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8002356: Add ForkJoin common pool and CountedCompleter

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Reviewed-by: chegar, mduigou
This commit is contained in:
Doug Lea 2012-12-20 13:44:06 +00:00
parent 199a9c32b2
commit a3f6c5ebeb
5 changed files with 3714 additions and 2486 deletions
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jdk/make/java/java/FILES_java.gmk
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@ -322,6 +322,7 @@ JAVA_JAVA_java = \
java/util/concurrent/CopyOnWriteArrayList.java \
java/util/concurrent/CopyOnWriteArraySet.java \
java/util/concurrent/CountDownLatch.java \
java/util/concurrent/CountedCompleter.java \
java/util/concurrent/CyclicBarrier.java \
java/util/concurrent/DelayQueue.java \
java/util/concurrent/Delayed.java \

743
jdk/src/share/classes/java/util/concurrent/CountedCompleter.java Normal file
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@ -0,0 +1,743 @@
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
/**
* A {@link ForkJoinTask} with a completion action performed when
* triggered and there are no remaining pending
* actions. CountedCompleters are in general more robust in the
* presence of subtask stalls and blockage than are other forms of
* ForkJoinTasks, but are less intuitive to program. Uses of
* CountedCompleter are similar to those of other completion based
* components (such as {@link java.nio.channels.CompletionHandler})
* except that multiple <em>pending</em> completions may be necessary
* to trigger the completion action {@link #onCompletion}, not just one.
* Unless initialized otherwise, the {@linkplain #getPendingCount pending
* count} starts at zero, but may be (atomically) changed using
* methods {@link #setPendingCount}, {@link #addToPendingCount}, and
* {@link #compareAndSetPendingCount}. Upon invocation of {@link
* #tryComplete}, if the pending action count is nonzero, it is
* decremented; otherwise, the completion action is performed, and if
* this completer itself has a completer, the process is continued
* with its completer. As is the case with related synchronization
* components such as {@link java.util.concurrent.Phaser Phaser} and
* {@link java.util.concurrent.Semaphore Semaphore}, these methods
* affect only internal counts; they do not establish any further
* internal bookkeeping. In particular, the identities of pending
* tasks are not maintained. As illustrated below, you can create
* subclasses that do record some or all pending tasks or their
* results when needed. As illustrated below, utility methods
* supporting customization of completion traversals are also
* provided. However, because CountedCompleters provide only basic
* synchronization mechanisms, it may be useful to create further
* abstract subclasses that maintain linkages, fields, and additional
* support methods appropriate for a set of related usages.
*
* <p>A concrete CountedCompleter class must define method {@link
* #compute}, that should in most cases (as illustrated below), invoke
* {@code tryComplete()} once before returning. The class may also
* optionally override method {@link #onCompletion} to perform an
* action upon normal completion, and method {@link
* #onExceptionalCompletion} to perform an action upon any exception.
*
* <p>CountedCompleters most often do not bear results, in which case
* they are normally declared as {@code CountedCompleter<Void>}, and
* will always return {@code null} as a result value. In other cases,
* you should override method {@link #getRawResult} to provide a
* result from {@code join(), invoke()}, and related methods. In
* general, this method should return the value of a field (or a
* function of one or more fields) of the CountedCompleter object that
* holds the result upon completion. Method {@link #setRawResult} by
* default plays no role in CountedCompleters. It is possible, but
* rarely applicable, to override this method to maintain other
* objects or fields holding result data.
*
* <p>A CountedCompleter that does not itself have a completer (i.e.,
* one for which {@link #getCompleter} returns {@code null}) can be
* used as a regular ForkJoinTask with this added functionality.
* However, any completer that in turn has another completer serves
* only as an internal helper for other computations, so its own task
* status (as reported in methods such as {@link ForkJoinTask#isDone})
* is arbitrary; this status changes only upon explicit invocations of
* {@link #complete}, {@link ForkJoinTask#cancel}, {@link
* ForkJoinTask#completeExceptionally} or upon exceptional completion
* of method {@code compute}. Upon any exceptional completion, the
* exception may be relayed to a task's completer (and its completer,
* and so on), if one exists and it has not otherwise already
* completed. Similarly, cancelling an internal CountedCompleter has
* only a local effect on that completer, so is not often useful.
*
* <p><b>Sample Usages.</b>
*
* <p><b>Parallel recursive decomposition.</b> CountedCompleters may
* be arranged in trees similar to those often used with {@link
* RecursiveAction}s, although the constructions involved in setting
* them up typically vary. Here, the completer of each task is its
* parent in the computation tree. Even though they entail a bit more
* bookkeeping, CountedCompleters may be better choices when applying
* a possibly time-consuming operation (that cannot be further
* subdivided) to each element of an array or collection; especially
* when the operation takes a significantly different amount of time
* to complete for some elements than others, either because of
* intrinsic variation (for example I/O) or auxiliary effects such as
* garbage collection. Because CountedCompleters provide their own
* continuations, other threads need not block waiting to perform
* them.
*
* <p>For example, here is an initial version of a class that uses
* divide-by-two recursive decomposition to divide work into single
* pieces (leaf tasks). Even when work is split into individual calls,
* tree-based techniques are usually preferable to directly forking
* leaf tasks, because they reduce inter-thread communication and
* improve load balancing. In the recursive case, the second of each
* pair of subtasks to finish triggers completion of its parent
* (because no result combination is performed, the default no-op
* implementation of method {@code onCompletion} is not overridden). A
* static utility method sets up the base task and invokes it
* (here, implicitly using the {@link ForkJoinPool#commonPool()}).
*
* <pre> {@code
* class MyOperation<E> { void apply(E e) { ... } }
*
* class ForEach<E> extends CountedCompleter<Void> {
*
* public static <E> void forEach(E[] array, MyOperation<E> op) {
* new ForEach<E>(null, array, op, 0, array.length).invoke();
* }
*
* final E[] array; final MyOperation<E> op; final int lo, hi;
* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) {
* super(p);
* this.array = array; this.op = op; this.lo = lo; this.hi = hi;
* }
*
* public void compute() { // version 1
* if (hi - lo >= 2) {
* int mid = (lo + hi) >>> 1;
* setPendingCount(2); // must set pending count before fork
* new ForEach(this, array, op, mid, hi).fork(); // right child
* new ForEach(this, array, op, lo, mid).fork(); // left child
* }
* else if (hi > lo)
* op.apply(array[lo]);
* tryComplete();
* }
* }}</pre>
*
* This design can be improved by noticing that in the recursive case,
* the task has nothing to do after forking its right task, so can
* directly invoke its left task before returning. (This is an analog
* of tail recursion removal.) Also, because the task returns upon
* executing its left task (rather than falling through to invoke
* {@code tryComplete}) the pending count is set to one:
*
* <pre> {@code
* class ForEach<E> ...
* public void compute() { // version 2
* if (hi - lo >= 2) {
* int mid = (lo + hi) >>> 1;
* setPendingCount(1); // only one pending
* new ForEach(this, array, op, mid, hi).fork(); // right child
* new ForEach(this, array, op, lo, mid).compute(); // direct invoke
* }
* else {
* if (hi > lo)
* op.apply(array[lo]);
* tryComplete();
* }
* }
* }</pre>
*
* As a further improvement, notice that the left task need not even
* exist. Instead of creating a new one, we can iterate using the
* original task, and add a pending count for each fork. Additionally,
* because no task in this tree implements an {@link #onCompletion}
* method, {@code tryComplete()} can be replaced with {@link
* #propagateCompletion}.
*
* <pre> {@code
* class ForEach<E> ...
* public void compute() { // version 3
* int l = lo, h = hi;
* while (h - l >= 2) {
* int mid = (l + h) >>> 1;
* addToPendingCount(1);
* new ForEach(this, array, op, mid, h).fork(); // right child
* h = mid;
* }
* if (h > l)
* op.apply(array[l]);
* propagateCompletion();
* }
* }</pre>
*
* Additional improvements of such classes might entail precomputing
* pending counts so that they can be established in constructors,
* specializing classes for leaf steps, subdividing by say, four,
* instead of two per iteration, and using an adaptive threshold
* instead of always subdividing down to single elements.
*
* <p><b>Searching.</b> A tree of CountedCompleters can search for a
* value or property in different parts of a data structure, and
* report a result in an {@link
* java.util.concurrent.atomic.AtomicReference AtomicReference} as
* soon as one is found. The others can poll the result to avoid
* unnecessary work. (You could additionally {@linkplain #cancel
* cancel} other tasks, but it is usually simpler and more efficient
* to just let them notice that the result is set and if so skip
* further processing.) Illustrating again with an array using full
* partitioning (again, in practice, leaf tasks will almost always
* process more than one element):
*
* <pre> {@code
* class Searcher<E> extends CountedCompleter<E> {
* final E[] array; final AtomicReference<E> result; final int lo, hi;
* Searcher(CountedCompleter<?> p, E[] array, AtomicReference<E> result, int lo, int hi) {
* super(p);
* this.array = array; this.result = result; this.lo = lo; this.hi = hi;
* }
* public E getRawResult() { return result.get(); }
* public void compute() { // similar to ForEach version 3
* int l = lo, h = hi;
* while (result.get() == null && h >= l) {
* if (h - l >= 2) {
* int mid = (l + h) >>> 1;
* addToPendingCount(1);
* new Searcher(this, array, result, mid, h).fork();
* h = mid;
* }
* else {
* E x = array[l];
* if (matches(x) && result.compareAndSet(null, x))
* quietlyCompleteRoot(); // root task is now joinable
* break;
* }
* }
* tryComplete(); // normally complete whether or not found
* }
* boolean matches(E e) { ... } // return true if found
*
* public static <E> E search(E[] array) {
* return new Searcher<E>(null, array, new AtomicReference<E>(), 0, array.length).invoke();
* }
*}}</pre>
*
* In this example, as well as others in which tasks have no other
* effects except to compareAndSet a common result, the trailing
* unconditional invocation of {@code tryComplete} could be made
* conditional ({@code if (result.get() == null) tryComplete();})
* because no further bookkeeping is required to manage completions
* once the root task completes.
*
* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine
* results of multiple subtasks usually need to access these results
* in method {@link #onCompletion}. As illustrated in the following
* class (that performs a simplified form of map-reduce where mappings
* and reductions are all of type {@code E}), one way to do this in
* divide and conquer designs is to have each subtask record its
* sibling, so that it can be accessed in method {@code onCompletion}.
* This technique applies to reductions in which the order of
* combining left and right results does not matter; ordered
* reductions require explicit left/right designations. Variants of
* other streamlinings seen in the above examples may also apply.
*
* <pre> {@code
* class MyMapper<E> { E apply(E v) { ... } }
* class MyReducer<E> { E apply(E x, E y) { ... } }
* class MapReducer<E> extends CountedCompleter<E> {
* final E[] array; final MyMapper<E> mapper;
* final MyReducer<E> reducer; final int lo, hi;
* MapReducer<E> sibling;
* E result;
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
* MyReducer<E> reducer, int lo, int hi) {
* super(p);
* this.array = array; this.mapper = mapper;
* this.reducer = reducer; this.lo = lo; this.hi = hi;
* }
* public void compute() {
* if (hi - lo >= 2) {
* int mid = (lo + hi) >>> 1;
* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid);
* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi);
* left.sibling = right;
* right.sibling = left;
* setPendingCount(1); // only right is pending
* right.fork();
* left.compute(); // directly execute left
* }
* else {
* if (hi > lo)
* result = mapper.apply(array[lo]);
* tryComplete();
* }
* }
* public void onCompletion(CountedCompleter<?> caller) {
* if (caller != this) {
* MapReducer<E> child = (MapReducer<E>)caller;
* MapReducer<E> sib = child.sibling;
* if (sib == null || sib.result == null)
* result = child.result;
* else
* result = reducer.apply(child.result, sib.result);
* }
* }
* public E getRawResult() { return result; }
*
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
* return new MapReducer<E>(null, array, mapper, reducer,
* 0, array.length).invoke();
* }
* }}</pre>
*
* Here, method {@code onCompletion} takes a form common to many
* completion designs that combine results. This callback-style method
* is triggered once per task, in either of the two different contexts
* in which the pending count is, or becomes, zero: (1) by a task
* itself, if its pending count is zero upon invocation of {@code
* tryComplete}, or (2) by any of its subtasks when they complete and
* decrement the pending count to zero. The {@code caller} argument
* distinguishes cases. Most often, when the caller is {@code this},
* no action is necessary. Otherwise the caller argument can be used
* (usually via a cast) to supply a value (and/or links to other
* values) to be combined. Assuming proper use of pending counts, the
* actions inside {@code onCompletion} occur (once) upon completion of
* a task and its subtasks. No additional synchronization is required
* within this method to ensure thread safety of accesses to fields of
* this task or other completed tasks.
*
* <p><b>Completion Traversals</b>. If using {@code onCompletion} to
* process completions is inapplicable or inconvenient, you can use
* methods {@link #firstComplete} and {@link #nextComplete} to create
* custom traversals. For example, to define a MapReducer that only
* splits out right-hand tasks in the form of the third ForEach
* example, the completions must cooperatively reduce along
* unexhausted subtask links, which can be done as follows:
*
* <pre> {@code
* class MapReducer<E> extends CountedCompleter<E> { // version 2
* final E[] array; final MyMapper<E> mapper;
* final MyReducer<E> reducer; final int lo, hi;
* MapReducer<E> forks, next; // record subtask forks in list
* E result;
* MapReducer(CountedCompleter<?> p, E[] array, MyMapper<E> mapper,
* MyReducer<E> reducer, int lo, int hi, MapReducer<E> next) {
* super(p);
* this.array = array; this.mapper = mapper;
* this.reducer = reducer; this.lo = lo; this.hi = hi;
* this.next = next;
* }
* public void compute() {
* int l = lo, h = hi;
* while (h - l >= 2) {
* int mid = (l + h) >>> 1;
* addToPendingCount(1);
* (forks = new MapReducer(this, array, mapper, reducer, mid, h, forks)).fork;
* h = mid;
* }
* if (h > l)
* result = mapper.apply(array[l]);
* // process completions by reducing along and advancing subtask links
* for (CountedCompleter<?> c = firstComplete(); c != null; c = c.nextComplete()) {
* for (MapReducer t = (MapReducer)c, s = t.forks; s != null; s = t.forks = s.next)
* t.result = reducer.apply(t.result, s.result);
* }
* }
* public E getRawResult() { return result; }
*
* public static <E> E mapReduce(E[] array, MyMapper<E> mapper, MyReducer<E> reducer) {
* return new MapReducer<E>(null, array, mapper, reducer,
* 0, array.length, null).invoke();
* }
* }}</pre>
*
* <p><b>Triggers.</b> Some CountedCompleters are themselves never
* forked, but instead serve as bits of plumbing in other designs;
* including those in which the completion of one of more async tasks
* triggers another async task. For example:
*
* <pre> {@code
* class HeaderBuilder extends CountedCompleter<...> { ... }
* class BodyBuilder extends CountedCompleter<...> { ... }
* class PacketSender extends CountedCompleter<...> {
* PacketSender(...) { super(null, 1); ... } // trigger on second completion
* public void compute() { } // never called
* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); }
* }
* // sample use:
* PacketSender p = new PacketSender();
* new HeaderBuilder(p, ...).fork();
* new BodyBuilder(p, ...).fork();
* }</pre>
*
* @since 1.8
* @author Doug Lea
*/
public abstract class CountedCompleter<T> extends ForkJoinTask<T> {
private static final long serialVersionUID = 5232453752276485070L;
/** This task's completer, or null if none */
final CountedCompleter<?> completer;
/** The number of pending tasks until completion */
volatile int pending;
/**
* Creates a new CountedCompleter with the given completer
* and initial pending count.
*
* @param completer this task's completer, or {@code null} if none
* @param initialPendingCount the initial pending count
*/
protected CountedCompleter(CountedCompleter<?> completer,
int initialPendingCount) {
this.completer = completer;
this.pending = initialPendingCount;
}
/**
* Creates a new CountedCompleter with the given completer
* and an initial pending count of zero.
*
* @param completer this task's completer, or {@code null} if none
*/
protected CountedCompleter(CountedCompleter<?> completer) {
this.completer = completer;
}
/**
* Creates a new CountedCompleter with no completer
* and an initial pending count of zero.
*/
protected CountedCompleter() {
this.completer = null;
}
/**
* The main computation performed by this task.
*/
public abstract void compute();
/**
* Performs an action when method {@link #tryComplete} is invoked
* and the pending count is zero, or when the unconditional
* method {@link #complete} is invoked. By default, this method
* does nothing. You can distinguish cases by checking the
* identity of the given caller argument. If not equal to {@code
* this}, then it is typically a subtask that may contain results
* (and/or links to other results) to combine.
*
* @param caller the task invoking this method (which may
* be this task itself).
*/
public void onCompletion(CountedCompleter<?> caller) {
}
/**
* Performs an action when method {@link #completeExceptionally}
* is invoked or method {@link #compute} throws an exception, and
* this task has not otherwise already completed normally. On
* entry to this method, this task {@link
* ForkJoinTask#isCompletedAbnormally}. The return value of this
* method controls further propagation: If {@code true} and this
* task has a completer, then this completer is also completed
* exceptionally. The default implementation of this method does
* nothing except return {@code true}.
*
* @param ex the exception
* @param caller the task invoking this method (which may
* be this task itself).
* @return true if this exception should be propagated to this
* task's completer, if one exists.
*/
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) {
return true;
}
/**
* Returns the completer established in this task's constructor,
* or {@code null} if none.
*
* @return the completer
*/
public final CountedCompleter<?> getCompleter() {
return completer;
}
/**
* Returns the current pending count.
*
* @return the current pending count
*/
public final int getPendingCount() {
return pending;
}
/**
* Sets the pending count to the given value.
*
* @param count the count
*/
public final void setPendingCount(int count) {
pending = count;
}
/**
* Adds (atomically) the given value to the pending count.
*
* @param delta the value to add
*/
public final void addToPendingCount(int delta) {
int c; // note: can replace with intrinsic in jdk8
do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta));
}
/**
* Sets (atomically) the pending count to the given count only if
* it currently holds the given expected value.
*
* @param expected the expected value
* @param count the new value
* @return true if successful
*/
public final boolean compareAndSetPendingCount(int expected, int count) {
return U.compareAndSwapInt(this, PENDING, expected, count);
}
/**
* If the pending count is nonzero, (atomically) decrements it.
*
* @return the initial (undecremented) pending count holding on entry
* to this method
*/
public final int decrementPendingCountUnlessZero() {
int c;
do {} while ((c = pending) != 0 &&
!U.compareAndSwapInt(this, PENDING, c, c - 1));
return c;
}
/**
* Returns the root of the current computation; i.e., this
* task if it has no completer, else its completer's root.
*
* @return the root of the current computation
*/
public final CountedCompleter<?> getRoot() {
CountedCompleter<?> a = this, p;
while ((p = a.completer) != null)
a = p;
return a;
}
/**
* If the pending count is nonzero, decrements the count;
* otherwise invokes {@link #onCompletion} and then similarly
* tries to complete this task's completer, if one exists,
* else marks this task as complete.
*/
public final void tryComplete() {
CountedCompleter<?> a = this, s = a;
for (int c;;) {
if ((c = a.pending) == 0) {
a.onCompletion(s);
if ((a = (s = a).completer) == null) {
s.quietlyComplete();
return;
}
}
else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
return;
}
}
/**
* Equivalent to {@link #tryComplete} but does not invoke {@link
* #onCompletion} along the completion path: If the pending count
* is nonzero, decrements the count; otherwise, similarly tries to
* complete this task's completer, if one exists, else marks this
* task as complete. This method may be useful in cases where
* {@code onCompletion} should not, or need not, be invoked for
* each completer in a computation.
*/
public final void propagateCompletion() {
CountedCompleter<?> a = this, s = a;
for (int c;;) {
if ((c = a.pending) == 0) {
if ((a = (s = a).completer) == null) {
s.quietlyComplete();
return;
}
}
else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
return;
}
}
/**
* Regardless of pending count, invokes {@link #onCompletion},
* marks this task as complete and further triggers {@link
* #tryComplete} on this task's completer, if one exists. The
* given rawResult is used as an argument to {@link #setRawResult}
* before invoking {@link #onCompletion} or marking this task as
* complete; its value is meaningful only for classes overriding
* {@code setRawResult}.
*
* <p>This method may be useful when forcing completion as soon as
* any one (versus all) of several subtask results are obtained.
* However, in the common (and recommended) case in which {@code
* setRawResult} is not overridden, this effect can be obtained
* more simply using {@code quietlyCompleteRoot();}.
*
* @param rawResult the raw result
*/
public void complete(T rawResult) {
CountedCompleter<?> p;
setRawResult(rawResult);
onCompletion(this);
quietlyComplete();
if ((p = completer) != null)
p.tryComplete();
}
/**
* If this task's pending count is zero, returns this task;
* otherwise decrements its pending count and returns {@code
* null}. This method is designed to be used with {@link
* #nextComplete} in completion traversal loops.
*
* @return this task, if pending count was zero, else {@code null}
*/
public final CountedCompleter<?> firstComplete() {
for (int c;;) {
if ((c = pending) == 0)
return this;
else if (U.compareAndSwapInt(this, PENDING, c, c - 1))
return null;
}
}
/**
* If this task does not have a completer, invokes {@link
* ForkJoinTask#quietlyComplete} and returns {@code null}. Or, if
* this task's pending count is non-zero, decrements its pending
* count and returns {@code null}. Otherwise, returns the
* completer. This method can be used as part of a completion
* traversal loop for homogeneous task hierarchies:
*
* <pre> {@code
* for (CountedCompleter<?> c = firstComplete();
* c != null;
* c = c.nextComplete()) {
* // ... process c ...
* }}</pre>
*
* @return the completer, or {@code null} if none
*/
public final CountedCompleter<?> nextComplete() {
CountedCompleter<?> p;
if ((p = completer) != null)
return p.firstComplete();
else {
quietlyComplete();
return null;
}
}
/**
* Equivalent to {@code getRoot().quietlyComplete()}.
*/
public final void quietlyCompleteRoot() {
for (CountedCompleter<?> a = this, p;;) {
if ((p = a.completer) == null) {
a.quietlyComplete();
return;
}
a = p;
}
}
/**
* Supports ForkJoinTask exception propagation.
*/
void internalPropagateException(Throwable ex) {
CountedCompleter<?> a = this, s = a;
while (a.onExceptionalCompletion(ex, s) &&
(a = (s = a).completer) != null && a.status >= 0)
a.recordExceptionalCompletion(ex);
}
/**
* Implements execution conventions for CountedCompleters.
*/
protected final boolean exec() {
compute();
return false;
}
/**
* Returns the result of the computation. By default
* returns {@code null}, which is appropriate for {@code Void}
* actions, but in other cases should be overridden, almost
* always to return a field or function of a field that
* holds the result upon completion.
*
* @return the result of the computation
*/
public T getRawResult() { return null; }
/**
* A method that result-bearing CountedCompleters may optionally
* use to help maintain result data. By default, does nothing.
* Overrides are not recommended. However, if this method is
* overridden to update existing objects or fields, then it must
* in general be defined to be thread-safe.
*/
protected void setRawResult(T t) { }
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long PENDING;
static {
try {
U = sun.misc.Unsafe.getUnsafe();
PENDING = U.objectFieldOffset
(CountedCompleter.class.getDeclaredField("pending"));
} catch (Exception e) {
throw new Error(e);
}
}
}

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jdk/src/share/classes/java/util/concurrent/ForkJoinPool.java
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jdk/src/share/classes/java/util/concurrent/ForkJoinTask.java
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jdk/src/share/classes/java/util/concurrent/ForkJoinWorkerThread.java
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@ -35,9 +35,6 @@
package java.util.concurrent;
import java.util.Collection;
import java.util.concurrent.RejectedExecutionException;
/**
* A thread managed by a {@link ForkJoinPool}, which executes
* {@link ForkJoinTask}s.
@ -54,238 +51,20 @@ import java.util.concurrent.RejectedExecutionException;
*/
public class ForkJoinWorkerThread extends Thread {
/*
* Overview:
*
* ForkJoinWorkerThreads are managed by ForkJoinPools and perform
* ForkJoinTasks. This class includes bookkeeping in support of
* worker activation, suspension, and lifecycle control described
* in more detail in the internal documentation of class
* ForkJoinPool. And as described further below, this class also
* includes special-cased support for some ForkJoinTask
* methods. But the main mechanics involve work-stealing:
* ForkJoinTasks. For explanation, see the internal documentation
* of class ForkJoinPool.
*
* Work-stealing queues are special forms of Deques that support
* only three of the four possible end-operations -- push, pop,
* and deq (aka steal), under the further constraints that push
* and pop are called only from the owning thread, while deq may
* be called from other threads. (If you are unfamiliar with
* them, you probably want to read Herlihy and Shavit's book "The
* Art of Multiprocessor programming", chapter 16 describing these
* in more detail before proceeding.) The main work-stealing
* queue design is roughly similar to those in the papers "Dynamic
* Circular Work-Stealing Deque" by Chase and Lev, SPAA 2005
* (http://research.sun.com/scalable/pubs/index.html) and
* "Idempotent work stealing" by Michael, Saraswat, and Vechev,
* PPoPP 2009 (http://portal.acm.org/citation.cfm?id=1504186).
* The main differences ultimately stem from gc requirements that
* we null out taken slots as soon as we can, to maintain as small
* a footprint as possible even in programs generating huge
* numbers of tasks. To accomplish this, we shift the CAS
* arbitrating pop vs deq (steal) from being on the indices
* ("queueBase" and "queueTop") to the slots themselves (mainly
* via method "casSlotNull()"). So, both a successful pop and deq
* mainly entail a CAS of a slot from non-null to null. Because
* we rely on CASes of references, we do not need tag bits on
* queueBase or queueTop. They are simple ints as used in any
* circular array-based queue (see for example ArrayDeque).
* Updates to the indices must still be ordered in a way that
* guarantees that queueTop == queueBase means the queue is empty,
* but otherwise may err on the side of possibly making the queue
* appear nonempty when a push, pop, or deq have not fully
* committed. Note that this means that the deq operation,
* considered individually, is not wait-free. One thief cannot
* successfully continue until another in-progress one (or, if
* previously empty, a push) completes. However, in the
* aggregate, we ensure at least probabilistic non-blockingness.
* If an attempted steal fails, a thief always chooses a different
* random victim target to try next. So, in order for one thief to
* progress, it suffices for any in-progress deq or new push on
* any empty queue to complete.
*
* This approach also enables support for "async mode" where local
* task processing is in FIFO, not LIFO order; simply by using a
* version of deq rather than pop when locallyFifo is true (as set
* by the ForkJoinPool). This allows use in message-passing
* frameworks in which tasks are never joined. However neither
* mode considers 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.)
*
* When a worker would otherwise be blocked waiting to join a
* task, it first tries a form of linear helping: Each worker
* records (in field currentSteal) the most recent task it stole
* from some other worker. Plus, it records (in field currentJoin)
* the task it is currently actively joining. Method joinTask uses
* these markers to try to find a worker to help (i.e., steal back
* a task from and execute it) that could hasten completion of the
* actively joined task. In essence, the joiner executes a task
* that would be on its own local deque had the to-be-joined task
* not been stolen. This may be seen as a conservative variant of
* the approach in Wagner & Calder "Leapfrogging: a portable
* technique for implementing efficient futures" SIGPLAN Notices,
* 1993 (http://portal.acm.org/citation.cfm?id=155354). It differs
* in that: (1) We only maintain dependency links across workers
* upon steals, rather than use per-task bookkeeping. This may
* require a linear scan of workers array to locate stealers, but
* usually doesn't because stealers leave hints (that may become
* stale/wrong) of where to locate them. This isolates cost to
* when it is needed, rather than adding to per-task overhead.
* (2) It is "shallow", ignoring nesting and potentially cyclic
* mutual steals. (3) It is intentionally racy: field currentJoin
* is updated only while actively joining, which means that we
* miss links in the chain during long-lived tasks, GC stalls etc
* (which is OK since blocking in such cases is usually a good
* idea). (4) We bound the number of attempts to find work (see
* MAX_HELP) and fall back to suspending the worker and if
* necessary replacing it with another.
*
* Efficient implementation of these algorithms currently relies
* on an uncomfortable amount of "Unsafe" mechanics. To maintain
* correct orderings, reads and writes of variable queueBase
* require volatile ordering. Variable queueTop need not be
* volatile because non-local reads always follow those of
* queueBase. Similarly, because they are protected by volatile
* queueBase reads, reads of the queue array and its slots by
* other threads do not need volatile load semantics, but writes
* (in push) require store order and CASes (in pop and deq)
* require (volatile) CAS semantics. (Michael, Saraswat, and
* Vechev's algorithm has similar properties, but without support
* for nulling slots.) Since these combinations aren't supported
* using ordinary volatiles, the only way to accomplish these
* efficiently is to use direct Unsafe calls. (Using external
* AtomicIntegers and AtomicReferenceArrays for the indices and
* array is significantly slower because of memory locality and
* indirection effects.)
*
* Further, performance on most platforms is very sensitive to
* placement and sizing of the (resizable) queue array. Even
* though these queues don't usually become all that big, the
* initial size must be large enough to counteract cache
* contention effects across multiple queues (especially in the
* presence of GC cardmarking). Also, to improve thread-locality,
* queues are initialized after starting.
* This class just maintains links to its pool and WorkQueue. The
* pool field is set immediately upon construction, but the
* workQueue field is not set until a call to registerWorker
* completes. This leads to a visibility race, that is tolerated
* by requiring that the workQueue field is only accessed by the
* owning thread.
*/
/**
* Mask for pool indices encoded as shorts
*/
private static final int SMASK = 0xffff;
/**
* Capacity of work-stealing queue array upon initialization.
* Must be a power of two. Initial size must be at least 4, but is
* padded to minimize cache effects.
*/
private static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
/**
* Maximum size for queue array. Must be a power of two
* less than or equal to 1 << (31 - width of array entry) to
* ensure lack of index wraparound, but is capped at a lower
* value to help users trap runaway computations.
*/
private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
/**
* The work-stealing queue array. Size must be a power of two.
* Initialized when started (as opposed to when constructed), to
* improve memory locality.
*/
ForkJoinTask<?>[] queue;
/**
* The pool this thread works in. Accessed directly by ForkJoinTask.
*/
final ForkJoinPool pool;
/**
* Index (mod queue.length) of next queue slot to push to or pop
* from. It is written only by owner thread, and accessed by other
* threads only after reading (volatile) queueBase. Both queueTop
* and queueBase are allowed to wrap around on overflow, but
* (queueTop - queueBase) still estimates size.
*/
int queueTop;
/**
* Index (mod queue.length) of least valid queue slot, which is
* always the next position to steal from if nonempty.
*/
volatile int queueBase;
/**
* The index of most recent stealer, used as a hint to avoid
* traversal in method helpJoinTask. This is only a hint because a
* worker might have had multiple steals and this only holds one
* of them (usually the most current). Declared non-volatile,
* relying on other prevailing sync to keep reasonably current.
*/
int stealHint;
/**
* Index of this worker in pool array. Set once by pool before
* running, and accessed directly by pool to locate this worker in
* its workers array.
*/
final int poolIndex;
/**
* Encoded record for pool task waits. Usages are always
* surrounded by volatile reads/writes
*/
int nextWait;
/**
* Complement of poolIndex, offset by count of entries of task
* waits. Accessed by ForkJoinPool to manage event waiters.
*/
volatile int eventCount;
/**
* Seed for random number generator for choosing steal victims.
* Uses Marsaglia xorshift. Must be initialized as nonzero.
*/
int seed;
/**
* Number of steals. Directly accessed (and reset) by pool when
* idle.
*/
int stealCount;
/**
* True if this worker should or did terminate
*/
volatile boolean terminate;
/**
* Set to true before LockSupport.park; false on return
*/
volatile boolean parked;
/**
* True if use local fifo, not default lifo, for local polling.
* Shadows value from ForkJoinPool.
*/
final boolean locallyFifo;
/**
* The task most recently stolen from another worker (or
* submission queue). All uses are surrounded by enough volatile
* reads/writes to maintain as non-volatile.
*/
ForkJoinTask<?> currentSteal;
/**
* The task currently being joined, set only when actively trying
* to help other stealers in helpJoinTask. All uses are surrounded
* by enough volatile reads/writes to maintain as non-volatile.
*/
ForkJoinTask<?> currentJoin;
final ForkJoinPool pool; // the pool this thread works in
final ForkJoinPool.WorkQueue workQueue; // work-stealing mechanics
/**
* Creates a ForkJoinWorkerThread operating in the given pool.
@ -294,20 +73,12 @@ public class ForkJoinWorkerThread extends Thread {
* @throws NullPointerException if pool is null
*/
protected ForkJoinWorkerThread(ForkJoinPool pool) {
super(pool.nextWorkerName());
// Use a placeholder until a useful name can be set in registerWorker
super("aForkJoinWorkerThread");
this.pool = pool;
int k = pool.registerWorker(this);
poolIndex = k;
eventCount = ~k & SMASK; // clear wait count
locallyFifo = pool.locallyFifo;
Thread.UncaughtExceptionHandler ueh = pool.ueh;
if (ueh != null)
setUncaughtExceptionHandler(ueh);
setDaemon(true);
this.workQueue = pool.registerWorker(this);
}
// Public methods
/**
* Returns the pool hosting this thread.
*
@ -327,28 +98,9 @@ public class ForkJoinWorkerThread extends Thread {
* @return the index number
*/
public int getPoolIndex() {
return poolIndex;
return workQueue.poolIndex;
}
// Randomization
/**
* Computes next value for random victim probes and backoffs.
* Scans don't require a very high quality generator, but also not
* a crummy one. Marsaglia xor-shift is cheap and works well
* enough. Note: This is manually inlined in FJP.scan() to avoid
* writes inside busy loops.
*/
private int nextSeed() {
int r = seed;
r ^= r << 13;
r ^= r >>> 17;
r ^= r << 5;
return seed = r;
}
// Run State management
/**
* Initializes internal state after construction but before
* processing any tasks. If you override this method, you must
@ -359,9 +111,6 @@ public class ForkJoinWorkerThread extends Thread {
* processing tasks.
*/
protected void onStart() {
queue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
int r = ForkJoinPool.workerSeedGenerator.nextInt();
seed = (r == 0) ? 1 : r; // must be nonzero
}
/**
@ -373,17 +122,6 @@ public class ForkJoinWorkerThread extends Thread {
* to an unrecoverable error, or {@code null} if completed normally
*/
protected void onTermination(Throwable exception) {
try {
terminate = true;
cancelTasks();
pool.deregisterWorker(this, exception);
} catch (Throwable ex) { // Shouldn't ever happen
if (exception == null) // but if so, at least rethrown
exception = ex;
} finally {
if (exception != null)
UNSAFE.throwException(exception);
}
}
/**
@ -395,604 +133,18 @@ public class ForkJoinWorkerThread extends Thread {
Throwable exception = null;
try {
onStart();
pool.work(this);
pool.runWorker(workQueue);
} catch (Throwable ex) {
exception = ex;
} finally {
onTermination(exception);
}
}
/*
* Intrinsics-based atomic writes for queue slots. These are
* basically the same as methods in AtomicReferenceArray, but
* specialized for (1) ForkJoinTask elements (2) requirement that
* nullness and bounds checks have already been performed by
* callers and (3) effective offsets are known not to overflow
* from int to long (because of MAXIMUM_QUEUE_CAPACITY). We don't
* need corresponding version for reads: plain array reads are OK
* because they are protected by other volatile reads and are
* confirmed by CASes.
*
* Most uses don't actually call these methods, but instead
* contain inlined forms that enable more predictable
* optimization. We don't define the version of write used in
* pushTask at all, but instead inline there a store-fenced array
* slot write.
*
* Also in most methods, as a performance (not correctness) issue,
* we'd like to encourage compilers not to arbitrarily postpone
* setting queueTop after writing slot. Currently there is no
* intrinsic for arranging this, but using Unsafe putOrderedInt
* may be a preferable strategy on some compilers even though its
* main effect is a pre-, not post- fence. To simplify possible
* changes, the option is left in comments next to the associated
* assignments.
*/
/**
* CASes slot i of array q from t to null. Caller must ensure q is
* non-null and index is in range.
*/
private static final boolean casSlotNull(ForkJoinTask<?>[] q, int i,
ForkJoinTask<?> t) {
return UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null);
}
/**
* Performs a volatile write of the given task at given slot of
* array q. Caller must ensure q is non-null and index is in
* range. This method is used only during resets and backouts.
*/
private static final void writeSlot(ForkJoinTask<?>[] q, int i,
ForkJoinTask<?> t) {
UNSAFE.putObjectVolatile(q, (i << ASHIFT) + ABASE, t);
}
// queue methods
/**
* Pushes a task. Call only from this thread.
*
* @param t the task. Caller must ensure non-null.
*/
final void pushTask(ForkJoinTask<?> t) {
ForkJoinTask<?>[] q; int s, m;
if ((q = queue) != null) { // ignore if queue removed
long u = (((s = queueTop) & (m = q.length - 1)) << ASHIFT) + ABASE;
UNSAFE.putOrderedObject(q, u, t);
queueTop = s + 1; // or use putOrderedInt
if ((s -= queueBase) <= 2)
pool.signalWork();
else if (s == m)
growQueue();
}
}
/**
* Creates or doubles queue array. Transfers elements by
* emulating steals (deqs) from old array and placing, oldest
* first, into new array.
*/
private void growQueue() {
ForkJoinTask<?>[] oldQ = queue;
int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
if (size > MAXIMUM_QUEUE_CAPACITY)
throw new RejectedExecutionException("Queue capacity exceeded");
if (size < INITIAL_QUEUE_CAPACITY)
size = INITIAL_QUEUE_CAPACITY;
ForkJoinTask<?>[] q = queue = new ForkJoinTask<?>[size];
int mask = size - 1;
int top = queueTop;
int oldMask;
if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
for (int b = queueBase; b != top; ++b) {
long u = ((b & oldMask) << ASHIFT) + ABASE;
Object x = UNSAFE.getObjectVolatile(oldQ, u);
if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
UNSAFE.putObjectVolatile
(q, ((b & mask) << ASHIFT) + ABASE, x);
try {
onTermination(exception);
} catch (Throwable ex) {
if (exception == null)
exception = ex;
} finally {
pool.deregisterWorker(this, exception);
}
}
}
/**
* Tries to take a task from the base of the queue, failing if
* empty or contended. Note: Specializations of this code appear
* in locallyDeqTask and elsewhere.
*
* @return a task, or null if none or contended
*/
final ForkJoinTask<?> deqTask() {
ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
if (queueTop != (b = queueBase) &&
(q = queue) != null && // must read q after b
(i = (q.length - 1) & b) >= 0 &&
(t = q[i]) != null && queueBase == b &&
UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE, t, null)) {
queueBase = b + 1;
return t;
}
return null;
}
/**
* Tries to take a task from the base of own queue. Called only
* by this thread.
*
* @return a task, or null if none
*/
final ForkJoinTask<?> locallyDeqTask() {
ForkJoinTask<?> t; int m, b, i;
ForkJoinTask<?>[] q = queue;
if (q != null && (m = q.length - 1) >= 0) {
while (queueTop != (b = queueBase)) {
if ((t = q[i = m & b]) != null &&
queueBase == b &&
UNSAFE.compareAndSwapObject(q, (i << ASHIFT) + ABASE,
t, null)) {
queueBase = b + 1;
return t;
}
}
}
return null;
}
/**
* Returns a popped task, or null if empty.
* Called only by this thread.
*/
private ForkJoinTask<?> popTask() {
int m;
ForkJoinTask<?>[] q = queue;
if (q != null && (m = q.length - 1) >= 0) {
for (int s; (s = queueTop) != queueBase;) {
int i = m & --s;
long u = (i << ASHIFT) + ABASE; // raw offset
ForkJoinTask<?> t = q[i];
if (t == null) // lost to stealer
break;
if (UNSAFE.compareAndSwapObject(q, u, t, null)) {
queueTop = s; // or putOrderedInt
return t;
}
}
}
return null;
}
/**
* Specialized version of popTask to pop only if topmost element
* is the given task. Called only by this thread.
*
* @param t the task. Caller must ensure non-null.
*/
final boolean unpushTask(ForkJoinTask<?> t) {
ForkJoinTask<?>[] q;
int s;
if ((q = queue) != null && (s = queueTop) != queueBase &&
UNSAFE.compareAndSwapObject
(q, (((q.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
queueTop = s; // or putOrderedInt
return true;
}
return false;
}
/**
* Returns next task, or null if empty or contended.
*/
final ForkJoinTask<?> peekTask() {
int m;
ForkJoinTask<?>[] q = queue;
if (q == null || (m = q.length - 1) < 0)
return null;
int i = locallyFifo ? queueBase : (queueTop - 1);
return q[i & m];
}
// Support methods for ForkJoinPool
/**
* Runs the given task, plus any local tasks until queue is empty
*/
final void execTask(ForkJoinTask<?> t) {
currentSteal = t;
for (;;) {
if (t != null)
t.doExec();
if (queueTop == queueBase)
break;
t = locallyFifo ? locallyDeqTask() : popTask();
}
++stealCount;
currentSteal = null;
}
/**
* Removes and cancels all tasks in queue. Can be called from any
* thread.
*/
final void cancelTasks() {
ForkJoinTask<?> cj = currentJoin; // try to cancel ongoing tasks
if (cj != null && cj.status >= 0)
cj.cancelIgnoringExceptions();
ForkJoinTask<?> cs = currentSteal;
if (cs != null && cs.status >= 0)
cs.cancelIgnoringExceptions();
while (queueBase != queueTop) {
ForkJoinTask<?> t = deqTask();
if (t != null)
t.cancelIgnoringExceptions();
}
}
/**
* Drains tasks to given collection c.
*
* @return the number of tasks drained
*/
final int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
int n = 0;
while (queueBase != queueTop) {
ForkJoinTask<?> t = deqTask();
if (t != null) {
c.add(t);
++n;
}
}
return n;
}
// Support methods for ForkJoinTask
/**
* Returns an estimate of the number of tasks in the queue.
*/
final int getQueueSize() {
return queueTop - queueBase;
}
/**
* Gets and removes a local task.
*
* @return a task, if available
*/
final ForkJoinTask<?> pollLocalTask() {
return locallyFifo ? locallyDeqTask() : popTask();
}
/**
* Gets and removes a local or stolen task.
*
* @return a task, if available
*/
final ForkJoinTask<?> pollTask() {
ForkJoinWorkerThread[] ws;
ForkJoinTask<?> t = pollLocalTask();
if (t != null || (ws = pool.workers) == null)
return t;
int n = ws.length; // cheap version of FJP.scan
int steps = n << 1;
int r = nextSeed();
int i = 0;
while (i < steps) {
ForkJoinWorkerThread w = ws[(i++ + r) & (n - 1)];
if (w != null && w.queueBase != w.queueTop && w.queue != null) {
if ((t = w.deqTask()) != null)
return t;
i = 0;
}
}
return null;
}
/**
* The maximum stolen->joining link depth allowed in helpJoinTask,
* as well as the maximum number of retries (allowing on average
* one staleness retry per level) per attempt to instead try
* compensation. Depths for legitimate chains are unbounded, but
* we use a fixed constant to avoid (otherwise unchecked) cycles
* and bound staleness of traversal parameters at the expense of
* sometimes blocking when we could be helping.
*/
private static final int MAX_HELP = 16;
/**
* Possibly runs some tasks and/or blocks, until joinMe is done.
*
* @param joinMe the task to join
* @return completion status on exit
*/
final int joinTask(ForkJoinTask<?> joinMe) {
ForkJoinTask<?> prevJoin = currentJoin;
currentJoin = joinMe;
for (int s, retries = MAX_HELP;;) {
if ((s = joinMe.status) < 0) {
currentJoin = prevJoin;
return s;
}
if (retries > 0) {
if (queueTop != queueBase) {
if (!localHelpJoinTask(joinMe))
retries = 0; // cannot help
}
else if (retries == MAX_HELP >>> 1) {
--retries; // check uncommon case
if (tryDeqAndExec(joinMe) >= 0)
Thread.yield(); // for politeness
}
else
retries = helpJoinTask(joinMe) ? MAX_HELP : retries - 1;
}
else {
retries = MAX_HELP; // restart if not done
pool.tryAwaitJoin(joinMe);
}
}
}
/**
* If present, pops and executes the given task, or any other
* cancelled task
*
* @return false if any other non-cancelled task exists in local queue
*/
private boolean localHelpJoinTask(ForkJoinTask<?> joinMe) {
int s, i; ForkJoinTask<?>[] q; ForkJoinTask<?> t;
if ((s = queueTop) != queueBase && (q = queue) != null &&
(i = (q.length - 1) & --s) >= 0 &&
(t = q[i]) != null) {
if (t != joinMe && t.status >= 0)
return false;
if (UNSAFE.compareAndSwapObject
(q, (i << ASHIFT) + ABASE, t, null)) {
queueTop = s; // or putOrderedInt
t.doExec();
}
}
return true;
}
/**
* Tries to locate and execute tasks for a stealer of the given
* task, or in turn one of its stealers, Traces
* currentSteal->currentJoin links looking for a thread working on
* a descendant of the given task and with a non-empty queue to
* steal back and execute tasks from. The implementation is very
* branchy to cope with potential inconsistencies or loops
* encountering chains that are stale, unknown, or of length
* greater than MAX_HELP links. All of these cases are dealt with
* by just retrying by caller.
*
* @param joinMe the task to join
* @param canSteal true if local queue is empty
* @return true if ran a task
*/
private boolean helpJoinTask(ForkJoinTask<?> joinMe) {
boolean helped = false;
int m = pool.scanGuard & SMASK;
ForkJoinWorkerThread[] ws = pool.workers;
if (ws != null && ws.length > m && joinMe.status >= 0) {
int levels = MAX_HELP; // remaining chain length
ForkJoinTask<?> task = joinMe; // base of chain
outer:for (ForkJoinWorkerThread thread = this;;) {
// Try to find v, the stealer of task, by first using hint
ForkJoinWorkerThread v = ws[thread.stealHint & m];
if (v == null || v.currentSteal != task) {
for (int j = 0; ;) { // search array
if ((v = ws[j]) != null && v.currentSteal == task) {
thread.stealHint = j;
break; // save hint for next time
}
if (++j > m)
break outer; // can't find stealer
}
}
// Try to help v, using specialized form of deqTask
for (;;) {
ForkJoinTask<?>[] q; int b, i;
if (joinMe.status < 0)
break outer;
if ((b = v.queueBase) == v.queueTop ||
(q = v.queue) == null ||
(i = (q.length-1) & b) < 0)
break; // empty
long u = (i << ASHIFT) + ABASE;
ForkJoinTask<?> t = q[i];
if (task.status < 0)
break outer; // stale
if (t != null && v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
v.queueBase = b + 1;
v.stealHint = poolIndex;
ForkJoinTask<?> ps = currentSteal;
currentSteal = t;
t.doExec();
currentSteal = ps;
helped = true;
}
}
// Try to descend to find v's stealer
ForkJoinTask<?> next = v.currentJoin;
if (--levels > 0 && task.status >= 0 &&
next != null && next != task) {
task = next;
thread = v;
}
else
break; // max levels, stale, dead-end, or cyclic
}
}
return helped;
}
/**
* Performs an uncommon case for joinTask: If task t is at base of
* some workers queue, steals and executes it.
*
* @param t the task
* @return t's status
*/
private int tryDeqAndExec(ForkJoinTask<?> t) {
int m = pool.scanGuard & SMASK;
ForkJoinWorkerThread[] ws = pool.workers;
if (ws != null && ws.length > m && t.status >= 0) {
for (int j = 0; j <= m; ++j) {
ForkJoinTask<?>[] q; int b, i;
ForkJoinWorkerThread v = ws[j];
if (v != null &&
(b = v.queueBase) != v.queueTop &&
(q = v.queue) != null &&
(i = (q.length - 1) & b) >= 0 &&
q[i] == t) {
long u = (i << ASHIFT) + ABASE;
if (v.queueBase == b &&
UNSAFE.compareAndSwapObject(q, u, t, null)) {
v.queueBase = b + 1;
v.stealHint = poolIndex;
ForkJoinTask<?> ps = currentSteal;
currentSteal = t;
t.doExec();
currentSteal = ps;
}
break;
}
}
}
return t.status;
}
/**
* Implements ForkJoinTask.getSurplusQueuedTaskCount(). Returns
* an estimate of the number of tasks, offset by a function of
* number of idle workers.
*
* This method provides a cheap heuristic guide for task
* partitioning when programmers, frameworks, tools, or languages
* have little or no idea about task granularity. In essence by
* offering this method, we ask users only about tradeoffs in
* overhead vs expected throughput and its variance, rather than
* how finely to partition tasks.
*
* In a steady state strict (tree-structured) computation, each
* thread makes available for stealing enough tasks for other
* threads to remain active. Inductively, if all threads play by
* the same rules, each thread should make available only a
* constant number of tasks.
*
* The minimum useful constant is just 1. But using a value of 1
* would require immediate replenishment upon each steal to
* maintain enough tasks, which is infeasible. Further,
* partitionings/granularities of offered tasks should minimize
* steal rates, which in general means that threads nearer the top
* of computation tree should generate more than those nearer the
* bottom. In perfect steady state, each thread is at
* approximately the same level of computation tree. However,
* producing extra tasks amortizes the uncertainty of progress and
* diffusion assumptions.
*
* So, users will want to use values larger, but not much larger
* than 1 to both smooth over transient shortages and hedge
* against uneven progress; as traded off against the cost of
* extra task overhead. We leave the user to pick a threshold
* value to compare with the results of this call to guide
* decisions, but recommend values such as 3.
*
* When all threads are active, it is on average OK to estimate
* surplus strictly locally. In steady-state, if one thread is
* maintaining say 2 surplus tasks, then so are others. So we can
* just use estimated queue length (although note that (queueTop -
* queueBase) can be an overestimate because of stealers lagging
* increments of queueBase). However, this strategy alone leads
* to serious mis-estimates in some non-steady-state conditions
* (ramp-up, ramp-down, other stalls). We can detect many of these
* by further considering the number of "idle" threads, that are
* known to have zero queued tasks, so compensate by a factor of
* (#idle/#active) threads.
*/
final int getEstimatedSurplusTaskCount() {
return queueTop - queueBase - pool.idlePerActive();
}
/**
* Runs tasks until {@code pool.isQuiescent()}. We piggyback on
* pool's active count ctl maintenance, but rather than blocking
* when tasks cannot be found, we rescan until all others cannot
* find tasks either. The bracketing by pool quiescerCounts
* updates suppresses pool auto-shutdown mechanics that could
* otherwise prematurely terminate the pool because all threads
* appear to be inactive.
*/
final void helpQuiescePool() {
boolean active = true;
ForkJoinTask<?> ps = currentSteal; // to restore below
ForkJoinPool p = pool;
p.addQuiescerCount(1);
for (;;) {
ForkJoinWorkerThread[] ws = p.workers;
ForkJoinWorkerThread v = null;
int n;
if (queueTop != queueBase)
v = this;
else if (ws != null && (n = ws.length) > 1) {
ForkJoinWorkerThread w;
int r = nextSeed(); // cheap version of FJP.scan
int steps = n << 1;
for (int i = 0; i < steps; ++i) {
if ((w = ws[(i + r) & (n - 1)]) != null &&
w.queueBase != w.queueTop) {
v = w;
break;
}
}
}
if (v != null) {
ForkJoinTask<?> t;
if (!active) {
active = true;
p.addActiveCount(1);
}
if ((t = (v != this) ? v.deqTask() :
locallyFifo ? locallyDeqTask() : popTask()) != null) {
currentSteal = t;
t.doExec();
currentSteal = ps;
}
}
else {
if (active) {
active = false;
p.addActiveCount(-1);
}
if (p.isQuiescent()) {
p.addActiveCount(1);
p.addQuiescerCount(-1);
break;
}
}
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long ABASE;
private static final int ASHIFT;
static {
int s;
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> a = ForkJoinTask[].class;
ABASE = UNSAFE.arrayBaseOffset(a);
s = UNSAFE.arrayIndexScale(a);
} catch (Exception e) {
throw new Error(e);
}
if ((s & (s-1)) != 0)
throw new Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
}
}