/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* 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
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*/
/*
* 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
*/
/**
* 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.
*
* <p>A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* <em>work-stealing</em>: 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 <em>asyncMode</em> to true in
* constructors, {@code ForkJoinPool}s may also be appropriate for use
* with event-style tasks that are never joined.
*
* <p>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.
*
* <p>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,
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
* <p> 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
* 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
* <em>NOT</em> use these pool execution methods, but instead use the
* within-computation forms listed in the table.
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <tr>
* <td></td>
* </tr>
* <tr>
* <td> <b>Arrange async execution</td>
* <td> {@link #execute(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork}</td>
* </tr>
* <tr>
* <td> <b>Await and obtain result</td>
* <td> {@link #invoke(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#invoke}</td>
* </tr>
* <tr>
* <td> <b>Arrange exec and obtain Future</td>
* <td> {@link #submit(ForkJoinTask)}</td>
* <td> {@link ForkJoinTask#fork} (ForkJoinTasks <em>are</em> Futures)</td>
* </tr>
* </table>
*
* <p><b>Sample Usage.</b> 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.
*
* <pre>
* static final ForkJoinPool mainPool = new ForkJoinPool();
* ...
* public void sort(long[] array) {
* mainPool.invoke(new SortTask(array, 0, array.length));
* }
* </pre>
*
* <p><b>Implementation notes</b>: 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}.
*
* <p>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
*/
/*
* 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
* 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
* there appear to be tasks available. On the other hand, we must
* quickly prod them into action when new tasks are submitted or
* 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
*/
}
/**
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
static class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
return new ForkJoinWorkerThread(pool);
}
}
/**
* Creates a new ForkJoinWorkerThread. This factory is used unless
* overridden in ForkJoinPool constructors.
*/
public static final ForkJoinWorkerThreadFactory
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
/**
* If there is a security manager, makes sure caller has
* permission to modify threads.
*/
private static void checkPermission() {
}
/**
* Generator for assigning sequence numbers as pool names.
*/
/**
* 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.
*/
/**
* 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.
*/
/**
* 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.
*/
/**
* 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.
*/
/**
* Array serving as submission queue. Initialized upon construction.
*/
/**
* Lock protecting submissions array for addSubmission
*/
/**
* Condition for awaitTermination, using submissionLock for
* convenience.
*/
/**
* Creation factory for worker threads.
*/
/**
* The uncaught exception handler used when any worker abruptly
* terminates.
*/
/**
* Prefix for assigning names to worker threads
*/
/**
* 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
* signed shifts to maintain signedness.
*/
volatile long ctl;
// bounds
// masks
// units for incrementing and decrementing
// masks and units for dealing with u = (int)(ctl >>> 32)
// masks and units for dealing with e = (int)ctl
/**
* 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;
/**
* 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
*
* @param w the worker
*/
boolean swept = false; // true on empty scans
long c;
int a; // active count
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))) &
if (e > 0) { // release a waiting worker
break;
((long)(u + UAC_UNIT) << 32));
if (w.eventCount == e &&
if (w.parked)
break;
}
}
else if (UNSAFE.compareAndSwapLong
(this, ctlOffset, c,
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() {
if ((e = (int)(c = ctl)) > 0 &&
(int)(c >> AC_SHIFT) < 0 &&
if (w.eventCount != e ||
return false;
if (w.parked)
}
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
*/
int g = scanGuard; // mask 0 avoids useless scans if only one active
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 (d != 0)
signalWork(); // propagate if nonempty
w.execTask(t);
}
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;
(q = submissionQueue) != 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
*/
int v = w.eventCount;
w.nextWait = (int)c; // w's successor record
long d = ctl; // return true if lost to a deq, to force scan
return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
}
long s = stealCount;
else if (w.eventCount != v)
return true; // update next time
}
if ((!shutdown || !tryTerminate(false)) &&
for (boolean rescanned = false;;) {
if (w.eventCount != v)
return true;
if (!rescanned) {
rescanned = true;
for (int i = 0; i <= m; ++i) {
ForkJoinWorkerThread u = ws[i];
if (u != null) {
!tryReleaseWaiter())
rescanned = false; // contended
if (w.eventCount != v)
return true;
}
}
}
if (scanGuard != g || // stale
rescanned = false;
if (!rescanned)
else
}
else {
w.parked = true; // must recheck
if (w.eventCount != v) {
w.parked = false;
return true;
}
LockSupport.park(this);
}
}
}
/**
* 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
*/
long prevCtl, int v) {
if (w.eventCount == v) {
if (shutdown)
tryTerminate(false);
while (ctl == currentCtl) {
w.parked = true;
if (w.eventCount == v) // must recheck
w.parked = false;
if (w.eventCount != v)
break;
currentCtl, prevCtl)) {
w.terminate = true; // restore previous
break;
}
}
}
}
// Submissions
/**
* Enqueues the given task in the submissionQueue. Same idea as
* ForkJoinWorkerThread.pushTask except for use of submissionLock.
*
* @param t the task
*/
try {
ForkJoinTask<?>[] q; int s, m;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1;
if (s - queueBase == m)
}
} finally {
}
signalWork();
}
// (pollSubmission is defined below with exported methods)
/**
* Creates or doubles submissionQueue array.
* Basically identical to ForkJoinWorkerThread version.
*/
private void growSubmissionQueue() {
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
if (size < INITIAL_QUEUE_CAPACITY)
int oldMask;
}
}
}
// 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;
int pc = parallelism;
do {
long c = ctl;
int u = (int)(c >>> 32);
if ((e = (int)c) < 0) {
// skip -- terminating
}
if (w.eventCount == e &&
if (w.parked)
return true; // release an idle worker
}
}
return true; // no compensation needed
}
addWorker();
return true; // create a replacement
}
}
// try to back out on any failure and let caller retry
b = blockedCount, b - 1));
}
return false;
}
/**
* Decrements blockedCount and increments active count
*/
private void postBlock() {
long c;
int b;
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
*/
int s;
if (tryPreBlock()) {
postBlock();
}
}
}
/**
* 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
*/
break;
}
if (tryPreBlock()) {
if (millis <= 0)
break;
break;
break;
}
}
postBlock();
break;
}
}
}
/**
* If necessary, compensates for blocker, and blocks
*/
throws InterruptedException {
while (!blocker.isReleasable()) {
if (tryPreBlock()) {
try {
} finally {
postBlock();
}
break;
}
}
}
// Creating, registering and deregistring workers
/**
* Tries to create and start a worker; minimally rolls back counts
* on failure.
*/
private void addWorker() {
ForkJoinWorkerThread t = null;
try {
} catch (Throwable e) {
ex = e;
}
if (t == null) { // null or exceptional factory return
long c; // adjust counts
do {} while (!UNSAFE.compareAndSwapLong
// Propagate exception if originating from an external caller
}
else
t.start();
}
/**
* Callback from ForkJoinWorkerThread constructor to assign a
* public name
*/
for (int n;;) {
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
*/
/*
* 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;;) {
g, g | SG_UNIT)) {
int k = nextWorkerIndex;
try {
;
if (k == n)
}
ws[k] = w;
nextWorkerIndex = k + 1;
int m = g & SMASK;
}
} finally {
scanGuard = g;
}
return k;
}
for (ForkJoinWorkerThread u : ws) {
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
*/
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;
g, g |= SG_UNIT)) {
steps = 1;
}
if (steps == 1 &&
steps = 2;
if (sc != 0 &&
s = stealCount, s + sc))
sc = 0;
if (!tryTerminate(false)) {
signalWork();
else
}
}
// Shutdown and 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
*/
long c;
if (!now) {
if ((int)(c >> AC_SHIFT) != -parallelism)
return false;
if (ctl == c) // staleness check
return false;
continue;
}
}
}
try {
} finally {
}
}
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() {
for (ForkJoinWorkerThread w : ws) {
if (w != null) {
w.terminate = true;
if (pass > 0) {
w.cancelTasks();
try {
w.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
}
}
}
}
/**
* Polls and cancels all submissions. Called only during termination.
*/
private void cancelSubmissions() {
try {
}
}
}
}
/**
* Tries to set the termination status of waiting workers, and
* then wakes them up (after which they will terminate).
*/
private void terminateWaiters() {
ForkJoinWorkerThread w; long c; int i, e;
w.terminate = true;
w.eventCount = e + EC_UNIT;
if (w.parked)
}
}
}
}
// 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
*/
int c;
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
*/
long c;
((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;
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() {
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")}
*/
}
/**
* 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")}
*/
boolean asyncMode) {
throw new NullPointerException();
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.locallyFifo = asyncMode;
// 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;
}
this.submissionLock = new ReentrantLock();
}
// 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
*/
throw new NullPointerException();
if (shutdown)
throw new RejectedExecutionException();
if ((t instanceof ForkJoinWorkerThread) &&
((ForkJoinWorkerThread)t).pool == this)
else {
}
}
/**
* Unless terminating, forks task if within an ongoing FJ
* computation in the current pool, else submits as external task.
*/
if (shutdown)
throw new RejectedExecutionException();
if ((t instanceof ForkJoinWorkerThread) &&
(w = (ForkJoinWorkerThread)t).pool == this)
else
}
/**
* 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
*/
throw new NullPointerException();
}
// AbstractExecutorService methods
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
throw new NullPointerException();
ForkJoinTask<?> job;
else
}
/**
* 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
*/
throw new NullPointerException();
return task;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
throw new NullPointerException();
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
throw new NullPointerException();
return job;
}
/**
* @throws NullPointerException if the task is null
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
*/
throw new NullPointerException();
ForkJoinTask<?> job;
else
return job;
}
/**
* @throws NullPointerException {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
return futures;
}
public void compute() {
}
}
/**
* Returns the factory used for constructing new workers.
*
* @return the factory used for constructing new workers
*/
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
*/
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() {
}
/**
* 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() {
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() {
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() {
}
/**
* 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
* 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;
for (ForkJoinWorkerThread w : ws)
if (w != null)
}
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() {
}
/**
* 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() {
}
/**
* 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
*/
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
(q = submissionQueue) != null &&
if ((t = q[i]) != null &&
queueBase == b &&
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
*/
int count = 0;
ForkJoinTask<?> t = pollSubmission();
if (t != null) {
c.add(t);
++count;
}
}
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
*/
long st = getStealCount();
long qt = getQueuedTaskCount();
long qs = getQueuedSubmissionCount();
int pc = parallelism;
long c = ctl;
rc = 0;
if ((c & STOP_BIT) != 0)
else
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() {
shutdown = true;
tryTerminate(false);
}
/**
* 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")}
*/
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() {
}
/**
* 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
*/
throws InterruptedException {
try {
for (;;) {
if (isTerminated())
return true;
if (nanos <= 0)
return false;
}
} finally {
}
}
/**
* Interface for extending managed parallelism for tasks running
* in {@link ForkJoinPool}s.
*
* <p>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.
*
* <p>For example, here is a ManagedBlocker based on a
* ReentrantLock:
* <pre> {@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());
* }
* }}</pre>
*
* <p>Here is a class that possibly blocks waiting for an
* item on a given queue:
* <pre> {@code
* class QueueTaker<E> implements ManagedBlocker {
* final BlockingQueue<E> queue;
* volatile E item = null;
* QueueTaker(BlockingQueue<E> 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;
* }
* }}</pre>
*/
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)
*/
/**
* 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.
*
* <p>If the caller is not a {@link ForkJoinTask}, this method is
* behaviorally equivalent to
* <pre> {@code
* while (!blocker.isReleasable())
* if (blocker.block())
* return;
* }</pre>
*
* 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
*/
throws InterruptedException {
if (t instanceof ForkJoinWorkerThread) {
ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
}
else {
}
}
// AbstractExecutorService overrides. These rely on undocumented
// fact that ForkJoinTask.adapt returns ForkJoinTasks that also
// implement RunnableFuture.
}
}
// Unsafe mechanics
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();
int s;
try {
Class k = ForkJoinPool.class;
(k.getDeclaredField("ctl"));
(k.getDeclaredField("stealCount"));
(k.getDeclaredField("blockedCount"));
(k.getDeclaredField("quiescerCount"));
(k.getDeclaredField("scanGuard"));
(k.getDeclaredField("nextWorkerNumber"));
Class a = ForkJoinTask[].class;
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");
}
}