/*
* 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.concurrent.*;
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}s, 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). A {@code ForkJoinPool} may also be used for mixed
* execution of some plain {@code Runnable}- or {@code Callable}-
* based activities along with {@code ForkJoinTask}s. When setting
* {@linkplain #setAsyncMode async mode}, a {@code ForkJoinPool} may
* also be appropriate for use with fine-grained tasks of any form
* that are never joined. Otherwise, other {@code ExecutorService}
* implementations are typically more appropriate choices.
*
*
A {@code ForkJoinPool} is constructed with a given target
* parallelism level; by default, equal to the number of available
* processors. Unless configured otherwise via {@link
* #setMaintainsParallelism}, the pool attempts to maintain this
* number of 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 performed in the face of blocked IO or other unmanaged
* synchronization. The nested {@link ManagedBlocker} interface
* enables extension of the kinds of synchronization accommodated.
* The target parallelism level may also be changed dynamically
* ({@link #setParallelism}). The total number of threads may be
* limited using method {@link #setMaximumPoolSize}, in which case it
* may become possible for the activities of a pool to stall due to
* the lack of available threads to process new tasks. When the pool
* is executing tasks, these and other configuration setting methods
* may only gradually affect actual pool sizes. It is normally best
* practice to invoke these methods only when the pool is known to be
* quiescent.
*
*
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.
*
*
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.
*
* @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.)
*
* 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 the management responsibilities of this class. 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 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.
*
* 2. Bookkeeping for dynamically adding and removing workers. We
* maintain a given level of parallelism (or, if
* maintainsParallelism is false, at least avoid starvation). 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 artifically 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. To support these decisions,
* updates to spare counts must be prospective (not
* retrospective). For example, the running count is decremented
* before blocking by a thread about to block as a spare, but
* incremented by the thread about to unblock it. Updates upon
* resumption ofr threads blocking in awaitJoin or awaitBlocker
* cannot usually be prospective, so the running count is in
* general an upper bound of the number of productively running
* threads Updates to the workerCounts field sometimes transiently
* encounter a fair amount of contention when join dependencies
* are such that many threads block or unblock at about the same
* time. We alleviate this by sometimes bundling updates (for
* example blocking one thread on join and resuming a spare cancel
* each other out), and in most other cases performing an
* alternative action like releasing waiters or locating spares.
*
* 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 termination and reconfiguration 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. To reduce delays
* in task diffusion, workers not otherwise occupied may invoke
* method releaseWaiters, that removes and signals (unparks)
* workers not waiting on current count. To minimize task
* production stalls associate with signalling, any worker pushing
* a task on an empty queue invokes the weaker method signalWork,
* that only releases idle workers until it detects interference
* by other threads trying to release, and lets them take
* over. 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.
* 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 is about
* to block waiting for a join (or via ManagedBlockers), we may
* create a new thread to maintain parallelism level, or at least
* avoid starvation (see below). 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. Methods awaitJoin and awaitBlocker look
* for suspended threads to resume before considering creating a
* new replacement. We don't need a special data structure to
* maintain spares; simply scanning the workers array looking for
* worker.isSuspended() is fine because the calling thread is
* otherwise not doing anything useful anyway; we are at least as
* happy if after locating a spare, the caller doesn't actually
* block because the join is ready before we try to adjust and
* compensate. Note that this 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 they should re-suspend. The only
* effective difference between "extra" and "core" threads is that
* we allow the "extra" ones to time out and die if they are not
* resumed within a keep-alive interval of a few seconds. This is
* implemented mainly within ForkJoinWorkerThread, but requires
* some coordination (isTrimmed() -- meaning killed while
* suspended) to correctly maintain pool counts.
*
* 6. Deciding when to create new workers. The main dynamic
* control in this class is deciding when to create extra threads,
* in methods awaitJoin and awaitBlocker. We always
* need to create one when the number of running threads becomes
* zero. But because blocked joins are typically dependent, we
* don't necessarily need or want one-to-one replacement. Using a
* one-to-one compensation rule often leads to enough useless
* overhead creating, suspending, resuming, and/or killing threads
* to signficantly degrade throughput. We use a rule reflecting
* the idea that, the more spare threads you already have, the
* more evidence you need to create another one. The "evidence"
* here takes two forms: (1) Using a creation threshold expressed
* in terms of the current deficit -- target minus running
* threads. To reduce flickering and drift around target values,
* the relation is quadratic: adding a spare if (dc*dc)>=(sc*pc)
* (where dc is deficit, sc is number of spare threads and pc is
* target parallelism.) (2) Using a form of adaptive
* spionning. requiring a number of threshold checks proportional
* to the number of spare threads. This effectively reduces churn
* at the price of systematically undershooting target parallelism
* when many threads are blocked. However, biasing toward
* undeshooting partially compensates for the above mechanics to
* suspend extra threads, that normally lead to overshoot because
* we can only suspend workers in-between top-level actions. It
* 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
* many 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.
*
* 7. Maintaining other configuration parameters and monitoring
* statistics. Updates to fields controlling parallelism level,
* max size, etc can only meaningfully take effect for individual
* threads upon their next top-level actions; i.e., between
* stealing/running tasks/submission, which are separated by calls
* to preStep. Memory ordering for these (assumed infrequent)
* reconfiguration calls is ensured by using reads and writes to
* volatile field workerCounts (that must be read in preStep anyway)
* as "fences" -- user-level reads are preceded by reads of
* workCounts, and writes are followed by no-op CAS to
* workerCounts. The values reported by other management and
* monitoring methods are either computed on demand, or are kept
* in fields that are only updated when threads are otherwise
* idle.
*
* 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 read orderings. 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 a few
* other coding oddities that help some methods perform reasonably
* even when interpreted (not compiled).
*
* 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();
/**
* Absolute bound for parallelism level. Twice this number must
* fit into a 16bit field to enable word-packing for some counts.
*/
private static final int MAX_THREADS = 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 CountDownLatch terminationLatch;
/**
* 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 word contains one plus the pool index of waiting worker
* thread.
*/
private volatile long eventWaiters;
private static final int EVENT_COUNT_SHIFT = 32;
private static final long WAITER_INDEX_MASK = (1L << EVENT_COUNT_SHIFT)-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 and reconfiguration
*/
private volatile int eventCount;
/**
* 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).
*/
private 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;
private static final int ONE_ACTIVE = 1; // active update delta
/**
* 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. This field is also used for memory-fencing
* configuration parameters.
*/
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;
/*
* Fields parallelism. maxPoolSize, and maintainsParallelism are
* non-volatile, but external reads/writes use workerCount fences
* to ensure visability.
*/
/**
* The target parallelism level.
*/
private int parallelism;
/**
* The maximum allowed pool size.
*/
private int maxPoolSize;
/**
* True if use local fifo, not default lifo, for local polling
* Replicated by ForkJoinWorkerThreads
*/
private volatile boolean locallyFifo;
/**
* Controls whether to add spares to maintain parallelism
*/
private boolean maintainsParallelism;
/**
* The uncaught exception handler used when any worker
* abruptly terminates
*/
private volatile Thread.UncaughtExceptionHandler ueh;
/**
* Pool number, just for assigning useful names to worker threads
*/
private final int poolNumber;
// utilities for updating fields
/**
* Adds delta to running count. Used mainly by ForkJoinTask.
*/
final void updateRunningCount(int delta) {
int wc;
do {} while (!UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc = workerCounts,
wc + delta));
}
/**
* Decrements 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);
}
/**
* Write fence for user modifications of pool parameters
* (parallelism. etc). Note that it doesn't matter if CAS fails.
*/
private void workerCountWriteFence() {
int wc;
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc = workerCounts, wc);
}
/**
* Read fence for external reads of pool parameters
* (parallelism. maxPoolSize, etc).
*/
private void workerCountReadFence() {
int ignore = workerCounts;
}
/**
* Tries incrementing active count; fails on contention.
* Called by workers before executing tasks.
*
* @return true on success
*/
final boolean tryIncrementActiveCount() {
int c;
return UNSAFE.compareAndSwapInt(this, runStateOffset,
c = runState, c + ONE_ACTIVE);
}
/**
* 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 - ONE_ACTIVE);
}
/**
* 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 nws = ws.length;
if (k < 0 || k >= nws || ws[k] != null) {
for (k = 0; k < nws && ws[k] != null; ++k)
;
if (k == nws)
ws = Arrays.copyOf(ws, nws << 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 unecessary 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();
}
}
// adding and removing workers
/**
* Tries to create and add new worker. Assumes that worker counts
* are already updated to accommodate the worker, so adjusts on
* failure.
*
* @return new worker or null if creation failed
*/
private ForkJoinWorkerThread addWorker() {
ForkJoinWorkerThread w = null;
try {
w = factory.newThread(this);
} finally { // Adjust on either null or exceptional factory return
if (w == null) {
onWorkerCreationFailure();
return null;
}
}
w.start(recordWorker(w), locallyFifo, ueh);
return w;
}
/**
* Adjusts counts upon failure to create worker
*/
private void onWorkerCreationFailure() {
for (;;) {
int wc = workerCounts;
if ((wc >>> TOTAL_COUNT_SHIFT) > 0 &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - (ONE_RUNNING|ONE_TOTAL)))
break;
}
tryTerminate(false); // in case of failure during shutdown
}
/**
* Create enough total workers to establish target parallelism,
* giving up if terminating or addWorker fails
*/
private void ensureEnoughTotalWorkers() {
int wc;
while (((wc = workerCounts) >>> TOTAL_COUNT_SHIFT) < parallelism &&
runState < TERMINATING) {
if ((UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc + (ONE_RUNNING|ONE_TOTAL)) &&
addWorker() == null))
break;
}
}
/**
* Final callback from terminating worker. Removes record of
* worker from array, and adjusts counts. If pool is shutting
* down, tries to complete terminatation, else possibly replaces
* the worker.
*
* @param w the worker
*/
final void workerTerminated(ForkJoinWorkerThread w) {
if (w.active) { // force inactive
w.active = false;
do {} while (!tryDecrementActiveCount());
}
forgetWorker(w);
// Decrement total count, and if was running, running count
// Spin (waiting for other updates) if either would be negative
int nr = w.isTrimmed() ? 0 : ONE_RUNNING;
int unit = ONE_TOTAL + nr;
for (;;) {
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
if (rc - nr < 0 || (wc >>> TOTAL_COUNT_SHIFT) == 0)
Thread.yield(); // back off if waiting for other updates
else if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - unit))
break;
}
accumulateStealCount(w); // collect final count
if (!tryTerminate(false))
ensureEnoughTotalWorkers();
}
// Waiting for and signalling events
/**
* Ensures eventCount on exit is different (mod 2^32) than on
* entry. CAS failures are OK -- any change in count suffices.
*/
private void advanceEventCount() {
int c;
UNSAFE.compareAndSwapInt(this, eventCountOffset, c = eventCount, c+1);
}
/**
* Releases workers blocked on a count not equal to current count.
*/
final void releaseWaiters() {
long top;
int id;
while ((id = (int)((top = eventWaiters) & WAITER_INDEX_MASK)) > 0 &&
(int)(top >>> EVENT_COUNT_SHIFT) != eventCount) {
ForkJoinWorkerThread[] ws = workers;
ForkJoinWorkerThread w;
if (ws.length >= id && (w = ws[id - 1]) != null &&
UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
top, w.nextWaiter))
LockSupport.unpark(w);
}
}
/**
* Advances eventCount and releases waiters until interference by
* other releasing threads is detected.
*/
final void signalWork() {
int ec;
UNSAFE.compareAndSwapInt(this, eventCountOffset, ec=eventCount, ec+1);
outer:for (;;) {
long top = eventWaiters;
ec = eventCount;
for (;;) {
ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
int id = (int)(top & WAITER_INDEX_MASK);
if (id <= 0 || (int)(top >>> EVENT_COUNT_SHIFT) == ec)
return;
if ((ws = workers).length < id || (w = ws[id - 1]) == null ||
!UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
top, top = w.nextWaiter))
continue outer; // possibly stale; reread
LockSupport.unpark(w);
if (top != eventWaiters) // let someone else take over
return;
}
}
}
/**
* If worker is inactive, blocks until terminating or event count
* advances from last value held by worker; in any case helps
* release others.
*
* @param w the calling worker thread
*/
private void eventSync(ForkJoinWorkerThread w) {
if (!w.active) {
int prev = w.lastEventCount;
long nextTop = (((long)prev << EVENT_COUNT_SHIFT) |
((long)(w.poolIndex + 1)));
long top;
while ((runState < SHUTDOWN || !tryTerminate(false)) &&
(((int)(top = eventWaiters) & WAITER_INDEX_MASK) == 0 ||
(int)(top >>> EVENT_COUNT_SHIFT) == prev) &&
eventCount == prev) {
if (UNSAFE.compareAndSwapLong(this, eventWaitersOffset,
w.nextWaiter = top, nextTop)) {
accumulateStealCount(w); // transfer steals while idle
Thread.interrupted(); // clear/ignore interrupt
while (eventCount == prev)
w.doPark();
break;
}
}
w.lastEventCount = eventCount;
}
releaseWaiters();
}
/**
* Callback from workers invoked upon each top-level action (i.e.,
* stealing a task or taking a submission and running
* it). Performs one or both of the following:
*
* * If the worker cannot find work, updates its active status to
* inactive and updates activeCount unless there is contention, in
* which case it may try again (either in this or a subsequent
* call). Additionally, awaits the next task event and/or helps
* wake up other releasable waiters.
*
* * If there are too many running threads, suspends this worker
* (first forcing inactivation if necessary). If it is not
* resumed before a keepAlive elapses, the worker may be "trimmed"
* -- killed while suspended within suspendAsSpare. Otherwise,
* upon resume it rechecks to make sure that it is still needed.
*
* @param w the worker
* @param worked false if the worker scanned for work but didn't
* find any (in which case it may block waiting for work).
*/
final void preStep(ForkJoinWorkerThread w, boolean worked) {
boolean active = w.active;
boolean inactivate = !worked & active;
for (;;) {
if (inactivate) {
int c = runState;
if (UNSAFE.compareAndSwapInt(this, runStateOffset,
c, c - ONE_ACTIVE))
inactivate = active = w.active = false;
}
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) <= parallelism) {
if (!worked)
eventSync(w);
return;
}
if (!(inactivate |= active) && // must inactivate to suspend
UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING) &&
!w.suspendAsSpare()) // false if trimmed
return;
}
}
/**
* Adjusts counts and creates or resumes compensating threads for
* a worker blocking on task joinMe. First tries resuming an
* existing spare (which usually also avoids any count
* adjustment), but must then decrement running count to determine
* whether a new thread is needed. See above for fuller
* explanation. This code is sprawled out non-modularly mainly
* because adaptive spinning works best if the entire method is
* either interpreted or compiled vs having only some pieces of it
* compiled.
*
* @param joinMe the task to join
* @return task status on exit (to simplify usage by callers)
*/
final int awaitJoin(ForkJoinTask> joinMe) {
int pc = parallelism;
boolean adj = false; // true when running count adjusted
int scans = 0;
while (joinMe.status >= 0) {
ForkJoinWorkerThread spare = null;
if ((workerCounts & RUNNING_COUNT_MASK) < pc) {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && w.isSuspended()) {
spare = w;
break;
}
}
if (joinMe.status < 0)
break;
}
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
int dc = pc - rc;
if (dc > 0 && spare != null && spare.tryUnsuspend()) {
if (adj) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
adj = true;
LockSupport.unpark(spare);
}
else if (adj) {
if (dc <= 0)
break;
int tc = wc >>> TOTAL_COUNT_SHIFT;
if (scans > tc) {
int ts = (tc - pc) * pc;
if (rc != 0 && (dc * dc < ts || !maintainsParallelism))
break;
if (scans > ts && tc < maxPoolSize &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc+(ONE_RUNNING|ONE_TOTAL))){
addWorker();
break;
}
}
}
else if (rc != 0)
adj = UNSAFE.compareAndSwapInt (this, workerCountsOffset,
wc, wc - ONE_RUNNING);
if ((scans++ & 1) == 0)
releaseWaiters(); // help others progress
else
Thread.yield(); // avoid starving productive threads
}
if (adj) {
joinMe.internalAwaitDone();
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
return joinMe.status;
}
/**
* Same idea as awaitJoin
*/
final void awaitBlocker(ManagedBlocker blocker, boolean maintainPar)
throws InterruptedException {
maintainPar &= maintainsParallelism;
int pc = parallelism;
boolean adj = false; // true when running count adjusted
int scans = 0;
boolean done;
for (;;) {
if (done = blocker.isReleasable())
break;
ForkJoinWorkerThread spare = null;
if ((workerCounts & RUNNING_COUNT_MASK) < pc) {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && w.isSuspended()) {
spare = w;
break;
}
}
if (done = blocker.isReleasable())
break;
}
int wc = workerCounts;
int rc = wc & RUNNING_COUNT_MASK;
int dc = pc - rc;
if (dc > 0 && spare != null && spare.tryUnsuspend()) {
if (adj) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
adj = true;
LockSupport.unpark(spare);
}
else if (adj) {
if (dc <= 0)
break;
int tc = wc >>> TOTAL_COUNT_SHIFT;
if (scans > tc) {
int ts = (tc - pc) * pc;
if (rc != 0 && (dc * dc < ts || !maintainPar))
break;
if (scans > ts && tc < maxPoolSize &&
UNSAFE.compareAndSwapInt(this, workerCountsOffset, wc,
wc+(ONE_RUNNING|ONE_TOTAL))){
addWorker();
break;
}
}
}
else if (rc != 0)
adj = UNSAFE.compareAndSwapInt (this, workerCountsOffset,
wc, wc - ONE_RUNNING);
if ((++scans & 1) == 0)
releaseWaiters(); // help others progress
else
Thread.yield(); // avoid starving productive threads
}
try {
if (!done)
do {} while (!blocker.isReleasable() && !blocker.block());
} finally {
if (adj) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
}
}
}
/**
* Unless there are not enough other running threads, adjusts
* counts and blocks a worker performing helpJoin that cannot find
* any work.
*
* @return true if joinMe now done
*/
final boolean tryAwaitBusyJoin(ForkJoinTask> joinMe) {
int pc = parallelism;
outer:for (;;) {
releaseWaiters();
if ((workerCounts & RUNNING_COUNT_MASK) < pc) {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && w.isSuspended()) {
if (joinMe.status < 0)
return true;
if ((workerCounts & RUNNING_COUNT_MASK) > pc)
break;
if (w.tryUnsuspend()) {
LockSupport.unpark(w);
break outer;
}
continue outer;
}
}
}
if (joinMe.status < 0)
return true;
int wc = workerCounts;
if ((wc & RUNNING_COUNT_MASK) <= 2 ||
(wc >>> TOTAL_COUNT_SHIFT) < pc)
return false; // keep this thread alive
if (UNSAFE.compareAndSwapInt(this, workerCountsOffset,
wc, wc - ONE_RUNNING))
break;
}
joinMe.internalAwaitDone();
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
return true;
}
/**
* 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);
terminationLatch.countDown();
}
return true;
}
/**
* Actions on transition to TERMINATING
*/
private void startTerminating() {
for (int i = 0; i < 2; ++i) { // twice to mop up newly created workers
cancelSubmissions();
shutdownWorkers();
cancelWorkerTasks();
advanceEventCount();
releaseWaiters();
interruptWorkers();
}
}
/**
* Clear out and cancel submissions, ignoring exceptions
*/
private void cancelSubmissions() {
ForkJoinTask> task;
while ((task = submissionQueue.poll()) != null) {
try {
task.cancel(false);
} catch (Throwable ignore) {
}
}
}
/**
* Sets all worker run states to at least shutdown,
* also resuming suspended workers
*/
private void shutdownWorkers() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.shutdown();
}
}
/**
* Clears out and cancels all locally queued tasks
*/
private void cancelWorkerTasks() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.cancelTasks();
}
}
/**
* Unsticks all workers blocked on joins etc
*/
private void interruptWorkers() {
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && !w.isTerminated()) {
try {
w.interrupt();
} catch (SecurityException ignore) {
}
}
}
}
// misc support for ForkJoinWorkerThread
/**
* Returns pool number
*/
final int getPoolNumber() {
return poolNumber;
}
/**
* Accumulates steal count from a worker, clearing
* the worker's value
*/
final void accumulateStealCount(ForkJoinWorkerThread w) {
int sc = w.stealCount;
if (sc != 0) {
long c;
w.stealCount = 0;
do {} while (!UNSAFE.compareAndSwapLong(this, stealCountOffset,
c = stealCount, c + sc));
}
}
/**
* Returns the approximate (non-atomic) number of idle threads per
* active thread.
*/
final int idlePerActive() {
int ac = runState; // no mask -- artifically boosts during shutdown
int pc = parallelism; // use targeted parallelism, not rc
// 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}, and using the {@linkplain
* #defaultForkJoinWorkerThreadFactory default thread factory}.
*
* @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);
}
/**
* Creates a {@code ForkJoinPool} with the indicated parallelism
* level and using the {@linkplain
* #defaultForkJoinWorkerThreadFactory default thread factory}.
*
* @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);
}
/**
* Creates a {@code ForkJoinPool} with parallelism equal to {@link
* java.lang.Runtime#availableProcessors}, and using the given
* thread factory.
*
* @param factory the factory for creating new threads
* @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(ForkJoinWorkerThreadFactory factory) {
this(Runtime.getRuntime().availableProcessors(), factory);
}
/**
* Creates a {@code ForkJoinPool} with the given parallelism and
* thread factory.
*
* @param parallelism the parallelism level
* @param factory the factory for creating new threads
* @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) {
checkPermission();
if (factory == null)
throw new NullPointerException();
if (parallelism <= 0 || parallelism > MAX_THREADS)
throw new IllegalArgumentException();
this.poolNumber = poolNumberGenerator.incrementAndGet();
int arraySize = initialArraySizeFor(parallelism);
this.parallelism = parallelism;
this.factory = factory;
this.maxPoolSize = MAX_THREADS;
this.maintainsParallelism = true;
this.workers = new ForkJoinWorkerThread[arraySize];
this.submissionQueue = new LinkedTransferQueue>();
this.workerLock = new ReentrantLock();
this.terminationLatch = new CountDownLatch(1);
}
/**
* Returns initial power of two size for workers array.
* @param pc the initial parallelism level
*/
private static int initialArraySizeFor(int pc) {
// See Hackers Delight, sec 3.2. We know MAX_THREADS < (1 >>> 16)
int size = pc < MAX_THREADS ? pc + 1 : MAX_THREADS;
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);
advanceEventCount();
releaseWaiters();
ensureEnoughTotalWorkers();
}
/**
* 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);
}
/**
* @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;
}
/**
* 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 {@inheritDoc}
* @throws RejectedExecutionException {@inheritDoc}
*/
public List> invokeAll(Collection extends Callable> 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() {
workerCountReadFence();
return ueh;
}
/**
* Sets the handler for internal worker threads that terminate due
* to unrecoverable errors encountered while executing tasks.
* Unless set, the current default or ThreadGroup handler is used
* as handler.
*
* @param h the new handler
* @return the old handler, or {@code null} if none
* @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 Thread.UncaughtExceptionHandler
setUncaughtExceptionHandler(Thread.UncaughtExceptionHandler h) {
checkPermission();
Thread.UncaughtExceptionHandler old = ueh;
if (h != old) {
ueh = h;
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.setUncaughtExceptionHandler(h);
}
}
return old;
}
/**
* Sets the target parallelism level of this pool.
*
* @param parallelism the target parallelism
* @throws IllegalArgumentException if parallelism less than or
* equal to zero or greater than maximum size bounds
* @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 setParallelism(int parallelism) {
checkPermission();
if (parallelism <= 0 || parallelism > maxPoolSize)
throw new IllegalArgumentException();
workerCountReadFence();
int pc = this.parallelism;
if (pc != parallelism) {
this.parallelism = parallelism;
workerCountWriteFence();
// Release spares. If too many, some will die after re-suspend
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null && w.tryUnsuspend()) {
int c;
do {} while (!UNSAFE.compareAndSwapInt
(this, workerCountsOffset,
c = workerCounts, c + ONE_RUNNING));
LockSupport.unpark(w);
}
}
ensureEnoughTotalWorkers();
advanceEventCount();
releaseWaiters(); // force config recheck by existing workers
}
}
/**
* Returns the targeted parallelism level of this pool.
*
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
// workerCountReadFence(); // inlined below
int ignore = workerCounts;
return parallelism;
}
/**
* Returns the number of worker threads that have started but not
* yet terminated. This 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 the maximum number of threads allowed to exist in the
* pool. Unless set using {@link #setMaximumPoolSize}, the
* maximum is an implementation-defined value designed only to
* prevent runaway growth.
*
* @return the maximum
*/
public int getMaximumPoolSize() {
workerCountReadFence();
return maxPoolSize;
}
/**
* Sets the maximum number of threads allowed to exist in the
* pool. The given value should normally be greater than or equal
* to the {@link #getParallelism parallelism} level. Setting this
* value has no effect on current pool size. It controls
* construction of new threads. The use of this method may cause
* tasks that intrinsically require extra threads for dependent
* computations to indefinitely stall. If you are instead trying
* to minimize internal thread creation, consider setting {@link
* #setMaintainsParallelism} as false.
*
* @throws IllegalArgumentException if negative or greater than
* internal implementation limit
*/
public void setMaximumPoolSize(int newMax) {
if (newMax < 0 || newMax > MAX_THREADS)
throw new IllegalArgumentException();
maxPoolSize = newMax;
workerCountWriteFence();
}
/**
* Returns {@code true} if this pool dynamically maintains its
* target parallelism level. If false, new threads are added only
* to avoid possible starvation. This setting is by default true.
*
* @return {@code true} if maintains parallelism
*/
public boolean getMaintainsParallelism() {
workerCountReadFence();
return maintainsParallelism;
}
/**
* Sets whether this pool dynamically maintains its target
* parallelism level. If false, new threads are added only to
* avoid possible starvation.
*
* @param enable {@code true} to maintain parallelism
*/
public void setMaintainsParallelism(boolean enable) {
maintainsParallelism = enable;
workerCountWriteFence();
}
/**
* 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 asynchronous tasks. This method is
* designed to be invoked only when the pool is quiescent, and
* typically only before any tasks are submitted. The effects of
* invocations at other times may be unpredictable.
*
* @param async if {@code true}, use locally FIFO scheduling
* @return the previous mode
* @see #getAsyncMode
*/
public boolean setAsyncMode(boolean async) {
workerCountReadFence();
boolean oldMode = locallyFifo;
if (oldMode != async) {
locallyFifo = async;
workerCountWriteFence();
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
w.setAsyncMode(async);
}
}
return oldMode;
}
/**
* 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
* @see #setAsyncMode
*/
public boolean getAsyncMode() {
workerCountReadFence();
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;
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
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 super ForkJoinTask>> c) {
int n = submissionQueue.drainTo(c);
ForkJoinWorkerThread[] ws = workers;
int nws = ws.length;
for (int i = 0; i < nws; ++i) {
ForkJoinWorkerThread w = ws[i];
if (w != null)
n += w.drainTasksTo(c);
}
return n;
}
/**
* 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 {
return terminationLatch.await(timeout, unit);
}
/**
* 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).
*
*
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());
* }
* }}
*/
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 parallelism while the current thread is blocked.
*
* If {@code maintainParallelism} is {@code true} and the pool
* supports it ({@link #getMaintainsParallelism}), this method
* attempts to maintain the pool's nominal parallelism. Otherwise
* it activates a thread only if necessary to avoid complete
* starvation. This option may be preferable when blockages use
* timeouts, or are almost always brief.
*
*
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
* @param maintainParallelism if {@code true} and supported by
* this pool, attempt to maintain the pool's nominal parallelism;
* otherwise activate a thread only if necessary to avoid
* complete starvation.
* @throws InterruptedException if blocker.block did so
*/
public static void managedBlock(ManagedBlocker blocker,
boolean maintainParallelism)
throws InterruptedException {
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread)
((ForkJoinWorkerThread) t).pool.
awaitBlocker(blocker, maintainParallelism);
else
awaitBlocker(blocker);
}
/**
* Performs Non-FJ blocking
*/
private static void awaitBlocker(ManagedBlocker blocker)
throws InterruptedException {
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 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());
}
}
}
}