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My first initial commit with all of mine experimental changes.

Dmitry Kabak (userdima committed 13 years ago

Commit d37b88d1 parent 3f5b46618352 files · +2598093 −-8690Browse files

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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+}