/*
* 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/licenses/publicdomain
*/
package jsr166y;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.locks.LockSupport;
import java.util.concurrent.locks.ReentrantLock;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.CountDownLatch;
/**
* 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.
* The main work-stealing mechanics implemented in class
* ForkJoinWorkerThread 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. These mechanics do not consider
* affinities, loads, cache localities, etc, so rarely provide the
* best possible performance on a given machine, but portably
* provide good throughput by averaging over these factors.
* (Further, even if we did try to use such information, we do not
* usually have a basis for exploiting it. For example, some sets
* of tasks profit from cache affinities, but others are harmed by
* cache pollution effects.)
*
* Beyond work-stealing support and essential bookkeeping, 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. Given that the creation costs of most threads on most
* systems mainly surrounds setting up runtime stacks, thread
* creation and switching is usually not much more expensive than
* stack creation and switching, and is more flexible). 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.helpJoinTask tracks joining->stealing
* links to try to find such a task.
*
* Compensating: Unless there are already enough live threads,
* method helpMaintainParallelism() may create or
* re-activate a spare thread to compensate for blocked
* joiners until they unblock.
*
* It is impossible to keep exactly the target (parallelism)
* number of threads running at any given time. Determining
* 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. Compensation
* occurs in slow-motion. It is triggered only upon timeouts of
* Object.wait used for joins. This reduces poor decisions that
* would otherwise be made when threads are waiting for others
* that are stalled because of unrelated activities such as
* garbage collection.
*
* The ManagedBlocker extension API can't use helping so relies
* only on compensation in method awaitBlocker.
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly steal tasks from each
* other. We do not want to negate this by creating bottlenecks
* implementing other management responsibilities. So we use a
* collection of techniques that avoid, reduce, or cope well with
* contention. These entail several instances of bit-packing into
* CASable fields to maintain only the minimally required
* atomicity. To enable such packing, we restrict maximum
* parallelism to (1<<15)-1 (enabling twice this (to accommodate
* unbalanced increments and decrements) to fit into a 16 bit
* field, which is far in excess of normal operating range. Even
* though updates to some of these bookkeeping fields do sometimes
* contend with each other, they don't normally cache-contend with
* updates to others enough to warrant memory padding or
* isolation. So they are all held as fields of ForkJoinPool
* objects. The main capabilities are as follows:
*
* 1. Creating and removing workers. Workers are recorded in the
* "workers" array. 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 lock
* (workerLock) 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. Currently, 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 unusual
* code constructions here). In essence, the workers array serves
* as a WeakReference mechanism. Thus for example the event queue
* stores worker indices, not worker references. Access to the
* workers in associated methods (for example releaseEventWaiters)
* 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 shutdown, in which case
* it is OK to give up. On termination, we just clobber these
* data structures without trying to use them.
*
* 2. Bookkeeping for dynamically adding and removing workers. We
* aim to approximately maintain the given level of parallelism.
* When some workers are known to be blocked (on joins or via
* ManagedBlocker), we may create or resume others to take their
* place until they unblock (see below). Implementing this
* requires counts of the number of "running" threads (i.e., those
* that are neither blocked nor artificially suspended) as well as
* the total number. These two values are packed into one field,
* "workerCounts" because we need accurate snapshots when deciding
* to create, resume or suspend. Note however that the
* correspondence of these counts to reality is not guaranteed. In
* particular updates for unblocked threads may lag until they
* actually wake up.
*
* 3. Maintaining global run state. The run state of the pool
* consists of a runLevel (SHUTDOWN, TERMINATING, etc) similar to
* those in other Executor implementations, as well as a count of
* "active" workers -- those that are, or soon will be, or
* recently were executing tasks. The runLevel and active count
* are packed together in order to correctly trigger shutdown and
* termination. Without care, active counts can be subject to very
* high contention. We substantially reduce this contention by
* relaxing update rules. A worker must claim active status
* prospectively, by activating if it sees that a submitted or
* stealable task exists (it may find after activating that the
* task no longer exists). It stays active while processing this
* task (if it exists) and any other local subtasks it produces,
* until it cannot find any other tasks. It then tries
* inactivating (see method preStep), but upon update contention
* instead scans for more tasks, later retrying inactivation if it
* doesn't find any.
*
* 4. Managing idle workers waiting for tasks. We cannot let
* workers spin indefinitely scanning for tasks when none are
* available. On the other hand, we must quickly prod them into
* action when new tasks are submitted or generated. We
* park/unpark these idle workers using an event-count scheme.
* Field eventCount is incremented upon events that may enable
* workers that previously could not find a task to now find one:
* Submission of a new task to the pool, or another worker pushing
* a task onto a previously empty queue. (We also use this
* mechanism for configuration and termination actions that
* require wakeups of idle workers). Each worker maintains its
* last known event count, and blocks when a scan for work did not
* find a task AND its lastEventCount matches the current
* eventCount. Waiting idle workers are recorded in a variant of
* Treiber stack headed by field eventWaiters which, when nonzero,
* encodes the thread index and count awaited for by the worker
* thread most recently calling eventSync. This thread in turn has
* a record (field nextEventWaiter) for the next waiting worker.
* In addition to allowing simpler decisions about need for
* wakeup, the event count bits in eventWaiters serve the role of
* tags to avoid ABA errors in Treiber stacks. Upon any wakeup,
* released threads also try to release at most two others. The
* net effect is a tree-like diffusion of signals, where released
* threads (and possibly others) help with unparks. To further
* reduce contention effects a bit, failed CASes to increment
* field eventCount are tolerated without retries in signalWork.
* Conceptually they are merged into the same event, which is OK
* when their only purpose is to enable workers to scan for work.
*
* 5. Managing suspension of extra workers. When a worker notices
* (usually upon timeout of a wait()) that there are too few
* running threads, we may create a new thread to maintain
* parallelism level, or at least avoid starvation. Usually, extra
* threads are needed for only very short periods, yet join
* dependencies are such that we sometimes need them in
* bursts. Rather than create new threads each time this happens,
* we suspend no-longer-needed extra ones as "spares". For most
* purposes, we don't distinguish "extra" spare threads from
* normal "core" threads: On each call to preStep (the only point
* at which we can do this) a worker checks to see if there are
* now too many running workers, and if so, suspends itself.
* Method helpMaintainParallelism looks for suspended threads to
* resume before considering creating a new replacement. The
* spares themselves are encoded on another variant of a Treiber
* Stack, headed at field "spareWaiters". Note that the use of
* spares is intrinsically racy. One thread may become a spare at
* about the same time as another is needlessly being created. We
* counteract this and related slop in part by requiring resumed
* spares to immediately recheck (in preStep) to see whether they
* should re-suspend.
*
* 6. Killing off unneeded workers. A timeout mechanism is used to
* shed unused workers: The oldest (first) event queue waiter uses
* a timed rather than hard wait. When this wait times out without
* a normal wakeup, it tries to shutdown any one (for convenience
* the newest) other spare or event waiter via
* tryShutdownUnusedWorker. This eventually reduces the number of
* worker threads to a minimum of one after a long enough period
* without use.
*
* 7. Deciding when to create new workers. The main dynamic
* control in this class is deciding when to create extra threads
* in method helpMaintainParallelism. We would like to keep
* exactly #parallelism threads running, which is an impossible
* task. We always need to create one when the number of running
* threads would become zero and all workers are busy. Beyond
* this, we must rely on heuristics that work well in the
* presence of transient phenomena such as GC stalls, dynamic
* compilation, and wake-up lags. These transients are extremely
* common -- we are normally trying to fully saturate the CPUs on
* a machine, so almost any activity other than running tasks
* impedes accuracy. Our main defense is to allow parallelism to
* lapse for a while during joins, and use a timeout to see if,
* after the resulting settling, there is still a need for
* additional workers. This also better copes with the fact that
* some of the methods in this class tend to never become compiled
* (but are interpreted), so some components of the entire set of
* controls might execute 100 times faster than others. And
* similarly for cases where the apparent lack of work is just due
* to GC stalls and other transient system activity.
*
* Beware that there is a lot of representation-level coupling
* among classes ForkJoinPool, ForkJoinWorkerThread, and
* ForkJoinTask. For example, direct 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.
*
* Style notes: 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). Also 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), at the expense of some messy constructions that
* reduce byte code counts.
*
* The order of declarations in this file is: (1) statics (2)
* fields (along with constants used when unpacking some of them)
* (3) internal control methods (4) callbacks and other support
* for ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
* methods (plus a few little helpers).
*/
/**
* 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 =
new DefaultForkJoinWorkerThreadFactory();
/**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission =
new RuntimePermission("modifyThread");
/**
* 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 =
new AtomicInteger();
/**
* The time to block in a join (see awaitJoin) before checking if
* a new worker should be (re)started to maintain parallelism
* level. The value should be short enough to maintain global
* responsiveness and progress but long enough to avoid
* counterproductive firings during GC stalls or unrelated system
* activity, and to not bog down systems with continual re-firings
* on GCs or legitimately long waits.
*/
private static final long JOIN_TIMEOUT_MILLIS = 250L; // 4 per second
/**
* The wakeup interval (in nanoseconds) for the oldest worker
* waiting for an event to invoke tryShutdownUnusedWorker 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_NANOS =
30L * 1000L * 1000L * 1000L; // 2 per minute
/**
* Absolute bound for parallelism level. Twice this number plus
* one (i.e., 0xfff) must fit into a 16bit field to enable
* word-packing for some counts and indices.
*/
private static final int MAX_WORKERS = 0x7fff;
/**
* Array holding all worker threads in the pool. Array size must
* be a power of two. Updates and replacements are protected by
* workerLock, 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. All readers must tolerate that some
* array slots may be null.
*/
volatile ForkJoinWorkerThread[] workers;
/**
* Queue for external submissions.
*/
private final LinkedTransferQueue> submissionQueue;
/**
* Lock protecting updates to workers array.
*/
private final ReentrantLock workerLock;
/**
* Latch released upon termination.
*/
private final Phaser termination;
/**
* Creation factory for worker threads.
*/
private final ForkJoinWorkerThreadFactory factory;
/**
* Sum of per-thread steal counts, updated only when threads are
* idle or terminating.
*/
private volatile long stealCount;
/**
* Encoded record of top of Treiber stack of threads waiting for
* events. The top 32 bits contain the count being waited for. The
* bottom 16 bits contains one plus the pool index of waiting
* worker thread. (Bits 16-31 are unused.)
*/
private volatile long eventWaiters;
private static final int EVENT_COUNT_SHIFT = 32;
private static final long WAITER_ID_MASK = (1L << 16) - 1L;
/**
* A counter for events that may wake up worker threads:
* - Submission of a new task to the pool
* - A worker pushing a task on an empty queue
* - termination
*/
private volatile int eventCount;
/**
* Encoded record of top of Treiber stack of spare threads waiting
* for resumption. The top 16 bits contain an arbitrary count to
* avoid ABA effects. The bottom 16bits contains one plus the pool
* index of waiting worker thread.
*/
private volatile int spareWaiters;
private static final int SPARE_COUNT_SHIFT = 16;
private static final int SPARE_ID_MASK = (1 << 16) - 1;
/**
* Lifecycle control. The low word contains the number of workers
* that are (probably) executing tasks. This value is atomically
* incremented before a worker gets a task to run, and decremented
* when worker has no tasks and cannot find any. Bits 16-18
* contain runLevel value. When all are zero, the pool is
* running. Level transitions are monotonic (running -> shutdown
* -> terminating -> terminated) so each transition adds a bit.
* These are bundled together to ensure consistent read for
* termination checks (i.e., that runLevel is at least SHUTDOWN
* and active threads is zero).
*
* Notes: Most direct CASes are dependent on these bitfield
* positions. Also, this field is non-private to enable direct
* performance-sensitive CASes in ForkJoinWorkerThread.
*/
volatile int runState;
// Note: The order among run level values matters.
private static final int RUNLEVEL_SHIFT = 16;
private static final int SHUTDOWN = 1 << RUNLEVEL_SHIFT;
private static final int TERMINATING = 1 << (RUNLEVEL_SHIFT + 1);
private static final int TERMINATED = 1 << (RUNLEVEL_SHIFT + 2);
private static final int ACTIVE_COUNT_MASK = (1 << RUNLEVEL_SHIFT) - 1;
/**
* Holds number of total (i.e., created and not yet terminated)
* and running (i.e., not blocked on joins or other managed sync)
* threads, packed together to ensure consistent snapshot when
* making decisions about creating and suspending spare
* threads. Updated only by CAS. Note that adding a new worker
* requires incrementing both counts, since workers start off in
* running state.
*/
private volatile int workerCounts;
private static final int TOTAL_COUNT_SHIFT = 16;
private static final int RUNNING_COUNT_MASK = (1 << TOTAL_COUNT_SHIFT) - 1;
private static final int ONE_RUNNING = 1;
private static final int ONE_TOTAL = 1 << TOTAL_COUNT_SHIFT;
/**
* The target parallelism level.
* Accessed directly by ForkJoinWorkerThreads.
*/
final int parallelism;
/**
* True if use local fifo, not default lifo, for local polling
* Read by, and replicated by ForkJoinWorkerThreads
*/
final boolean locallyFifo;
/**
* The uncaught exception handler used when any worker abruptly
* terminates.
*/
private final Thread.UncaughtExceptionHandler ueh;
/**
* Pool number, just for assigning useful names to worker threads
*/
private final int poolNumber;
// Utilities for CASing fields. Note that most of these
// are usually manually inlined by callers
/**
* Increments running count part of workerCounts
*/
final void incrementRunningCount() {
int c;
do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
c = workerCounts,
c + ONE_RUNNING));
}
/**
* Tries to decrement running count unless already zero
*/
final boolean tryDecrementRunningCount() {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) == 0)
return false;
return UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING);
}
/**
* Forces decrement of encoded workerCounts, awaiting nonzero if
* (rarely) necessary when other count updates lag.
*
* @param dr -- either zero or ONE_RUNNING
* @param dt -- either zero or ONE_TOTAL
*/
private void decrementWorkerCounts(int dr, int dt) {
for (;;) {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) - dr < 0 ||
(wc >>> TOTAL_COUNT_SHIFT) - dt < 0) {
if ((runState & TERMINATED) != 0)
return; // lagging termination on a backout
Thread.yield();
}
if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - (dr + dt)))
return;
}
}
/**
* Tries decrementing active count; fails on contention.
* Called when workers cannot find tasks to run.
*/
final boolean tryDecrementActiveCount() {
int c;
return UNSAFE.compareAndSwapInt(this, runStateOffset,
c = runState, c - 1);
}
/**
* Advances to at least the given level. Returns true if not
* already in at least the given level.
*/
private boolean advanceRunLevel(int level) {
for (;;) {
int s = runState;
if ((s & level) != 0)
return false;
if (UNSAFE.compareAndSwapInt(this, runStateOffset, s, s | level))
return true;
}
}
// workers array maintenance
/**
* Records and returns a workers array index for new worker.
*/
private int recordWorker(ForkJoinWorkerThread w) {
// Try using slot totalCount-1. If not available, scan and/or resize
int k = (workerCounts >>> TOTAL_COUNT_SHIFT) - 1;
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
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 = Arrays.copyOf(ws, n << 1);
}
ws[k] = w;
workers = ws; // volatile array write ensures slot visibility
} finally {
lock.unlock();
}
return k;
}
/**
* Nulls out record of worker in workers array.
*/
private void forgetWorker(ForkJoinWorkerThread w) {
int idx = w.poolIndex;
// Locking helps method recordWorker avoid unnecessary expansion
final ReentrantLock lock = this.workerLock;
lock.lock();
try {
ForkJoinWorkerThread[] ws = workers;
if (idx >= 0 && idx < ws.length && ws[idx] == w) // verify
ws[idx] = null;
} finally {
lock.unlock();
}
}
/**
* 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 workerTerminated(ForkJoinWorkerThread w) {
forgetWorker(w);
decrementWorkerCounts(w.isTrimmed()? 0 : ONE_RUNNING, ONE_TOTAL);
while (w.stealCount != 0) // collect final count
tryAccumulateStealCount(w);
tryTerminate(false);
}
// Waiting for and signalling events
/**
* Releases workers blocked on a count not equal to current count.
* Normally called after precheck that eventWaiters isn't zero to
* avoid wasted array checks. Gives up upon a change in count or
* upon releasing two workers, letting others take over.
*/
private void releaseEventWaiters() {
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
long h = eventWaiters;
int ec = eventCount;
boolean releasedOne = false;
ForkJoinWorkerThread w; int id;
while ((id = ((int)(h & WAITER_ID_MASK)) - 1) >= 0 &&
(int)(h >>> EVENT_COUNT_SHIFT) != ec &&
id < n && (w = ws[id]) != null) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
h, w.nextWaiter)) {
LockSupport.unpark(w);
if (releasedOne) // exit on second release
break;
releasedOne = true;
}
if (eventCount != ec)
break;
h = eventWaiters;
}
}
/**
* Tries to advance eventCount and releases waiters. Called only
* from workers.
*/
final void signalWork() {
int c; // try to increment event count -- CAS failure OK
UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
if (eventWaiters != 0L)
releaseEventWaiters();
}
/**
* Adds the given worker to event queue and blocks until
* terminating or event count advances from the given value
*
* @param w the calling worker thread
* @param ec the count
*/
private void eventSync(ForkJoinWorkerThread w, int ec) {
long nh = (((long)ec) << EVENT_COUNT_SHIFT) | ((long)(w.poolIndex+1));
long h;
while ((runState < SHUTDOWN || !tryTerminate(false)) &&
(((int)((h = eventWaiters) & WAITER_ID_MASK)) == 0 ||
(int)(h >>> EVENT_COUNT_SHIFT) == ec) &&
eventCount == ec) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
w.nextWaiter = h, nh)) {
awaitEvent(w, ec);
break;
}
}
}
/**
* Blocks the given worker (that has already been entered as an
* event waiter) until terminating or event count advances from
* the given value. The oldest (first) waiter uses a timed wait to
* occasionally one-by-one shrink the number of workers (to a
* minimum of one) if the pool has not been used for extended
* periods.
*
* @param w the calling worker thread
* @param ec the count
*/
private void awaitEvent(ForkJoinWorkerThread w, int ec) {
while (eventCount == ec) {
if (tryAccumulateStealCount(w)) { // transfer while idle
boolean untimed = (w.nextWaiter != 0L ||
(workerCounts & RUNNING_COUNT_MASK) <= 1);
long startTime = untimed? 0 : System.nanoTime();
Thread.interrupted(); // clear/ignore interrupt
if (eventCount != ec || w.runState != 0 ||
runState >= TERMINATING) // recheck after clear
break;
if (untimed)
LockSupport.park(w);
else {
LockSupport.parkNanos(w, SHRINK_RATE_NANOS);
if (eventCount != ec || w.runState != 0 ||
runState >= TERMINATING)
break;
if (System.nanoTime() - startTime >= SHRINK_RATE_NANOS)
tryShutdownUnusedWorker(ec);
}
}
}
}
// Maintaining parallelism
/**
* Pushes worker onto the spare stack.
*/
final void pushSpare(ForkJoinWorkerThread w) {
int ns = (++w.spareCount << SPARE_COUNT_SHIFT) | (w.poolIndex + 1);
do {} while (!UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
w.nextSpare = spareWaiters,ns));
}
/**
* Tries (once) to resume a spare if the number of running
* threads is less than target.
*/
private void tryResumeSpare() {
int sw, id;
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
ForkJoinWorkerThread w;
if ((sw = spareWaiters) != 0 &&
(id = (sw & SPARE_ID_MASK) - 1) >= 0 &&
id < n && (w = ws[id]) != null &&
(workerCounts & RUNNING_COUNT_MASK) < parallelism &&
spareWaiters == sw &&
UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
sw, w.nextSpare)) {
int c; // increment running count before resume
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
if (w.tryUnsuspend())
LockSupport.unpark(w);
else // back out if w was shutdown
decrementWorkerCounts(ONE_RUNNING, 0);
}
}
/**
* Tries to increase the number of running workers if below target
* parallelism: If a spare exists tries to resume it via
* tryResumeSpare. Otherwise, if not enough total workers or all
* existing workers are busy, adds a new worker. In all cases also
* helps wake up releasable workers waiting for work.
*/
private void helpMaintainParallelism() {
int pc = parallelism;
int wc, rs, tc;
while (((wc = workerCounts) & RUNNING_COUNT_MASK) < pc &&
(rs = runState) < TERMINATING) {
if (spareWaiters != 0)
tryResumeSpare();
else if ((tc = wc >>> TOTAL_COUNT_SHIFT) >= MAX_WORKERS ||
(tc >= pc && (rs & ACTIVE_COUNT_MASK) != tc))
break; // enough total
else if (runState == rs && workerCounts == wc &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc + (ONE_RUNNING|ONE_TOTAL))) {
ForkJoinWorkerThread w = null;
try {
w = factory.newThread(this);
} finally { // adjust on null or exceptional factory return
if (w == null) {
decrementWorkerCounts(ONE_RUNNING, ONE_TOTAL);
tryTerminate(false); // handle failure during shutdown
}
}
if (w == null)
break;
w.start(recordWorker(w), ueh);
if ((workerCounts >>> TOTAL_COUNT_SHIFT) >= pc) {
int c; // advance event count
UNSAFE.compareAndSwapInt(this, eventCountOffset,
c = eventCount, c+1);
break; // add at most one unless total below target
}
}
}
if (eventWaiters != 0L)
releaseEventWaiters();
}
/**
* Callback from the oldest waiter in awaitEvent waking up after a
* period of non-use. If all workers are idle, tries (once) to
* shutdown an event waiter or a spare, if one exists. Note that
* we don't need CAS or locks here because the method is called
* only from one thread occasionally waking (and even misfires are
* OK). Note that until the shutdown worker fully terminates,
* workerCounts will overestimate total count, which is tolerable.
*
* @param ec the event count waited on by caller (to abort
* attempt if count has since changed).
*/
private void tryShutdownUnusedWorker(int ec) {
if (runState == 0 && eventCount == ec) { // only trigger if all idle
ForkJoinWorkerThread[] ws = workers;
int n = ws.length;
ForkJoinWorkerThread w = null;
boolean shutdown = false;
int sw;
long h;
if ((sw = spareWaiters) != 0) { // prefer killing spares
int id = (sw & SPARE_ID_MASK) - 1;
if (id >= 0 && id < n && (w = ws[id]) != null &&
UNSAFE.compareAndSwapInt(this, spareWaitersOffset,
sw, w.nextSpare))
shutdown = true;
}
else if ((h = eventWaiters) != 0L) {
long nh;
int id = ((int)(h & WAITER_ID_MASK)) - 1;
if (id >= 0 && id < n && (w = ws[id]) != null &&
(nh = w.nextWaiter) != 0L && // keep at least one worker
UNSAFE.compareAndSwapLong(this, eventWaitersOffset, h, nh))
shutdown = true;
}
if (w != null && shutdown) {
w.shutdown();
LockSupport.unpark(w);
}
}
releaseEventWaiters(); // in case of interference
}
/**
* Callback from workers invoked upon each top-level action (i.e.,
* stealing a task or taking a submission and running it).
* Performs one or more of the following:
*
* 1. If the worker is active and either did not run a task
* or there are too many workers, try to set its active status
* to inactive and update activeCount. On contention, we may
* try again in this or a subsequent call.
*
* 2. If not enough total workers, help create some.
*
* 3. If there are too many running workers, suspend this worker
* (first forcing inactive if necessary). If it is not needed,
* it may be shutdown while suspended (via
* tryShutdownUnusedWorker). Otherwise, upon resume it
* rechecks running thread count and need for event sync.
*
* 4. If worker did not run a task, await the next task event via
* eventSync if necessary (first forcing inactivation), upon
* which the worker may be shutdown via
* tryShutdownUnusedWorker. Otherwise, help release any
* existing event waiters that are now releasable,
*
* @param w the worker
* @param ran true if worker ran a task since last call to this method
*/
final void preStep(ForkJoinWorkerThread w, boolean ran) {
int wec = w.lastEventCount;
boolean active = w.active;
boolean inactivate = false;
int pc = parallelism;
int rs;
while (w.runState == 0 && (rs = runState) < TERMINATING) {
if ((inactivate || (active && (rs & ACTIVE_COUNT_MASK) >= pc)) &&
UNSAFE.compareAndSwapInt(this, runStateOffset, rs, rs - 1))
inactivate = active = w.active = false;
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) > pc) {
if (!(inactivate |= active) && // must inactivate to suspend
workerCounts == wc && // try to suspend as spare
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
w.suspendAsSpare();
}
else if ((wc >>> TOTAL_COUNT_SHIFT) < pc)
helpMaintainParallelism(); // not enough workers
else if (!ran) {
long h = eventWaiters;
int ec = eventCount;
if (h != 0L && (int)(h >>> EVENT_COUNT_SHIFT) != ec)
releaseEventWaiters(); // release others before waiting
else if (ec != wec) {
w.lastEventCount = ec; // no need to wait
break;
}
else if (!(inactivate |= active))
eventSync(w, wec); // must inactivate before sync
}
else
break;
}
}
/**
* Helps and/or blocks awaiting join of the given task.
* See above for explanation.
*
* @param joinMe the task to join
* @param worker the current worker thread
*/
final void awaitJoin(ForkJoinTask joinMe, ForkJoinWorkerThread worker) {
int retries = 2 + (parallelism >> 2); // #helpJoins before blocking
while (joinMe.status >= 0) {
int wc;
worker.helpJoinTask(joinMe);
if (joinMe.status < 0)
break;
else if (retries > 0)
--retries;
else if (((wc = workerCounts) & RUNNING_COUNT_MASK) != 0 &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING)) {
int stat, c; long h;
while ((stat = joinMe.status) >= 0 &&
(h = eventWaiters) != 0L && // help release others
(int)(h >>> EVENT_COUNT_SHIFT) != eventCount)
releaseEventWaiters();
if (stat >= 0 &&
((workerCounts & RUNNING_COUNT_MASK) == 0 ||
(stat =
joinMe.internalAwaitDone(JOIN_TIMEOUT_MILLIS)) >= 0))
helpMaintainParallelism(); // timeout or no running workers
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
if (stat < 0)
break; // else restart
}
}
}
/**
* Same idea as awaitJoin, but no helping, retries, or timeouts.
*/
final void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
while (!blocker.isReleasable()) {
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) != 0 &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING)) {
try {
while (!blocker.isReleasable()) {
long h = eventWaiters;
if (h != 0L &&
(int)(h >>> EVENT_COUNT_SHIFT) != eventCount)
releaseEventWaiters();
else if ((workerCounts & RUNNING_COUNT_MASK) == 0 &&
runState < TERMINATING)
helpMaintainParallelism();
else if (blocker.block())
break;
}
} finally {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
break;
}
}
}
/**
* 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) {
if (now)
advanceRunLevel(SHUTDOWN); // ensure at least SHUTDOWN
else if (runState < SHUTDOWN ||
!submissionQueue.isEmpty() ||
(runState & ACTIVE_COUNT_MASK) != 0)
return false;
if (advanceRunLevel(TERMINATING))
startTerminating();
// Finish now if all threads terminated; else in some subsequent call
if ((workerCounts >>> TOTAL_COUNT_SHIFT) == 0) {
advanceRunLevel(TERMINATED);
termination.arrive();
}
return true;
}
/**
* Actions on transition to TERMINATING
*
* Runs up to four passes through workers: (0) shutting down each
* (without waking up if parked) to quickly spread notifications
* without unnecessary bouncing around event queues etc (1) wake
* up and help cancel tasks (2) interrupt (3) mop up races with
* interrupted workers
*/
private void startTerminating() {
cancelSubmissions();
for (int passes = 0; passes < 4 && workerCounts != 0; ++passes) {
int c; // advance event count
UNSAFE.compareAndSwapInt(this, eventCountOffset,
c = eventCount, c+1);
eventWaiters = 0L; // clobber lists
spareWaiters = 0;
for (ForkJoinWorkerThread w : workers) {
if (w != null) {
w.shutdown();
if (passes > 0 && !w.isTerminated()) {
w.cancelTasks();
LockSupport.unpark(w);
if (passes > 1) {
try {
w.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
}
}
}
/**
* Clears out and cancels submissions, ignoring exceptions.
*/
private void cancelSubmissions() {
ForkJoinTask task;
while ((task = submissionQueue.poll()) != null) {
try {
task.cancel(false);
} catch (Throwable ignore) {
}
}
}
// misc support for ForkJoinWorkerThread
/**
* Returns pool number.
*/
final int getPoolNumber() {
return poolNumber;
}
/**
* Tries to accumulate steal count from a worker, clearing
* the worker's value if successful.
*
* @return true if worker steal count now zero
*/
final boolean tryAccumulateStealCount(ForkJoinWorkerThread w) {
int sc = w.stealCount;
long c = stealCount;
// CAS even if zero, for fence effects
if (UNSAFE.compareAndSwapLong(this, stealCountOffset, c, c + sc)) {
if (sc != 0)
w.stealCount = 0;
return true;
}
return sc == 0;
}
/**
* Returns the approximate (non-atomic) number of idle threads per
* active thread.
*/
final int idlePerActive() {
int pc = parallelism; // use parallelism, not rc
int ac = runState; // no mask -- artificially boosts during shutdown
// Use exact results for small values, saturate past 4
return ((pc <= ac) ? 0 :
(pc >>> 1 <= ac) ? 1 :
(pc >>> 2 <= ac) ? 3 :
pc >>> 3);
}
// Public and protected 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_WORKERS)
throw new IllegalArgumentException();
this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
this.locallyFifo = asyncMode;
int arraySize = initialArraySizeFor(parallelism);
this.workers = new ForkJoinWorkerThread[arraySize];
this.submissionQueue = new LinkedTransferQueue>();
this.workerLock = new ReentrantLock();
this.termination = new Phaser(1);
this.poolNumber = poolNumberGenerator.incrementAndGet();
}
/**
* Returns initial power of two size for workers array.
* @param pc the initial parallelism level
*/
private static int initialArraySizeFor(int pc) {
// If possible, initially allocate enough space for one spare
int size = pc < MAX_WORKERS ? pc + 1 : MAX_WORKERS;
// See Hackers Delight, sec 3.2. We know MAX_WORKERS < (1 >>> 16)
size |= size >>> 1;
size |= size >>> 2;
size |= size >>> 4;
size |= size >>> 8;
return size + 1;
}
// Execution methods
/**
* Common code for execute, invoke and submit
*/
private void doSubmit(ForkJoinTask task) {
if (task == null)
throw new NullPointerException();
if (runState >= SHUTDOWN)
throw new RejectedExecutionException();
submissionQueue.offer(task);
int c; // try to increment event count -- CAS failure OK
UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
helpMaintainParallelism(); // create, start, or resume some workers
}
/**
* Performs the given task, returning its result upon completion.
*
* @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) {
doSubmit(task);
return task.join();
}
/**
* 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) {
doSubmit(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) {
ForkJoinTask job;
if (task instanceof ForkJoinTask) // avoid re-wrap
job = (ForkJoinTask) task;
else
job = ForkJoinTask.adapt(task, null);
doSubmit(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) {
doSubmit(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) {
ForkJoinTask job = ForkJoinTask.adapt(task);
doSubmit(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) {
ForkJoinTask job = ForkJoinTask.adapt(task, result);
doSubmit(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) {
ForkJoinTask job;
if (task instanceof ForkJoinTask) // avoid re-wrap
job = (ForkJoinTask) task;
else
job = ForkJoinTask.adapt(task, null);
doSubmit(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 workerCounts >>> TOTAL_COUNT_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() {
return workerCounts & RUNNING_COUNT_MASK;
}
/**
* 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 runState & ACTIVE_COUNT_MASK;
}
/**
* 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 (runState & ACTIVE_COUNT_MASK) == 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;
for (ForkJoinWorkerThread w : workers)
if (w != null)
count += w.getQueueSize();
return count;
}
/**
* Returns an estimate of the number of tasks submitted to this
* pool that have not yet begun executing. This method takes time
* proportional to the number of submissions.
*
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
return submissionQueue.size();
}
/**
* 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 !submissionQueue.isEmpty();
}
/**
* 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() {
return submissionQueue.poll();
}
/**
* 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 = submissionQueue.drainTo(c);
for (ForkJoinWorkerThread w : workers)
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 wc = workerCounts;
int tc = wc >>> TOTAL_COUNT_SHIFT;
int rc = wc & RUNNING_COUNT_MASK;
int pc = parallelism;
int rs = runState;
int ac = rs & ACTIVE_COUNT_MASK;
return super.toString() +
"[" + runLevelToString(rs) +
", parallelism = " + pc +
", size = " + tc +
", active = " + ac +
", running = " + rc +
", steals = " + st +
", tasks = " + qt +
", submissions = " + qs +
"]";
}
private static String runLevelToString(int s) {
return ((s & TERMINATED) != 0 ? "Terminated" :
((s & TERMINATING) != 0 ? "Terminating" :
((s & SHUTDOWN) != 0 ? "Shutting down" :
"Running")));
}
/**
* 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();
advanceRunLevel(SHUTDOWN);
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();
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() {
return runState >= TERMINATED;
}
/**
* 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, causing this executor not
* to properly terminate.
*
* @return {@code true} if terminating but not yet terminated
*/
public boolean isTerminating() {
return (runState & (TERMINATING|TERMINATED)) == TERMINATING;
}
/**
* Returns {@code true} if this pool has been shut down.
*
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
return runState >= 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 {
try {
return termination.awaitAdvanceInterruptibly(0, timeout, unit) > 0;
} catch (TimeoutException ex) {
return false;
}
}
/**
* 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). 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 = getUnsafe();
private static final long workerCountsOffset =
objectFieldOffset("workerCounts", ForkJoinPool.class);
private static final long runStateOffset =
objectFieldOffset("runState", ForkJoinPool.class);
private static final long eventCountOffset =
objectFieldOffset("eventCount", ForkJoinPool.class);
private static final long eventWaitersOffset =
objectFieldOffset("eventWaiters", ForkJoinPool.class);
private static final long stealCountOffset =
objectFieldOffset("stealCount", ForkJoinPool.class);
private static final long spareWaitersOffset =
objectFieldOffset("spareWaiters", ForkJoinPool.class);
private static long objectFieldOffset(String field, Class klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
} catch (NoSuchFieldException e) {
// Convert Exception to corresponding Error
NoSuchFieldError error = new NoSuchFieldError(field);
error.initCause(e);
throw error;
}
}
/**
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package.
* Replace with a simple call to Unsafe.getUnsafe when integrating
* into a jdk.
*
* @return a sun.misc.Unsafe
*/
private static sun.misc.Unsafe getUnsafe() {
try {
return sun.misc.Unsafe.getUnsafe();
} catch (SecurityException se) {
try {
return java.security.AccessController.doPrivileged
(new java.security
.PrivilegedExceptionAction() {
public sun.misc.Unsafe run() throws Exception {
java.lang.reflect.Field f = sun.misc
.Unsafe.class.getDeclaredField("theUnsafe");
f.setAccessible(true);
return (sun.misc.Unsafe) f.get(null);
}});
} catch (java.security.PrivilegedActionException e) {
throw new RuntimeException("Could not initialize intrinsics",
e.getCause());
}
}
}
}