/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent; import java.util.ArrayList; import java.util.Arrays; import java.util.Collection; import java.util.Collections; import java.util.List; import java.util.Random; import java.util.concurrent.AbstractExecutorService; import java.util.concurrent.Callable; import java.util.concurrent.ExecutorService; import java.util.concurrent.Future; import java.util.concurrent.RejectedExecutionException; import java.util.concurrent.RunnableFuture; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; import java.util.concurrent.atomic.AtomicInteger; import java.util.concurrent.locks.LockSupport; import java.util.concurrent.locks.ReentrantLock; import java.util.concurrent.locks.Condition; /** * An {@link ExecutorService} for running {@link ForkJoinTask}s. * A {@code ForkJoinPool} provides the entry point for submissions * from non-{@code ForkJoinTask} clients, as well as management and * monitoring operations. * *

A {@code ForkJoinPool} differs from other kinds of {@link * ExecutorService} mainly by virtue of employing * work-stealing: all threads in the pool attempt to find and * execute subtasks created by other active tasks (eventually blocking * waiting for work if none exist). This enables efficient processing * when most tasks spawn other subtasks (as do most {@code * ForkJoinTask}s). When setting asyncMode to true in * constructors, {@code ForkJoinPool}s may also be appropriate for use * with event-style tasks that are never joined. * *

A {@code ForkJoinPool} is constructed with a given target * parallelism level; by default, equal to the number of available * processors. The pool attempts to maintain enough active (or * available) threads by dynamically adding, suspending, or resuming * internal worker threads, even if some tasks are stalled waiting to * join others. However, no such adjustments are guaranteed in the * face of blocked IO or other unmanaged synchronization. The nested * {@link ManagedBlocker} interface enables extension of the kinds of * synchronization accommodated. * *

In addition to execution and lifecycle control methods, this * class provides status check methods (for example * {@link #getStealCount}) that are intended to aid in developing, * tuning, and monitoring fork/join applications. Also, method * {@link #toString} returns indications of pool state in a * convenient form for informal monitoring. * *

As is the case with other ExecutorServices, there are three * main task execution methods summarized in the following * table. These are designed to be used by clients not already engaged * in fork/join computations in the current pool. The main forms of * these methods accept instances of {@code ForkJoinTask}, but * overloaded forms also allow mixed execution of plain {@code * Runnable}- or {@code Callable}- based activities as well. However, * tasks that are already executing in a pool should normally * NOT use these pool execution methods, but instead use the * within-computation forms listed in the table. * * * * * * * * * * * * * * * * * * * * * * *
Call from non-fork/join clients Call from within fork/join computations
Arrange async execution {@link #execute(ForkJoinTask)} {@link ForkJoinTask#fork}
Await and obtain result {@link #invoke(ForkJoinTask)} {@link ForkJoinTask#invoke}
Arrange exec and obtain Future {@link #submit(ForkJoinTask)} {@link ForkJoinTask#fork} (ForkJoinTasks are Futures)
* *

Sample Usage. Normally a single {@code ForkJoinPool} is * used for all parallel task execution in a program or subsystem. * Otherwise, use would not usually outweigh the construction and * bookkeeping overhead of creating a large set of threads. For * example, a common pool could be used for the {@code SortTasks} * illustrated in {@link RecursiveAction}. Because {@code * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon * daemon} mode, there is typically no need to explicitly {@link * #shutdown} such a pool upon program exit. * *

 * static final ForkJoinPool mainPool = new ForkJoinPool();
 * ...
 * public void sort(long[] array) {
 *   mainPool.invoke(new SortTask(array, 0, array.length));
 * }
 * 
* *

Implementation notes: This implementation restricts the * maximum number of running threads to 32767. Attempts to create * pools with greater than the maximum number result in * {@code IllegalArgumentException}. * *

This implementation rejects submitted tasks (that is, by throwing * {@link RejectedExecutionException}) only when the pool is shut down * or internal resources have been exhausted. * * @since 1.7 * @author Doug Lea */ public class ForkJoinPool extends AbstractExecutorService { /* * Implementation Overview * * This class provides the central bookkeeping and control for a * set of worker threads: Submissions from non-FJ threads enter * into a submission queue. Workers take these tasks and typically * split them into subtasks that may be stolen by other workers. * Preference rules give first priority to processing tasks from * their own queues (LIFO or FIFO, depending on mode), then to * randomized FIFO steals of tasks in other worker queues, and * lastly to new submissions. * * The main throughput advantages of work-stealing stem from * decentralized control -- workers mostly take tasks from * themselves or each other. We cannot negate this in the * implementation of other management responsibilities. The main * tactic for avoiding bottlenecks is packing nearly all * essentially atomic control state into a single 64bit volatile * variable ("ctl"). This variable is read on the order of 10-100 * times as often as it is modified (always via CAS). (There is * some additional control state, for example variable "shutdown" * for which we can cope with uncoordinated updates.) This * streamlines synchronization and control at the expense of messy * constructions needed to repack status bits upon updates. * Updates tend not to contend with each other except during * bursts while submitted tasks begin or end. In some cases when * they do contend, threads can instead do something else * (usually, scan for tasks) until contention subsides. * * To enable packing, we restrict maximum parallelism to (1<<15)-1 * (which is far in excess of normal operating range) to allow * ids, counts, and their negations (used for thresholding) to fit * into 16bit fields. * * Recording Workers. Workers are recorded in the "workers" array * that is created upon pool construction and expanded if (rarely) * necessary. This is an array as opposed to some other data * structure to support index-based random steals by workers. * Updates to the array recording new workers and unrecording * terminated ones are protected from each other by a seqLock * (scanGuard) but the array is otherwise concurrently readable, * and accessed directly by workers. To simplify index-based * operations, the array size is always a power of two, and all * readers must tolerate null slots. To avoid flailing during * start-up, the array is presized to hold twice #parallelism * workers (which is unlikely to need further resizing during * execution). But to avoid dealing with so many null slots, * variable scanGuard includes a mask for the nearest power of two * that contains all current workers. All worker thread creation * is on-demand, triggered by task submissions, replacement of * terminated workers, and/or compensation for blocked * workers. However, all other support code is set up to work with * other policies. To ensure that we do not hold on to worker * references that would prevent GC, ALL accesses to workers are * via indices into the workers array (which is one source of some * of the messy code constructions here). In essence, the workers * array serves as a weak reference mechanism. Thus for example * the wait queue field of ctl stores worker indices, not worker * references. Access to the workers in associated methods (for * example signalWork) must both index-check and null-check the * IDs. All such accesses ignore bad IDs by returning out early * from what they are doing, since this can only be associated * with termination, in which case it is OK to give up. * * All uses of the workers array, as well as queue arrays, check * that the array is non-null (even if previously non-null). This * allows nulling during termination, which is currently not * necessary, but remains an option for resource-revocation-based * shutdown schemes. * * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot * let workers spin indefinitely scanning for tasks when none can * be found immediately, and we cannot start/resume workers unless * there appear to be tasks available. On the other hand, we must * quickly prod them into action when new tasks are submitted or * generated. We park/unpark workers after placing in an event * wait queue when they cannot find work. This "queue" is actually * a simple Treiber stack, headed by the "id" field of ctl, plus a * 15bit counter value to both wake up waiters (by advancing their * count) and avoid ABA effects. Successors are held in worker * field "nextWait". Queuing deals with several intrinsic races, * mainly that a task-producing thread can miss seeing (and * signalling) another thread that gave up looking for work but * has not yet entered the wait queue. We solve this by requiring * a full sweep of all workers both before (in scan()) and after * (in tryAwaitWork()) a newly waiting worker is added to the wait * queue. During a rescan, the worker might release some other * queued worker rather than itself, which has the same net * effect. Because enqueued workers may actually be rescanning * rather than waiting, we set and clear the "parked" field of * ForkJoinWorkerThread to reduce unnecessary calls to unpark. * (Use of the parked field requires a secondary recheck to avoid * missed signals.) * * Signalling. We create or wake up workers only when there * appears to be at least one task they might be able to find and * execute. When a submission is added or another worker adds a * task to a queue that previously had two or fewer tasks, they * signal waiting workers (or trigger creation of new ones if * fewer than the given parallelism level -- see signalWork). * These primary signals are buttressed by signals during rescans * as well as those performed when a worker steals a task and * notices that there are more tasks too; together these cover the * signals needed in cases when more than two tasks are pushed * but untaken. * * Trimming workers. To release resources after periods of lack of * use, a worker starting to wait when the pool is quiescent will * time out and terminate if the pool has remained quiescent for * SHRINK_RATE nanosecs. This will slowly propagate, eventually * terminating all workers after long periods of non-use. * * Submissions. External submissions are maintained in an * array-based queue that is structured identically to * ForkJoinWorkerThread queues except for the use of * submissionLock in method addSubmission. Unlike the case for * worker queues, multiple external threads can add new * submissions, so adding requires a lock. * * Compensation. Beyond work-stealing support and lifecycle * control, the main responsibility of this framework is to take * actions when one worker is waiting to join a task stolen (or * always held by) another. Because we are multiplexing many * tasks on to a pool of workers, we can't just let them block (as * in Thread.join). We also cannot just reassign the joiner's * run-time stack with another and replace it later, which would * be a form of "continuation", that even if possible is not * necessarily a good idea since we sometimes need both an * unblocked task and its continuation to progress. Instead we * combine two tactics: * * Helping: Arranging for the joiner to execute some task that it * would be running if the steal had not occurred. Method * ForkJoinWorkerThread.joinTask tracks joining->stealing * links to try to find such a task. * * Compensating: Unless there are already enough live threads, * method tryPreBlock() may create or re-activate a spare * thread to compensate for blocked joiners until they * unblock. * * The ManagedBlocker extension API can't use helping so relies * only on compensation in method awaitBlocker. * * It is impossible to keep exactly the target parallelism number * of threads running at any given time. Determining the * existence of conservatively safe helping targets, the * availability of already-created spares, and the apparent need * to create new spares are all racy and require heuristic * guidance, so we rely on multiple retries of each. Currently, * in keeping with on-demand signalling policy, we compensate only * if blocking would leave less than one active (non-waiting, * non-blocked) worker. Additionally, to avoid some false alarms * due to GC, lagging counters, system activity, etc, compensated * blocking for joins is only attempted after rechecks stabilize * (retries are interspersed with Thread.yield, for good * citizenship). The variable blockedCount, incremented before * blocking and decremented after, is sometimes needed to * distinguish cases of waiting for work vs blocking on joins or * other managed sync. Both cases are equivalent for most pool * control, so we can update non-atomically. (Additionally, * contention on blockedCount alleviates some contention on ctl). * * Shutdown and Termination. A call to shutdownNow atomically sets * the ctl stop bit and then (non-atomically) sets each workers * "terminate" status, cancels all unprocessed tasks, and wakes up * all waiting workers. Detecting whether termination should * commence after a non-abrupt shutdown() call requires more work * and bookkeeping. We need consensus about quiesence (i.e., that * there is no more work) which is reflected in active counts so * long as there are no current blockers, as well as possible * re-evaluations during independent changes in blocking or * quiescing workers. * * Style notes: There is a lot of representation-level coupling * among classes ForkJoinPool, ForkJoinWorkerThread, and * ForkJoinTask. Most fields of ForkJoinWorkerThread maintain * data structures managed by ForkJoinPool, so are directly * accessed. Conversely we allow access to "workers" array by * workers, and direct access to ForkJoinTask.status by both * ForkJoinPool and ForkJoinWorkerThread. There is little point * trying to reduce this, since any associated future changes in * representations will need to be accompanied by algorithmic * changes anyway. All together, these low-level implementation * choices produce as much as a factor of 4 performance * improvement compared to naive implementations, and enable the * processing of billions of tasks per second, at the expense of * some ugliness. * * Methods signalWork() and scan() are the main bottlenecks so are * especially heavily micro-optimized/mangled. There are lots of * inline assignments (of form "while ((local = field) != 0)") * which are usually the simplest way to ensure the required read * orderings (which are sometimes critical). This leads to a * "C"-like style of listing declarations of these locals at the * heads of methods or blocks. There are several occurrences of * the unusual "do {} while (!cas...)" which is the simplest way * to force an update of a CAS'ed variable. There are also other * coding oddities that help some methods perform reasonably even * when interpreted (not compiled). * * The order of declarations in this file is: (1) declarations of * statics (2) fields (along with constants used when unpacking * some of them), listed in an order that tends to reduce * contention among them a bit under most JVMs. (3) internal * control methods (4) callbacks and other support for * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported * methods (plus a few little helpers). (6) static block * initializing all statics in a minimally dependent order. */ /** * Factory for creating new {@link ForkJoinWorkerThread}s. * A {@code ForkJoinWorkerThreadFactory} must be defined and used * for {@code ForkJoinWorkerThread} subclasses that extend base * functionality or initialize threads with different contexts. */ public static interface ForkJoinWorkerThreadFactory { /** * Returns a new worker thread operating in the given pool. * * @param pool the pool this thread works in * @throws NullPointerException if the pool is null */ public ForkJoinWorkerThread newThread(ForkJoinPool pool); } /** * Default ForkJoinWorkerThreadFactory implementation; creates a * new ForkJoinWorkerThread. */ static class DefaultForkJoinWorkerThreadFactory implements ForkJoinWorkerThreadFactory { public ForkJoinWorkerThread newThread(ForkJoinPool pool) { return new ForkJoinWorkerThread(pool); } } /** * Creates a new ForkJoinWorkerThread. This factory is used unless * overridden in ForkJoinPool constructors. */ public static final ForkJoinWorkerThreadFactory defaultForkJoinWorkerThreadFactory; /** * Permission required for callers of methods that may start or * kill threads. */ private static final RuntimePermission modifyThreadPermission; /** * If there is a security manager, makes sure caller has * permission to modify threads. */ private static void checkPermission() { SecurityManager security = System.getSecurityManager(); if (security != null) security.checkPermission(modifyThreadPermission); } /** * Generator for assigning sequence numbers as pool names. */ private static final AtomicInteger poolNumberGenerator; /** * Generator for initial random seeds for worker victim * selection. This is used only to create initial seeds. Random * steals use a cheaper xorshift generator per steal attempt. We * don't expect much contention on seedGenerator, so just use a * plain Random. */ static final Random workerSeedGenerator; /** * Array holding all worker threads in the pool. Initialized upon * construction. Array size must be a power of two. Updates and * replacements are protected by scanGuard, but the array is * always kept in a consistent enough state to be randomly * accessed without locking by workers performing work-stealing, * as well as other traversal-based methods in this class, so long * as reads memory-acquire by first reading ctl. All readers must * tolerate that some array slots may be null. */ ForkJoinWorkerThread[] workers; /** * Initial size for submission queue array. Must be a power of * two. In many applications, these always stay small so we use a * small initial cap. */ private static final int INITIAL_QUEUE_CAPACITY = 8; /** * Maximum size for submission queue array. Must be a power of two * less than or equal to 1 << (31 - width of array entry) to * ensure lack of index wraparound, but is capped at a lower * value to help users trap runaway computations. */ private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M /** * Array serving as submission queue. Initialized upon construction. */ private ForkJoinTask[] submissionQueue; /** * Lock protecting submissions array for addSubmission */ private final ReentrantLock submissionLock; /** * Condition for awaitTermination, using submissionLock for * convenience. */ private final Condition termination; /** * Creation factory for worker threads. */ private final ForkJoinWorkerThreadFactory factory; /** * The uncaught exception handler used when any worker abruptly * terminates. */ final Thread.UncaughtExceptionHandler ueh; /** * Prefix for assigning names to worker threads */ private final String workerNamePrefix; /** * Sum of per-thread steal counts, updated only when threads are * idle or terminating. */ private volatile long stealCount; /** * Main pool control -- a long packed with: * AC: Number of active running workers minus target parallelism (16 bits) * TC: Number of total workers minus target parallelism (16bits) * ST: true if pool is terminating (1 bit) * EC: the wait count of top waiting thread (15 bits) * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits) * * When convenient, we can extract the upper 32 bits of counts and * the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e = * (int)ctl. The ec field is never accessed alone, but always * together with id and st. The offsets of counts by the target * parallelism and the positionings of fields makes it possible to * perform the most common checks via sign tests of fields: When * ac is negative, there are not enough active workers, when tc is * negative, there are not enough total workers, when id is * negative, there is at least one waiting worker, and when e is * negative, the pool is terminating. To deal with these possibly * negative fields, we use casts in and out of "short" and/or * signed shifts to maintain signedness. */ volatile long ctl; // bit positions/shifts for fields private static final int AC_SHIFT = 48; private static final int TC_SHIFT = 32; private static final int ST_SHIFT = 31; private static final int EC_SHIFT = 16; // bounds private static final int MAX_ID = 0x7fff; // max poolIndex private static final int SMASK = 0xffff; // mask short bits private static final int SHORT_SIGN = 1 << 15; private static final int INT_SIGN = 1 << 31; // masks private static final long STOP_BIT = 0x0001L << ST_SHIFT; private static final long AC_MASK = ((long)SMASK) << AC_SHIFT; private static final long TC_MASK = ((long)SMASK) << TC_SHIFT; // units for incrementing and decrementing private static final long TC_UNIT = 1L << TC_SHIFT; private static final long AC_UNIT = 1L << AC_SHIFT; // masks and units for dealing with u = (int)(ctl >>> 32) private static final int UAC_SHIFT = AC_SHIFT - 32; private static final int UTC_SHIFT = TC_SHIFT - 32; private static final int UAC_MASK = SMASK << UAC_SHIFT; private static final int UTC_MASK = SMASK << UTC_SHIFT; private static final int UAC_UNIT = 1 << UAC_SHIFT; private static final int UTC_UNIT = 1 << UTC_SHIFT; // masks and units for dealing with e = (int)ctl private static final int E_MASK = 0x7fffffff; // no STOP_BIT private static final int EC_UNIT = 1 << EC_SHIFT; /** * The target parallelism level. */ final int parallelism; /** * Index (mod submission queue length) of next element to take * from submission queue. Usage is identical to that for * per-worker queues -- see ForkJoinWorkerThread internal * documentation. */ volatile int queueBase; /** * Index (mod submission queue length) of next element to add * in submission queue. Usage is identical to that for * per-worker queues -- see ForkJoinWorkerThread internal * documentation. */ int queueTop; /** * True when shutdown() has been called. */ volatile boolean shutdown; /** * True if use local fifo, not default lifo, for local polling * Read by, and replicated by ForkJoinWorkerThreads */ final boolean locallyFifo; /** * The number of threads in ForkJoinWorkerThreads.helpQuiescePool. * When non-zero, suppresses automatic shutdown when active * counts become zero. */ volatile int quiescerCount; /** * The number of threads blocked in join. */ volatile int blockedCount; /** * Counter for worker Thread names (unrelated to their poolIndex) */ private volatile int nextWorkerNumber; /** * The index for the next created worker. Accessed under scanGuard. */ private int nextWorkerIndex; /** * SeqLock and index masking for updates to workers array. Locked * when SG_UNIT is set. Unlocking clears bit by adding * SG_UNIT. Staleness of read-only operations can be checked by * comparing scanGuard to value before the reads. The low 16 bits * (i.e, anding with SMASK) hold (the smallest power of two * covering all worker indices, minus one, and is used to avoid * dealing with large numbers of null slots when the workers array * is overallocated. */ volatile int scanGuard; private static final int SG_UNIT = 1 << 16; /** * The wakeup interval (in nanoseconds) for a worker waiting for a * task when the pool is quiescent to instead try to shrink the * number of workers. The exact value does not matter too * much. It must be short enough to release resources during * sustained periods of idleness, but not so short that threads * are continually re-created. */ private static final long SHRINK_RATE = 4L * 1000L * 1000L * 1000L; // 4 seconds /** * Top-level loop for worker threads: On each step: if the * previous step swept through all queues and found no tasks, or * there are excess threads, then possibly blocks. Otherwise, * scans for and, if found, executes a task. Returns when pool * and/or worker terminate. * * @param w the worker */ final void work(ForkJoinWorkerThread w) { boolean swept = false; // true on empty scans long c; while (!w.terminate && (int)(c = ctl) >= 0) { int a; // active count if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0) swept = scan(w, a); else if (tryAwaitWork(w, c)) swept = false; } } // Signalling /** * Wakes up or creates a worker. */ final void signalWork() { /* * The while condition is true if: (there is are too few total * workers OR there is at least one waiter) AND (there are too * few active workers OR the pool is terminating). The value * of e distinguishes the remaining cases: zero (no waiters) * for create, negative if terminating (in which case do * nothing), else release a waiter. The secondary checks for * release (non-null array etc) can fail if the pool begins * terminating after the test, and don't impose any added cost * because JVMs must perform null and bounds checks anyway. */ long c; int e, u; while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) & (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) { if (e > 0) { // release a waiting worker int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; if ((ws = workers) == null || (i = ~e & SMASK) >= ws.length || (w = ws[i]) == null) break; long nc = (((long)(w.nextWait & E_MASK)) | ((long)(u + UAC_UNIT) << 32)); if (w.eventCount == e && UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { w.eventCount = (e + EC_UNIT) & E_MASK; if (w.parked) UNSAFE.unpark(w); break; } } else if (UNSAFE.compareAndSwapLong (this, ctlOffset, c, (long)(((u + UTC_UNIT) & UTC_MASK) | ((u + UAC_UNIT) & UAC_MASK)) << 32)) { addWorker(); break; } } } /** * Variant of signalWork to help release waiters on rescans. * Tries once to release a waiter if active count < 0. * * @return false if failed due to contention, else true */ private boolean tryReleaseWaiter() { long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws; if ((e = (int)(c = ctl)) > 0 && (int)(c >> AC_SHIFT) < 0 && (ws = workers) != null && (i = ~e & SMASK) < ws.length && (w = ws[i]) != null) { long nc = ((long)(w.nextWait & E_MASK) | ((c + AC_UNIT) & (AC_MASK|TC_MASK))); if (w.eventCount != e || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) return false; w.eventCount = (e + EC_UNIT) & E_MASK; if (w.parked) UNSAFE.unpark(w); } return true; } // Scanning for tasks /** * Scans for and, if found, executes one task. Scans start at a * random index of workers array, and randomly select the first * (2*#workers)-1 probes, and then, if all empty, resort to 2 * circular sweeps, which is necessary to check quiescence. and * taking a submission only if no stealable tasks were found. The * steal code inside the loop is a specialized form of * ForkJoinWorkerThread.deqTask, followed bookkeeping to support * helpJoinTask and signal propagation. The code for submission * queues is almost identical. On each steal, the worker completes * not only the task, but also all local tasks that this task may * have generated. On detecting staleness or contention when * trying to take a task, this method returns without finishing * sweep, which allows global state rechecks before retry. * * @param w the worker * @param a the number of active workers * @return true if swept all queues without finding a task */ private boolean scan(ForkJoinWorkerThread w, int a) { int g = scanGuard; // mask 0 avoids useless scans if only one active int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK; ForkJoinWorkerThread[] ws = workers; if (ws == null || ws.length <= m) // staleness check return false; for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) { ForkJoinTask t; ForkJoinTask[] q; int b, i; ForkJoinWorkerThread v = ws[k & m]; if (v != null && (b = v.queueBase) != v.queueTop && (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) { long u = (i << ASHIFT) + ABASE; if ((t = q[i]) != null && v.queueBase == b && UNSAFE.compareAndSwapObject(q, u, t, null)) { int d = (v.queueBase = b + 1) - v.queueTop; v.stealHint = w.poolIndex; if (d != 0) signalWork(); // propagate if nonempty w.execTask(t); } r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5); return false; // store next seed } else if (j < 0) { // xorshift r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5; } else ++k; } if (scanGuard != g) // staleness check return false; else { // try to take submission ForkJoinTask t; ForkJoinTask[] q; int b, i; if ((b = queueBase) != queueTop && (q = submissionQueue) != null && (i = (q.length - 1) & b) >= 0) { long u = (i << ASHIFT) + ABASE; if ((t = q[i]) != null && queueBase == b && UNSAFE.compareAndSwapObject(q, u, t, null)) { queueBase = b + 1; w.execTask(t); } return false; } return true; // all queues empty } } /** * Tries to enqueue worker w in wait queue and await change in * worker's eventCount. If the pool is quiescent and there is * more than one worker, possibly terminates worker upon exit. * Otherwise, before blocking, rescans queues to avoid missed * signals. Upon finding work, releases at least one worker * (which may be the current worker). Rescans restart upon * detected staleness or failure to release due to * contention. Note the unusual conventions about Thread.interrupt * here and elsewhere: Because interrupts are used solely to alert * threads to check termination, which is checked here anyway, we * clear status (using Thread.interrupted) before any call to * park, so that park does not immediately return due to status * being set via some other unrelated call to interrupt in user * code. * * @param w the calling worker * @param c the ctl value on entry * @return true if waited or another thread was released upon enq */ private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) { int v = w.eventCount; w.nextWait = (int)c; // w's successor record long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK)); if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { long d = ctl; // return true if lost to a deq, to force scan return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L; } for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount long s = stealCount; if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc)) sc = w.stealCount = 0; else if (w.eventCount != v) return true; // update next time } if ((!shutdown || !tryTerminate(false)) && (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 && blockedCount == 0 && quiescerCount == 0) idleAwaitWork(w, nc, c, v); // quiescent for (boolean rescanned = false;;) { if (w.eventCount != v) return true; if (!rescanned) { int g = scanGuard, m = g & SMASK; ForkJoinWorkerThread[] ws = workers; if (ws != null && m < ws.length) { rescanned = true; for (int i = 0; i <= m; ++i) { ForkJoinWorkerThread u = ws[i]; if (u != null) { if (u.queueBase != u.queueTop && !tryReleaseWaiter()) rescanned = false; // contended if (w.eventCount != v) return true; } } } if (scanGuard != g || // stale (queueBase != queueTop && !tryReleaseWaiter())) rescanned = false; if (!rescanned) Thread.yield(); // reduce contention else Thread.interrupted(); // clear before park } else { w.parked = true; // must recheck if (w.eventCount != v) { w.parked = false; return true; } LockSupport.park(this); rescanned = w.parked = false; } } } /** * If inactivating worker w has caused pool to become * quiescent, check for pool termination, and wait for event * for up to SHRINK_RATE nanosecs (rescans are unnecessary in * this case because quiescence reflects consensus about lack * of work). On timeout, if ctl has not changed, terminate the * worker. Upon its termination (see deregisterWorker), it may * wake up another worker to possibly repeat this process. * * @param w the calling worker * @param currentCtl the ctl value after enqueuing w * @param prevCtl the ctl value if w terminated * @param v the eventCount w awaits change */ private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl, long prevCtl, int v) { if (w.eventCount == v) { if (shutdown) tryTerminate(false); ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs while (ctl == currentCtl) { long startTime = System.nanoTime(); w.parked = true; if (w.eventCount == v) // must recheck LockSupport.parkNanos(this, SHRINK_RATE); w.parked = false; if (w.eventCount != v) break; else if (System.nanoTime() - startTime < SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop Thread.interrupted(); // spurious wakeup else if (UNSAFE.compareAndSwapLong(this, ctlOffset, currentCtl, prevCtl)) { w.terminate = true; // restore previous w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK; break; } } } } // Submissions /** * Enqueues the given task in the submissionQueue. Same idea as * ForkJoinWorkerThread.pushTask except for use of submissionLock. * * @param t the task */ private void addSubmission(ForkJoinTask t) { final ReentrantLock lock = this.submissionLock; lock.lock(); try { ForkJoinTask[] q; int s, m; if ((q = submissionQueue) != null) { // ignore if queue removed long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE; UNSAFE.putOrderedObject(q, u, t); queueTop = s + 1; if (s - queueBase == m) growSubmissionQueue(); } } finally { lock.unlock(); } signalWork(); } // (pollSubmission is defined below with exported methods) /** * Creates or doubles submissionQueue array. * Basically identical to ForkJoinWorkerThread version. */ private void growSubmissionQueue() { ForkJoinTask[] oldQ = submissionQueue; int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY; if (size > MAXIMUM_QUEUE_CAPACITY) throw new RejectedExecutionException("Queue capacity exceeded"); if (size < INITIAL_QUEUE_CAPACITY) size = INITIAL_QUEUE_CAPACITY; ForkJoinTask[] q = submissionQueue = new ForkJoinTask[size]; int mask = size - 1; int top = queueTop; int oldMask; if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) { for (int b = queueBase; b != top; ++b) { long u = ((b & oldMask) << ASHIFT) + ABASE; Object x = UNSAFE.getObjectVolatile(oldQ, u); if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null)) UNSAFE.putObjectVolatile (q, ((b & mask) << ASHIFT) + ABASE, x); } } } // Blocking support /** * Tries to increment blockedCount, decrement active count * (sometimes implicitly) and possibly release or create a * compensating worker in preparation for blocking. Fails * on contention or termination. * * @return true if the caller can block, else should recheck and retry */ private boolean tryPreBlock() { int b = blockedCount; if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) { int pc = parallelism; do { ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w; int e, ac, tc, rc, i; long c = ctl; int u = (int)(c >>> 32); if ((e = (int)c) < 0) { // skip -- terminating } else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 && (ws = workers) != null && (i = ~e & SMASK) < ws.length && (w = ws[i]) != null) { long nc = ((long)(w.nextWait & E_MASK) | (c & (AC_MASK|TC_MASK))); if (w.eventCount == e && UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { w.eventCount = (e + EC_UNIT) & E_MASK; if (w.parked) UNSAFE.unpark(w); return true; // release an idle worker } } else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) { long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK); if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) return true; // no compensation needed } else if (tc + pc < MAX_ID) { long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK); if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) { addWorker(); return true; // create a replacement } } // try to back out on any failure and let caller retry } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, b = blockedCount, b - 1)); } return false; } /** * Decrements blockedCount and increments active count */ private void postBlock() { long c; do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask c = ctl, c + AC_UNIT)); int b; do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset, b = blockedCount, b - 1)); } /** * Possibly blocks waiting for the given task to complete, or * cancels the task if terminating. Fails to wait if contended. * * @param joinMe the task */ final void tryAwaitJoin(ForkJoinTask joinMe) { int s; Thread.interrupted(); // clear interrupts before checking termination if (joinMe.status >= 0) { if (tryPreBlock()) { joinMe.tryAwaitDone(0L); postBlock(); } else if ((ctl & STOP_BIT) != 0L) joinMe.cancelIgnoringExceptions(); } } /** * Possibly blocks the given worker waiting for joinMe to * complete or timeout * * @param joinMe the task * @param millis the wait time for underlying Object.wait */ final void timedAwaitJoin(ForkJoinTask joinMe, long nanos) { while (joinMe.status >= 0) { Thread.interrupted(); if ((ctl & STOP_BIT) != 0L) { joinMe.cancelIgnoringExceptions(); break; } if (tryPreBlock()) { long last = System.nanoTime(); while (joinMe.status >= 0) { long millis = TimeUnit.NANOSECONDS.toMillis(nanos); if (millis <= 0) break; joinMe.tryAwaitDone(millis); if (joinMe.status < 0) break; if ((ctl & STOP_BIT) != 0L) { joinMe.cancelIgnoringExceptions(); break; } long now = System.nanoTime(); nanos -= now - last; last = now; } postBlock(); break; } } } /** * If necessary, compensates for blocker, and blocks */ private void awaitBlocker(ManagedBlocker blocker) throws InterruptedException { while (!blocker.isReleasable()) { if (tryPreBlock()) { try { do {} while (!blocker.isReleasable() && !blocker.block()); } finally { postBlock(); } break; } } } // Creating, registering and deregistring workers /** * Tries to create and start a worker; minimally rolls back counts * on failure. */ private void addWorker() { Throwable ex = null; ForkJoinWorkerThread t = null; try { t = factory.newThread(this); } catch (Throwable e) { ex = e; } if (t == null) { // null or exceptional factory return long c; // adjust counts do {} while (!UNSAFE.compareAndSwapLong (this, ctlOffset, c = ctl, (((c - AC_UNIT) & AC_MASK) | ((c - TC_UNIT) & TC_MASK) | (c & ~(AC_MASK|TC_MASK))))); // Propagate exception if originating from an external caller if (!tryTerminate(false) && ex != null && !(Thread.currentThread() instanceof ForkJoinWorkerThread)) UNSAFE.throwException(ex); } else t.start(); } /** * Callback from ForkJoinWorkerThread constructor to assign a * public name */ final String nextWorkerName() { for (int n;;) { if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset, n = nextWorkerNumber, ++n)) return workerNamePrefix + n; } } /** * Callback from ForkJoinWorkerThread constructor to * determine its poolIndex and record in workers array. * * @param w the worker * @return the worker's pool index */ final int registerWorker(ForkJoinWorkerThread w) { /* * In the typical case, a new worker acquires the lock, uses * next available index and returns quickly. Since we should * not block callers (ultimately from signalWork or * tryPreBlock) waiting for the lock needed to do this, we * instead help release other workers while waiting for the * lock. */ for (int g;;) { ForkJoinWorkerThread[] ws; if (((g = scanGuard) & SG_UNIT) == 0 && UNSAFE.compareAndSwapInt(this, scanGuardOffset, g, g | SG_UNIT)) { int k = nextWorkerIndex; try { if ((ws = workers) != null) { // ignore on shutdown int n = ws.length; if (k < 0 || k >= n || ws[k] != null) { for (k = 0; k < n && ws[k] != null; ++k) ; if (k == n) ws = workers = Arrays.copyOf(ws, n << 1); } ws[k] = w; nextWorkerIndex = k + 1; int m = g & SMASK; g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1); } } finally { scanGuard = g; } return k; } else if ((ws = workers) != null) { // help release others for (ForkJoinWorkerThread u : ws) { if (u != null && u.queueBase != u.queueTop) { if (tryReleaseWaiter()) break; } } } } } /** * Final callback from terminating worker. Removes record of * worker from array, and adjusts counts. If pool is shutting * down, tries to complete termination. * * @param w the worker */ final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) { int idx = w.poolIndex; int sc = w.stealCount; int steps = 0; // Remove from array, adjust worker counts and collect steal count. // We can intermix failed removes or adjusts with steal updates do { long s, c; int g; if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 && UNSAFE.compareAndSwapInt(this, scanGuardOffset, g, g |= SG_UNIT)) { ForkJoinWorkerThread[] ws = workers; if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w) ws[idx] = null; // verify nextWorkerIndex = idx; scanGuard = g + SG_UNIT; steps = 1; } if (steps == 1 && UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl, (((c - AC_UNIT) & AC_MASK) | ((c - TC_UNIT) & TC_MASK) | (c & ~(AC_MASK|TC_MASK))))) steps = 2; if (sc != 0 && UNSAFE.compareAndSwapLong(this, stealCountOffset, s = stealCount, s + sc)) sc = 0; } while (steps != 2 || sc != 0); if (!tryTerminate(false)) { if (ex != null) // possibly replace if died abnormally signalWork(); else tryReleaseWaiter(); } } // Shutdown and termination /** * Possibly initiates and/or completes termination. * * @param now if true, unconditionally terminate, else only * if shutdown and empty queue and no active workers * @return true if now terminating or terminated */ private boolean tryTerminate(boolean now) { long c; while (((c = ctl) & STOP_BIT) == 0) { if (!now) { if ((int)(c >> AC_SHIFT) != -parallelism) return false; if (!shutdown || blockedCount != 0 || quiescerCount != 0 || queueBase != queueTop) { if (ctl == c) // staleness check return false; continue; } } if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT)) startTerminating(); } if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers final ReentrantLock lock = this.submissionLock; lock.lock(); try { termination.signalAll(); } finally { lock.unlock(); } } return true; } /** * Runs up to three passes through workers: (0) Setting * termination status for each worker, followed by wakeups up to * queued workers; (1) helping cancel tasks; (2) interrupting * lagging threads (likely in external tasks, but possibly also * blocked in joins). Each pass repeats previous steps because of * potential lagging thread creation. */ private void startTerminating() { cancelSubmissions(); for (int pass = 0; pass < 3; ++pass) { ForkJoinWorkerThread[] ws = workers; if (ws != null) { for (ForkJoinWorkerThread w : ws) { if (w != null) { w.terminate = true; if (pass > 0) { w.cancelTasks(); if (pass > 1 && !w.isInterrupted()) { try { w.interrupt(); } catch (SecurityException ignore) { } } } } } terminateWaiters(); } } } /** * Polls and cancels all submissions. Called only during termination. */ private void cancelSubmissions() { while (queueBase != queueTop) { ForkJoinTask task = pollSubmission(); if (task != null) { try { task.cancel(false); } catch (Throwable ignore) { } } } } /** * Tries to set the termination status of waiting workers, and * then wakes them up (after which they will terminate). */ private void terminateWaiters() { ForkJoinWorkerThread[] ws = workers; if (ws != null) { ForkJoinWorkerThread w; long c; int i, e; int n = ws.length; while ((i = ~(e = (int)(c = ctl)) & SMASK) < n && (w = ws[i]) != null && w.eventCount == (e & E_MASK)) { if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, (long)(w.nextWait & E_MASK) | ((c + AC_UNIT) & AC_MASK) | (c & (TC_MASK|STOP_BIT)))) { w.terminate = true; w.eventCount = e + EC_UNIT; if (w.parked) UNSAFE.unpark(w); } } } } // misc ForkJoinWorkerThread support /** * Increment or decrement quiescerCount. Needed only to prevent * triggering shutdown if a worker is transiently inactive while * checking quiescence. * * @param delta 1 for increment, -1 for decrement */ final void addQuiescerCount(int delta) { int c; do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset, c = quiescerCount, c + delta)); } /** * Directly increment or decrement active count without * queuing. This method is used to transiently assert inactivation * while checking quiescence. * * @param delta 1 for increment, -1 for decrement */ final void addActiveCount(int delta) { long d = delta < 0 ? -AC_UNIT : AC_UNIT; long c; do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl, ((c + d) & AC_MASK) | (c & ~AC_MASK))); } /** * Returns the approximate (non-atomic) number of idle threads per * active thread. */ final int idlePerActive() { // Approximate at powers of two for small values, saturate past 4 int p = parallelism; int a = p + (int)(ctl >> AC_SHIFT); return (a > (p >>>= 1) ? 0 : a > (p >>>= 1) ? 1 : a > (p >>>= 1) ? 2 : a > (p >>>= 1) ? 4 : 8); } // Exported methods // Constructors /** * Creates a {@code ForkJoinPool} with parallelism equal to {@link * java.lang.Runtime#availableProcessors}, using the {@linkplain * #defaultForkJoinWorkerThreadFactory default thread factory}, * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool() { this(Runtime.getRuntime().availableProcessors(), defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the indicated parallelism * level, the {@linkplain * #defaultForkJoinWorkerThreadFactory default thread factory}, * no UncaughtExceptionHandler, and non-async LIFO processing mode. * * @param parallelism the parallelism level * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism) { this(parallelism, defaultForkJoinWorkerThreadFactory, null, false); } /** * Creates a {@code ForkJoinPool} with the given parameters. * * @param parallelism the parallelism level. For default value, * use {@link java.lang.Runtime#availableProcessors}. * @param factory the factory for creating new threads. For default value, * use {@link #defaultForkJoinWorkerThreadFactory}. * @param handler the handler for internal worker threads that * terminate due to unrecoverable errors encountered while executing * tasks. For default value, use {@code null}. * @param asyncMode if true, * establishes local first-in-first-out scheduling mode for forked * tasks that are never joined. This mode may be more appropriate * than default locally stack-based mode in applications in which * worker threads only process event-style asynchronous tasks. * For default value, use {@code false}. * @throws IllegalArgumentException if parallelism less than or * equal to zero, or greater than implementation limit * @throws NullPointerException if the factory is null * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public ForkJoinPool(int parallelism, ForkJoinWorkerThreadFactory factory, Thread.UncaughtExceptionHandler handler, boolean asyncMode) { checkPermission(); if (factory == null) throw new NullPointerException(); if (parallelism <= 0 || parallelism > MAX_ID) throw new IllegalArgumentException(); this.parallelism = parallelism; this.factory = factory; this.ueh = handler; this.locallyFifo = asyncMode; long np = (long)(-parallelism); // offset ctl counts this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK); this.submissionQueue = new ForkJoinTask[INITIAL_QUEUE_CAPACITY]; // initialize workers array with room for 2*parallelism if possible int n = parallelism << 1; if (n >= MAX_ID) n = MAX_ID; else { // See Hackers Delight, sec 3.2, where n < (1 << 16) n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; } workers = new ForkJoinWorkerThread[n + 1]; this.submissionLock = new ReentrantLock(); this.termination = submissionLock.newCondition(); StringBuilder sb = new StringBuilder("ForkJoinPool-"); sb.append(poolNumberGenerator.incrementAndGet()); sb.append("-worker-"); this.workerNamePrefix = sb.toString(); } // Execution methods /** * Performs the given task, returning its result upon completion. * If the computation encounters an unchecked Exception or Error, * it is rethrown as the outcome of this invocation. Rethrown * exceptions behave in the same way as regular exceptions, but, * when possible, contain stack traces (as displayed for example * using {@code ex.printStackTrace()}) of both the current thread * as well as the thread actually encountering the exception; * minimally only the latter. * * @param task the task * @return the task's result * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public T invoke(ForkJoinTask task) { Thread t = Thread.currentThread(); if (task == null) throw new NullPointerException(); if (shutdown) throw new RejectedExecutionException(); if ((t instanceof ForkJoinWorkerThread) && ((ForkJoinWorkerThread)t).pool == this) return task.invoke(); // bypass submit if in same pool else { addSubmission(task); return task.join(); } } /** * Unless terminating, forks task if within an ongoing FJ * computation in the current pool, else submits as external task. */ private void forkOrSubmit(ForkJoinTask task) { ForkJoinWorkerThread w; Thread t = Thread.currentThread(); if (shutdown) throw new RejectedExecutionException(); if ((t instanceof ForkJoinWorkerThread) && (w = (ForkJoinWorkerThread)t).pool == this) w.pushTask(task); else addSubmission(task); } /** * Arranges for (asynchronous) execution of the given task. * * @param task the task * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public void execute(ForkJoinTask task) { if (task == null) throw new NullPointerException(); forkOrSubmit(task); } // AbstractExecutorService methods /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public void execute(Runnable task) { if (task == null) throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else job = ForkJoinTask.adapt(task, null); forkOrSubmit(job); } /** * Submits a ForkJoinTask for execution. * * @param task the task to submit * @return the task * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(ForkJoinTask task) { if (task == null) throw new NullPointerException(); forkOrSubmit(task); return task; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Callable task) { if (task == null) throw new NullPointerException(); ForkJoinTask job = ForkJoinTask.adapt(task); forkOrSubmit(job); return job; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Runnable task, T result) { if (task == null) throw new NullPointerException(); ForkJoinTask job = ForkJoinTask.adapt(task, result); forkOrSubmit(job); return job; } /** * @throws NullPointerException if the task is null * @throws RejectedExecutionException if the task cannot be * scheduled for execution */ public ForkJoinTask submit(Runnable task) { if (task == null) throw new NullPointerException(); ForkJoinTask job; if (task instanceof ForkJoinTask) // avoid re-wrap job = (ForkJoinTask) task; else job = ForkJoinTask.adapt(task, null); forkOrSubmit(job); return job; } /** * @throws NullPointerException {@inheritDoc} * @throws RejectedExecutionException {@inheritDoc} */ public List> invokeAll(Collection> tasks) { ArrayList> forkJoinTasks = new ArrayList>(tasks.size()); for (Callable task : tasks) forkJoinTasks.add(ForkJoinTask.adapt(task)); invoke(new InvokeAll(forkJoinTasks)); @SuppressWarnings({"unchecked", "rawtypes"}) List> futures = (List>) (List) forkJoinTasks; return futures; } static final class InvokeAll extends RecursiveAction { final ArrayList> tasks; InvokeAll(ArrayList> tasks) { this.tasks = tasks; } public void compute() { try { invokeAll(tasks); } catch (Exception ignore) {} } private static final long serialVersionUID = -7914297376763021607L; } /** * Returns the factory used for constructing new workers. * * @return the factory used for constructing new workers */ public ForkJoinWorkerThreadFactory getFactory() { return factory; } /** * Returns the handler for internal worker threads that terminate * due to unrecoverable errors encountered while executing tasks. * * @return the handler, or {@code null} if none */ public Thread.UncaughtExceptionHandler getUncaughtExceptionHandler() { return ueh; } /** * Returns the targeted parallelism level of this pool. * * @return the targeted parallelism level of this pool */ public int getParallelism() { return parallelism; } /** * Returns the number of worker threads that have started but not * yet terminated. The result returned by this method may differ * from {@link #getParallelism} when threads are created to * maintain parallelism when others are cooperatively blocked. * * @return the number of worker threads */ public int getPoolSize() { return parallelism + (short)(ctl >>> TC_SHIFT); } /** * Returns {@code true} if this pool uses local first-in-first-out * scheduling mode for forked tasks that are never joined. * * @return {@code true} if this pool uses async mode */ public boolean getAsyncMode() { return locallyFifo; } /** * Returns an estimate of the number of worker threads that are * not blocked waiting to join tasks or for other managed * synchronization. This method may overestimate the * number of running threads. * * @return the number of worker threads */ public int getRunningThreadCount() { int r = parallelism + (int)(ctl >> AC_SHIFT); return (r <= 0) ? 0 : r; // suppress momentarily negative values } /** * Returns an estimate of the number of threads that are currently * stealing or executing tasks. This method may overestimate the * number of active threads. * * @return the number of active threads */ public int getActiveThreadCount() { int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount; return (r <= 0) ? 0 : r; // suppress momentarily negative values } /** * Returns {@code true} if all worker threads are currently idle. * An idle worker is one that cannot obtain a task to execute * because none are available to steal from other threads, and * there are no pending submissions to the pool. This method is * conservative; it might not return {@code true} immediately upon * idleness of all threads, but will eventually become true if * threads remain inactive. * * @return {@code true} if all threads are currently idle */ public boolean isQuiescent() { return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0; } /** * Returns an estimate of the total number of tasks stolen from * one thread's work queue by another. The reported value * underestimates the actual total number of steals when the pool * is not quiescent. This value may be useful for monitoring and * tuning fork/join programs: in general, steal counts should be * high enough to keep threads busy, but low enough to avoid * overhead and contention across threads. * * @return the number of steals */ public long getStealCount() { return stealCount; } /** * Returns an estimate of the total number of tasks currently held * in queues by worker threads (but not including tasks submitted * to the pool that have not begun executing). This value is only * an approximation, obtained by iterating across all threads in * the pool. This method may be useful for tuning task * granularities. * * @return the number of queued tasks */ public long getQueuedTaskCount() { long count = 0; ForkJoinWorkerThread[] ws; if ((short)(ctl >>> TC_SHIFT) > -parallelism && (ws = workers) != null) { for (ForkJoinWorkerThread w : ws) if (w != null) count -= w.queueBase - w.queueTop; // must read base first } return count; } /** * Returns an estimate of the number of tasks submitted to this * pool that have not yet begun executing. This method may take * time proportional to the number of submissions. * * @return the number of queued submissions */ public int getQueuedSubmissionCount() { return -queueBase + queueTop; } /** * Returns {@code true} if there are any tasks submitted to this * pool that have not yet begun executing. * * @return {@code true} if there are any queued submissions */ public boolean hasQueuedSubmissions() { return queueBase != queueTop; } /** * Removes and returns the next unexecuted submission if one is * available. This method may be useful in extensions to this * class that re-assign work in systems with multiple pools. * * @return the next submission, or {@code null} if none */ protected ForkJoinTask pollSubmission() { ForkJoinTask t; ForkJoinTask[] q; int b, i; while ((b = queueBase) != queueTop && (q = submissionQueue) != null && (i = (q.length - 1) & b) >= 0) { long u = (i << ASHIFT) + ABASE; if ((t = q[i]) != null && queueBase == b && UNSAFE.compareAndSwapObject(q, u, t, null)) { queueBase = b + 1; return t; } } return null; } /** * Removes all available unexecuted submitted and forked tasks * from scheduling queues and adds them to the given collection, * without altering their execution status. These may include * artificially generated or wrapped tasks. This method is * designed to be invoked only when the pool is known to be * quiescent. Invocations at other times may not remove all * tasks. A failure encountered while attempting to add elements * to collection {@code c} may result in elements being in * neither, either or both collections when the associated * exception is thrown. The behavior of this operation is * undefined if the specified collection is modified while the * operation is in progress. * * @param c the collection to transfer elements into * @return the number of elements transferred */ protected int drainTasksTo(Collection> c) { int count = 0; while (queueBase != queueTop) { ForkJoinTask t = pollSubmission(); if (t != null) { c.add(t); ++count; } } ForkJoinWorkerThread[] ws; if ((short)(ctl >>> TC_SHIFT) > -parallelism && (ws = workers) != null) { for (ForkJoinWorkerThread w : ws) if (w != null) count += w.drainTasksTo(c); } return count; } /** * Returns a string identifying this pool, as well as its state, * including indications of run state, parallelism level, and * worker and task counts. * * @return a string identifying this pool, as well as its state */ public String toString() { long st = getStealCount(); long qt = getQueuedTaskCount(); long qs = getQueuedSubmissionCount(); int pc = parallelism; long c = ctl; int tc = pc + (short)(c >>> TC_SHIFT); int rc = pc + (int)(c >> AC_SHIFT); if (rc < 0) // ignore transient negative rc = 0; int ac = rc + blockedCount; String level; if ((c & STOP_BIT) != 0) level = (tc == 0) ? "Terminated" : "Terminating"; else level = shutdown ? "Shutting down" : "Running"; return super.toString() + "[" + level + ", parallelism = " + pc + ", size = " + tc + ", active = " + ac + ", running = " + rc + ", steals = " + st + ", tasks = " + qt + ", submissions = " + qs + "]"; } /** * Initiates an orderly shutdown in which previously submitted * tasks are executed, but no new tasks will be accepted. * Invocation has no additional effect if already shut down. * Tasks that are in the process of being submitted concurrently * during the course of this method may or may not be rejected. * * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public void shutdown() { checkPermission(); shutdown = true; tryTerminate(false); } /** * Attempts to cancel and/or stop all tasks, and reject all * subsequently submitted tasks. Tasks that are in the process of * being submitted or executed concurrently during the course of * this method may or may not be rejected. This method cancels * both existing and unexecuted tasks, in order to permit * termination in the presence of task dependencies. So the method * always returns an empty list (unlike the case for some other * Executors). * * @return an empty list * @throws SecurityException if a security manager exists and * the caller is not permitted to modify threads * because it does not hold {@link * java.lang.RuntimePermission}{@code ("modifyThread")} */ public List shutdownNow() { checkPermission(); shutdown = true; tryTerminate(true); return Collections.emptyList(); } /** * Returns {@code true} if all tasks have completed following shut down. * * @return {@code true} if all tasks have completed following shut down */ public boolean isTerminated() { long c = ctl; return ((c & STOP_BIT) != 0L && (short)(c >>> TC_SHIFT) == -parallelism); } /** * Returns {@code true} if the process of termination has * commenced but not yet completed. This method may be useful for * debugging. A return of {@code true} reported a sufficient * period after shutdown may indicate that submitted tasks have * ignored or suppressed interruption, or are waiting for IO, * causing this executor not to properly terminate. (See the * advisory notes for class {@link ForkJoinTask} stating that * tasks should not normally entail blocking operations. But if * they do, they must abort them on interrupt.) * * @return {@code true} if terminating but not yet terminated */ public boolean isTerminating() { long c = ctl; return ((c & STOP_BIT) != 0L && (short)(c >>> TC_SHIFT) != -parallelism); } /** * Returns true if terminating or terminated. Used by ForkJoinWorkerThread. */ final boolean isAtLeastTerminating() { return (ctl & STOP_BIT) != 0L; } /** * Returns {@code true} if this pool has been shut down. * * @return {@code true} if this pool has been shut down */ public boolean isShutdown() { return shutdown; } /** * Blocks until all tasks have completed execution after a shutdown * request, or the timeout occurs, or the current thread is * interrupted, whichever happens first. * * @param timeout the maximum time to wait * @param unit the time unit of the timeout argument * @return {@code true} if this executor terminated and * {@code false} if the timeout elapsed before termination * @throws InterruptedException if interrupted while waiting */ public boolean awaitTermination(long timeout, TimeUnit unit) throws InterruptedException { long nanos = unit.toNanos(timeout); final ReentrantLock lock = this.submissionLock; lock.lock(); try { for (;;) { if (isTerminated()) return true; if (nanos <= 0) return false; nanos = termination.awaitNanos(nanos); } } finally { lock.unlock(); } } /** * Interface for extending managed parallelism for tasks running * in {@link ForkJoinPool}s. * *

A {@code ManagedBlocker} provides two methods. Method * {@code isReleasable} must return {@code true} if blocking is * not necessary. Method {@code block} blocks the current thread * if necessary (perhaps internally invoking {@code isReleasable} * before actually blocking). These actions are performed by any * thread invoking {@link ForkJoinPool#managedBlock}. The * unusual methods in this API accommodate synchronizers that may, * but don't usually, block for long periods. Similarly, they * allow more efficient internal handling of cases in which * additional workers may be, but usually are not, needed to * ensure sufficient parallelism. Toward this end, * implementations of method {@code isReleasable} must be amenable * to repeated invocation. * *

For example, here is a ManagedBlocker based on a * ReentrantLock: *

 {@code
     * class ManagedLocker implements ManagedBlocker {
     *   final ReentrantLock lock;
     *   boolean hasLock = false;
     *   ManagedLocker(ReentrantLock lock) { this.lock = lock; }
     *   public boolean block() {
     *     if (!hasLock)
     *       lock.lock();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return hasLock || (hasLock = lock.tryLock());
     *   }
     * }}
* *

Here is a class that possibly blocks waiting for an * item on a given queue: *

 {@code
     * class QueueTaker implements ManagedBlocker {
     *   final BlockingQueue queue;
     *   volatile E item = null;
     *   QueueTaker(BlockingQueue q) { this.queue = q; }
     *   public boolean block() throws InterruptedException {
     *     if (item == null)
     *       item = queue.take();
     *     return true;
     *   }
     *   public boolean isReleasable() {
     *     return item != null || (item = queue.poll()) != null;
     *   }
     *   public E getItem() { // call after pool.managedBlock completes
     *     return item;
     *   }
     * }}
*/ public static interface ManagedBlocker { /** * Possibly blocks the current thread, for example waiting for * a lock or condition. * * @return {@code true} if no additional blocking is necessary * (i.e., if isReleasable would return true) * @throws InterruptedException if interrupted while waiting * (the method is not required to do so, but is allowed to) */ boolean block() throws InterruptedException; /** * Returns {@code true} if blocking is unnecessary. */ boolean isReleasable(); } /** * Blocks in accord with the given blocker. If the current thread * is a {@link ForkJoinWorkerThread}, this method possibly * arranges for a spare thread to be activated if necessary to * ensure sufficient parallelism while the current thread is blocked. * *

If the caller is not a {@link ForkJoinTask}, this method is * behaviorally equivalent to *

 {@code
     * while (!blocker.isReleasable())
     *   if (blocker.block())
     *     return;
     * }
* * If the caller is a {@code ForkJoinTask}, then the pool may * first be expanded to ensure parallelism, and later adjusted. * * @param blocker the blocker * @throws InterruptedException if blocker.block did so */ public static void managedBlock(ManagedBlocker blocker) throws InterruptedException { Thread t = Thread.currentThread(); if (t instanceof ForkJoinWorkerThread) { ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; w.pool.awaitBlocker(blocker); } else { do {} while (!blocker.isReleasable() && !blocker.block()); } } // AbstractExecutorService overrides. These rely on undocumented // fact that ForkJoinTask.adapt returns ForkJoinTasks that also // implement RunnableFuture. protected RunnableFuture newTaskFor(Runnable runnable, T value) { return (RunnableFuture) ForkJoinTask.adapt(runnable, value); } protected RunnableFuture newTaskFor(Callable callable) { return (RunnableFuture) ForkJoinTask.adapt(callable); } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE; private static final long ctlOffset; private static final long stealCountOffset; private static final long blockedCountOffset; private static final long quiescerCountOffset; private static final long scanGuardOffset; private static final long nextWorkerNumberOffset; private static final long ABASE; private static final int ASHIFT; static { poolNumberGenerator = new AtomicInteger(); workerSeedGenerator = new Random(); modifyThreadPermission = new RuntimePermission("modifyThread"); defaultForkJoinWorkerThreadFactory = new DefaultForkJoinWorkerThreadFactory(); int s; try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class k = ForkJoinPool.class; ctlOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("ctl")); stealCountOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("stealCount")); blockedCountOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("blockedCount")); quiescerCountOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("quiescerCount")); scanGuardOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("scanGuard")); nextWorkerNumberOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("nextWorkerNumber")); Class a = ForkJoinTask[].class; ABASE = UNSAFE.arrayBaseOffset(a); s = UNSAFE.arrayIndexScale(a); } catch (Exception e) { throw new Error(e); } if ((s & (s-1)) != 0) throw new Error("data type scale not a power of two"); ASHIFT = 31 - Integer.numberOfLeadingZeros(s); } }