return null;
1827 }
1828
1829 /**
1830 * Removes all available unexecuted submitted and forked tasks
1831 * from scheduling queues and adds them to the given collection,
1832 * without altering their execution status. These may include
1833 * artificially generated or wrapped tasks. This method is
1834 * designed to be invoked only when the pool is known to be
1835 * quiescent. Invocations at other times may not remove all
1836 * tasks. A failure encountered while attempting to add elements
1837 * to collection {@code c} may result in elements being in
1838 * neither, either or both collections when the associated
1839 * exception is thrown. The behavior of this operation is
1840 * undefined if the specified collection is modified while the
1841 * operation is in progress.
1842 *
1843 * @param c the collection to transfer elements into
1844 * @return the number of elements transferred
1845 */
1846 protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
1847 int count = 0;
1848 while (queueBase != queueTop) {
1849 ForkJoinTask<?> t = pollSubmission();
1850 if (t != null) {
1851 c.add(t);
1852 ++count;
1853 }
1854 }
1855 ForkJoinWorkerThread[] ws;
1856 if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
1857 (ws = workers) != null) {
1858 for (ForkJoinWorkerThread w : ws)
1859 if (w != null)
1860 count += w.drainTasksTo(c);
1861 }
1862 return count;
1863 }
1864
1865 /**
1866 * Returns a string identifying this pool, as well as its state,
1867 * including indications of run state, parallelism level, and
1868 * worker and task counts.
1869 *
1870 * @return a string identifying this pool, as well as its state
1871 */
1872 public String toString() {
1873 long st = getStealCount();
1874 long qt = getQueuedTaskCount();
1875 long qs = getQueuedSubmissionCount();
1876 int pc = parallelism;
1877 long c = ctl;
1878 int tc = pc + (short)(c >>> TC_SHIFT);
1879 int rc = pc + (int)(c >> AC_SHIFT);
1880 if (rc < 0) // ignore transient negative
1881 rc = 0;
1882 int ac = rc + blockedCount;
1883 String level;
1884 if ((c & STOP_BIT) != 0)
1885 level = (tc == 0) ? "Terminated" : "Terminating";
1886 else
1887 level = shutdown ? "Shutting down" : "Running";
1888 return super.toString() +
1889 "[" + level +
1890 ", parallelism = " + pc +
1891 ", size = " + tc +
1892 ", active = " + ac +
1893 ", running = " + rc +
1894 ", steals = " + st +
1895 ", tasks = " + qt +
1896 ", submissions = " + qs +
1897 "]";
1898 }
1899
1900 /**
1901 * Initiates an orderly shutdown in which previously submitted
1902 * tasks are executed, but no new tasks will be accepted.
1903 * Invocation has no additional effect if already shut down.
1904 * Tasks that are in the process of being submitted concurrently
1905 * during the course of this method may or may not be rejected.
1906 *
1907 * @throws SecurityException if a security manager exists and
1908 * the caller is not permitted to modify threads
1909 * because it does not hold {@link
1910 * java.lang.RuntimePermission}{@code ("modifyThread")}
1911 */
1912 public void shutdown() {
1913 checkPermission();
1914 shutdown = true;
1915 tryTerminate(false);
1916 }
1917
1918 /**
1919 * Attempts to cancel and/or stop all tasks, and reject all
1920 * subsequently submitted tasks. Tasks that are in the process of
1921 * being submitted or executed concurrently during the course of
1922 * this method may or may not be rejected. This method cancels
1923 * both existing and unexecuted tasks, in order to permit
1924 * termination in the presence of task dependencies. So the method
1925 * always returns an empty list (unlike the case for some other
1926 * Executors).
1927 *
1928 * @return an empty list
1929 * @throws SecurityException if a security manager exists and
1930 * the caller is not permitted to modify threads
1931 * because it does not hold {@link
1932 * java.lang.RuntimePermission}{@code ("modifyThread")}
1933 */
1934 public List<Runnable> shutdownNow() {
1935 checkPermission();
1936 shutdown = true;
1937 tryTerminate(true);
1938 return Collections.emptyList();
1939 }
1940
1941 /**
1942 * Returns {@code true} if all tasks have completed following shut down.
1943 *
1944 * @return {@code true} if all tasks have completed following shut down
1945 */
1946 public boolean isTerminated() {
1947 long c = ctl;
1948 return ((c & STOP_BIT) != 0L &&
1949 (short)(c >>> TC_SHIFT) == -parallelism);
1950 }
1951
1952 /**
1953 * Returns {@code true} if the process of termination has
1954 * commenced but not yet completed. This method may be useful for
1955 * debugging. A return of {@code true} reported a sufficient
1956 * period after shutdown may indicate that submitted tasks have
1957 * ignored or suppressed interruption, or are waiting for IO,
1958 * causing this executor not to properly terminate. (See the
1959 * advisory notes for class {@link ForkJoinTask} stating that
1960 * tasks should not normally entail blocking operations. But if
1961 * they do, they must abort them on interrupt.)
1962 *
1963 * @return {@code true} if terminating but not yet terminated
1964 */
1965 public boolean isTerminating() {
1966 long c = ctl;
1967 return ((c & STOP_BIT) != 0L &&
1968 (short)(c >>> TC_SHIFT) != -parallelism);
1969 }
1970
1971 /**
1972 * Returns true if terminating or terminated. Used by ForkJoinWorkerThread.
1973 */
1974 final boolean isAtLeastTerminating() {
1975 return (ctl & STOP_BIT) != 0L;
1976 }
1977
1978 /**
1979 * Returns {@code true} if this pool has been shut down.
1980 *
1981 * @return {@code true} if this pool has been shut down
1982 */
1983 public boolean isShutdown() {
1984 return shutdown;
1985 }
1986
1987 /**
1988 * Blocks until all tasks have completed execution after a shutdown
1989 * request, or the timeout occurs, or the current thread is
1990 * interrupted, whichever happens first.
1991 *
1992 * @param timeout the maximum time to wait
1993 * @param unit the time unit of the timeout argument
1994 * @return {@code true} if this executor terminated and
1995 * {@code false} if the timeout elapsed before termination
1996 * @throws InterruptedException if interrupted while waiting
1997 */
1998 public boolean awaitTermination(long timeout, TimeUnit unit)
1999 throws InterruptedException {
2000 long nanos = unit.toNanos(timeout);
2001 final ReentrantLock lock = this.submissionLock;
2002 lock.lock();
2003 try {
2004 for (;;) {
2005 if (isTerminated())
2006 return true;
2007 if (nanos <= 0)
2008 return false;
2009 nanos = termination.awaitNanos(nanos);
2010 }
2011 } finally {
2012 lock.unlock();
2013 }
2014 }
2015
2016 /**
2017 * Interface for extending managed parallelism for tasks running
2018 * in {@link ForkJoinPool}s.
2019 *
2020 * <p>A {@code ManagedBlocker} provides two methods. Method
2021 * {@code isReleasable} must return {@code true} if blocking is
2022 * not necessary. Method {@code block} blocks the current thread
2023 * if necessary (perhaps internally invoking {@code isReleasable}
2024 * before actually blocking). These actions are performed by any
2025 * thread invoking {@link ForkJoinPool#managedBlock}. The
2026 * unusual methods in this API accommodate synchronizers that may,
2027 * but don't usually, block for long periods. Similarly, they
2028 * allow more efficient internal handling of cases in which
2029 * additional workers may be, but usually are not, needed to
2030 * ensure sufficient parallelism. Toward this end,
2031 * implementations of method {@code isReleasable} must be amenable
2032 * to repeated invocation.
2033 *
2034 * <p>For example, here is a ManagedBlocker based on a
2035 * ReentrantLock:
2036 * <pre> {@code
2037 * class ManagedLocker implements ManagedBlocker {
2038 * final ReentrantLock lock;
2039 * boolean hasLock = false;
2040 * ManagedLocker(ReentrantLock lock) { this.lock = lock; }
2041 * public boolean block() {
2042 * if (!hasLock)
2043 * lock.lock();
2044 * return true;
2045 * }
2046 * public boolean isReleasable() {
2047 * return hasLock || (hasLock = lock.tryLock());
2048 * }
2049 * }}</pre>
2050 *
2051 * <p>Here is a class that possibly blocks waiting for an
2052 * item on a given queue:
2053 * <pre> {@code
2054 * class QueueTaker<E> implements ManagedBlocker {
2055 * final BlockingQueue<E> queue;
2056 * volatile E item = null;
2057 * QueueTaker(BlockingQueue<E> q) { this.queue = q; }
2058 * public boolean block() throws InterruptedException {
2059 * if (item == null)
2060 * item = queue.take();
2061 * return true;
2062 * }
2063 * public boolean isReleasable() {
2064 * return item != null || (item = queue.poll()) != null;
2065 * }
2066 * public E getItem() { // call after pool.managedBlock completes
2067 * return item;
2068 * }
2069 * }}</pre>
2070 */
2071 public static interface ManagedBlocker {
2072 /**
2073 * Possibly blocks the current thread, for example waiting for
2074 * a lock or condition.
2075 *
2076 * @return {@code true} if no additional blocking is necessary
2077 * (i.e., if isReleasable would return true)
2078 * @throws InterruptedException if interrupted while waiting
2079 * (the method is not required to do so, but is allowed to)
2080 */
2081 boolean block() throws InterruptedException;
2082
2083 /**
2084 * Returns {@code true} if blocking is unnecessary.
2085 */
2086 boolean isReleasable();
2087 }
2088
2089 /**
2090 * Blocks in accord with the given blocker. If the current thread
2091 * is a {@link ForkJoinWorkerThread}, this method possibly
2092 * arranges for a spare thread to be activated if necessary to
2093 * ensure sufficient parallelism while the current thread is blocked.
2094 *
2095 * <p>If the caller is not a {@link ForkJoinTask}, this method is
2096 * behaviorally equivalent to
2097 * <pre> {@code
2098 * while (!blocker.isReleasable())
2099 * if (blocker.block())
2100 * return;
2101 * }</pre>
2102 *
2103 * If the caller is a {@code ForkJoinTask}, then the pool may
2104 * first be expanded to ensure parallelism, and later adjusted.
2105 *
2106 * @param blocker the blocker
2107 * @throws InterruptedException if blocker.block did so
2108 */
2109 public static void managedBlock(ManagedBlocker blocker)
2110 throws InterruptedException {
2111 Thread t = Thread.currentThread();
2112 if (t instanceof ForkJoinWorkerThread) {
2113 ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
2114 w.pool.awaitBlocker(blocker);
2115 }
2116 else {
2117 do {} while (!blocker.isReleasable() && !blocker.block());
2118 }
2119 }
2120
2121 // AbstractExecutorService overrides. These rely on undocumented
2122 // fact that ForkJoinTask.adapt returns ForkJoinTasks that also
2123 // implement RunnableFuture.
2124
2125 protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
2126 return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
2127 }
2128
2129 protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
2130 return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
2131 }
2132
2133 // Unsafe mechanics
2134 private static final sun.misc.Unsafe UNSAFE;
2135 private static final long ctlOffset;
2136 private static final long stealCountOffset;
2137 private static final long blockedCountOffset;
2138 private static final long quiescerCountOffset;
2139 private static final long scanGuardOffset;
2140 private static final long nextWorkerNumberOffset;
2141 private static final long ABASE;
2142 private static final int ASHIFT;
2143
2144 static {
2145 poolNumberGenerator = new AtomicInteger();
2146 workerSeedGenerator = new Random();
2147 modifyThreadPermission = new RuntimePermission("modifyThread");
2148 defaultForkJoinWorkerThreadFactory =
2149 new DefaultForkJoinWorkerThreadFactory();
2150 int s;
2151 try {
2152 UNSAFE = sun.misc.Unsafe.getUnsafe();
2153 Class k = ForkJoinPool.class;
2154 ctlOffset = UNSAFE.objectFieldOffset
2155 (k.getDeclaredField("ctl"));
2156 stealCountOffset = UNSAFE.objectFieldOffset
2157 (k.getDeclaredField("stealCount"));
2158 blockedCountOffset = UNSAFE.objectFieldOffset
2159 (k.getDeclaredField("blockedCount"));
2160 quiescerCountOffset = UNSAFE.objectFieldOffset
2161 (k.getDeclaredField("quiescerCount"));
2162 scanGuardOffset = UNSAFE.objectFieldOffset
2163 (k.getDeclaredField("scanGuard"));
2164 nextWorkerNumberOffset = UNSAFE.objectFieldOffset
2165 (k.getDeclaredField("nextWorkerNumber"));
2166 Class a = ForkJoinTask[].class;
2167 ABASE = UNSAFE.arrayBaseOffset(a);
2168 s = UNSAFE.arrayIndexScale(a);
2169 } catch (Exception e) {
2170 throw new Error(e);
2171 }
2172 if ((s & (s-1)) != 0)
2173 throw new Error("data type scale not a power of two");
2174 ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
2175 }
2176
2177 }