--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/java/util/concurrent/CountedCompleter.java Thu Dec 20 13:44:06 2012 +0000
@@ -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);
+ }
+ }
+}
--- a/jdk/src/share/classes/java/util/concurrent/ForkJoinPool.java Thu Dec 20 17:24:56 2012 +0400
+++ b/jdk/src/share/classes/java/util/concurrent/ForkJoinPool.java Thu Dec 20 13:44:06 2012 +0000
@@ -40,7 +40,6 @@
import java.util.Collection;
import java.util.Collections;
import java.util.List;
-import java.util.Random;
import java.util.concurrent.AbstractExecutorService;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
@@ -48,11 +47,6 @@
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
import java.util.concurrent.TimeUnit;
-import java.util.concurrent.TimeoutException;
-import java.util.concurrent.atomic.AtomicInteger;
-import java.util.concurrent.locks.LockSupport;
-import java.util.concurrent.locks.ReentrantLock;
-import java.util.concurrent.locks.Condition;
/**
* An {@link ExecutorService} for running {@link ForkJoinTask}s.
@@ -63,21 +57,31 @@
* <p>A {@code ForkJoinPool} differs from other kinds of {@link
* ExecutorService} mainly by virtue of employing
* <em>work-stealing</em>: all threads in the pool attempt to find and
- * execute subtasks created by other active tasks (eventually blocking
- * waiting for work if none exist). This enables efficient processing
- * when most tasks spawn other subtasks (as do most {@code
- * ForkJoinTask}s). When setting <em>asyncMode</em> to true in
- * constructors, {@code ForkJoinPool}s may also be appropriate for use
- * with event-style tasks that are never joined.
+ * execute tasks submitted to the pool and/or 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), as well as when many small
+ * tasks are submitted to the pool from external clients. Especially
+ * when setting <em>asyncMode</em> to true in constructors, {@code
+ * ForkJoinPool}s may also be appropriate for use with event-style
+ * tasks that are never joined.
*
- * <p>A {@code ForkJoinPool} is constructed with a given target
- * parallelism level; by default, equal to the number of available
- * processors. The pool attempts to maintain enough active (or
- * available) threads by dynamically adding, suspending, or resuming
- * internal worker threads, even if some tasks are stalled waiting to
- * join others. However, no such adjustments are guaranteed in the
- * face of blocked IO or other unmanaged synchronization. The nested
- * {@link ManagedBlocker} interface enables extension of the kinds of
+ * <p>A static {@link #commonPool()} is available and appropriate for
+ * most applications. The common pool is used by any ForkJoinTask that
+ * is not explicitly submitted to a specified pool. Using the common
+ * pool normally reduces resource usage (its threads are slowly
+ * reclaimed during periods of non-use, and reinstated upon subsequent
+ * use).
+ *
+ * <p>For applications that require separate or custom pools, a {@code
+ * ForkJoinPool} may be constructed with a given target parallelism
+ * level; by default, equal to the number of available processors. The
+ * pool attempts to maintain enough active (or available) threads by
+ * dynamically adding, suspending, or resuming internal worker
+ * threads, even if some tasks are stalled waiting to join
+ * others. However, no such adjustments are guaranteed in the face of
+ * blocked I/O or other unmanaged synchronization. The nested {@link
+ * ManagedBlocker} interface enables extension of the kinds of
* synchronization accommodated.
*
* <p>In addition to execution and lifecycle control methods, this
@@ -87,16 +91,17 @@
* {@link #toString} returns indications of pool state in a
* convenient form for informal monitoring.
*
- * <p> As is the case with other ExecutorServices, there are three
- * main task execution methods summarized in the following
- * table. These are designed to be used by clients not already engaged
- * in fork/join computations in the current pool. The main forms of
- * these methods accept instances of {@code ForkJoinTask}, but
- * overloaded forms also allow mixed execution of plain {@code
+ * <p>As is the case with other ExecutorServices, there are three
+ * main task execution methods summarized in the following table.
+ * These are designed to be used primarily by clients not already
+ * engaged in fork/join computations in the current pool. The main
+ * forms of these methods accept instances of {@code ForkJoinTask},
+ * but overloaded forms also allow mixed execution of plain {@code
* Runnable}- or {@code Callable}- based activities as well. However,
- * tasks that are already executing in a pool should normally
- * <em>NOT</em> use these pool execution methods, but instead use the
- * within-computation forms listed in the table.
+ * tasks that are already executing in a pool should normally instead
+ * use the within-computation forms listed in the table unless using
+ * async event-style tasks that are not usually joined, in which case
+ * there is little difference among choice of methods.
*
* <table BORDER CELLPADDING=3 CELLSPACING=1>
* <tr>
@@ -121,23 +126,16 @@
* </tr>
* </table>
*
- * <p><b>Sample Usage.</b> Normally a single {@code ForkJoinPool} is
- * used for all parallel task execution in a program or subsystem.
- * Otherwise, use would not usually outweigh the construction and
- * bookkeeping overhead of creating a large set of threads. For
- * example, a common pool could be used for the {@code SortTasks}
- * illustrated in {@link RecursiveAction}. Because {@code
- * ForkJoinPool} uses threads in {@linkplain java.lang.Thread#isDaemon
- * daemon} mode, there is typically no need to explicitly {@link
- * #shutdown} such a pool upon program exit.
- *
- * <pre>
- * static final ForkJoinPool mainPool = new ForkJoinPool();
- * ...
- * public void sort(long[] array) {
- * mainPool.invoke(new SortTask(array, 0, array.length));
- * }
- * </pre>
+ * <p>The common pool is by default constructed with default
+ * parameters, but these may be controlled by setting three {@link
+ * System#getProperty system properties} with prefix {@code
+ * java.util.concurrent.ForkJoinPool.common}: {@code parallelism} --
+ * an integer greater than zero, {@code threadFactory} -- the class
+ * name of a {@link ForkJoinWorkerThreadFactory}, and {@code
+ * exceptionHandler} -- the class name of a {@link
+ * java.lang.Thread.UncaughtExceptionHandler
+ * Thread.UncaughtExceptionHandler}. Upon any error in establishing
+ * these settings, default parameters are used.
*
* <p><b>Implementation notes</b>: This implementation restricts the
* maximum number of running threads to 32767. Attempts to create
@@ -156,214 +154,388 @@
/*
* Implementation Overview
*
- * This class provides the central bookkeeping and control for a
- * set of worker threads: Submissions from non-FJ threads enter
- * into a submission queue. Workers take these tasks and typically
- * split them into subtasks that may be stolen by other workers.
- * Preference rules give first priority to processing tasks from
- * their own queues (LIFO or FIFO, depending on mode), then to
- * randomized FIFO steals of tasks in other worker queues, and
- * lastly to new submissions.
+ * This class and its nested classes provide the main
+ * functionality and control for a set of worker threads:
+ * Submissions from non-FJ threads enter into submission queues.
+ * Workers take these tasks and typically split them into subtasks
+ * that may be stolen by other workers. Preference rules give
+ * first priority to processing tasks from their own queues (LIFO
+ * or FIFO, depending on mode), then to randomized FIFO steals of
+ * tasks in other queues.
+ *
+ * WorkQueues
+ * ==========
+ *
+ * Most operations occur within work-stealing queues (in nested
+ * class WorkQueue). These are special forms of Deques that
+ * support only three of the four possible end-operations -- push,
+ * pop, and poll (aka steal), under the further constraints that
+ * push and pop are called only from the owning thread (or, as
+ * extended here, under a lock), while poll 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 poll (steal) from being on the indices
+ * ("base" and "top") to the slots themselves. So, both a
+ * successful pop and poll 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 base or top. 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 top == base means the queue is empty,
+ * but otherwise may err on the side of possibly making the queue
+ * appear nonempty when a push, pop, or poll have not fully
+ * committed. Note that this means that the poll 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 poll or new push on
+ * any empty queue to complete. (This is why we normally use
+ * method pollAt and its variants that try once at the apparent
+ * base index, else consider alternative actions, rather than
+ * method poll.)
+ *
+ * This approach also enables support of a user mode in which local
+ * task processing is in FIFO, not LIFO order, simply by using
+ * poll rather than pop. This can be useful 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.)
+ *
+ * WorkQueues are also used in a similar way for tasks submitted
+ * to the pool. We cannot mix these tasks in the same queues used
+ * for work-stealing (this would contaminate lifo/fifo
+ * processing). Instead, we randomly associate submission queues
+ * with submitting threads, using a form of hashing. The
+ * ThreadLocal Submitter class contains a value initially used as
+ * a hash code for choosing existing queues, but may be randomly
+ * repositioned upon contention with other submitters. In
+ * essence, submitters act like workers except that they are
+ * restricted to executing local tasks that they submitted (or in
+ * the case of CountedCompleters, others with the same root task).
+ * However, because most shared/external queue operations are more
+ * expensive than internal, and because, at steady state, external
+ * submitters will compete for CPU with workers, ForkJoinTask.join
+ * and related methods disable them from repeatedly helping to
+ * process tasks if all workers are active. Insertion of tasks in
+ * shared mode requires a lock (mainly to protect in the case of
+ * resizing) but we use only a simple spinlock (using bits in
+ * field qlock), because submitters encountering a busy queue move
+ * on to try or create other queues -- they block only when
+ * creating and registering new queues.
+ *
+ * Management
+ * ==========
*
* The main throughput advantages of work-stealing stem from
* decentralized control -- workers mostly take tasks from
* themselves or each other. We cannot negate this in the
* implementation of other management responsibilities. The main
* tactic for avoiding bottlenecks is packing nearly all
- * essentially atomic control state into a single 64bit volatile
- * variable ("ctl"). This variable is read on the order of 10-100
- * times as often as it is modified (always via CAS). (There is
- * some additional control state, for example variable "shutdown"
- * for which we can cope with uncoordinated updates.) This
- * streamlines synchronization and control at the expense of messy
- * constructions needed to repack status bits upon updates.
- * Updates tend not to contend with each other except during
- * bursts while submitted tasks begin or end. In some cases when
- * they do contend, threads can instead do something else
- * (usually, scan for tasks) until contention subsides.
+ * essentially atomic control state into two volatile variables
+ * that are by far most often read (not written) as status and
+ * consistency checks.
*
- * To enable packing, we restrict maximum parallelism to (1<<15)-1
- * (which is far in excess of normal operating range) to allow
- * ids, counts, and their negations (used for thresholding) to fit
- * into 16bit fields.
+ * Field "ctl" contains 64 bits holding all the information needed
+ * to atomically decide to add, inactivate, enqueue (on an event
+ * queue), dequeue, and/or re-activate workers. To enable this
+ * packing, we restrict maximum parallelism to (1<<15)-1 (which is
+ * far in excess of normal operating range) to allow ids, counts,
+ * and their negations (used for thresholding) to fit into 16bit
+ * fields.
+ *
+ * Field "plock" is a form of sequence lock with a saturating
+ * shutdown bit (similarly for per-queue "qlocks"), mainly
+ * protecting updates to the workQueues array, as well as to
+ * enable shutdown. When used as a lock, it is normally only very
+ * briefly held, so is nearly always available after at most a
+ * brief spin, but we use a monitor-based backup strategy to
+ * block when needed.
*
- * Recording Workers. Workers are recorded in the "workers" array
- * that is created upon pool construction and expanded if (rarely)
- * necessary. This is an array as opposed to some other data
- * structure to support index-based random steals by workers.
- * Updates to the array recording new workers and unrecording
- * terminated ones are protected from each other by a seqLock
- * (scanGuard) but the array is otherwise concurrently readable,
- * and accessed directly by workers. To simplify index-based
- * operations, the array size is always a power of two, and all
- * readers must tolerate null slots. To avoid flailing during
- * start-up, the array is presized to hold twice #parallelism
- * workers (which is unlikely to need further resizing during
- * execution). But to avoid dealing with so many null slots,
- * variable scanGuard includes a mask for the nearest power of two
- * that contains all current workers. All worker thread creation
- * is on-demand, triggered by task submissions, replacement of
- * terminated workers, and/or compensation for blocked
- * workers. However, all other support code is set up to work with
- * other policies. To ensure that we do not hold on to worker
- * references that would prevent GC, ALL accesses to workers are
- * via indices into the workers array (which is one source of some
- * of the messy code constructions here). In essence, the workers
- * array serves as a weak reference mechanism. Thus for example
- * the wait queue field of ctl stores worker indices, not worker
- * references. Access to the workers in associated methods (for
- * example signalWork) must both index-check and null-check the
- * IDs. All such accesses ignore bad IDs by returning out early
- * from what they are doing, since this can only be associated
- * with termination, in which case it is OK to give up.
+ * Recording WorkQueues. WorkQueues are recorded in the
+ * "workQueues" array that is created upon first use and expanded
+ * if necessary. Updates to the array while recording new workers
+ * and unrecording terminated ones are protected from each other
+ * by a lock but the array is otherwise concurrently readable, and
+ * accessed directly. To simplify index-based operations, the
+ * array size is always a power of two, and all readers must
+ * tolerate null slots. Worker queues are at odd indices. Shared
+ * (submission) queues are at even indices, up to a maximum of 64
+ * slots, to limit growth even if array needs to expand to add
+ * more workers. Grouping them together in this way simplifies and
+ * speeds up task scanning.
*
- * All uses of the workers array, as well as queue arrays, check
- * that the array is non-null (even if previously non-null). This
- * allows nulling during termination, which is currently not
- * necessary, but remains an option for resource-revocation-based
- * shutdown schemes.
+ * All worker thread creation is on-demand, triggered by task
+ * submissions, replacement of terminated workers, and/or
+ * compensation for blocked workers. However, all other support
+ * code is set up to work with other policies. To ensure that we
+ * do not hold on to worker references that would prevent GC, ALL
+ * accesses to workQueues are via indices into the workQueues
+ * array (which is one source of some of the messy code
+ * constructions here). In essence, the workQueues array serves as
+ * a weak reference mechanism. Thus for example the wait queue
+ * field of ctl stores indices, not references. Access to the
+ * workQueues in associated methods (for example signalWork) must
+ * both index-check and null-check the IDs. All such accesses
+ * ignore bad IDs by returning out early from what they are doing,
+ * since this can only be associated with termination, in which
+ * case it is OK to give up. All uses of the workQueues array
+ * also check that it is non-null (even if previously
+ * non-null). This allows nulling during termination, which is
+ * currently not necessary, but remains an option for
+ * resource-revocation-based shutdown schemes. It also helps
+ * reduce JIT issuance of uncommon-trap code, which tends to
+ * unnecessarily complicate control flow in some methods.
*
- * Wait Queuing. Unlike HPC work-stealing frameworks, we cannot
+ * Event Queuing. Unlike HPC work-stealing frameworks, we cannot
* let workers spin indefinitely scanning for tasks when none can
* be found immediately, and we cannot start/resume workers unless
* there appear to be tasks available. On the other hand, we must
* quickly prod them into action when new tasks are submitted or
- * generated. We park/unpark workers after placing in an event
- * wait queue when they cannot find work. This "queue" is actually
- * a simple Treiber stack, headed by the "id" field of ctl, plus a
- * 15bit counter value to both wake up waiters (by advancing their
- * count) and avoid ABA effects. Successors are held in worker
- * field "nextWait". Queuing deals with several intrinsic races,
- * mainly that a task-producing thread can miss seeing (and
+ * generated. In many usages, ramp-up time to activate workers is
+ * the main limiting factor in overall performance (this is
+ * compounded at program start-up by JIT compilation and
+ * allocation). So we try to streamline this as much as possible.
+ * We park/unpark workers after placing in an event wait queue
+ * when they cannot find work. This "queue" is actually a simple
+ * Treiber stack, headed by the "id" field of ctl, plus a 15bit
+ * counter value (that reflects the number of times a worker has
+ * been inactivated) to avoid ABA effects (we need only as many
+ * version numbers as worker threads). Successors are held in
+ * field WorkQueue.nextWait. Queuing deals with several intrinsic
+ * races, mainly that a task-producing thread can miss seeing (and
* signalling) another thread that gave up looking for work but
* has not yet entered the wait queue. We solve this by requiring
- * a full sweep of all workers both before (in scan()) and after
- * (in tryAwaitWork()) a newly waiting worker is added to the wait
- * queue. During a rescan, the worker might release some other
- * queued worker rather than itself, which has the same net
- * effect. Because enqueued workers may actually be rescanning
- * rather than waiting, we set and clear the "parked" field of
- * ForkJoinWorkerThread to reduce unnecessary calls to unpark.
- * (Use of the parked field requires a secondary recheck to avoid
- * missed signals.)
+ * a full sweep of all workers (via repeated calls to method
+ * scan()) both before and after a newly waiting worker is added
+ * to the wait queue. During a rescan, the worker might release
+ * some other queued worker rather than itself, which has the same
+ * net effect. Because enqueued workers may actually be rescanning
+ * rather than waiting, we set and clear the "parker" field of
+ * WorkQueues to reduce unnecessary calls to unpark. (This
+ * requires a secondary recheck to avoid missed signals.) Note
+ * the unusual conventions about Thread.interrupts surrounding
+ * parking and other blocking: Because interrupts are used solely
+ * to alert threads to check termination, which is checked anyway
+ * upon blocking, we clear status (using Thread.interrupted)
+ * before any call to park, so that park does not immediately
+ * return due to status being set via some other unrelated call to
+ * interrupt in user code.
*
* Signalling. We create or wake up workers only when there
* appears to be at least one task they might be able to find and
- * execute. When a submission is added or another worker adds a
- * task to a queue that previously had two or fewer tasks, they
- * signal waiting workers (or trigger creation of new ones if
- * fewer than the given parallelism level -- see signalWork).
- * These primary signals are buttressed by signals during rescans
- * as well as those performed when a worker steals a task and
- * notices that there are more tasks too; together these cover the
- * signals needed in cases when more than two tasks are pushed
- * but untaken.
+ * execute. However, many other threads may notice the same task
+ * and each signal to wake up a thread that might take it. So in
+ * general, pools will be over-signalled. When a submission is
+ * added or another worker adds a task to a queue that has fewer
+ * than two tasks, they signal waiting workers (or trigger
+ * creation of new ones if fewer than the given parallelism level
+ * -- signalWork), and may leave a hint to the unparked worker to
+ * help signal others upon wakeup). These primary signals are
+ * buttressed by others (see method helpSignal) whenever other
+ * threads scan for work or do not have a task to process. On
+ * most platforms, signalling (unpark) overhead time is noticeably
+ * long, and the time between signalling a thread and it actually
+ * making progress can be very noticeably long, so it is worth
+ * offloading these delays from critical paths as much as
+ * possible.
*
* Trimming workers. To release resources after periods of lack of
* use, a worker starting to wait when the pool is quiescent will
- * time out and terminate if the pool has remained quiescent for
- * SHRINK_RATE nanosecs. This will slowly propagate, eventually
- * terminating all workers after long periods of non-use.
- *
- * Submissions. External submissions are maintained in an
- * array-based queue that is structured identically to
- * ForkJoinWorkerThread queues except for the use of
- * submissionLock in method addSubmission. Unlike the case for
- * worker queues, multiple external threads can add new
- * submissions, so adding requires a lock.
+ * time out and terminate if the pool has remained quiescent for a
+ * given period -- a short period if there are more threads than
+ * parallelism, longer as the number of threads decreases. This
+ * will slowly propagate, eventually terminating all workers after
+ * periods of non-use.
*
- * Compensation. Beyond work-stealing support and lifecycle
- * control, the main responsibility of this framework is to take
- * actions when one worker is waiting to join a task stolen (or
- * always held by) another. Because we are multiplexing many
- * tasks on to a pool of workers, we can't just let them block (as
- * in Thread.join). We also cannot just reassign the joiner's
- * run-time stack with another and replace it later, which would
- * be a form of "continuation", that even if possible is not
- * necessarily a good idea since we sometimes need both an
- * unblocked task and its continuation to progress. Instead we
- * combine two tactics:
+ * Shutdown and Termination. A call to shutdownNow atomically sets
+ * a plock bit and then (non-atomically) sets each worker's
+ * qlock status, cancels all unprocessed tasks, and wakes up
+ * all waiting workers. Detecting whether termination should
+ * commence after a non-abrupt shutdown() call requires more work
+ * and bookkeeping. We need consensus about quiescence (i.e., that
+ * there is no more work). The active count provides a primary
+ * indication but non-abrupt shutdown still requires a rechecking
+ * scan for any workers that are inactive but not queued.
+ *
+ * Joining Tasks
+ * =============
+ *
+ * Any of several actions may be taken when one worker is waiting
+ * to join a task stolen (or always held) by another. Because we
+ * are multiplexing many tasks on to a pool of workers, we can't
+ * just let them block (as in Thread.join). We also cannot just
+ * reassign the joiner's run-time stack with another and replace
+ * it later, which would be a form of "continuation", that even if
+ * possible is not necessarily a good idea since we sometimes need
+ * both an unblocked task and its continuation to progress.
+ * Instead we combine two tactics:
*
* Helping: Arranging for the joiner to execute some task that it
- * would be running if the steal had not occurred. Method
- * ForkJoinWorkerThread.joinTask tracks joining->stealing
- * links to try to find such a task.
+ * would be running if the steal had not occurred.
*
* Compensating: Unless there are already enough live threads,
- * method tryPreBlock() may create or re-activate a spare
- * thread to compensate for blocked joiners until they
- * unblock.
+ * method tryCompensate() may create or re-activate a spare
+ * thread to compensate for blocked joiners until they unblock.
+ *
+ * A third form (implemented in tryRemoveAndExec) amounts to
+ * helping a hypothetical compensator: If we can readily tell that
+ * a possible action of a compensator is to steal and execute the
+ * task being joined, the joining thread can do so directly,
+ * without the need for a compensation thread (although at the
+ * expense of larger run-time stacks, but the tradeoff is
+ * typically worthwhile).
*
* The ManagedBlocker extension API can't use helping so relies
* only on compensation in method awaitBlocker.
*
+ * The algorithm in tryHelpStealer entails 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 tryHelpStealer 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 sometimes
+ * requires a linear scan of workQueues array to locate stealers,
+ * but often doesn't because stealers leave hints (that may become
+ * stale/wrong) of where to locate them. It is only a hint
+ * because a worker might have had multiple steals and the hint
+ * records only one of them (usually the most current). Hinting
+ * 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.
+ *
+ * Helping actions for CountedCompleters are much simpler: Method
+ * helpComplete can take and execute any task with the same root
+ * as the task being waited on. However, this still entails some
+ * traversal of completer chains, so is less efficient than using
+ * CountedCompleters without explicit joins.
+ *
* It is impossible to keep exactly the target parallelism number
* of threads running at any given time. Determining the
* existence of conservatively safe helping targets, the
* availability of already-created spares, and the apparent need
- * to create new spares are all racy and require heuristic
- * guidance, so we rely on multiple retries of each. Currently,
- * in keeping with on-demand signalling policy, we compensate only
- * if blocking would leave less than one active (non-waiting,
- * non-blocked) worker. Additionally, to avoid some false alarms
- * due to GC, lagging counters, system activity, etc, compensated
- * blocking for joins is only attempted after rechecks stabilize
- * (retries are interspersed with Thread.yield, for good
- * citizenship). The variable blockedCount, incremented before
- * blocking and decremented after, is sometimes needed to
- * distinguish cases of waiting for work vs blocking on joins or
- * other managed sync. Both cases are equivalent for most pool
- * control, so we can update non-atomically. (Additionally,
- * contention on blockedCount alleviates some contention on ctl).
+ * to create new spares are all racy, so we rely on multiple
+ * retries of each. Compensation in the apparent absence of
+ * helping opportunities is challenging to control on JVMs, where
+ * GC and other activities can stall progress of tasks that in
+ * turn stall out many other dependent tasks, without us being
+ * able to determine whether they will ever require compensation.
+ * Even though work-stealing otherwise encounters little
+ * degradation in the presence of more threads than cores,
+ * aggressively adding new threads in such cases entails risk of
+ * unwanted positive feedback control loops in which more threads
+ * cause more dependent stalls (as well as delayed progress of
+ * unblocked threads to the point that we know they are available)
+ * leading to more situations requiring more threads, and so
+ * on. This aspect of control can be seen as an (analytically
+ * intractable) game with an opponent that may choose the worst
+ * (for us) active thread to stall at any time. We take several
+ * precautions to bound losses (and thus bound gains), mainly in
+ * methods tryCompensate and awaitJoin.
+ *
+ * Common Pool
+ * ===========
*
- * Shutdown and Termination. A call to shutdownNow atomically sets
- * the ctl stop bit and then (non-atomically) sets each workers
- * "terminate" status, cancels all unprocessed tasks, and wakes up
- * all waiting workers. Detecting whether termination should
- * commence after a non-abrupt shutdown() call requires more work
- * and bookkeeping. We need consensus about quiesence (i.e., that
- * there is no more work) which is reflected in active counts so
- * long as there are no current blockers, as well as possible
- * re-evaluations during independent changes in blocking or
- * quiescing workers.
+ * The static commonPool always exists after static
+ * initialization. Since it (or any other created pool) need
+ * never be used, we minimize initial construction overhead and
+ * footprint to the setup of about a dozen fields, with no nested
+ * allocation. Most bootstrapping occurs within method
+ * fullExternalPush during the first submission to the pool.
*
- * Style notes: There is a lot of representation-level coupling
- * among classes ForkJoinPool, ForkJoinWorkerThread, and
- * ForkJoinTask. Most fields of ForkJoinWorkerThread maintain
- * data structures managed by ForkJoinPool, so are directly
- * accessed. Conversely we allow access to "workers" array by
- * workers, and direct access to ForkJoinTask.status by both
- * ForkJoinPool and ForkJoinWorkerThread. There is little point
+ * When external threads submit to the common pool, they can
+ * perform some subtask processing (see externalHelpJoin and
+ * related methods). We do not need to record whether these
+ * submissions are to the common pool -- if not, externalHelpJoin
+ * returns quickly (at the most helping to signal some common pool
+ * workers). These submitters would otherwise be blocked waiting
+ * for completion, so the extra effort (with liberally sprinkled
+ * task status checks) in inapplicable cases amounts to an odd
+ * form of limited spin-wait before blocking in ForkJoinTask.join.
+ *
+ * Style notes
+ * ===========
+ *
+ * There is a lot of representation-level coupling among classes
+ * ForkJoinPool, ForkJoinWorkerThread, and ForkJoinTask. The
+ * fields of WorkQueue maintain data structures managed by
+ * ForkJoinPool, so are directly accessed. There is little point
* trying to reduce this, since any associated future changes in
* representations will need to be accompanied by algorithmic
- * changes anyway. All together, these low-level implementation
- * choices produce as much as a factor of 4 performance
- * improvement compared to naive implementations, and enable the
- * processing of billions of tasks per second, at the expense of
- * some ugliness.
+ * changes anyway. Several methods intrinsically sprawl because
+ * they must accumulate sets of consistent reads of volatiles held
+ * in local variables. Methods signalWork() and scan() are the
+ * main bottlenecks, so are especially heavily
+ * micro-optimized/mangled. There are lots of inline assignments
+ * (of form "while ((local = field) != 0)") which are usually the
+ * simplest way to ensure the required read orderings (which are
+ * sometimes critical). This leads to a "C"-like style of listing
+ * declarations of these locals at the heads of methods or blocks.
+ * There are several occurrences of the unusual "do {} while
+ * (!cas...)" which is the simplest way to force an update of a
+ * CAS'ed variable. There are also other coding oddities (including
+ * several unnecessary-looking hoisted null checks) that help
+ * some methods perform reasonably even when interpreted (not
+ * compiled).
*
- * Methods signalWork() and scan() are the main bottlenecks so are
- * especially heavily micro-optimized/mangled. There are lots of
- * inline assignments (of form "while ((local = field) != 0)")
- * which are usually the simplest way to ensure the required read
- * orderings (which are sometimes critical). This leads to a
- * "C"-like style of listing declarations of these locals at the
- * heads of methods or blocks. There are several occurrences of
- * the unusual "do {} while (!cas...)" which is the simplest way
- * to force an update of a CAS'ed variable. There are also other
- * coding oddities that help some methods perform reasonably even
- * when interpreted (not compiled).
- *
- * The order of declarations in this file is: (1) declarations of
- * statics (2) fields (along with constants used when unpacking
- * some of them), listed in an order that tends to reduce
- * contention among them a bit under most JVMs. (3) internal
- * control methods (4) callbacks and other support for
- * ForkJoinTask and ForkJoinWorkerThread classes, (5) exported
- * methods (plus a few little helpers). (6) static block
- * initializing all statics in a minimally dependent order.
+ * The order of declarations in this file is:
+ * (1) Static utility functions
+ * (2) Nested (static) classes
+ * (3) Static fields
+ * (4) Fields, along with constants used when unpacking some of them
+ * (5) Internal control methods
+ * (6) Callbacks and other support for ForkJoinTask methods
+ * (7) Exported methods
+ * (8) Static block initializing statics in minimally dependent order
*/
+ // Static utilities
+
+ /**
+ * 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);
+ }
+
+ // Nested classes
+
/**
* Factory for creating new {@link ForkJoinWorkerThread}s.
* A {@code ForkJoinWorkerThreadFactory} must be defined and used
@@ -384,14 +556,526 @@
* Default ForkJoinWorkerThreadFactory implementation; creates a
* new ForkJoinWorkerThread.
*/
- static class DefaultForkJoinWorkerThreadFactory
+ static final class DefaultForkJoinWorkerThreadFactory
implements ForkJoinWorkerThreadFactory {
- public ForkJoinWorkerThread newThread(ForkJoinPool pool) {
+ public final ForkJoinWorkerThread newThread(ForkJoinPool pool) {
return new ForkJoinWorkerThread(pool);
}
}
/**
+ * Per-thread records for threads that submit to pools. Currently
+ * holds only pseudo-random seed / index that is used to choose
+ * submission queues in method externalPush. In the future, this may
+ * also incorporate a means to implement different task rejection
+ * and resubmission policies.
+ *
+ * Seeds for submitters and workers/workQueues work in basically
+ * the same way but are initialized and updated using slightly
+ * different mechanics. Both are initialized using the same
+ * approach as in class ThreadLocal, where successive values are
+ * unlikely to collide with previous values. Seeds are then
+ * randomly modified upon collisions using xorshifts, which
+ * requires a non-zero seed.
+ */
+ static final class Submitter {
+ int seed;
+ Submitter(int s) { seed = s; }
+ }
+
+ /**
+ * Class for artificial tasks that are used to replace the target
+ * of local joins if they are removed from an interior queue slot
+ * in WorkQueue.tryRemoveAndExec. We don't need the proxy to
+ * actually do anything beyond having a unique identity.
+ */
+ static final class EmptyTask extends ForkJoinTask<Void> {
+ private static final long serialVersionUID = -7721805057305804111L;
+ EmptyTask() { status = ForkJoinTask.NORMAL; } // force done
+ public final Void getRawResult() { return null; }
+ public final void setRawResult(Void x) {}
+ public final boolean exec() { return true; }
+ }
+
+ /**
+ * Queues supporting work-stealing as well as external task
+ * submission. See above for main rationale and algorithms.
+ * Implementation relies heavily on "Unsafe" intrinsics
+ * and selective use of "volatile":
+ *
+ * Field "base" is the index (mod array.length) of the least valid
+ * queue slot, which is always the next position to steal (poll)
+ * from if nonempty. Reads and writes require volatile orderings
+ * but not CAS, because updates are only performed after slot
+ * CASes.
+ *
+ * Field "top" is the index (mod array.length) of the next queue
+ * slot to push to or pop from. It is written only by owner thread
+ * for push, or under lock for external/shared push, and accessed
+ * by other threads only after reading (volatile) base. Both top
+ * and base are allowed to wrap around on overflow, but (top -
+ * base) (or more commonly -(base - top) to force volatile read of
+ * base before top) still estimates size. The lock ("qlock") is
+ * forced to -1 on termination, causing all further lock attempts
+ * to fail. (Note: we don't need CAS for termination state because
+ * upon pool shutdown, all shared-queues will stop being used
+ * anyway.) Nearly all lock bodies are set up so that exceptions
+ * within lock bodies are "impossible" (modulo JVM errors that
+ * would cause failure anyway.)
+ *
+ * The array slots are read and written using the emulation of
+ * volatiles/atomics provided by Unsafe. Insertions must in
+ * general use putOrderedObject as a form of releasing store to
+ * ensure that all writes to the task object are ordered before
+ * its publication in the queue. All removals entail a CAS to
+ * null. The array is always a power of two. To ensure safety of
+ * Unsafe array operations, all accesses perform explicit null
+ * checks and implicit bounds checks via power-of-two masking.
+ *
+ * In addition to basic queuing support, this class contains
+ * fields described elsewhere to control execution. It turns out
+ * to work better memory-layout-wise to include them in this class
+ * rather than a separate class.
+ *
+ * Performance on most platforms is very sensitive to placement of
+ * instances of both WorkQueues and their arrays -- we absolutely
+ * do not want multiple WorkQueue instances or multiple queue
+ * arrays sharing cache lines. (It would be best for queue objects
+ * and their arrays to share, but there is nothing available to
+ * help arrange that). Unfortunately, because they are recorded
+ * in a common array, WorkQueue instances are often moved to be
+ * adjacent by garbage collectors. To reduce impact, we use field
+ * padding that works OK on common platforms; this effectively
+ * trades off slightly slower average field access for the sake of
+ * avoiding really bad worst-case access. (Until better JVM
+ * support is in place, this padding is dependent on transient
+ * properties of JVM field layout rules.) We also take care in
+ * allocating, sizing and resizing the array. Non-shared queue
+ * arrays are initialized by workers before use. Others are
+ * allocated on first use.
+ */
+ static final class WorkQueue {
+ /**
+ * Capacity of work-stealing queue array upon initialization.
+ * Must be a power of two; at least 4, but should be larger to
+ * reduce or eliminate cacheline sharing among queues.
+ * Currently, it is much larger, as a partial workaround for
+ * the fact that JVMs often place arrays in locations that
+ * share GC bookkeeping (especially cardmarks) such that
+ * per-write accesses encounter serious memory contention.
+ */
+ static final int INITIAL_QUEUE_CAPACITY = 1 << 13;
+
+ /**
+ * Maximum size for queue arrays. Must be a power of two less
+ * than or equal to 1 << (31 - width of array entry) to ensure
+ * lack of wraparound of index calculations, but defined to a
+ * value a bit less than this to help users trap runaway
+ * programs before saturating systems.
+ */
+ static final int MAXIMUM_QUEUE_CAPACITY = 1 << 26; // 64M
+
+ // Heuristic padding to ameliorate unfortunate memory placements
+ volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
+
+ int seed; // for random scanning; initialize nonzero
+ volatile int eventCount; // encoded inactivation count; < 0 if inactive
+ int nextWait; // encoded record of next event waiter
+ int hint; // steal or signal hint (index)
+ int poolIndex; // index of this queue in pool (or 0)
+ final int mode; // 0: lifo, > 0: fifo, < 0: shared
+ int nsteals; // number of steals
+ volatile int qlock; // 1: locked, -1: terminate; else 0
+ volatile int base; // index of next slot for poll
+ int top; // index of next slot for push
+ ForkJoinTask<?>[] array; // the elements (initially unallocated)
+ final ForkJoinPool pool; // the containing pool (may be null)
+ final ForkJoinWorkerThread owner; // owning thread or null if shared
+ volatile Thread parker; // == owner during call to park; else null
+ volatile ForkJoinTask<?> currentJoin; // task being joined in awaitJoin
+ ForkJoinTask<?> currentSteal; // current non-local task being executed
+
+ volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+ volatile Object pad18, pad19, pad1a, pad1b, pad1c, pad1d;
+
+ WorkQueue(ForkJoinPool pool, ForkJoinWorkerThread owner, int mode,
+ int seed) {
+ this.pool = pool;
+ this.owner = owner;
+ this.mode = mode;
+ this.seed = seed;
+ // Place indices in the center of array (that is not yet allocated)
+ base = top = INITIAL_QUEUE_CAPACITY >>> 1;
+ }
+
+ /**
+ * Returns the approximate number of tasks in the queue.
+ */
+ final int queueSize() {
+ int n = base - top; // non-owner callers must read base first
+ return (n >= 0) ? 0 : -n; // ignore transient negative
+ }
+
+ /**
+ * Provides a more accurate estimate of whether this queue has
+ * any tasks than does queueSize, by checking whether a
+ * near-empty queue has at least one unclaimed task.
+ */
+ final boolean isEmpty() {
+ ForkJoinTask<?>[] a; int m, s;
+ int n = base - (s = top);
+ return (n >= 0 ||
+ (n == -1 &&
+ ((a = array) == null ||
+ (m = a.length - 1) < 0 ||
+ U.getObject
+ (a, (long)((m & (s - 1)) << ASHIFT) + ABASE) == null)));
+ }
+
+ /**
+ * Pushes a task. Call only by owner in unshared queues. (The
+ * shared-queue version is embedded in method externalPush.)
+ *
+ * @param task the task. Caller must ensure non-null.
+ * @throw RejectedExecutionException if array cannot be resized
+ */
+ final void push(ForkJoinTask<?> task) {
+ ForkJoinTask<?>[] a; ForkJoinPool p;
+ int s = top, m, n;
+ if ((a = array) != null) { // ignore if queue removed
+ int j = (((m = a.length - 1) & s) << ASHIFT) + ABASE;
+ U.putOrderedObject(a, j, task);
+ if ((n = (top = s + 1) - base) <= 2) {
+ if ((p = pool) != null)
+ p.signalWork(this);
+ }
+ else if (n >= m)
+ growArray();
+ }
+ }
+
+ /**
+ * Initializes or doubles the capacity of array. Call either
+ * by owner or with lock held -- it is OK for base, but not
+ * top, to move while resizings are in progress.
+ */
+ final ForkJoinTask<?>[] growArray() {
+ ForkJoinTask<?>[] oldA = array;
+ int size = oldA != null ? oldA.length << 1 : INITIAL_QUEUE_CAPACITY;
+ if (size > MAXIMUM_QUEUE_CAPACITY)
+ throw new RejectedExecutionException("Queue capacity exceeded");
+ int oldMask, t, b;
+ ForkJoinTask<?>[] a = array = new ForkJoinTask<?>[size];
+ if (oldA != null && (oldMask = oldA.length - 1) >= 0 &&
+ (t = top) - (b = base) > 0) {
+ int mask = size - 1;
+ do {
+ ForkJoinTask<?> x;
+ int oldj = ((b & oldMask) << ASHIFT) + ABASE;
+ int j = ((b & mask) << ASHIFT) + ABASE;
+ x = (ForkJoinTask<?>)U.getObjectVolatile(oldA, oldj);
+ if (x != null &&
+ U.compareAndSwapObject(oldA, oldj, x, null))
+ U.putObjectVolatile(a, j, x);
+ } while (++b != t);
+ }
+ return a;
+ }
+
+ /**
+ * Takes next task, if one exists, in LIFO order. Call only
+ * by owner in unshared queues.
+ */
+ final ForkJoinTask<?> pop() {
+ ForkJoinTask<?>[] a; ForkJoinTask<?> t; int m;
+ if ((a = array) != null && (m = a.length - 1) >= 0) {
+ for (int s; (s = top - 1) - base >= 0;) {
+ long j = ((m & s) << ASHIFT) + ABASE;
+ if ((t = (ForkJoinTask<?>)U.getObject(a, j)) == null)
+ break;
+ if (U.compareAndSwapObject(a, j, t, null)) {
+ top = s;
+ return t;
+ }
+ }
+ }
+ return null;
+ }
+
+ /**
+ * Takes a task in FIFO order if b is base of queue and a task
+ * can be claimed without contention. Specialized versions
+ * appear in ForkJoinPool methods scan and tryHelpStealer.
+ */
+ final ForkJoinTask<?> pollAt(int b) {
+ ForkJoinTask<?> t; ForkJoinTask<?>[] a;
+ if ((a = array) != null) {
+ int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+ if ((t = (ForkJoinTask<?>)U.getObjectVolatile(a, j)) != null &&
+ base == b &&
+ U.compareAndSwapObject(a, j, t, null)) {
+ base = b + 1;
+ return t;
+ }
+ }
+ return null;
+ }
+
+ /**
+ * Takes next task, if one exists, in FIFO order.
+ */
+ final ForkJoinTask<?> poll() {
+ ForkJoinTask<?>[] a; int b; ForkJoinTask<?> t;
+ while ((b = base) - top < 0 && (a = array) != null) {
+ int j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+ t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+ if (t != null) {
+ if (base == b &&
+ U.compareAndSwapObject(a, j, t, null)) {
+ base = b + 1;
+ return t;
+ }
+ }
+ else if (base == b) {
+ if (b + 1 == top)
+ break;
+ Thread.yield(); // wait for lagging update (very rare)
+ }
+ }
+ return null;
+ }
+
+ /**
+ * Takes next task, if one exists, in order specified by mode.
+ */
+ final ForkJoinTask<?> nextLocalTask() {
+ return mode == 0 ? pop() : poll();
+ }
+
+ /**
+ * Returns next task, if one exists, in order specified by mode.
+ */
+ final ForkJoinTask<?> peek() {
+ ForkJoinTask<?>[] a = array; int m;
+ if (a == null || (m = a.length - 1) < 0)
+ return null;
+ int i = mode == 0 ? top - 1 : base;
+ int j = ((i & m) << ASHIFT) + ABASE;
+ return (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+ }
+
+ /**
+ * Pops the given task only if it is at the current top.
+ * (A shared version is available only via FJP.tryExternalUnpush)
+ */
+ final boolean tryUnpush(ForkJoinTask<?> t) {
+ ForkJoinTask<?>[] a; int s;
+ if ((a = array) != null && (s = top) != base &&
+ U.compareAndSwapObject
+ (a, (((a.length - 1) & --s) << ASHIFT) + ABASE, t, null)) {
+ top = s;
+ return true;
+ }
+ return false;
+ }
+
+ /**
+ * Removes and cancels all known tasks, ignoring any exceptions.
+ */
+ final void cancelAll() {
+ ForkJoinTask.cancelIgnoringExceptions(currentJoin);
+ ForkJoinTask.cancelIgnoringExceptions(currentSteal);
+ for (ForkJoinTask<?> t; (t = poll()) != null; )
+ ForkJoinTask.cancelIgnoringExceptions(t);
+ }
+
+ /**
+ * Computes next value for random probes. 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 its usages in ForkJoinPool to
+ * avoid writes inside busy scan loops.
+ */
+ final int nextSeed() {
+ int r = seed;
+ r ^= r << 13;
+ r ^= r >>> 17;
+ return seed = r ^= r << 5;
+ }
+
+ // Specialized execution methods
+
+ /**
+ * Pops and runs tasks until empty.
+ */
+ private void popAndExecAll() {
+ // A bit faster than repeated pop calls
+ ForkJoinTask<?>[] a; int m, s; long j; ForkJoinTask<?> t;
+ while ((a = array) != null && (m = a.length - 1) >= 0 &&
+ (s = top - 1) - base >= 0 &&
+ (t = ((ForkJoinTask<?>)
+ U.getObject(a, j = ((m & s) << ASHIFT) + ABASE)))
+ != null) {
+ if (U.compareAndSwapObject(a, j, t, null)) {
+ top = s;
+ t.doExec();
+ }
+ }
+ }
+
+ /**
+ * Polls and runs tasks until empty.
+ */
+ private void pollAndExecAll() {
+ for (ForkJoinTask<?> t; (t = poll()) != null;)
+ t.doExec();
+ }
+
+ /**
+ * If present, removes from queue and executes the given task,
+ * or any other cancelled task. Returns (true) on any CAS
+ * or consistency check failure so caller can retry.
+ *
+ * @return false if no progress can be made, else true;
+ */
+ final boolean tryRemoveAndExec(ForkJoinTask<?> task) {
+ boolean stat = true, removed = false, empty = true;
+ ForkJoinTask<?>[] a; int m, s, b, n;
+ if ((a = array) != null && (m = a.length - 1) >= 0 &&
+ (n = (s = top) - (b = base)) > 0) {
+ for (ForkJoinTask<?> t;;) { // traverse from s to b
+ int j = ((--s & m) << ASHIFT) + ABASE;
+ t = (ForkJoinTask<?>)U.getObjectVolatile(a, j);
+ if (t == null) // inconsistent length
+ break;
+ else if (t == task) {
+ if (s + 1 == top) { // pop
+ if (!U.compareAndSwapObject(a, j, task, null))
+ break;
+ top = s;
+ removed = true;
+ }
+ else if (base == b) // replace with proxy
+ removed = U.compareAndSwapObject(a, j, task,
+ new EmptyTask());
+ break;
+ }
+ else if (t.status >= 0)
+ empty = false;
+ else if (s + 1 == top) { // pop and throw away
+ if (U.compareAndSwapObject(a, j, t, null))
+ top = s;
+ break;
+ }
+ if (--n == 0) {
+ if (!empty && base == b)
+ stat = false;
+ break;
+ }
+ }
+ }
+ if (removed)
+ task.doExec();
+ return stat;
+ }
+
+ /**
+ * Polls for and executes the given task or any other task in
+ * its CountedCompleter computation
+ */
+ final boolean pollAndExecCC(ForkJoinTask<?> root) {
+ ForkJoinTask<?>[] a; int b; Object o;
+ outer: while ((b = base) - top < 0 && (a = array) != null) {
+ long j = (((a.length - 1) & b) << ASHIFT) + ABASE;
+ if ((o = U.getObject(a, j)) == null ||
+ !(o instanceof CountedCompleter))
+ break;
+ for (CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;;) {
+ if (r == root) {
+ if (base == b &&
+ U.compareAndSwapObject(a, j, t, null)) {
+ base = b + 1;
+ t.doExec();
+ return true;
+ }
+ else
+ break; // restart
+ }
+ if ((r = r.completer) == null)
+ break outer; // not part of root computation
+ }
+ }
+ return false;
+ }
+
+ /**
+ * Executes a top-level task and any local tasks remaining
+ * after execution.
+ */
+ final void runTask(ForkJoinTask<?> t) {
+ if (t != null) {
+ (currentSteal = t).doExec();
+ currentSteal = null;
+ ++nsteals;
+ if (base - top < 0) { // process remaining local tasks
+ if (mode == 0)
+ popAndExecAll();
+ else
+ pollAndExecAll();
+ }
+ }
+ }
+
+ /**
+ * Executes a non-top-level (stolen) task.
+ */
+ final void runSubtask(ForkJoinTask<?> t) {
+ if (t != null) {
+ ForkJoinTask<?> ps = currentSteal;
+ (currentSteal = t).doExec();
+ currentSteal = ps;
+ }
+ }
+
+ /**
+ * Returns true if owned and not known to be blocked.
+ */
+ final boolean isApparentlyUnblocked() {
+ Thread wt; Thread.State s;
+ return (eventCount >= 0 &&
+ (wt = owner) != null &&
+ (s = wt.getState()) != Thread.State.BLOCKED &&
+ s != Thread.State.WAITING &&
+ s != Thread.State.TIMED_WAITING);
+ }
+
+ // Unsafe mechanics
+ private static final sun.misc.Unsafe U;
+ private static final long QLOCK;
+ private static final int ABASE;
+ private static final int ASHIFT;
+ static {
+ int s;
+ try {
+ U = sun.misc.Unsafe.getUnsafe();
+ Class<?> k = WorkQueue.class;
+ Class<?> ak = ForkJoinTask[].class;
+ QLOCK = U.objectFieldOffset
+ (k.getDeclaredField("qlock"));
+ ABASE = U.arrayBaseOffset(ak);
+ s = U.arrayIndexScale(ak);
+ } 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);
+ }
+ }
+
+ // static fields (initialized in static initializer below)
+
+ /**
* Creates a new ForkJoinWorkerThread. This factory is used unless
* overridden in ForkJoinPool constructors.
*/
@@ -399,107 +1083,93 @@
defaultForkJoinWorkerThreadFactory;
/**
+ * Per-thread submission bookkeeping. Shared across all pools
+ * to reduce ThreadLocal pollution and because random motion
+ * to avoid contention in one pool is likely to hold for others.
+ * Lazily initialized on first submission (but null-checked
+ * in other contexts to avoid unnecessary initialization).
+ */
+ static final ThreadLocal<Submitter> submitters;
+
+ /**
* Permission required for callers of methods that may start or
* kill threads.
*/
private static final RuntimePermission modifyThreadPermission;
/**
- * If there is a security manager, makes sure caller has
- * permission to modify threads.
+ * Common (static) pool. Non-null for public use unless a static
+ * construction exception, but internal usages null-check on use
+ * to paranoically avoid potential initialization circularities
+ * as well as to simplify generated code.
*/
- private static void checkPermission() {
- SecurityManager security = System.getSecurityManager();
- if (security != null)
- security.checkPermission(modifyThreadPermission);
- }
+ static final ForkJoinPool commonPool;
/**
- * Generator for assigning sequence numbers as pool names.
+ * Common pool parallelism. Must equal commonPool.parallelism.
*/
- private static final AtomicInteger poolNumberGenerator;
+ static final int commonPoolParallelism;
/**
- * Generator for initial random seeds for worker victim
- * selection. This is used only to create initial seeds. Random
- * steals use a cheaper xorshift generator per steal attempt. We
- * don't expect much contention on seedGenerator, so just use a
- * plain Random.
+ * Sequence number for creating workerNamePrefix.
*/
- static final Random workerSeedGenerator;
+ private static int poolNumberSequence;
/**
- * Array holding all worker threads in the pool. Initialized upon
- * construction. Array size must be a power of two. Updates and
- * replacements are protected by scanGuard, but the array is
- * always kept in a consistent enough state to be randomly
- * accessed without locking by workers performing work-stealing,
- * as well as other traversal-based methods in this class, so long
- * as reads memory-acquire by first reading ctl. All readers must
- * tolerate that some array slots may be null.
+ * Return the next sequence number. We don't expect this to
+ * ever contend so use simple builtin sync.
*/
- ForkJoinWorkerThread[] workers;
+ private static final synchronized int nextPoolId() {
+ return ++poolNumberSequence;
+ }
- /**
- * Initial size for submission queue array. Must be a power of
- * two. In many applications, these always stay small so we use a
- * small initial cap.
- */
- private static final int INITIAL_QUEUE_CAPACITY = 8;
+ // static constants
/**
- * Maximum size for submission queue array. Must be a power of two
- * less than or equal to 1 << (31 - width of array entry) to
- * ensure lack of index wraparound, but is capped at a lower
- * value to help users trap runaway computations.
+ * Initial timeout value (in nanoseconds) for the thread
+ * triggering quiescence to park waiting for new work. On timeout,
+ * the thread will instead try to shrink the number of
+ * workers. The value should be large enough to avoid overly
+ * aggressive shrinkage during most transient stalls (long GCs
+ * etc).
*/
- private static final int MAXIMUM_QUEUE_CAPACITY = 1 << 24; // 16M
+ private static final long IDLE_TIMEOUT = 2000L * 1000L * 1000L; // 2sec
/**
- * Array serving as submission queue. Initialized upon construction.
+ * Timeout value when there are more threads than parallelism level
*/
- private ForkJoinTask<?>[] submissionQueue;
+ private static final long FAST_IDLE_TIMEOUT = 200L * 1000L * 1000L;
/**
- * Lock protecting submissions array for addSubmission
+ * Tolerance for idle timeouts, to cope with timer undershoots
*/
- private final ReentrantLock submissionLock;
-
- /**
- * Condition for awaitTermination, using submissionLock for
- * convenience.
- */
- private final Condition termination;
+ private static final long TIMEOUT_SLOP = 2000000L;
/**
- * Creation factory for worker threads.
+ * The maximum stolen->joining link depth allowed in method
+ * tryHelpStealer. Must be a power of two. Depths for legitimate
+ * chains are unbounded, but we use a fixed constant to avoid
+ * (otherwise unchecked) cycles and to bound staleness of
+ * traversal parameters at the expense of sometimes blocking when
+ * we could be helping.
*/
- private final ForkJoinWorkerThreadFactory factory;
-
- /**
- * The uncaught exception handler used when any worker abruptly
- * terminates.
- */
- final Thread.UncaughtExceptionHandler ueh;
+ private static final int MAX_HELP = 64;
/**
- * Prefix for assigning names to worker threads
+ * Increment for seed generators. See class ThreadLocal for
+ * explanation.
*/
- private final String workerNamePrefix;
+ private static final int SEED_INCREMENT = 0x61c88647;
/**
- * Sum of per-thread steal counts, updated only when threads are
- * idle or terminating.
- */
- private volatile long stealCount;
-
- /**
- * Main pool control -- a long packed with:
+ * Bits and masks for control variables
+ *
+ * Field ctl is a long packed with:
* AC: Number of active running workers minus target parallelism (16 bits)
- * TC: Number of total workers minus target parallelism (16bits)
+ * TC: Number of total workers minus target parallelism (16 bits)
* ST: true if pool is terminating (1 bit)
* EC: the wait count of top waiting thread (15 bits)
- * ID: ~poolIndex of top of Treiber stack of waiting threads (16 bits)
+ * ID: poolIndex of top of Treiber stack of waiters (16 bits)
*
* When convenient, we can extract the upper 32 bits of counts and
* the lower 32 bits of queue state, u = (int)(ctl >>> 32) and e =
@@ -508,13 +1178,26 @@
* parallelism and the positionings of fields makes it possible to
* perform the most common checks via sign tests of fields: When
* ac is negative, there are not enough active workers, when tc is
- * negative, there are not enough total workers, when id is
- * negative, there is at least one waiting worker, and when e is
+ * negative, there are not enough total workers, and when e is
* negative, the pool is terminating. To deal with these possibly
* negative fields, we use casts in and out of "short" and/or
* signed shifts to maintain signedness.
+ *
+ * When a thread is queued (inactivated), its eventCount field is
+ * set negative, which is the only way to tell if a worker is
+ * prevented from executing tasks, even though it must continue to
+ * scan for them to avoid queuing races. Note however that
+ * eventCount updates lag releases so usage requires care.
+ *
+ * Field plock is an int packed with:
+ * SHUTDOWN: true if shutdown is enabled (1 bit)
+ * SEQ: a sequence lock, with PL_LOCK bit set if locked (30 bits)
+ * SIGNAL: set when threads may be waiting on the lock (1 bit)
+ *
+ * The sequence number enables simple consistency checks:
+ * Staleness of read-only operations on the workQueues array can
+ * be checked by comparing plock before vs after the reads.
*/
- volatile long ctl;
// bit positions/shifts for fields
private static final int AC_SHIFT = 48;
@@ -523,8 +1206,10 @@
private static final int EC_SHIFT = 16;
// bounds
- private static final int MAX_ID = 0x7fff; // max poolIndex
- private static final int SMASK = 0xffff; // mask short bits
+ private static final int SMASK = 0xffff; // short bits
+ private static final int MAX_CAP = 0x7fff; // max #workers - 1
+ private static final int EVENMASK = 0xfffe; // even short bits
+ private static final int SQMASK = 0x007e; // max 64 (even) slots
private static final int SHORT_SIGN = 1 << 15;
private static final int INT_SIGN = 1 << 31;
@@ -546,649 +1231,648 @@
private static final int UTC_UNIT = 1 << UTC_SHIFT;
// masks and units for dealing with e = (int)ctl
- private static final int E_MASK = 0x7fffffff; // no STOP_BIT
- private static final int EC_UNIT = 1 << EC_SHIFT;
-
- /**
- * The target parallelism level.
- */
- final int parallelism;
+ private static final int E_MASK = 0x7fffffff; // no STOP_BIT
+ private static final int E_SEQ = 1 << EC_SHIFT;
- /**
- * Index (mod submission queue length) of next element to take
- * from submission queue. Usage is identical to that for
- * per-worker queues -- see ForkJoinWorkerThread internal
- * documentation.
- */
- volatile int queueBase;
-
- /**
- * Index (mod submission queue length) of next element to add
- * in submission queue. Usage is identical to that for
- * per-worker queues -- see ForkJoinWorkerThread internal
- * documentation.
- */
- int queueTop;
+ // plock bits
+ private static final int SHUTDOWN = 1 << 31;
+ private static final int PL_LOCK = 2;
+ private static final int PL_SIGNAL = 1;
+ private static final int PL_SPINS = 1 << 8;
- /**
- * True when shutdown() has been called.
- */
- volatile boolean shutdown;
-
- /**
- * True if use local fifo, not default lifo, for local polling
- * Read by, and replicated by ForkJoinWorkerThreads
- */
- final boolean locallyFifo;
+ // access mode for WorkQueue
+ static final int LIFO_QUEUE = 0;
+ static final int FIFO_QUEUE = 1;
+ static final int SHARED_QUEUE = -1;
- /**
- * The number of threads in ForkJoinWorkerThreads.helpQuiescePool.
- * When non-zero, suppresses automatic shutdown when active
- * counts become zero.
- */
- volatile int quiescerCount;
+ // bounds for #steps in scan loop -- must be power 2 minus 1
+ private static final int MIN_SCAN = 0x1ff; // cover estimation slop
+ private static final int MAX_SCAN = 0x1ffff; // 4 * max workers
+
+ // Instance fields
- /**
- * The number of threads blocked in join.
- */
- volatile int blockedCount;
-
- /**
- * Counter for worker Thread names (unrelated to their poolIndex)
+ /*
+ * Field layout of this class tends to matter more than one would
+ * like. Runtime layout order is only loosely related to
+ * declaration order and may differ across JVMs, but the following
+ * empirically works OK on current JVMs.
*/
- private volatile int nextWorkerNumber;
- /**
- * The index for the next created worker. Accessed under scanGuard.
- */
- private int nextWorkerIndex;
+ // Heuristic padding to ameliorate unfortunate memory placements
+ volatile long pad00, pad01, pad02, pad03, pad04, pad05, pad06;
- /**
- * SeqLock and index masking for updates to workers array. Locked
- * when SG_UNIT is set. Unlocking clears bit by adding
- * SG_UNIT. Staleness of read-only operations can be checked by
- * comparing scanGuard to value before the reads. The low 16 bits
- * (i.e, anding with SMASK) hold (the smallest power of two
- * covering all worker indices, minus one, and is used to avoid
- * dealing with large numbers of null slots when the workers array
- * is overallocated.
- */
- volatile int scanGuard;
+ volatile long stealCount; // collects worker counts
+ volatile long ctl; // main pool control
+ volatile int plock; // shutdown status and seqLock
+ volatile int indexSeed; // worker/submitter index seed
+ final int config; // mode and parallelism level
+ WorkQueue[] workQueues; // main registry
+ final ForkJoinWorkerThreadFactory factory;
+ final Thread.UncaughtExceptionHandler ueh; // per-worker UEH
+ final String workerNamePrefix; // to create worker name string
- private static final int SG_UNIT = 1 << 16;
+ volatile Object pad10, pad11, pad12, pad13, pad14, pad15, pad16, pad17;
+ volatile Object pad18, pad19, pad1a, pad1b;
- /**
- * The wakeup interval (in nanoseconds) for a worker waiting for a
- * task when the pool is quiescent to instead try to shrink the
- * number of workers. The exact value does not matter too
- * much. It must be short enough to release resources during
- * sustained periods of idleness, but not so short that threads
- * are continually re-created.
+ /*
+ * Acquires the plock lock to protect worker array and related
+ * updates. This method is called only if an initial CAS on plock
+ * fails. This acts as a spinLock for normal cases, but falls back
+ * to builtin monitor to block when (rarely) needed. This would be
+ * a terrible idea for a highly contended lock, but works fine as
+ * a more conservative alternative to a pure spinlock.
*/
- private static final long SHRINK_RATE =
- 4L * 1000L * 1000L * 1000L; // 4 seconds
-
- /**
- * Top-level loop for worker threads: On each step: if the
- * previous step swept through all queues and found no tasks, or
- * there are excess threads, then possibly blocks. Otherwise,
- * scans for and, if found, executes a task. Returns when pool
- * and/or worker terminate.
- *
- * @param w the worker
- */
- final void work(ForkJoinWorkerThread w) {
- boolean swept = false; // true on empty scans
- long c;
- while (!w.terminate && (int)(c = ctl) >= 0) {
- int a; // active count
- if (!swept && (a = (int)(c >> AC_SHIFT)) <= 0)
- swept = scan(w, a);
- else if (tryAwaitWork(w, c))
- swept = false;
+ private int acquirePlock() {
+ int spins = PL_SPINS, r = 0, ps, nps;
+ for (;;) {
+ if (((ps = plock) & PL_LOCK) == 0 &&
+ U.compareAndSwapInt(this, PLOCK, ps, nps = ps + PL_LOCK))
+ return nps;
+ else if (r == 0) { // randomize spins if possible
+ Thread t = Thread.currentThread(); WorkQueue w; Submitter z;
+ if ((t instanceof ForkJoinWorkerThread) &&
+ (w = ((ForkJoinWorkerThread)t).workQueue) != null)
+ r = w.seed;
+ else if ((z = submitters.get()) != null)
+ r = z.seed;
+ else
+ r = 1;
+ }
+ else if (spins >= 0) {
+ r ^= r << 1; r ^= r >>> 3; r ^= r << 10; // xorshift
+ if (r >= 0)
+ --spins;
+ }
+ else if (U.compareAndSwapInt(this, PLOCK, ps, ps | PL_SIGNAL)) {
+ synchronized (this) {
+ if ((plock & PL_SIGNAL) != 0) {
+ try {
+ wait();
+ } catch (InterruptedException ie) {
+ try {
+ Thread.currentThread().interrupt();
+ } catch (SecurityException ignore) {
+ }
+ }
+ }
+ else
+ notifyAll();
+ }
+ }
}
}
- // Signalling
+ /**
+ * Unlocks and signals any thread waiting for plock. Called only
+ * when CAS of seq value for unlock fails.
+ */
+ private void releasePlock(int ps) {
+ plock = ps;
+ synchronized (this) { notifyAll(); }
+ }
/**
- * Wakes up or creates a worker.
+ * Performs secondary initialization, called when plock is zero.
+ * Creates workQueue array and sets plock to a valid value. The
+ * lock body must be exception-free (so no try/finally) so we
+ * optimistically allocate new array outside the lock and throw
+ * away if (very rarely) not needed. (A similar tactic is used in
+ * fullExternalPush.) Because the plock seq value can eventually
+ * wrap around zero, this method harmlessly fails to reinitialize
+ * if workQueues exists, while still advancing plock.
+ *
+ * Additionally tries to create the first worker.
*/
- final void signalWork() {
- /*
- * The while condition is true if: (there is are too few total
- * workers OR there is at least one waiter) AND (there are too
- * few active workers OR the pool is terminating). The value
- * of e distinguishes the remaining cases: zero (no waiters)
- * for create, negative if terminating (in which case do
- * nothing), else release a waiter. The secondary checks for
- * release (non-null array etc) can fail if the pool begins
- * terminating after the test, and don't impose any added cost
- * because JVMs must perform null and bounds checks anyway.
- */
- long c; int e, u;
- while ((((e = (int)(c = ctl)) | (u = (int)(c >>> 32))) &
- (INT_SIGN|SHORT_SIGN)) == (INT_SIGN|SHORT_SIGN) && e >= 0) {
- if (e > 0) { // release a waiting worker
- int i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
- if ((ws = workers) == null ||
- (i = ~e & SMASK) >= ws.length ||
- (w = ws[i]) == null)
- break;
- long nc = (((long)(w.nextWait & E_MASK)) |
- ((long)(u + UAC_UNIT) << 32));
- if (w.eventCount == e &&
- UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
- w.eventCount = (e + EC_UNIT) & E_MASK;
- if (w.parked)
- UNSAFE.unpark(w);
- break;
+ private void initWorkers() {
+ WorkQueue[] ws, nws; int ps;
+ int p = config & SMASK; // find power of two table size
+ int n = (p > 1) ? p - 1 : 1; // ensure at least 2 slots
+ n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8; n |= n >>> 16;
+ n = (n + 1) << 1;
+ if ((ws = workQueues) == null || ws.length == 0)
+ nws = new WorkQueue[n];
+ else
+ nws = null;
+ if (((ps = plock) & PL_LOCK) != 0 ||
+ !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+ ps = acquirePlock();
+ if (((ws = workQueues) == null || ws.length == 0) && nws != null)
+ workQueues = nws;
+ int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+ if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+ releasePlock(nps);
+ tryAddWorker();
+ }
+
+ /**
+ * Tries to create and start one worker if fewer than target
+ * parallelism level exist. Adjusts counts etc on failure.
+ */
+ private void tryAddWorker() {
+ long c; int u;
+ while ((u = (int)((c = ctl) >>> 32)) < 0 &&
+ (u & SHORT_SIGN) != 0 && (int)c == 0) {
+ long nc = (long)(((u + UTC_UNIT) & UTC_MASK) |
+ ((u + UAC_UNIT) & UAC_MASK)) << 32;
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ ForkJoinWorkerThreadFactory fac;
+ Throwable ex = null;
+ ForkJoinWorkerThread wt = null;
+ try {
+ if ((fac = factory) != null &&
+ (wt = fac.newThread(this)) != null) {
+ wt.start();
+ break;
+ }
+ } catch (Throwable e) {
+ ex = e;
}
- }
- else if (UNSAFE.compareAndSwapLong
- (this, ctlOffset, c,
- (long)(((u + UTC_UNIT) & UTC_MASK) |
- ((u + UAC_UNIT) & UAC_MASK)) << 32)) {
- addWorker();
+ deregisterWorker(wt, ex);
break;
}
}
}
+ // Registering and deregistering workers
+
/**
- * Variant of signalWork to help release waiters on rescans.
- * Tries once to release a waiter if active count < 0.
+ * Callback from ForkJoinWorkerThread to establish and record its
+ * WorkQueue. To avoid scanning bias due to packing entries in
+ * front of the workQueues array, we treat the array as a simple
+ * power-of-two hash table using per-thread seed as hash,
+ * expanding as needed.
+ *
+ * @param wt the worker thread
+ * @return the worker's queue
+ */
+ final WorkQueue registerWorker(ForkJoinWorkerThread wt) {
+ Thread.UncaughtExceptionHandler handler; WorkQueue[] ws; int s, ps;
+ wt.setDaemon(true);
+ if ((handler = ueh) != null)
+ wt.setUncaughtExceptionHandler(handler);
+ do {} while (!U.compareAndSwapInt(this, INDEXSEED, s = indexSeed,
+ s += SEED_INCREMENT) ||
+ s == 0); // skip 0
+ WorkQueue w = new WorkQueue(this, wt, config >>> 16, s);
+ if (((ps = plock) & PL_LOCK) != 0 ||
+ !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+ ps = acquirePlock();
+ int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+ try {
+ if ((ws = workQueues) != null) { // skip if shutting down
+ int n = ws.length, m = n - 1;
+ int r = (s << 1) | 1; // use odd-numbered indices
+ if (ws[r &= m] != null) { // collision
+ int probes = 0; // step by approx half size
+ int step = (n <= 4) ? 2 : ((n >>> 1) & EVENMASK) + 2;
+ while (ws[r = (r + step) & m] != null) {
+ if (++probes >= n) {
+ workQueues = ws = Arrays.copyOf(ws, n <<= 1);
+ m = n - 1;
+ probes = 0;
+ }
+ }
+ }
+ w.eventCount = w.poolIndex = r; // volatile write orders
+ ws[r] = w;
+ }
+ } finally {
+ if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+ releasePlock(nps);
+ }
+ wt.setName(workerNamePrefix.concat(Integer.toString(w.poolIndex)));
+ return w;
+ }
+
+ /**
+ * Final callback from terminating worker, as well as upon failure
+ * to construct or start a worker. Removes record of worker from
+ * array, and adjusts counts. If pool is shutting down, tries to
+ * complete termination.
*
- * @return false if failed due to contention, else true
+ * @param wt the worker thread or null if construction failed
+ * @param ex the exception causing failure, or null if none
+ */
+ final void deregisterWorker(ForkJoinWorkerThread wt, Throwable ex) {
+ WorkQueue w = null;
+ if (wt != null && (w = wt.workQueue) != null) {
+ int ps;
+ w.qlock = -1; // ensure set
+ long ns = w.nsteals, sc; // collect steal count
+ do {} while (!U.compareAndSwapLong(this, STEALCOUNT,
+ sc = stealCount, sc + ns));
+ if (((ps = plock) & PL_LOCK) != 0 ||
+ !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+ ps = acquirePlock();
+ int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+ try {
+ int idx = w.poolIndex;
+ WorkQueue[] ws = workQueues;
+ if (ws != null && idx >= 0 && idx < ws.length && ws[idx] == w)
+ ws[idx] = null;
+ } finally {
+ if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+ releasePlock(nps);
+ }
+ }
+
+ long c; // adjust ctl counts
+ do {} while (!U.compareAndSwapLong
+ (this, CTL, c = ctl, (((c - AC_UNIT) & AC_MASK) |
+ ((c - TC_UNIT) & TC_MASK) |
+ (c & ~(AC_MASK|TC_MASK)))));
+
+ if (!tryTerminate(false, false) && w != null && w.array != null) {
+ w.cancelAll(); // cancel remaining tasks
+ WorkQueue[] ws; WorkQueue v; Thread p; int u, i, e;
+ while ((u = (int)((c = ctl) >>> 32)) < 0 && (e = (int)c) >= 0) {
+ if (e > 0) { // activate or create replacement
+ if ((ws = workQueues) == null ||
+ (i = e & SMASK) >= ws.length ||
+ (v = ws[i]) != null)
+ break;
+ long nc = (((long)(v.nextWait & E_MASK)) |
+ ((long)(u + UAC_UNIT) << 32));
+ if (v.eventCount != (e | INT_SIGN))
+ break;
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ v.eventCount = (e + E_SEQ) & E_MASK;
+ if ((p = v.parker) != null)
+ U.unpark(p);
+ break;
+ }
+ }
+ else {
+ if ((short)u < 0)
+ tryAddWorker();
+ break;
+ }
+ }
+ }
+ if (ex == null) // help clean refs on way out
+ ForkJoinTask.helpExpungeStaleExceptions();
+ else // rethrow
+ ForkJoinTask.rethrow(ex);
+ }
+
+ // Submissions
+
+ /**
+ * Unless shutting down, adds the given task to a submission queue
+ * at submitter's current queue index (modulo submission
+ * range). Only the most common path is directly handled in this
+ * method. All others are relayed to fullExternalPush.
+ *
+ * @param task the task. Caller must ensure non-null.
*/
- private boolean tryReleaseWaiter() {
- long c; int e, i; ForkJoinWorkerThread w; ForkJoinWorkerThread[] ws;
- if ((e = (int)(c = ctl)) > 0 &&
- (int)(c >> AC_SHIFT) < 0 &&
- (ws = workers) != null &&
- (i = ~e & SMASK) < ws.length &&
- (w = ws[i]) != null) {
- long nc = ((long)(w.nextWait & E_MASK) |
- ((c + AC_UNIT) & (AC_MASK|TC_MASK)));
- if (w.eventCount != e ||
- !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
- return false;
- w.eventCount = (e + EC_UNIT) & E_MASK;
- if (w.parked)
- UNSAFE.unpark(w);
+ final void externalPush(ForkJoinTask<?> task) {
+ WorkQueue[] ws; WorkQueue q; Submitter z; int m; ForkJoinTask<?>[] a;
+ if ((z = submitters.get()) != null && plock > 0 &&
+ (ws = workQueues) != null && (m = (ws.length - 1)) >= 0 &&
+ (q = ws[m & z.seed & SQMASK]) != null &&
+ U.compareAndSwapInt(q, QLOCK, 0, 1)) { // lock
+ int b = q.base, s = q.top, n, an;
+ if ((a = q.array) != null && (an = a.length) > (n = s + 1 - b)) {
+ int j = (((an - 1) & s) << ASHIFT) + ABASE;
+ U.putOrderedObject(a, j, task);
+ q.top = s + 1; // push on to deque
+ q.qlock = 0;
+ if (n <= 2)
+ signalWork(q);
+ return;
+ }
+ q.qlock = 0;
}
- return true;
+ fullExternalPush(task);
+ }
+
+ /**
+ * Full version of externalPush. This method is called, among
+ * other times, upon the first submission of the first task to the
+ * pool, so must perform secondary initialization (via
+ * initWorkers). It also detects first submission by an external
+ * thread by looking up its ThreadLocal, and creates a new shared
+ * queue if the one at index if empty or contended. The plock lock
+ * body must be exception-free (so no try/finally) so we
+ * optimistically allocate new queues outside the lock and throw
+ * them away if (very rarely) not needed.
+ */
+ private void fullExternalPush(ForkJoinTask<?> task) {
+ int r = 0; // random index seed
+ for (Submitter z = submitters.get();;) {
+ WorkQueue[] ws; WorkQueue q; int ps, m, k;
+ if (z == null) {
+ if (U.compareAndSwapInt(this, INDEXSEED, r = indexSeed,
+ r += SEED_INCREMENT) && r != 0)
+ submitters.set(z = new Submitter(r));
+ }
+ else if (r == 0) { // move to a different index
+ r = z.seed;
+ r ^= r << 13; // same xorshift as WorkQueues
+ r ^= r >>> 17;
+ z.seed = r ^ (r << 5);
+ }
+ else if ((ps = plock) < 0)
+ throw new RejectedExecutionException();
+ else if (ps == 0 || (ws = workQueues) == null ||
+ (m = ws.length - 1) < 0)
+ initWorkers();
+ else if ((q = ws[k = r & m & SQMASK]) != null) {
+ if (q.qlock == 0 && U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+ ForkJoinTask<?>[] a = q.array;
+ int s = q.top;
+ boolean submitted = false;
+ try { // locked version of push
+ if ((a != null && a.length > s + 1 - q.base) ||
+ (a = q.growArray()) != null) { // must presize
+ int j = (((a.length - 1) & s) << ASHIFT) + ABASE;
+ U.putOrderedObject(a, j, task);
+ q.top = s + 1;
+ submitted = true;
+ }
+ } finally {
+ q.qlock = 0; // unlock
+ }
+ if (submitted) {
+ signalWork(q);
+ return;
+ }
+ }
+ r = 0; // move on failure
+ }
+ else if (((ps = plock) & PL_LOCK) == 0) { // create new queue
+ q = new WorkQueue(this, null, SHARED_QUEUE, r);
+ if (((ps = plock) & PL_LOCK) != 0 ||
+ !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+ ps = acquirePlock();
+ if ((ws = workQueues) != null && k < ws.length && ws[k] == null)
+ ws[k] = q;
+ int nps = (ps & SHUTDOWN) | ((ps + PL_LOCK) & ~SHUTDOWN);
+ if (!U.compareAndSwapInt(this, PLOCK, ps, nps))
+ releasePlock(nps);
+ }
+ else
+ r = 0; // try elsewhere while lock held
+ }
+ }
+
+ // Maintaining ctl counts
+
+ /**
+ * Increments active count; mainly called upon return from blocking.
+ */
+ final void incrementActiveCount() {
+ long c;
+ do {} while (!U.compareAndSwapLong(this, CTL, c = ctl, c + AC_UNIT));
+ }
+
+ /**
+ * Tries to create or activate a worker if too few are active.
+ *
+ * @param q the (non-null) queue holding tasks to be signalled
+ */
+ final void signalWork(WorkQueue q) {
+ int hint = q.poolIndex;
+ long c; int e, u, i, n; WorkQueue[] ws; WorkQueue w; Thread p;
+ while ((u = (int)((c = ctl) >>> 32)) < 0) {
+ if ((e = (int)c) > 0) {
+ if ((ws = workQueues) != null && ws.length > (i = e & SMASK) &&
+ (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+ long nc = (((long)(w.nextWait & E_MASK)) |
+ ((long)(u + UAC_UNIT) << 32));
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ w.hint = hint;
+ w.eventCount = (e + E_SEQ) & E_MASK;
+ if ((p = w.parker) != null)
+ U.unpark(p);
+ break;
+ }
+ if (q.top - q.base <= 0)
+ break;
+ }
+ else
+ break;
+ }
+ else {
+ if ((short)u < 0)
+ tryAddWorker();
+ break;
+ }
+ }
}
// Scanning for tasks
/**
- * Scans for and, if found, executes one task. Scans start at a
- * random index of workers array, and randomly select the first
- * (2*#workers)-1 probes, and then, if all empty, resort to 2
- * circular sweeps, which is necessary to check quiescence. and
- * taking a submission only if no stealable tasks were found. The
- * steal code inside the loop is a specialized form of
- * ForkJoinWorkerThread.deqTask, followed bookkeeping to support
- * helpJoinTask and signal propagation. The code for submission
- * queues is almost identical. On each steal, the worker completes
- * not only the task, but also all local tasks that this task may
- * have generated. On detecting staleness or contention when
- * trying to take a task, this method returns without finishing
- * sweep, which allows global state rechecks before retry.
- *
- * @param w the worker
- * @param a the number of active workers
- * @return true if swept all queues without finding a task
+ * Top-level runloop for workers, called by ForkJoinWorkerThread.run.
*/
- private boolean scan(ForkJoinWorkerThread w, int a) {
- int g = scanGuard; // mask 0 avoids useless scans if only one active
- int m = (parallelism == 1 - a && blockedCount == 0) ? 0 : g & SMASK;
- ForkJoinWorkerThread[] ws = workers;
- if (ws == null || ws.length <= m) // staleness check
- return false;
- for (int r = w.seed, k = r, j = -(m + m); j <= m + m; ++j) {
- ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
- ForkJoinWorkerThread v = ws[k & m];
- if (v != null && (b = v.queueBase) != v.queueTop &&
- (q = v.queue) != null && (i = (q.length - 1) & b) >= 0) {
- long u = (i << ASHIFT) + ABASE;
- if ((t = q[i]) != null && v.queueBase == b &&
- UNSAFE.compareAndSwapObject(q, u, t, null)) {
- int d = (v.queueBase = b + 1) - v.queueTop;
- v.stealHint = w.poolIndex;
- if (d != 0)
- signalWork(); // propagate if nonempty
- w.execTask(t);
- }
- r ^= r << 13; r ^= r >>> 17; w.seed = r ^ (r << 5);
- return false; // store next seed
- }
- else if (j < 0) { // xorshift
- r ^= r << 13; r ^= r >>> 17; k = r ^= r << 5;
- }
- else
- ++k;
- }
- if (scanGuard != g) // staleness check
- return false;
- else { // try to take submission
- ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
- if ((b = queueBase) != queueTop &&
- (q = submissionQueue) != null &&
- (i = (q.length - 1) & b) >= 0) {
- long u = (i << ASHIFT) + ABASE;
- if ((t = q[i]) != null && queueBase == b &&
- UNSAFE.compareAndSwapObject(q, u, t, null)) {
- queueBase = b + 1;
- w.execTask(t);
- }
- return false;
- }
- return true; // all queues empty
- }
+ final void runWorker(WorkQueue w) {
+ w.growArray(); // allocate queue
+ do { w.runTask(scan(w)); } while (w.qlock >= 0);
}
/**
- * Tries to enqueue worker w in wait queue and await change in
- * worker's eventCount. If the pool is quiescent and there is
- * more than one worker, possibly terminates worker upon exit.
- * Otherwise, before blocking, rescans queues to avoid missed
- * signals. Upon finding work, releases at least one worker
- * (which may be the current worker). Rescans restart upon
- * detected staleness or failure to release due to
- * contention. Note the unusual conventions about Thread.interrupt
- * here and elsewhere: Because interrupts are used solely to alert
- * threads to check termination, which is checked here anyway, we
- * clear status (using Thread.interrupted) before any call to
- * park, so that park does not immediately return due to status
- * being set via some other unrelated call to interrupt in user
- * code.
+ * Scans for and, if found, returns one task, else possibly
+ * inactivates the worker. This method operates on single reads of
+ * volatile state and is designed to be re-invoked continuously,
+ * in part because it returns upon detecting inconsistencies,
+ * contention, or state changes that indicate possible success on
+ * re-invocation.
*
- * @param w the calling worker
- * @param c the ctl value on entry
- * @return true if waited or another thread was released upon enq
+ * The scan searches for tasks across queues (starting at a random
+ * index, and relying on registerWorker to irregularly scatter
+ * them within array to avoid bias), checking each at least twice.
+ * The scan terminates upon either finding a non-empty queue, or
+ * completing the sweep. If the worker is not inactivated, it
+ * takes and returns a task from this queue. Otherwise, if not
+ * activated, it signals workers (that may include itself) and
+ * returns so caller can retry. Also returns for true if the
+ * worker array may have changed during an empty scan. On failure
+ * to find a task, we take one of the following actions, after
+ * which the caller will retry calling this method unless
+ * terminated.
+ *
+ * * If pool is terminating, terminate the worker.
+ *
+ * * If not already enqueued, try to inactivate and enqueue the
+ * worker on wait queue. Or, if inactivating has caused the pool
+ * to be quiescent, relay to idleAwaitWork to possibly shrink
+ * pool.
+ *
+ * * If already enqueued and none of the above apply, possibly
+ * park awaiting signal, else lingering to help scan and signal.
+ *
+ * * If a non-empty queue discovered or left as a hint,
+ * help wake up other workers before return
+ *
+ * @param w the worker (via its WorkQueue)
+ * @return a task or null if none found
*/
- private boolean tryAwaitWork(ForkJoinWorkerThread w, long c) {
- int v = w.eventCount;
- w.nextWait = (int)c; // w's successor record
- long nc = (long)(v & E_MASK) | ((c - AC_UNIT) & (AC_MASK|TC_MASK));
- if (ctl != c || !UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
- long d = ctl; // return true if lost to a deq, to force scan
- return (int)d != (int)c && ((d - c) & AC_MASK) >= 0L;
- }
- for (int sc = w.stealCount; sc != 0;) { // accumulate stealCount
- long s = stealCount;
- if (UNSAFE.compareAndSwapLong(this, stealCountOffset, s, s + sc))
- sc = w.stealCount = 0;
- else if (w.eventCount != v)
- return true; // update next time
- }
- if ((!shutdown || !tryTerminate(false)) &&
- (int)c != 0 && parallelism + (int)(nc >> AC_SHIFT) == 0 &&
- blockedCount == 0 && quiescerCount == 0)
- idleAwaitWork(w, nc, c, v); // quiescent
- for (boolean rescanned = false;;) {
- if (w.eventCount != v)
- return true;
- if (!rescanned) {
- int g = scanGuard, m = g & SMASK;
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null && m < ws.length) {
- rescanned = true;
- for (int i = 0; i <= m; ++i) {
- ForkJoinWorkerThread u = ws[i];
- if (u != null) {
- if (u.queueBase != u.queueTop &&
- !tryReleaseWaiter())
- rescanned = false; // contended
- if (w.eventCount != v)
- return true;
- }
+ private final ForkJoinTask<?> scan(WorkQueue w) {
+ WorkQueue[] ws; int m;
+ int ps = plock; // read plock before ws
+ if (w != null && (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+ int ec = w.eventCount; // ec is negative if inactive
+ int r = w.seed; r ^= r << 13; r ^= r >>> 17; w.seed = r ^= r << 5;
+ w.hint = -1; // update seed and clear hint
+ int j = ((m + m + 1) | MIN_SCAN) & MAX_SCAN;
+ do {
+ WorkQueue q; ForkJoinTask<?>[] a; int b;
+ if ((q = ws[(r + j) & m]) != null && (b = q.base) - q.top < 0 &&
+ (a = q.array) != null) { // probably nonempty
+ int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+ ForkJoinTask<?> t = (ForkJoinTask<?>)
+ U.getObjectVolatile(a, i);
+ if (q.base == b && ec >= 0 && t != null &&
+ U.compareAndSwapObject(a, i, t, null)) {
+ if ((q.base = b + 1) - q.top < 0)
+ signalWork(q);
+ return t; // taken
+ }
+ else if ((ec < 0 || j < m) && (int)(ctl >> AC_SHIFT) <= 0) {
+ w.hint = (r + j) & m; // help signal below
+ break; // cannot take
}
}
- if (scanGuard != g || // stale
- (queueBase != queueTop && !tryReleaseWaiter()))
- rescanned = false;
- if (!rescanned)
- Thread.yield(); // reduce contention
- else
- Thread.interrupted(); // clear before park
+ } while (--j >= 0);
+
+ int h, e, ns; long c, sc; WorkQueue q;
+ if ((ns = w.nsteals) != 0) {
+ if (U.compareAndSwapLong(this, STEALCOUNT,
+ sc = stealCount, sc + ns))
+ w.nsteals = 0; // collect steals and rescan
}
+ else if (plock != ps) // consistency check
+ ; // skip
+ else if ((e = (int)(c = ctl)) < 0)
+ w.qlock = -1; // pool is terminating
else {
- w.parked = true; // must recheck
- if (w.eventCount != v) {
- w.parked = false;
- return true;
+ if ((h = w.hint) < 0) {
+ if (ec >= 0) { // try to enqueue/inactivate
+ long nc = (((long)ec |
+ ((c - AC_UNIT) & (AC_MASK|TC_MASK))));
+ w.nextWait = e; // link and mark inactive
+ w.eventCount = ec | INT_SIGN;
+ if (ctl != c || !U.compareAndSwapLong(this, CTL, c, nc))
+ w.eventCount = ec; // unmark on CAS failure
+ else if ((int)(c >> AC_SHIFT) == 1 - (config & SMASK))
+ idleAwaitWork(w, nc, c);
+ }
+ else if (w.eventCount < 0 && !tryTerminate(false, false) &&
+ ctl == c) { // block
+ Thread wt = Thread.currentThread();
+ Thread.interrupted(); // clear status
+ U.putObject(wt, PARKBLOCKER, this);
+ w.parker = wt; // emulate LockSupport.park
+ if (w.eventCount < 0) // recheck
+ U.park(false, 0L);
+ w.parker = null;
+ U.putObject(wt, PARKBLOCKER, null);
+ }
}
- LockSupport.park(this);
- rescanned = w.parked = false;
+ if ((h >= 0 || (h = w.hint) >= 0) &&
+ (ws = workQueues) != null && h < ws.length &&
+ (q = ws[h]) != null) { // signal others before retry
+ WorkQueue v; Thread p; int u, i, s;
+ for (int n = (config & SMASK) >>> 1;;) {
+ int idleCount = (w.eventCount < 0) ? 0 : -1;
+ if (((s = idleCount - q.base + q.top) <= n &&
+ (n = s) <= 0) ||
+ (u = (int)((c = ctl) >>> 32)) >= 0 ||
+ (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+ (v = ws[i]) == null)
+ break;
+ long nc = (((long)(v.nextWait & E_MASK)) |
+ ((long)(u + UAC_UNIT) << 32));
+ if (v.eventCount != (e | INT_SIGN) ||
+ !U.compareAndSwapLong(this, CTL, c, nc))
+ break;
+ v.hint = h;
+ v.eventCount = (e + E_SEQ) & E_MASK;
+ if ((p = v.parker) != null)
+ U.unpark(p);
+ if (--n <= 0)
+ break;
+ }
+ }
}
}
+ return null;
}
/**
- * If inactivating worker w has caused pool to become
- * quiescent, check for pool termination, and wait for event
- * for up to SHRINK_RATE nanosecs (rescans are unnecessary in
- * this case because quiescence reflects consensus about lack
- * of work). On timeout, if ctl has not changed, terminate the
- * worker. Upon its termination (see deregisterWorker), it may
- * wake up another worker to possibly repeat this process.
+ * If inactivating worker w has caused the pool to become
+ * quiescent, checks for pool termination, and, so long as this is
+ * not the only worker, waits for event for up to a given
+ * duration. On timeout, if ctl has not changed, terminates the
+ * worker, which will in turn wake up another worker to possibly
+ * repeat this process.
*
* @param w the calling worker
- * @param currentCtl the ctl value after enqueuing w
- * @param prevCtl the ctl value if w terminated
- * @param v the eventCount w awaits change
+ * @param currentCtl the ctl value triggering possible quiescence
+ * @param prevCtl the ctl value to restore if thread is terminated
*/
- private void idleAwaitWork(ForkJoinWorkerThread w, long currentCtl,
- long prevCtl, int v) {
- if (w.eventCount == v) {
- if (shutdown)
- tryTerminate(false);
- ForkJoinTask.helpExpungeStaleExceptions(); // help clean weak refs
+ private void idleAwaitWork(WorkQueue w, long currentCtl, long prevCtl) {
+ if (w != null && w.eventCount < 0 &&
+ !tryTerminate(false, false) && (int)prevCtl != 0) {
+ int dc = -(short)(currentCtl >>> TC_SHIFT);
+ long parkTime = dc < 0 ? FAST_IDLE_TIMEOUT: (dc + 1) * IDLE_TIMEOUT;
+ long deadline = System.nanoTime() + parkTime - TIMEOUT_SLOP;
+ Thread wt = Thread.currentThread();
while (ctl == currentCtl) {
- long startTime = System.nanoTime();
- w.parked = true;
- if (w.eventCount == v) // must recheck
- LockSupport.parkNanos(this, SHRINK_RATE);
- w.parked = false;
- if (w.eventCount != v)
+ Thread.interrupted(); // timed variant of version in scan()
+ U.putObject(wt, PARKBLOCKER, this);
+ w.parker = wt;
+ if (ctl == currentCtl)
+ U.park(false, parkTime);
+ w.parker = null;
+ U.putObject(wt, PARKBLOCKER, null);
+ if (ctl != currentCtl)
break;
- else if (System.nanoTime() - startTime <
- SHRINK_RATE - (SHRINK_RATE / 10)) // timing slop
- Thread.interrupted(); // spurious wakeup
- else if (UNSAFE.compareAndSwapLong(this, ctlOffset,
- currentCtl, prevCtl)) {
- w.terminate = true; // restore previous
- w.eventCount = ((int)currentCtl + EC_UNIT) & E_MASK;
+ if (deadline - System.nanoTime() <= 0L &&
+ U.compareAndSwapLong(this, CTL, currentCtl, prevCtl)) {
+ w.eventCount = (w.eventCount + E_SEQ) | E_MASK;
+ w.qlock = -1; // shrink
break;
}
}
}
}
- // Submissions
-
/**
- * Enqueues the given task in the submissionQueue. Same idea as
- * ForkJoinWorkerThread.pushTask except for use of submissionLock.
- *
- * @param t the task
- */
- private void addSubmission(ForkJoinTask<?> t) {
- final ReentrantLock lock = this.submissionLock;
- lock.lock();
- try {
- ForkJoinTask<?>[] q; int s, m;
- if ((q = submissionQueue) != null) { // ignore if queue removed
- long u = (((s = queueTop) & (m = q.length-1)) << ASHIFT)+ABASE;
- UNSAFE.putOrderedObject(q, u, t);
- queueTop = s + 1;
- if (s - queueBase == m)
- growSubmissionQueue();
- }
- } finally {
- lock.unlock();
- }
- signalWork();
- }
-
- // (pollSubmission is defined below with exported methods)
-
- /**
- * Creates or doubles submissionQueue array.
- * Basically identical to ForkJoinWorkerThread version.
- */
- private void growSubmissionQueue() {
- ForkJoinTask<?>[] oldQ = submissionQueue;
- int size = oldQ != null ? oldQ.length << 1 : INITIAL_QUEUE_CAPACITY;
- if (size > MAXIMUM_QUEUE_CAPACITY)
- throw new RejectedExecutionException("Queue capacity exceeded");
- if (size < INITIAL_QUEUE_CAPACITY)
- size = INITIAL_QUEUE_CAPACITY;
- ForkJoinTask<?>[] q = submissionQueue = new ForkJoinTask<?>[size];
- int mask = size - 1;
- int top = queueTop;
- int oldMask;
- if (oldQ != null && (oldMask = oldQ.length - 1) >= 0) {
- for (int b = queueBase; b != top; ++b) {
- long u = ((b & oldMask) << ASHIFT) + ABASE;
- Object x = UNSAFE.getObjectVolatile(oldQ, u);
- if (x != null && UNSAFE.compareAndSwapObject(oldQ, u, x, null))
- UNSAFE.putObjectVolatile
- (q, ((b & mask) << ASHIFT) + ABASE, x);
- }
- }
- }
-
- // Blocking support
-
- /**
- * Tries to increment blockedCount, decrement active count
- * (sometimes implicitly) and possibly release or create a
- * compensating worker in preparation for blocking. Fails
- * on contention or termination.
+ * Scans through queues looking for work while joining a task; if
+ * any present, signals. May return early if more signalling is
+ * detectably unneeded.
*
- * @return true if the caller can block, else should recheck and retry
- */
- private boolean tryPreBlock() {
- int b = blockedCount;
- if (UNSAFE.compareAndSwapInt(this, blockedCountOffset, b, b + 1)) {
- int pc = parallelism;
- do {
- ForkJoinWorkerThread[] ws; ForkJoinWorkerThread w;
- int e, ac, tc, rc, i;
- long c = ctl;
- int u = (int)(c >>> 32);
- if ((e = (int)c) < 0) {
- // skip -- terminating
- }
- else if ((ac = (u >> UAC_SHIFT)) <= 0 && e != 0 &&
- (ws = workers) != null &&
- (i = ~e & SMASK) < ws.length &&
- (w = ws[i]) != null) {
- long nc = ((long)(w.nextWait & E_MASK) |
- (c & (AC_MASK|TC_MASK)));
- if (w.eventCount == e &&
- UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
- w.eventCount = (e + EC_UNIT) & E_MASK;
- if (w.parked)
- UNSAFE.unpark(w);
- return true; // release an idle worker
- }
- }
- else if ((tc = (short)(u >>> UTC_SHIFT)) >= 0 && ac + pc > 1) {
- long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
- if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc))
- return true; // no compensation needed
- }
- else if (tc + pc < MAX_ID) {
- long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
- if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, nc)) {
- addWorker();
- return true; // create a replacement
- }
- }
- // try to back out on any failure and let caller retry
- } while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
- b = blockedCount, b - 1));
- }
- return false;
- }
-
- /**
- * Decrements blockedCount and increments active count
- */
- private void postBlock() {
- long c;
- do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, // no mask
- c = ctl, c + AC_UNIT));
- int b;
- do {} while (!UNSAFE.compareAndSwapInt(this, blockedCountOffset,
- b = blockedCount, b - 1));
- }
-
- /**
- * Possibly blocks waiting for the given task to complete, or
- * cancels the task if terminating. Fails to wait if contended.
- *
- * @param joinMe the task
+ * @param task return early if done
+ * @param origin an index to start scan
*/
- final void tryAwaitJoin(ForkJoinTask<?> joinMe) {
- int s;
- Thread.interrupted(); // clear interrupts before checking termination
- if (joinMe.status >= 0) {
- if (tryPreBlock()) {
- joinMe.tryAwaitDone(0L);
- postBlock();
- }
- else if ((ctl & STOP_BIT) != 0L)
- joinMe.cancelIgnoringExceptions();
- }
- }
-
- /**
- * Possibly blocks the given worker waiting for joinMe to
- * complete or timeout
- *
- * @param joinMe the task
- * @param millis the wait time for underlying Object.wait
- */
- final void timedAwaitJoin(ForkJoinTask<?> joinMe, long nanos) {
- while (joinMe.status >= 0) {
- Thread.interrupted();
- if ((ctl & STOP_BIT) != 0L) {
- joinMe.cancelIgnoringExceptions();
- break;
- }
- if (tryPreBlock()) {
- long last = System.nanoTime();
- while (joinMe.status >= 0) {
- long millis = TimeUnit.NANOSECONDS.toMillis(nanos);
- if (millis <= 0)
- break;
- joinMe.tryAwaitDone(millis);
- if (joinMe.status < 0)
- break;
- if ((ctl & STOP_BIT) != 0L) {
- joinMe.cancelIgnoringExceptions();
+ private void helpSignal(ForkJoinTask<?> task, int origin) {
+ WorkQueue[] ws; WorkQueue w; Thread p; long c; int m, u, e, i, s;
+ if (task != null && task.status >= 0 &&
+ (u = (int)(ctl >>> 32)) < 0 && (u >> UAC_SHIFT) < 0 &&
+ (ws = workQueues) != null && (m = ws.length - 1) >= 0) {
+ outer: for (int k = origin, j = m; j >= 0; --j) {
+ WorkQueue q = ws[k++ & m];
+ for (int n = m;;) { // limit to at most m signals
+ if (task.status < 0)
+ break outer;
+ if (q == null ||
+ ((s = -q.base + q.top) <= n && (n = s) <= 0))
break;
- }
- long now = System.nanoTime();
- nanos -= now - last;
- last = now;
- }
- postBlock();
- break;
- }
- }
- }
-
- /**
- * If necessary, compensates for blocker, and blocks
- */
- private void awaitBlocker(ManagedBlocker blocker)
- throws InterruptedException {
- while (!blocker.isReleasable()) {
- if (tryPreBlock()) {
- try {
- do {} while (!blocker.isReleasable() && !blocker.block());
- } finally {
- postBlock();
- }
- break;
- }
- }
- }
-
- // Creating, registering and deregistring workers
-
- /**
- * Tries to create and start a worker; minimally rolls back counts
- * on failure.
- */
- private void addWorker() {
- Throwable ex = null;
- ForkJoinWorkerThread t = null;
- try {
- t = factory.newThread(this);
- } catch (Throwable e) {
- ex = e;
- }
- if (t == null) { // null or exceptional factory return
- long c; // adjust counts
- do {} while (!UNSAFE.compareAndSwapLong
- (this, ctlOffset, c = ctl,
- (((c - AC_UNIT) & AC_MASK) |
- ((c - TC_UNIT) & TC_MASK) |
- (c & ~(AC_MASK|TC_MASK)))));
- // Propagate exception if originating from an external caller
- if (!tryTerminate(false) && ex != null &&
- !(Thread.currentThread() instanceof ForkJoinWorkerThread))
- UNSAFE.throwException(ex);
- }
- else
- t.start();
- }
-
- /**
- * Callback from ForkJoinWorkerThread constructor to assign a
- * public name
- */
- final String nextWorkerName() {
- for (int n;;) {
- if (UNSAFE.compareAndSwapInt(this, nextWorkerNumberOffset,
- n = nextWorkerNumber, ++n))
- return workerNamePrefix + n;
- }
- }
-
- /**
- * Callback from ForkJoinWorkerThread constructor to
- * determine its poolIndex and record in workers array.
- *
- * @param w the worker
- * @return the worker's pool index
- */
- final int registerWorker(ForkJoinWorkerThread w) {
- /*
- * In the typical case, a new worker acquires the lock, uses
- * next available index and returns quickly. Since we should
- * not block callers (ultimately from signalWork or
- * tryPreBlock) waiting for the lock needed to do this, we
- * instead help release other workers while waiting for the
- * lock.
- */
- for (int g;;) {
- ForkJoinWorkerThread[] ws;
- if (((g = scanGuard) & SG_UNIT) == 0 &&
- UNSAFE.compareAndSwapInt(this, scanGuardOffset,
- g, g | SG_UNIT)) {
- int k = nextWorkerIndex;
- try {
- if ((ws = workers) != null) { // ignore on shutdown
- int n = ws.length;
- if (k < 0 || k >= n || ws[k] != null) {
- for (k = 0; k < n && ws[k] != null; ++k)
- ;
- if (k == n)
- ws = workers = Arrays.copyOf(ws, n << 1);
- }
- ws[k] = w;
- nextWorkerIndex = k + 1;
- int m = g & SMASK;
- g = (k > m) ? ((m << 1) + 1) & SMASK : g + (SG_UNIT<<1);
- }
- } finally {
- scanGuard = g;
- }
- return k;
- }
- else if ((ws = workers) != null) { // help release others
- for (ForkJoinWorkerThread u : ws) {
- if (u != null && u.queueBase != u.queueTop) {
- if (tryReleaseWaiter())
+ if ((u = (int)((c = ctl) >>> 32)) >= 0 ||
+ (e = (int)c) <= 0 || m < (i = e & SMASK) ||
+ (w = ws[i]) == null)
+ break outer;
+ long nc = (((long)(w.nextWait & E_MASK)) |
+ ((long)(u + UAC_UNIT) << 32));
+ if (w.eventCount != (e | INT_SIGN))
+ break outer;
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ w.eventCount = (e + E_SEQ) & E_MASK;
+ if ((p = w.parker) != null)
+ U.unpark(p);
+ if (--n <= 0)
break;
}
}
@@ -1197,202 +1881,631 @@
}
/**
- * Final callback from terminating worker. Removes record of
- * worker from array, and adjusts counts. If pool is shutting
- * down, tries to complete termination.
- *
- * @param w the worker
- */
- final void deregisterWorker(ForkJoinWorkerThread w, Throwable ex) {
- int idx = w.poolIndex;
- int sc = w.stealCount;
- int steps = 0;
- // Remove from array, adjust worker counts and collect steal count.
- // We can intermix failed removes or adjusts with steal updates
- do {
- long s, c;
- int g;
- if (steps == 0 && ((g = scanGuard) & SG_UNIT) == 0 &&
- UNSAFE.compareAndSwapInt(this, scanGuardOffset,
- g, g |= SG_UNIT)) {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null && idx >= 0 &&
- idx < ws.length && ws[idx] == w)
- ws[idx] = null; // verify
- nextWorkerIndex = idx;
- scanGuard = g + SG_UNIT;
- steps = 1;
- }
- if (steps == 1 &&
- UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
- (((c - AC_UNIT) & AC_MASK) |
- ((c - TC_UNIT) & TC_MASK) |
- (c & ~(AC_MASK|TC_MASK)))))
- steps = 2;
- if (sc != 0 &&
- UNSAFE.compareAndSwapLong(this, stealCountOffset,
- s = stealCount, s + sc))
- sc = 0;
- } while (steps != 2 || sc != 0);
- if (!tryTerminate(false)) {
- if (ex != null) // possibly replace if died abnormally
- signalWork();
- else
- tryReleaseWaiter();
- }
- }
-
- // Shutdown and termination
-
- /**
- * Possibly initiates and/or completes termination.
+ * 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 first call to this method upon a
+ * waiting join will often entail scanning/search, (which is OK
+ * because the joiner has nothing better to do), but this method
+ * leaves hints in workers to speed up subsequent calls. The
+ * implementation is very branchy to cope with potential
+ * inconsistencies or loops encountering chains that are stale,
+ * unknown, or so long that they are likely cyclic.
*
- * @param now if true, unconditionally terminate, else only
- * if shutdown and empty queue and no active workers
- * @return true if now terminating or terminated
+ * @param joiner the joining worker
+ * @param task the task to join
+ * @return 0 if no progress can be made, negative if task
+ * known complete, else positive
*/
- private boolean tryTerminate(boolean now) {
- long c;
- while (((c = ctl) & STOP_BIT) == 0) {
- if (!now) {
- if ((int)(c >> AC_SHIFT) != -parallelism)
- return false;
- if (!shutdown || blockedCount != 0 || quiescerCount != 0 ||
- queueBase != queueTop) {
- if (ctl == c) // staleness check
- return false;
- continue;
- }
- }
- if (UNSAFE.compareAndSwapLong(this, ctlOffset, c, c | STOP_BIT))
- startTerminating();
- }
- if ((short)(c >>> TC_SHIFT) == -parallelism) { // signal when 0 workers
- final ReentrantLock lock = this.submissionLock;
- lock.lock();
- try {
- termination.signalAll();
- } finally {
- lock.unlock();
- }
- }
- return true;
- }
-
- /**
- * Runs up to three passes through workers: (0) Setting
- * termination status for each worker, followed by wakeups up to
- * queued workers; (1) helping cancel tasks; (2) interrupting
- * lagging threads (likely in external tasks, but possibly also
- * blocked in joins). Each pass repeats previous steps because of
- * potential lagging thread creation.
- */
- private void startTerminating() {
- cancelSubmissions();
- for (int pass = 0; pass < 3; ++pass) {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- for (ForkJoinWorkerThread w : ws) {
- if (w != null) {
- w.terminate = true;
- if (pass > 0) {
- w.cancelTasks();
- if (pass > 1 && !w.isInterrupted()) {
- try {
- w.interrupt();
- } catch (SecurityException ignore) {
- }
+ private int tryHelpStealer(WorkQueue joiner, ForkJoinTask<?> task) {
+ int stat = 0, steps = 0; // bound to avoid cycles
+ if (joiner != null && task != null) { // hoist null checks
+ restart: for (;;) {
+ ForkJoinTask<?> subtask = task; // current target
+ for (WorkQueue j = joiner, v;;) { // v is stealer of subtask
+ WorkQueue[] ws; int m, s, h;
+ if ((s = task.status) < 0) {
+ stat = s;
+ break restart;
+ }
+ if ((ws = workQueues) == null || (m = ws.length - 1) <= 0)
+ break restart; // shutting down
+ if ((v = ws[h = (j.hint | 1) & m]) == null ||
+ v.currentSteal != subtask) {
+ for (int origin = h;;) { // find stealer
+ if (((h = (h + 2) & m) & 15) == 1 &&
+ (subtask.status < 0 || j.currentJoin != subtask))
+ continue restart; // occasional staleness check
+ if ((v = ws[h]) != null &&
+ v.currentSteal == subtask) {
+ j.hint = h; // save hint
+ break;
+ }
+ if (h == origin)
+ break restart; // cannot find stealer
+ }
+ }
+ for (;;) { // help stealer or descend to its stealer
+ ForkJoinTask[] a; int b;
+ if (subtask.status < 0) // surround probes with
+ continue restart; // consistency checks
+ if ((b = v.base) - v.top < 0 && (a = v.array) != null) {
+ int i = (((a.length - 1) & b) << ASHIFT) + ABASE;
+ ForkJoinTask<?> t =
+ (ForkJoinTask<?>)U.getObjectVolatile(a, i);
+ if (subtask.status < 0 || j.currentJoin != subtask ||
+ v.currentSteal != subtask)
+ continue restart; // stale
+ stat = 1; // apparent progress
+ if (t != null && v.base == b &&
+ U.compareAndSwapObject(a, i, t, null)) {
+ v.base = b + 1; // help stealer
+ joiner.runSubtask(t);
+ }
+ else if (v.base == b && ++steps == MAX_HELP)
+ break restart; // v apparently stalled
+ }
+ else { // empty -- try to descend
+ ForkJoinTask<?> next = v.currentJoin;
+ if (subtask.status < 0 || j.currentJoin != subtask ||
+ v.currentSteal != subtask)
+ continue restart; // stale
+ else if (next == null || ++steps == MAX_HELP)
+ break restart; // dead-end or maybe cyclic
+ else {
+ subtask = next;
+ j = v;
+ break;
}
}
}
}
- terminateWaiters();
+ }
+ }
+ return stat;
+ }
+
+ /**
+ * Analog of tryHelpStealer for CountedCompleters. Tries to steal
+ * and run tasks within the target's computation.
+ *
+ * @param task the task to join
+ * @param mode if shared, exit upon completing any task
+ * if all workers are active
+ *
+ */
+ private int helpComplete(ForkJoinTask<?> task, int mode) {
+ WorkQueue[] ws; WorkQueue q; int m, n, s, u;
+ if (task != null && (ws = workQueues) != null &&
+ (m = ws.length - 1) >= 0) {
+ for (int j = 1, origin = j;;) {
+ if ((s = task.status) < 0)
+ return s;
+ if ((q = ws[j & m]) != null && q.pollAndExecCC(task)) {
+ origin = j;
+ if (mode == SHARED_QUEUE &&
+ ((u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0))
+ break;
+ }
+ else if ((j = (j + 2) & m) == origin)
+ break;
+ }
+ }
+ return 0;
+ }
+
+ /**
+ * Tries to decrement active count (sometimes implicitly) and
+ * possibly release or create a compensating worker in preparation
+ * for blocking. Fails on contention or termination. Otherwise,
+ * adds a new thread if no idle workers are available and pool
+ * may become starved.
+ */
+ final boolean tryCompensate() {
+ int pc = config & SMASK, e, i, tc; long c;
+ WorkQueue[] ws; WorkQueue w; Thread p;
+ if ((ws = workQueues) != null && (e = (int)(c = ctl)) >= 0) {
+ if (e != 0 && (i = e & SMASK) < ws.length &&
+ (w = ws[i]) != null && w.eventCount == (e | INT_SIGN)) {
+ long nc = ((long)(w.nextWait & E_MASK) |
+ (c & (AC_MASK|TC_MASK)));
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ w.eventCount = (e + E_SEQ) & E_MASK;
+ if ((p = w.parker) != null)
+ U.unpark(p);
+ return true; // replace with idle worker
+ }
+ }
+ else if ((tc = (short)(c >>> TC_SHIFT)) >= 0 &&
+ (int)(c >> AC_SHIFT) + pc > 1) {
+ long nc = ((c - AC_UNIT) & AC_MASK) | (c & ~AC_MASK);
+ if (U.compareAndSwapLong(this, CTL, c, nc))
+ return true; // no compensation
+ }
+ else if (tc + pc < MAX_CAP) {
+ long nc = ((c + TC_UNIT) & TC_MASK) | (c & ~TC_MASK);
+ if (U.compareAndSwapLong(this, CTL, c, nc)) {
+ ForkJoinWorkerThreadFactory fac;
+ Throwable ex = null;
+ ForkJoinWorkerThread wt = null;
+ try {
+ if ((fac = factory) != null &&
+ (wt = fac.newThread(this)) != null) {
+ wt.start();
+ return true;
+ }
+ } catch (Throwable rex) {
+ ex = rex;
+ }
+ deregisterWorker(wt, ex); // clean up and return false
+ }
+ }
+ }
+ return false;
+ }
+
+ /**
+ * Helps and/or blocks until the given task is done.
+ *
+ * @param joiner the joining worker
+ * @param task the task
+ * @return task status on exit
+ */
+ final int awaitJoin(WorkQueue joiner, ForkJoinTask<?> task) {
+ int s = 0;
+ if (joiner != null && task != null && (s = task.status) >= 0) {
+ ForkJoinTask<?> prevJoin = joiner.currentJoin;
+ joiner.currentJoin = task;
+ do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+ joiner.tryRemoveAndExec(task)); // process local tasks
+ if (s >= 0 && (s = task.status) >= 0) {
+ helpSignal(task, joiner.poolIndex);
+ if ((s = task.status) >= 0 &&
+ (task instanceof CountedCompleter))
+ s = helpComplete(task, LIFO_QUEUE);
+ }
+ while (s >= 0 && (s = task.status) >= 0) {
+ if ((!joiner.isEmpty() || // try helping
+ (s = tryHelpStealer(joiner, task)) == 0) &&
+ (s = task.status) >= 0) {
+ helpSignal(task, joiner.poolIndex);
+ if ((s = task.status) >= 0 && tryCompensate()) {
+ if (task.trySetSignal() && (s = task.status) >= 0) {
+ synchronized (task) {
+ if (task.status >= 0) {
+ try { // see ForkJoinTask
+ task.wait(); // for explanation
+ } catch (InterruptedException ie) {
+ }
+ }
+ else
+ task.notifyAll();
+ }
+ }
+ long c; // re-activate
+ do {} while (!U.compareAndSwapLong
+ (this, CTL, c = ctl, c + AC_UNIT));
+ }
+ }
+ }
+ joiner.currentJoin = prevJoin;
+ }
+ return s;
+ }
+
+ /**
+ * Stripped-down variant of awaitJoin used by timed joins. Tries
+ * to help join only while there is continuous progress. (Caller
+ * will then enter a timed wait.)
+ *
+ * @param joiner the joining worker
+ * @param task the task
+ */
+ final void helpJoinOnce(WorkQueue joiner, ForkJoinTask<?> task) {
+ int s;
+ if (joiner != null && task != null && (s = task.status) >= 0) {
+ ForkJoinTask<?> prevJoin = joiner.currentJoin;
+ joiner.currentJoin = task;
+ do {} while ((s = task.status) >= 0 && !joiner.isEmpty() &&
+ joiner.tryRemoveAndExec(task));
+ if (s >= 0 && (s = task.status) >= 0) {
+ helpSignal(task, joiner.poolIndex);
+ if ((s = task.status) >= 0 &&
+ (task instanceof CountedCompleter))
+ s = helpComplete(task, LIFO_QUEUE);
+ }
+ if (s >= 0 && joiner.isEmpty()) {
+ do {} while (task.status >= 0 &&
+ tryHelpStealer(joiner, task) > 0);
+ }
+ joiner.currentJoin = prevJoin;
+ }
+ }
+
+ /**
+ * Returns a (probably) non-empty steal queue, if one is found
+ * during a random, then cyclic scan, else null. This method must
+ * be retried by caller if, by the time it tries to use the queue,
+ * it is empty.
+ * @param r a (random) seed for scanning
+ */
+ private WorkQueue findNonEmptyStealQueue(int r) {
+ for (WorkQueue[] ws;;) {
+ int ps = plock, m, n;
+ if ((ws = workQueues) == null || (m = ws.length - 1) < 1)
+ return null;
+ for (int j = (m + 1) << 2; ;) {
+ WorkQueue q = ws[(((r + j) << 1) | 1) & m];
+ if (q != null && (n = q.base - q.top) < 0) {
+ if (n < -1)
+ signalWork(q);
+ return q;
+ }
+ else if (--j < 0) {
+ if (plock == ps)
+ return null;
+ break;
+ }
}
}
}
/**
- * Polls and cancels all submissions. Called only during termination.
+ * Runs tasks until {@code isQuiescent()}. We piggyback on
+ * active count ctl maintenance, but rather than blocking
+ * when tasks cannot be found, we rescan until all others cannot
+ * find tasks either.
*/
- private void cancelSubmissions() {
- while (queueBase != queueTop) {
- ForkJoinTask<?> task = pollSubmission();
- if (task != null) {
- try {
- task.cancel(false);
- } catch (Throwable ignore) {
+ final void helpQuiescePool(WorkQueue w) {
+ for (boolean active = true;;) {
+ ForkJoinTask<?> localTask; // exhaust local queue
+ while ((localTask = w.nextLocalTask()) != null)
+ localTask.doExec();
+ // Similar to loop in scan(), but ignoring submissions
+ WorkQueue q = findNonEmptyStealQueue(w.nextSeed());
+ if (q != null) {
+ ForkJoinTask<?> t; int b;
+ if (!active) { // re-establish active count
+ long c;
+ active = true;
+ do {} while (!U.compareAndSwapLong
+ (this, CTL, c = ctl, c + AC_UNIT));
+ }
+ if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
+ w.runSubtask(t);
+ }
+ else {
+ long c;
+ if (active) { // decrement active count without queuing
+ active = false;
+ do {} while (!U.compareAndSwapLong
+ (this, CTL, c = ctl, c -= AC_UNIT));
+ }
+ else
+ c = ctl; // re-increment on exit
+ if ((int)(c >> AC_SHIFT) + (config & SMASK) == 0) {
+ do {} while (!U.compareAndSwapLong
+ (this, CTL, c = ctl, c + AC_UNIT));
+ break;
}
}
}
}
/**
- * Tries to set the termination status of waiting workers, and
- * then wakes them up (after which they will terminate).
+ * Gets and removes a local or stolen task for the given worker.
+ *
+ * @return a task, if available
+ */
+ final ForkJoinTask<?> nextTaskFor(WorkQueue w) {
+ for (ForkJoinTask<?> t;;) {
+ WorkQueue q; int b;
+ if ((t = w.nextLocalTask()) != null)
+ return t;
+ if ((q = findNonEmptyStealQueue(w.nextSeed())) == null)
+ return null;
+ if ((b = q.base) - q.top < 0 && (t = q.pollAt(b)) != null)
+ return t;
+ }
+ }
+
+ /**
+ * Returns 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. 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.
+ *
+ * Note: The approximation of #busy workers as #active workers is
+ * not very good under current signalling scheme, and should be
+ * improved.
*/
- private void terminateWaiters() {
- ForkJoinWorkerThread[] ws = workers;
- if (ws != null) {
- ForkJoinWorkerThread w; long c; int i, e;
- int n = ws.length;
- while ((i = ~(e = (int)(c = ctl)) & SMASK) < n &&
- (w = ws[i]) != null && w.eventCount == (e & E_MASK)) {
- if (UNSAFE.compareAndSwapLong(this, ctlOffset, c,
- (long)(w.nextWait & E_MASK) |
- ((c + AC_UNIT) & AC_MASK) |
- (c & (TC_MASK|STOP_BIT)))) {
- w.terminate = true;
- w.eventCount = e + EC_UNIT;
- if (w.parked)
- UNSAFE.unpark(w);
+ static int getSurplusQueuedTaskCount() {
+ Thread t; ForkJoinWorkerThread wt; ForkJoinPool pool; WorkQueue q;
+ if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) {
+ int p = (pool = (wt = (ForkJoinWorkerThread)t).pool).config & SMASK;
+ int n = (q = wt.workQueue).top - q.base;
+ int a = (int)(pool.ctl >> AC_SHIFT) + p;
+ return n - (a > (p >>>= 1) ? 0 :
+ a > (p >>>= 1) ? 1 :
+ a > (p >>>= 1) ? 2 :
+ a > (p >>>= 1) ? 4 :
+ 8);
+ }
+ return 0;
+ }
+
+ // Termination
+
+ /**
+ * Possibly initiates and/or completes termination. The caller
+ * triggering termination runs three passes through workQueues:
+ * (0) Setting termination status, followed by wakeups of queued
+ * workers; (1) cancelling all tasks; (2) interrupting lagging
+ * threads (likely in external tasks, but possibly also blocked in
+ * joins). Each pass repeats previous steps because of potential
+ * lagging thread creation.
+ *
+ * @param now if true, unconditionally terminate, else only
+ * if no work and no active workers
+ * @param enable if true, enable shutdown when next possible
+ * @return true if now terminating or terminated
+ */
+ private boolean tryTerminate(boolean now, boolean enable) {
+ if (this == commonPool) // cannot shut down
+ return false;
+ for (long c;;) {
+ if (((c = ctl) & STOP_BIT) != 0) { // already terminating
+ if ((short)(c >>> TC_SHIFT) == -(config & SMASK)) {
+ synchronized (this) {
+ notifyAll(); // signal when 0 workers
+ }
+ }
+ return true;
+ }
+ if (plock >= 0) { // not yet enabled
+ int ps;
+ if (!enable)
+ return false;
+ if (((ps = plock) & PL_LOCK) != 0 ||
+ !U.compareAndSwapInt(this, PLOCK, ps, ps += PL_LOCK))
+ ps = acquirePlock();
+ if (!U.compareAndSwapInt(this, PLOCK, ps, SHUTDOWN))
+ releasePlock(SHUTDOWN);
+ }
+ if (!now) { // check if idle & no tasks
+ if ((int)(c >> AC_SHIFT) != -(config & SMASK) ||
+ hasQueuedSubmissions())
+ return false;
+ // Check for unqueued inactive workers. One pass suffices.
+ WorkQueue[] ws = workQueues; WorkQueue w;
+ if (ws != null) {
+ for (int i = 1; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null && w.eventCount >= 0)
+ return false;
+ }
+ }
+ }
+ if (U.compareAndSwapLong(this, CTL, c, c | STOP_BIT)) {
+ for (int pass = 0; pass < 3; ++pass) {
+ WorkQueue[] ws = workQueues;
+ if (ws != null) {
+ WorkQueue w; Thread wt;
+ int n = ws.length;
+ for (int i = 0; i < n; ++i) {
+ if ((w = ws[i]) != null) {
+ w.qlock = -1;
+ if (pass > 0) {
+ w.cancelAll();
+ if (pass > 1 && (wt = w.owner) != null) {
+ if (!wt.isInterrupted()) {
+ try {
+ wt.interrupt();
+ } catch (SecurityException ignore) {
+ }
+ }
+ U.unpark(wt);
+ }
+ }
+ }
+ }
+ // Wake up workers parked on event queue
+ int i, e; long cc; Thread p;
+ while ((e = (int)(cc = ctl) & E_MASK) != 0 &&
+ (i = e & SMASK) < n &&
+ (w = ws[i]) != null) {
+ long nc = ((long)(w.nextWait & E_MASK) |
+ ((cc + AC_UNIT) & AC_MASK) |
+ (cc & (TC_MASK|STOP_BIT)));
+ if (w.eventCount == (e | INT_SIGN) &&
+ U.compareAndSwapLong(this, CTL, cc, nc)) {
+ w.eventCount = (e + E_SEQ) & E_MASK;
+ w.qlock = -1;
+ if ((p = w.parker) != null)
+ U.unpark(p);
+ }
+ }
+ }
}
}
}
}
- // misc ForkJoinWorkerThread support
+ // external operations on common pool
+
+ /**
+ * Returns common pool queue for a thread that has submitted at
+ * least one task.
+ */
+ static WorkQueue commonSubmitterQueue() {
+ ForkJoinPool p; WorkQueue[] ws; int m; Submitter z;
+ return ((z = submitters.get()) != null &&
+ (p = commonPool) != null &&
+ (ws = p.workQueues) != null &&
+ (m = ws.length - 1) >= 0) ?
+ ws[m & z.seed & SQMASK] : null;
+ }
/**
- * Increment or decrement quiescerCount. Needed only to prevent
- * triggering shutdown if a worker is transiently inactive while
- * checking quiescence.
- *
- * @param delta 1 for increment, -1 for decrement
+ * Tries to pop the given task from submitter's queue in common pool.
*/
- final void addQuiescerCount(int delta) {
- int c;
- do {} while (!UNSAFE.compareAndSwapInt(this, quiescerCountOffset,
- c = quiescerCount, c + delta));
+ static boolean tryExternalUnpush(ForkJoinTask<?> t) {
+ ForkJoinPool p; WorkQueue[] ws; WorkQueue q; Submitter z;
+ ForkJoinTask<?>[] a; int m, s;
+ if (t != null &&
+ (z = submitters.get()) != null &&
+ (p = commonPool) != null &&
+ (ws = p.workQueues) != null &&
+ (m = ws.length - 1) >= 0 &&
+ (q = ws[m & z.seed & SQMASK]) != null &&
+ (s = q.top) != q.base &&
+ (a = q.array) != null) {
+ long j = (((a.length - 1) & (s - 1)) << ASHIFT) + ABASE;
+ if (U.getObject(a, j) == t &&
+ U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+ if (q.array == a && q.top == s && // recheck
+ U.compareAndSwapObject(a, j, t, null)) {
+ q.top = s - 1;
+ q.qlock = 0;
+ return true;
+ }
+ q.qlock = 0;
+ }
+ }
+ return false;
}
/**
- * Directly increment or decrement active count without
- * queuing. This method is used to transiently assert inactivation
- * while checking quiescence.
- *
- * @param delta 1 for increment, -1 for decrement
+ * Tries to pop and run local tasks within the same computation
+ * as the given root. On failure, tries to help complete from
+ * other queues via helpComplete.
*/
- final void addActiveCount(int delta) {
- long d = delta < 0 ? -AC_UNIT : AC_UNIT;
- long c;
- do {} while (!UNSAFE.compareAndSwapLong(this, ctlOffset, c = ctl,
- ((c + d) & AC_MASK) |
- (c & ~AC_MASK)));
+ private void externalHelpComplete(WorkQueue q, ForkJoinTask<?> root) {
+ ForkJoinTask<?>[] a; int m;
+ if (q != null && (a = q.array) != null && (m = (a.length - 1)) >= 0 &&
+ root != null && root.status >= 0) {
+ for (;;) {
+ int s, u; Object o; CountedCompleter<?> task = null;
+ if ((s = q.top) - q.base > 0) {
+ long j = ((m & (s - 1)) << ASHIFT) + ABASE;
+ if ((o = U.getObject(a, j)) != null &&
+ (o instanceof CountedCompleter)) {
+ CountedCompleter<?> t = (CountedCompleter<?>)o, r = t;
+ do {
+ if (r == root) {
+ if (U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+ if (q.array == a && q.top == s &&
+ U.compareAndSwapObject(a, j, t, null)) {
+ q.top = s - 1;
+ task = t;
+ }
+ q.qlock = 0;
+ }
+ break;
+ }
+ } while ((r = r.completer) != null);
+ }
+ }
+ if (task != null)
+ task.doExec();
+ if (root.status < 0 ||
+ (u = (int)(ctl >>> 32)) >= 0 || (u >> UAC_SHIFT) >= 0)
+ break;
+ if (task == null) {
+ helpSignal(root, q.poolIndex);
+ if (root.status >= 0)
+ helpComplete(root, SHARED_QUEUE);
+ break;
+ }
+ }
+ }
}
/**
- * Returns the approximate (non-atomic) number of idle threads per
- * active thread.
+ * Tries to help execute or signal availability of the given task
+ * from submitter's queue in common pool.
*/
- final int idlePerActive() {
- // Approximate at powers of two for small values, saturate past 4
- int p = parallelism;
- int a = p + (int)(ctl >> AC_SHIFT);
- return (a > (p >>>= 1) ? 0 :
- a > (p >>>= 1) ? 1 :
- a > (p >>>= 1) ? 2 :
- a > (p >>>= 1) ? 4 :
- 8);
+ static void externalHelpJoin(ForkJoinTask<?> t) {
+ // Some hard-to-avoid overlap with tryExternalUnpush
+ ForkJoinPool p; WorkQueue[] ws; WorkQueue q, w; Submitter z;
+ ForkJoinTask<?>[] a; int m, s, n;
+ if (t != null &&
+ (z = submitters.get()) != null &&
+ (p = commonPool) != null &&
+ (ws = p.workQueues) != null &&
+ (m = ws.length - 1) >= 0 &&
+ (q = ws[m & z.seed & SQMASK]) != null &&
+ (a = q.array) != null) {
+ int am = a.length - 1;
+ if ((s = q.top) != q.base) {
+ long j = ((am & (s - 1)) << ASHIFT) + ABASE;
+ if (U.getObject(a, j) == t &&
+ U.compareAndSwapInt(q, QLOCK, 0, 1)) {
+ if (q.array == a && q.top == s &&
+ U.compareAndSwapObject(a, j, t, null)) {
+ q.top = s - 1;
+ q.qlock = 0;
+ t.doExec();
+ }
+ else
+ q.qlock = 0;
+ }
+ }
+ if (t.status >= 0) {
+ if (t instanceof CountedCompleter)
+ p.externalHelpComplete(q, t);
+ else
+ p.helpSignal(t, q.poolIndex);
+ }
+ }
+ }
+
+ /**
+ * Restricted version of helpQuiescePool for external callers
+ */
+ static void externalHelpQuiescePool() {
+ ForkJoinPool p; ForkJoinTask<?> t; WorkQueue q; int b;
+ if ((p = commonPool) != null &&
+ (q = p.findNonEmptyStealQueue(1)) != null &&
+ (b = q.base) - q.top < 0 &&
+ (t = q.pollAt(b)) != null)
+ t.doExec();
}
// Exported methods
@@ -1464,31 +2577,46 @@
checkPermission();
if (factory == null)
throw new NullPointerException();
- if (parallelism <= 0 || parallelism > MAX_ID)
+ if (parallelism <= 0 || parallelism > MAX_CAP)
throw new IllegalArgumentException();
- this.parallelism = parallelism;
this.factory = factory;
this.ueh = handler;
- this.locallyFifo = asyncMode;
+ this.config = parallelism | (asyncMode ? (FIFO_QUEUE << 16) : 0);
long np = (long)(-parallelism); // offset ctl counts
this.ctl = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
- this.submissionQueue = new ForkJoinTask<?>[INITIAL_QUEUE_CAPACITY];
- // initialize workers array with room for 2*parallelism if possible
- int n = parallelism << 1;
- if (n >= MAX_ID)
- n = MAX_ID;
- else { // See Hackers Delight, sec 3.2, where n < (1 << 16)
- n |= n >>> 1; n |= n >>> 2; n |= n >>> 4; n |= n >>> 8;
- }
- workers = new ForkJoinWorkerThread[n + 1];
- this.submissionLock = new ReentrantLock();
- this.termination = submissionLock.newCondition();
+ int pn = nextPoolId();
StringBuilder sb = new StringBuilder("ForkJoinPool-");
- sb.append(poolNumberGenerator.incrementAndGet());
+ sb.append(Integer.toString(pn));
sb.append("-worker-");
this.workerNamePrefix = sb.toString();
}
+ /**
+ * Constructor for common pool, suitable only for static initialization.
+ * Basically the same as above, but uses smallest possible initial footprint.
+ */
+ ForkJoinPool(int parallelism, long ctl,
+ ForkJoinWorkerThreadFactory factory,
+ Thread.UncaughtExceptionHandler handler) {
+ this.config = parallelism;
+ this.ctl = ctl;
+ this.factory = factory;
+ this.ueh = handler;
+ this.workerNamePrefix = "ForkJoinPool.commonPool-worker-";
+ }
+
+ /**
+ * Returns the common pool instance. This pool is statically
+ * constructed; its run state is unaffected by attempts to
+ * {@link #shutdown} or {@link #shutdownNow}.
+ *
+ * @return the common pool instance
+ */
+ public static ForkJoinPool commonPool() {
+ // assert commonPool != null : "static init error";
+ return commonPool;
+ }
+
// Execution methods
/**
@@ -1508,34 +2636,10 @@
* scheduled for execution
*/
public <T> T invoke(ForkJoinTask<T> task) {
- Thread t = Thread.currentThread();
if (task == null)
throw new NullPointerException();
- if (shutdown)
- throw new RejectedExecutionException();
- if ((t instanceof ForkJoinWorkerThread) &&
- ((ForkJoinWorkerThread)t).pool == this)
- return task.invoke(); // bypass submit if in same pool
- else {
- addSubmission(task);
- return task.join();
- }
- }
-
- /**
- * Unless terminating, forks task if within an ongoing FJ
- * computation in the current pool, else submits as external task.
- */
- private <T> void forkOrSubmit(ForkJoinTask<T> task) {
- ForkJoinWorkerThread w;
- Thread t = Thread.currentThread();
- if (shutdown)
- throw new RejectedExecutionException();
- if ((t instanceof ForkJoinWorkerThread) &&
- (w = (ForkJoinWorkerThread)t).pool == this)
- w.pushTask(task);
- else
- addSubmission(task);
+ externalPush(task);
+ return task.join();
}
/**
@@ -1549,7 +2653,7 @@
public void execute(ForkJoinTask<?> task) {
if (task == null)
throw new NullPointerException();
- forkOrSubmit(task);
+ externalPush(task);
}
// AbstractExecutorService methods
@@ -1566,8 +2670,8 @@
if (task instanceof ForkJoinTask<?>) // avoid re-wrap
job = (ForkJoinTask<?>) task;
else
- job = ForkJoinTask.adapt(task, null);
- forkOrSubmit(job);
+ job = new ForkJoinTask.AdaptedRunnableAction(task);
+ externalPush(job);
}
/**
@@ -1582,7 +2686,7 @@
public <T> ForkJoinTask<T> submit(ForkJoinTask<T> task) {
if (task == null)
throw new NullPointerException();
- forkOrSubmit(task);
+ externalPush(task);
return task;
}
@@ -1592,10 +2696,8 @@
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Callable<T> task) {
- if (task == null)
- throw new NullPointerException();
- ForkJoinTask<T> job = ForkJoinTask.adapt(task);
- forkOrSubmit(job);
+ ForkJoinTask<T> job = new ForkJoinTask.AdaptedCallable<T>(task);
+ externalPush(job);
return job;
}
@@ -1605,10 +2707,8 @@
* scheduled for execution
*/
public <T> ForkJoinTask<T> submit(Runnable task, T result) {
- if (task == null)
- throw new NullPointerException();
- ForkJoinTask<T> job = ForkJoinTask.adapt(task, result);
- forkOrSubmit(job);
+ ForkJoinTask<T> job = new ForkJoinTask.AdaptedRunnable<T>(task, result);
+ externalPush(job);
return job;
}
@@ -1624,8 +2724,8 @@
if (task instanceof ForkJoinTask<?>) // avoid re-wrap
job = (ForkJoinTask<?>) task;
else
- job = ForkJoinTask.adapt(task, null);
- forkOrSubmit(job);
+ job = new ForkJoinTask.AdaptedRunnableAction(task);
+ externalPush(job);
return job;
}
@@ -1634,25 +2734,31 @@
* @throws RejectedExecutionException {@inheritDoc}
*/
public <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks) {
- ArrayList<ForkJoinTask<T>> forkJoinTasks =
- new ArrayList<ForkJoinTask<T>>(tasks.size());
- for (Callable<T> task : tasks)
- forkJoinTasks.add(ForkJoinTask.adapt(task));
- invoke(new InvokeAll<T>(forkJoinTasks));
-
+ // In previous versions of this class, this method constructed
+ // a task to run ForkJoinTask.invokeAll, but now external
+ // invocation of multiple tasks is at least as efficient.
+ List<ForkJoinTask<T>> fs = new ArrayList<ForkJoinTask<T>>(tasks.size());
+ // Workaround needed because method wasn't declared with
+ // wildcards in return type but should have been.
@SuppressWarnings({"unchecked", "rawtypes"})
- List<Future<T>> futures = (List<Future<T>>) (List) forkJoinTasks;
- return futures;
- }
+ List<Future<T>> futures = (List<Future<T>>) (List) fs;
- static final class InvokeAll<T> extends RecursiveAction {
- final ArrayList<ForkJoinTask<T>> tasks;
- InvokeAll(ArrayList<ForkJoinTask<T>> tasks) { this.tasks = tasks; }
- public void compute() {
- try { invokeAll(tasks); }
- catch (Exception ignore) {}
+ boolean done = false;
+ try {
+ for (Callable<T> t : tasks) {
+ ForkJoinTask<T> f = new ForkJoinTask.AdaptedCallable<T>(t);
+ externalPush(f);
+ fs.add(f);
+ }
+ for (ForkJoinTask<T> f : fs)
+ f.quietlyJoin();
+ done = true;
+ return futures;
+ } finally {
+ if (!done)
+ for (ForkJoinTask<T> f : fs)
+ f.cancel(false);
}
- private static final long serialVersionUID = -7914297376763021607L;
}
/**
@@ -1680,7 +2786,16 @@
* @return the targeted parallelism level of this pool
*/
public int getParallelism() {
- return parallelism;
+ return config & SMASK;
+ }
+
+ /**
+ * Returns the targeted parallelism level of the common pool.
+ *
+ * @return the targeted parallelism level of the common pool
+ */
+ public static int getCommonPoolParallelism() {
+ return commonPoolParallelism;
}
/**
@@ -1692,7 +2807,7 @@
* @return the number of worker threads
*/
public int getPoolSize() {
- return parallelism + (short)(ctl >>> TC_SHIFT);
+ return (config & SMASK) + (short)(ctl >>> TC_SHIFT);
}
/**
@@ -1702,7 +2817,7 @@
* @return {@code true} if this pool uses async mode
*/
public boolean getAsyncMode() {
- return locallyFifo;
+ return (config >>> 16) == FIFO_QUEUE;
}
/**
@@ -1714,8 +2829,15 @@
* @return the number of worker threads
*/
public int getRunningThreadCount() {
- int r = parallelism + (int)(ctl >> AC_SHIFT);
- return (r <= 0) ? 0 : r; // suppress momentarily negative values
+ int rc = 0;
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 1; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null && w.isApparentlyUnblocked())
+ ++rc;
+ }
+ }
+ return rc;
}
/**
@@ -1726,7 +2848,7 @@
* @return the number of active threads
*/
public int getActiveThreadCount() {
- int r = parallelism + (int)(ctl >> AC_SHIFT) + blockedCount;
+ int r = (config & SMASK) + (int)(ctl >> AC_SHIFT);
return (r <= 0) ? 0 : r; // suppress momentarily negative values
}
@@ -1742,7 +2864,7 @@
* @return {@code true} if all threads are currently idle
*/
public boolean isQuiescent() {
- return parallelism + (int)(ctl >> AC_SHIFT) + blockedCount == 0;
+ return (int)(ctl >> AC_SHIFT) + (config & SMASK) == 0;
}
/**
@@ -1757,7 +2879,15 @@
* @return the number of steals
*/
public long getStealCount() {
- return stealCount;
+ long count = stealCount;
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 1; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null)
+ count += w.nsteals;
+ }
+ }
+ return count;
}
/**
@@ -1772,12 +2902,12 @@
*/
public long getQueuedTaskCount() {
long count = 0;
- ForkJoinWorkerThread[] ws;
- if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
- (ws = workers) != null) {
- for (ForkJoinWorkerThread w : ws)
- if (w != null)
- count -= w.queueBase - w.queueTop; // must read base first
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 1; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null)
+ count += w.queueSize();
+ }
}
return count;
}
@@ -1790,7 +2920,15 @@
* @return the number of queued submissions
*/
public int getQueuedSubmissionCount() {
- return -queueBase + queueTop;
+ int count = 0;
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 0; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null)
+ count += w.queueSize();
+ }
+ }
+ return count;
}
/**
@@ -1800,7 +2938,14 @@
* @return {@code true} if there are any queued submissions
*/
public boolean hasQueuedSubmissions() {
- return queueBase != queueTop;
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 0; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null && !w.isEmpty())
+ return true;
+ }
+ }
+ return false;
}
/**
@@ -1811,16 +2956,11 @@
* @return the next submission, or {@code null} if none
*/
protected ForkJoinTask<?> pollSubmission() {
- ForkJoinTask<?> t; ForkJoinTask<?>[] q; int b, i;
- while ((b = queueBase) != queueTop &&
- (q = submissionQueue) != null &&
- (i = (q.length - 1) & b) >= 0) {
- long u = (i << ASHIFT) + ABASE;
- if ((t = q[i]) != null &&
- queueBase == b &&
- UNSAFE.compareAndSwapObject(q, u, t, null)) {
- queueBase = b + 1;
- return t;
+ WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
+ if ((ws = workQueues) != null) {
+ for (int i = 0; i < ws.length; i += 2) {
+ if ((w = ws[i]) != null && (t = w.poll()) != null)
+ return t;
}
}
return null;
@@ -1845,20 +2985,17 @@
*/
protected int drainTasksTo(Collection<? super ForkJoinTask<?>> c) {
int count = 0;
- while (queueBase != queueTop) {
- ForkJoinTask<?> t = pollSubmission();
- if (t != null) {
- c.add(t);
- ++count;
+ WorkQueue[] ws; WorkQueue w; ForkJoinTask<?> t;
+ if ((ws = workQueues) != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ if ((w = ws[i]) != null) {
+ while ((t = w.poll()) != null) {
+ c.add(t);
+ ++count;
+ }
+ }
}
}
- ForkJoinWorkerThread[] ws;
- if ((short)(ctl >>> TC_SHIFT) > -parallelism &&
- (ws = workers) != null) {
- for (ForkJoinWorkerThread w : ws)
- if (w != null)
- count += w.drainTasksTo(c);
- }
return count;
}
@@ -1870,21 +3007,36 @@
* @return a string identifying this pool, as well as its state
*/
public String toString() {
- long st = getStealCount();
- long qt = getQueuedTaskCount();
- long qs = getQueuedSubmissionCount();
- int pc = parallelism;
+ // Use a single pass through workQueues to collect counts
+ long qt = 0L, qs = 0L; int rc = 0;
+ long st = stealCount;
long c = ctl;
+ WorkQueue[] ws; WorkQueue w;
+ if ((ws = workQueues) != null) {
+ for (int i = 0; i < ws.length; ++i) {
+ if ((w = ws[i]) != null) {
+ int size = w.queueSize();
+ if ((i & 1) == 0)
+ qs += size;
+ else {
+ qt += size;
+ st += w.nsteals;
+ if (w.isApparentlyUnblocked())
+ ++rc;
+ }
+ }
+ }
+ }
+ int pc = (config & SMASK);
int tc = pc + (short)(c >>> TC_SHIFT);
- int rc = pc + (int)(c >> AC_SHIFT);
- if (rc < 0) // ignore transient negative
- rc = 0;
- int ac = rc + blockedCount;
+ int ac = pc + (int)(c >> AC_SHIFT);
+ if (ac < 0) // ignore transient negative
+ ac = 0;
String level;
if ((c & STOP_BIT) != 0)
level = (tc == 0) ? "Terminated" : "Terminating";
else
- level = shutdown ? "Shutting down" : "Running";
+ level = plock < 0 ? "Shutting down" : "Running";
return super.toString() +
"[" + level +
", parallelism = " + pc +
@@ -1898,11 +3050,13 @@
}
/**
- * 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.
+ * Possibly initiates an orderly shutdown in which previously
+ * submitted tasks are executed, but no new tasks will be
+ * accepted. Invocation has no effect on execution state if this
+ * is the {@link #commonPool}, and 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
@@ -1911,19 +3065,20 @@
*/
public void shutdown() {
checkPermission();
- shutdown = true;
- tryTerminate(false);
+ tryTerminate(false, true);
}
/**
- * 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).
+ * Possibly attempts to cancel and/or stop all tasks, and reject
+ * all subsequently submitted tasks. Invocation has no effect on
+ * execution state if this is the {@link #commonPool}, and no
+ * additional effect if already shut down. Otherwise, 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
@@ -1933,8 +3088,7 @@
*/
public List<Runnable> shutdownNow() {
checkPermission();
- shutdown = true;
- tryTerminate(true);
+ tryTerminate(true, true);
return Collections.emptyList();
}
@@ -1946,7 +3100,7 @@
public boolean isTerminated() {
long c = ctl;
return ((c & STOP_BIT) != 0L &&
- (short)(c >>> TC_SHIFT) == -parallelism);
+ (short)(c >>> TC_SHIFT) == -(config & SMASK));
}
/**
@@ -1954,7 +3108,7 @@
* commenced but not yet completed. This method may be useful for
* debugging. A return of {@code true} reported a sufficient
* period after shutdown may indicate that submitted tasks have
- * ignored or suppressed interruption, or are waiting for IO,
+ * ignored or suppressed interruption, or are waiting for I/O,
* causing this executor not to properly terminate. (See the
* advisory notes for class {@link ForkJoinTask} stating that
* tasks should not normally entail blocking operations. But if
@@ -1965,14 +3119,7 @@
public boolean isTerminating() {
long c = ctl;
return ((c & STOP_BIT) != 0L &&
- (short)(c >>> TC_SHIFT) != -parallelism);
- }
-
- /**
- * Returns true if terminating or terminated. Used by ForkJoinWorkerThread.
- */
- final boolean isAtLeastTerminating() {
- return (ctl & STOP_BIT) != 0L;
+ (short)(c >>> TC_SHIFT) != -(config & SMASK));
}
/**
@@ -1981,13 +3128,15 @@
* @return {@code true} if this pool has been shut down
*/
public boolean isShutdown() {
- return shutdown;
+ return plock < 0;
}
/**
- * Blocks until all tasks have completed execution after a shutdown
- * request, or the timeout occurs, or the current thread is
- * interrupted, whichever happens first.
+ * Blocks until all tasks have completed execution after a
+ * shutdown request, or the timeout occurs, or the current thread
+ * is interrupted, whichever happens first. Note that the {@link
+ * #commonPool()} never terminates until program shutdown so
+ * this method will always time out.
*
* @param timeout the maximum time to wait
* @param unit the time unit of the timeout argument
@@ -1998,19 +3147,21 @@
public boolean awaitTermination(long timeout, TimeUnit unit)
throws InterruptedException {
long nanos = unit.toNanos(timeout);
- final ReentrantLock lock = this.submissionLock;
- lock.lock();
- try {
- for (;;) {
- if (isTerminated())
- return true;
- if (nanos <= 0)
- return false;
- nanos = termination.awaitNanos(nanos);
+ if (isTerminated())
+ return true;
+ long startTime = System.nanoTime();
+ boolean terminated = false;
+ synchronized (this) {
+ for (long waitTime = nanos, millis = 0L;;) {
+ if (terminated = isTerminated() ||
+ waitTime <= 0L ||
+ (millis = unit.toMillis(waitTime)) <= 0L)
+ break;
+ wait(millis);
+ waitTime = nanos - (System.nanoTime() - startTime);
}
- } finally {
- lock.unlock();
}
+ return terminated;
}
/**
@@ -2110,11 +3261,35 @@
throws InterruptedException {
Thread t = Thread.currentThread();
if (t instanceof ForkJoinWorkerThread) {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
- w.pool.awaitBlocker(blocker);
+ ForkJoinPool p = ((ForkJoinWorkerThread)t).pool;
+ while (!blocker.isReleasable()) { // variant of helpSignal
+ WorkQueue[] ws; WorkQueue q; int m, u;
+ if ((ws = p.workQueues) != null && (m = ws.length - 1) >= 0) {
+ for (int i = 0; i <= m; ++i) {
+ if (blocker.isReleasable())
+ return;
+ if ((q = ws[i]) != null && q.base - q.top < 0) {
+ p.signalWork(q);
+ if ((u = (int)(p.ctl >>> 32)) >= 0 ||
+ (u >> UAC_SHIFT) >= 0)
+ break;
+ }
+ }
+ }
+ if (p.tryCompensate()) {
+ try {
+ do {} while (!blocker.isReleasable() &&
+ !blocker.block());
+ } finally {
+ p.incrementActiveCount();
+ }
+ break;
+ }
+ }
}
else {
- do {} while (!blocker.isReleasable() && !blocker.block());
+ do {} while (!blocker.isReleasable() &&
+ !blocker.block());
}
}
@@ -2123,55 +3298,93 @@
// implement RunnableFuture.
protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
- return (RunnableFuture<T>) ForkJoinTask.adapt(runnable, value);
+ return new ForkJoinTask.AdaptedRunnable<T>(runnable, value);
}
protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
- return (RunnableFuture<T>) ForkJoinTask.adapt(callable);
+ return new ForkJoinTask.AdaptedCallable<T>(callable);
}
// Unsafe mechanics
- private static final sun.misc.Unsafe UNSAFE;
- private static final long ctlOffset;
- private static final long stealCountOffset;
- private static final long blockedCountOffset;
- private static final long quiescerCountOffset;
- private static final long scanGuardOffset;
- private static final long nextWorkerNumberOffset;
- private static final long ABASE;
+ private static final sun.misc.Unsafe U;
+ private static final long CTL;
+ private static final long PARKBLOCKER;
+ private static final int ABASE;
private static final int ASHIFT;
+ private static final long STEALCOUNT;
+ private static final long PLOCK;
+ private static final long INDEXSEED;
+ private static final long QLOCK;
static {
- poolNumberGenerator = new AtomicInteger();
- workerSeedGenerator = new Random();
- modifyThreadPermission = new RuntimePermission("modifyThread");
- defaultForkJoinWorkerThreadFactory =
- new DefaultForkJoinWorkerThreadFactory();
- int s;
+ int s; // initialize field offsets for CAS etc
try {
- UNSAFE = sun.misc.Unsafe.getUnsafe();
+ U = sun.misc.Unsafe.getUnsafe();
Class<?> k = ForkJoinPool.class;
- ctlOffset = UNSAFE.objectFieldOffset
+ CTL = U.objectFieldOffset
(k.getDeclaredField("ctl"));
- stealCountOffset = UNSAFE.objectFieldOffset
+ STEALCOUNT = U.objectFieldOffset
(k.getDeclaredField("stealCount"));
- blockedCountOffset = UNSAFE.objectFieldOffset
- (k.getDeclaredField("blockedCount"));
- quiescerCountOffset = UNSAFE.objectFieldOffset
- (k.getDeclaredField("quiescerCount"));
- scanGuardOffset = UNSAFE.objectFieldOffset
- (k.getDeclaredField("scanGuard"));
- nextWorkerNumberOffset = UNSAFE.objectFieldOffset
- (k.getDeclaredField("nextWorkerNumber"));
- Class<?> a = ForkJoinTask[].class;
- ABASE = UNSAFE.arrayBaseOffset(a);
- s = UNSAFE.arrayIndexScale(a);
+ PLOCK = U.objectFieldOffset
+ (k.getDeclaredField("plock"));
+ INDEXSEED = U.objectFieldOffset
+ (k.getDeclaredField("indexSeed"));
+ Class<?> tk = Thread.class;
+ PARKBLOCKER = U.objectFieldOffset
+ (tk.getDeclaredField("parkBlocker"));
+ Class<?> wk = WorkQueue.class;
+ QLOCK = U.objectFieldOffset
+ (wk.getDeclaredField("qlock"));
+ Class<?> ak = ForkJoinTask[].class;
+ ABASE = U.arrayBaseOffset(ak);
+ s = U.arrayIndexScale(ak);
+ ASHIFT = 31 - Integer.numberOfLeadingZeros(s);
} 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);
+
+ submitters = new ThreadLocal<Submitter>();
+ ForkJoinWorkerThreadFactory fac = defaultForkJoinWorkerThreadFactory =
+ new DefaultForkJoinWorkerThreadFactory();
+ modifyThreadPermission = new RuntimePermission("modifyThread");
+
+ /*
+ * Establish common pool parameters. For extra caution,
+ * computations to set up common pool state are here; the
+ * constructor just assigns these values to fields.
+ */
+
+ int par = 0;
+ Thread.UncaughtExceptionHandler handler = null;
+ try { // TBD: limit or report ignored exceptions?
+ String pp = System.getProperty
+ ("java.util.concurrent.ForkJoinPool.common.parallelism");
+ String hp = System.getProperty
+ ("java.util.concurrent.ForkJoinPool.common.exceptionHandler");
+ String fp = System.getProperty
+ ("java.util.concurrent.ForkJoinPool.common.threadFactory");
+ if (fp != null)
+ fac = ((ForkJoinWorkerThreadFactory)ClassLoader.
+ getSystemClassLoader().loadClass(fp).newInstance());
+ if (hp != null)
+ handler = ((Thread.UncaughtExceptionHandler)ClassLoader.
+ getSystemClassLoader().loadClass(hp).newInstance());
+ if (pp != null)
+ par = Integer.parseInt(pp);
+ } catch (Exception ignore) {
+ }
+
+ if (par <= 0)
+ par = Runtime.getRuntime().availableProcessors();
+ if (par > MAX_CAP)
+ par = MAX_CAP;
+ commonPoolParallelism = par;
+ long np = (long)(-par); // precompute initial ctl value
+ long ct = ((np << AC_SHIFT) & AC_MASK) | ((np << TC_SHIFT) & TC_MASK);
+
+ commonPool = new ForkJoinPool(par, ct, fac, handler);
}
}
--- a/jdk/src/share/classes/java/util/concurrent/ForkJoinTask.java Thu Dec 20 17:24:56 2012 +0400
+++ b/jdk/src/share/classes/java/util/concurrent/ForkJoinTask.java Thu Dec 20 13:44:06 2012 +0000
@@ -37,17 +37,13 @@
import java.io.Serializable;
import java.util.Collection;
-import java.util.Collections;
import java.util.List;
import java.util.RandomAccess;
-import java.util.Map;
import java.lang.ref.WeakReference;
import java.lang.ref.ReferenceQueue;
import java.util.concurrent.Callable;
import java.util.concurrent.CancellationException;
import java.util.concurrent.ExecutionException;
-import java.util.concurrent.Executor;
-import java.util.concurrent.ExecutorService;
import java.util.concurrent.Future;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.RunnableFuture;
@@ -63,46 +59,59 @@
* subtasks may be hosted by a small number of actual threads in a
* ForkJoinPool, at the price of some usage limitations.
*
- * <p>A "main" {@code ForkJoinTask} begins execution when submitted
- * to a {@link ForkJoinPool}. Once started, it will usually in turn
- * start other subtasks. As indicated by the name of this class,
- * many programs using {@code ForkJoinTask} employ only methods
- * {@link #fork} and {@link #join}, or derivatives such as {@link
+ * <p>A "main" {@code ForkJoinTask} begins execution when it is
+ * explicitly submitted to a {@link ForkJoinPool}, or, if not already
+ * engaged in a ForkJoin computation, commenced in the {@link
+ * ForkJoinPool#commonPool()} via {@link #fork}, {@link #invoke}, or
+ * related methods. Once started, it will usually in turn start other
+ * subtasks. As indicated by the name of this class, many programs
+ * using {@code ForkJoinTask} employ only methods {@link #fork} and
+ * {@link #join}, or derivatives such as {@link
* #invokeAll(ForkJoinTask...) invokeAll}. However, this class also
* provides a number of other methods that can come into play in
- * advanced usages, as well as extension mechanics that allow
- * support of new forms of fork/join processing.
+ * advanced usages, as well as extension mechanics that allow support
+ * of new forms of fork/join processing.
*
* <p>A {@code ForkJoinTask} is a lightweight form of {@link Future}.
* The efficiency of {@code ForkJoinTask}s stems from a set of
* restrictions (that are only partially statically enforceable)
- * reflecting their intended use as computational tasks calculating
- * pure functions or operating on purely isolated objects. The
- * primary coordination mechanisms are {@link #fork}, that arranges
+ * reflecting their main use as computational tasks calculating pure
+ * functions or operating on purely isolated objects. The primary
+ * coordination mechanisms are {@link #fork}, that arranges
* asynchronous execution, and {@link #join}, that doesn't proceed
* until the task's result has been computed. Computations should
- * avoid {@code synchronized} methods or blocks, and should minimize
- * other blocking synchronization apart from joining other tasks or
- * using synchronizers such as Phasers that are advertised to
- * cooperate with fork/join scheduling. Tasks should also not perform
- * blocking IO, and should ideally access variables that are
- * completely independent of those accessed by other running
- * tasks. Minor breaches of these restrictions, for example using
- * shared output streams, may be tolerable in practice, but frequent
- * use may result in poor performance, and the potential to
- * indefinitely stall if the number of threads not waiting for IO or
- * other external synchronization becomes exhausted. This usage
- * restriction is in part enforced by not permitting checked
- * exceptions such as {@code IOExceptions} to be thrown. However,
- * computations may still encounter unchecked exceptions, that are
- * rethrown to callers attempting to join them. These exceptions may
- * additionally include {@link RejectedExecutionException} stemming
- * from internal resource exhaustion, such as failure to allocate
- * internal task queues. Rethrown exceptions behave in the same way as
- * regular exceptions, but, when possible, contain stack traces (as
- * displayed for example using {@code ex.printStackTrace()}) of both
- * the thread that initiated the computation as well as the thread
- * actually encountering the exception; minimally only the latter.
+ * ideally avoid {@code synchronized} methods or blocks, and should
+ * minimize other blocking synchronization apart from joining other
+ * tasks or using synchronizers such as Phasers that are advertised to
+ * cooperate with fork/join scheduling. Subdividable tasks should also
+ * not perform blocking I/O, and should ideally access variables that
+ * are completely independent of those accessed by other running
+ * tasks. These guidelines are loosely enforced by not permitting
+ * checked exceptions such as {@code IOExceptions} to be
+ * thrown. However, computations may still encounter unchecked
+ * exceptions, that are rethrown to callers attempting to join
+ * them. These exceptions may additionally include {@link
+ * RejectedExecutionException} stemming from internal resource
+ * exhaustion, such as failure to allocate internal task
+ * queues. Rethrown exceptions behave in the same way as regular
+ * exceptions, but, when possible, contain stack traces (as displayed
+ * for example using {@code ex.printStackTrace()}) of both the thread
+ * that initiated the computation as well as the thread actually
+ * encountering the exception; minimally only the latter.
+ *
+ * <p>It is possible to define and use ForkJoinTasks that may block,
+ * but doing do requires three further considerations: (1) Completion
+ * of few if any <em>other</em> tasks should be dependent on a task
+ * that blocks on external synchronization or I/O. Event-style async
+ * tasks that are never joined (for example, those subclassing {@link
+ * CountedCompleter}) often fall into this category. (2) To minimize
+ * resource impact, tasks should be small; ideally performing only the
+ * (possibly) blocking action. (3) Unless the {@link
+ * ForkJoinPool.ManagedBlocker} API is used, or the number of possibly
+ * blocked tasks is known to be less than the pool's {@link
+ * ForkJoinPool#getParallelism} level, the pool cannot guarantee that
+ * enough threads will be available to ensure progress or good
+ * performance.
*
* <p>The primary method for awaiting completion and extracting
* results of a task is {@link #join}, but there are several variants:
@@ -118,6 +127,13 @@
* performs the most common form of parallel invocation: forking a set
* of tasks and joining them all.
*
+ * <p>In the most typical usages, a fork-join pair act like a call
+ * (fork) and return (join) from a parallel recursive function. As is
+ * the case with other forms of recursive calls, returns (joins)
+ * should be performed innermost-first. For example, {@code a.fork();
+ * b.fork(); b.join(); a.join();} is likely to be substantially more
+ * efficient than joining {@code a} before {@code b}.
+ *
* <p>The execution status of tasks may be queried at several levels
* of detail: {@link #isDone} is true if a task completed in any way
* (including the case where a task was cancelled without executing);
@@ -133,18 +149,13 @@
* <p>The ForkJoinTask class is not usually directly subclassed.
* Instead, you subclass one of the abstract classes that support a
* particular style of fork/join processing, typically {@link
- * RecursiveAction} for computations that do not return results, or
- * {@link RecursiveTask} for those that do. Normally, a concrete
- * ForkJoinTask subclass declares fields comprising its parameters,
- * established in a constructor, and then defines a {@code compute}
- * method that somehow uses the control methods supplied by this base
- * class. While these methods have {@code public} access (to allow
- * instances of different task subclasses to call each other's
- * methods), some of them may only be called from within other
- * ForkJoinTasks (as may be determined using method {@link
- * #inForkJoinPool}). Attempts to invoke them in other contexts
- * result in exceptions or errors, possibly including
- * {@code ClassCastException}.
+ * RecursiveAction} for most computations that do not return results,
+ * {@link RecursiveTask} for those that do, and {@link
+ * CountedCompleter} for those in which completed actions trigger
+ * other actions. Normally, a concrete ForkJoinTask subclass declares
+ * fields comprising its parameters, established in a constructor, and
+ * then defines a {@code compute} method that somehow uses the control
+ * methods supplied by this base class.
*
* <p>Method {@link #join} and its variants are appropriate for use
* only when completion dependencies are acyclic; that is, the
@@ -154,7 +165,17 @@
* supports other methods and techniques (for example the use of
* {@link Phaser}, {@link #helpQuiesce}, and {@link #complete}) that
* may be of use in constructing custom subclasses for problems that
- * are not statically structured as DAGs.
+ * are not statically structured as DAGs. To support such usages a
+ * ForkJoinTask may be atomically <em>tagged</em> with a {@code short}
+ * value using {@link #setForkJoinTaskTag} or {@link
+ * #compareAndSetForkJoinTaskTag} and checked using {@link
+ * #getForkJoinTaskTag}. The ForkJoinTask implementation does not use
+ * these {@code protected} methods or tags for any purpose, but they
+ * may be of use in the construction of specialized subclasses. For
+ * example, parallel graph traversals can use the supplied methods to
+ * avoid revisiting nodes/tasks that have already been processed.
+ * (Method names for tagging are bulky in part to encourage definition
+ * of methods that reflect their usage patterns.)
*
* <p>Most base support methods are {@code final}, to prevent
* overriding of implementations that are intrinsically tied to the
@@ -194,41 +215,50 @@
* See the internal documentation of class ForkJoinPool for a
* general implementation overview. ForkJoinTasks are mainly
* responsible for maintaining their "status" field amidst relays
- * to methods in ForkJoinWorkerThread and ForkJoinPool. The
- * methods of this class are more-or-less layered into (1) basic
- * status maintenance (2) execution and awaiting completion (3)
- * user-level methods that additionally report results. This is
- * sometimes hard to see because this file orders exported methods
- * in a way that flows well in javadocs.
+ * to methods in ForkJoinWorkerThread and ForkJoinPool.
+ *
+ * The methods of this class are more-or-less layered into
+ * (1) basic status maintenance
+ * (2) execution and awaiting completion
+ * (3) user-level methods that additionally report results.
+ * This is sometimes hard to see because this file orders exported
+ * methods in a way that flows well in javadocs.
*/
/*
* The status field holds run control status bits packed into a
* single int to minimize footprint and to ensure atomicity (via
* CAS). Status is initially zero, and takes on nonnegative
- * values until completed, upon which status holds value
- * NORMAL, CANCELLED, or EXCEPTIONAL. Tasks undergoing blocking
- * waits by other threads have the SIGNAL bit set. Completion of
- * a stolen task with SIGNAL set awakens any waiters via
- * notifyAll. Even though suboptimal for some purposes, we use
- * basic builtin wait/notify to take advantage of "monitor
- * inflation" in JVMs that we would otherwise need to emulate to
- * avoid adding further per-task bookkeeping overhead. We want
- * these monitors to be "fat", i.e., not use biasing or thin-lock
- * techniques, so use some odd coding idioms that tend to avoid
- * them.
+ * values until completed, upon which status (anded with
+ * DONE_MASK) holds value NORMAL, CANCELLED, or EXCEPTIONAL. Tasks
+ * undergoing blocking waits by other threads have the SIGNAL bit
+ * set. Completion of a stolen task with SIGNAL set awakens any
+ * waiters via notifyAll. Even though suboptimal for some
+ * purposes, we use basic builtin wait/notify to take advantage of
+ * "monitor inflation" in JVMs that we would otherwise need to
+ * emulate to avoid adding further per-task bookkeeping overhead.
+ * We want these monitors to be "fat", i.e., not use biasing or
+ * thin-lock techniques, so use some odd coding idioms that tend
+ * to avoid them, mainly by arranging that every synchronized
+ * block performs a wait, notifyAll or both.
+ *
+ * These control bits occupy only (some of) the upper half (16
+ * bits) of status field. The lower bits are used for user-defined
+ * tags.
*/
/** The run status of this task */
volatile int status; // accessed directly by pool and workers
- private static final int NORMAL = -1;
- private static final int CANCELLED = -2;
- private static final int EXCEPTIONAL = -3;
- private static final int SIGNAL = 1;
+ static final int DONE_MASK = 0xf0000000; // mask out non-completion bits
+ static final int NORMAL = 0xf0000000; // must be negative
+ static final int CANCELLED = 0xc0000000; // must be < NORMAL
+ static final int EXCEPTIONAL = 0x80000000; // must be < CANCELLED
+ static final int SIGNAL = 0x00010000; // must be >= 1 << 16
+ static final int SMASK = 0x0000ffff; // short bits for tags
/**
- * Marks completion and wakes up threads waiting to join this task,
- * also clearing signal request bits.
+ * Marks completion and wakes up threads waiting to join this
+ * task.
*
* @param completion one of NORMAL, CANCELLED, EXCEPTIONAL
* @return completion status on exit
@@ -237,8 +267,8 @@
for (int s;;) {
if ((s = status) < 0)
return s;
- if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) {
- if (s != 0)
+ if (U.compareAndSwapInt(this, STATUS, s, s | completion)) {
+ if ((s >>> 16) != 0)
synchronized (this) { notifyAll(); }
return completion;
}
@@ -246,27 +276,36 @@
}
/**
- * Tries to block a worker thread until completed or timed out.
- * Uses Object.wait time argument conventions.
- * May fail on contention or interrupt.
+ * Primary execution method for stolen tasks. Unless done, calls
+ * exec and records status if completed, but doesn't wait for
+ * completion otherwise.
*
- * @param millis if > 0, wait time.
+ * @return status on exit from this method
*/
- final void tryAwaitDone(long millis) {
- int s;
- try {
- if (((s = status) > 0 ||
- (s == 0 &&
- UNSAFE.compareAndSwapInt(this, statusOffset, 0, SIGNAL))) &&
- status > 0) {
- synchronized (this) {
- if (status > 0)
- wait(millis);
- }
+ final int doExec() {
+ int s; boolean completed;
+ if ((s = status) >= 0) {
+ try {
+ completed = exec();
+ } catch (Throwable rex) {
+ return setExceptionalCompletion(rex);
}
- } catch (InterruptedException ie) {
- // caller must check termination
+ if (completed)
+ s = setCompletion(NORMAL);
}
+ return s;
+ }
+
+ /**
+ * Tries to set SIGNAL status unless already completed. Used by
+ * ForkJoinPool. Other variants are directly incorporated into
+ * externalAwaitDone etc.
+ *
+ * @return true if successful
+ */
+ final boolean trySetSignal() {
+ int s = status;
+ return s >= 0 && U.compareAndSwapInt(this, STATUS, s, s | SIGNAL);
}
/**
@@ -275,113 +314,78 @@
*/
private int externalAwaitDone() {
int s;
- if ((s = status) >= 0) {
- boolean interrupted = false;
- synchronized (this) {
- while ((s = status) >= 0) {
- if (s == 0)
- UNSAFE.compareAndSwapInt(this, statusOffset,
- 0, SIGNAL);
- else {
+ ForkJoinPool.externalHelpJoin(this);
+ boolean interrupted = false;
+ while ((s = status) >= 0) {
+ if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+ synchronized (this) {
+ if (status >= 0) {
try {
wait();
} catch (InterruptedException ie) {
interrupted = true;
}
}
+ else
+ notifyAll();
}
}
- if (interrupted)
- Thread.currentThread().interrupt();
}
+ if (interrupted)
+ Thread.currentThread().interrupt();
return s;
}
/**
- * Blocks a non-worker-thread until completion or interruption or timeout.
+ * Blocks a non-worker-thread until completion or interruption.
*/
- private int externalInterruptibleAwaitDone(long millis)
- throws InterruptedException {
+ private int externalInterruptibleAwaitDone() throws InterruptedException {
int s;
if (Thread.interrupted())
throw new InterruptedException();
- if ((s = status) >= 0) {
- synchronized (this) {
- while ((s = status) >= 0) {
- if (s == 0)
- UNSAFE.compareAndSwapInt(this, statusOffset,
- 0, SIGNAL);
- else {
- wait(millis);
- if (millis > 0L)
- break;
- }
+ ForkJoinPool.externalHelpJoin(this);
+ while ((s = status) >= 0) {
+ if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+ synchronized (this) {
+ if (status >= 0)
+ wait();
+ else
+ notifyAll();
}
}
}
return s;
}
- /**
- * Primary execution method for stolen tasks. Unless done, calls
- * exec and records status if completed, but doesn't wait for
- * completion otherwise.
- */
- final void doExec() {
- if (status >= 0) {
- boolean completed;
- try {
- completed = exec();
- } catch (Throwable rex) {
- setExceptionalCompletion(rex);
- return;
- }
- if (completed)
- setCompletion(NORMAL); // must be outside try block
- }
- }
/**
- * Primary mechanics for join, get, quietlyJoin.
+ * Implementation for join, get, quietlyJoin. Directly handles
+ * only cases of already-completed, external wait, and
+ * unfork+exec. Others are relayed to ForkJoinPool.awaitJoin.
+ *
* @return status upon completion
*/
private int doJoin() {
- Thread t; ForkJoinWorkerThread w; int s; boolean completed;
- if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
- if ((s = status) < 0)
- return s;
- if ((w = (ForkJoinWorkerThread)t).unpushTask(this)) {
- try {
- completed = exec();
- } catch (Throwable rex) {
- return setExceptionalCompletion(rex);
- }
- if (completed)
- return setCompletion(NORMAL);
- }
- return w.joinTask(this);
- }
- else
- return externalAwaitDone();
+ int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w;
+ return (s = status) < 0 ? s :
+ ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+ (w = (wt = (ForkJoinWorkerThread)t).workQueue).
+ tryUnpush(this) && (s = doExec()) < 0 ? s :
+ wt.pool.awaitJoin(w, this) :
+ externalAwaitDone();
}
/**
- * Primary mechanics for invoke, quietlyInvoke.
+ * Implementation for invoke, quietlyInvoke.
+ *
* @return status upon completion
*/
private int doInvoke() {
- int s; boolean completed;
- if ((s = status) < 0)
- return s;
- try {
- completed = exec();
- } catch (Throwable rex) {
- return setExceptionalCompletion(rex);
- }
- if (completed)
- return setCompletion(NORMAL);
- else
- return doJoin();
+ int s; Thread t; ForkJoinWorkerThread wt;
+ return (s = doExec()) < 0 ? s :
+ ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+ (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, this) :
+ externalAwaitDone();
}
// Exception table support
@@ -416,7 +420,7 @@
* any ForkJoinPool will call helpExpungeStaleExceptions when its
* pool becomes isQuiescent.
*/
- static final class ExceptionNode extends WeakReference<ForkJoinTask<?>>{
+ static final class ExceptionNode extends WeakReference<ForkJoinTask<?>> {
final Throwable ex;
ExceptionNode next;
final long thrower; // use id not ref to avoid weak cycles
@@ -429,30 +433,67 @@
}
/**
- * Records exception and sets exceptional completion.
+ * Records exception and sets status.
+ *
+ * @return status on exit
+ */
+ final int recordExceptionalCompletion(Throwable ex) {
+ int s;
+ if ((s = status) >= 0) {
+ int h = System.identityHashCode(this);
+ final ReentrantLock lock = exceptionTableLock;
+ lock.lock();
+ try {
+ expungeStaleExceptions();
+ ExceptionNode[] t = exceptionTable;
+ int i = h & (t.length - 1);
+ for (ExceptionNode e = t[i]; ; e = e.next) {
+ if (e == null) {
+ t[i] = new ExceptionNode(this, ex, t[i]);
+ break;
+ }
+ if (e.get() == this) // already present
+ break;
+ }
+ } finally {
+ lock.unlock();
+ }
+ s = setCompletion(EXCEPTIONAL);
+ }
+ return s;
+ }
+
+ /**
+ * Records exception and possibly propagates
*
* @return status on exit
*/
private int setExceptionalCompletion(Throwable ex) {
- int h = System.identityHashCode(this);
- final ReentrantLock lock = exceptionTableLock;
- lock.lock();
- try {
- expungeStaleExceptions();
- ExceptionNode[] t = exceptionTable;
- int i = h & (t.length - 1);
- for (ExceptionNode e = t[i]; ; e = e.next) {
- if (e == null) {
- t[i] = new ExceptionNode(this, ex, t[i]);
- break;
- }
- if (e.get() == this) // already present
- break;
+ int s = recordExceptionalCompletion(ex);
+ if ((s & DONE_MASK) == EXCEPTIONAL)
+ internalPropagateException(ex);
+ return s;
+ }
+
+ /**
+ * Hook for exception propagation support for tasks with completers.
+ */
+ void internalPropagateException(Throwable ex) {
+ }
+
+ /**
+ * Cancels, ignoring any exceptions thrown by cancel. Used during
+ * worker and pool shutdown. Cancel is spec'ed not to throw any
+ * exceptions, but if it does anyway, we have no recourse during
+ * shutdown, so guard against this case.
+ */
+ static final void cancelIgnoringExceptions(ForkJoinTask<?> t) {
+ if (t != null && t.status >= 0) {
+ try {
+ t.cancel(false);
+ } catch (Throwable ignore) {
}
- } finally {
- lock.unlock();
}
- return setCompletion(EXCEPTIONAL);
}
/**
@@ -501,7 +542,7 @@
* @return the exception, or null if none
*/
private Throwable getThrowableException() {
- if (status != EXCEPTIONAL)
+ if ((status & DONE_MASK) != EXCEPTIONAL)
return null;
int h = System.identityHashCode(this);
ExceptionNode e;
@@ -519,7 +560,7 @@
Throwable ex;
if (e == null || (ex = e.ex) == null)
return null;
- if (e.thrower != Thread.currentThread().getId()) {
+ if (false && e.thrower != Thread.currentThread().getId()) {
Class<? extends Throwable> ec = ex.getClass();
try {
Constructor<?> noArgCtor = null;
@@ -586,41 +627,61 @@
}
/**
- * Report the result of invoke or join; called only upon
- * non-normal return of internal versions.
+ * A version of "sneaky throw" to relay exceptions
*/
- private V reportResult() {
- int s; Throwable ex;
- if ((s = status) == CANCELLED)
+ static void rethrow(final Throwable ex) {
+ if (ex != null) {
+ if (ex instanceof Error)
+ throw (Error)ex;
+ if (ex instanceof RuntimeException)
+ throw (RuntimeException)ex;
+ throw uncheckedThrowable(ex, RuntimeException.class);
+ }
+ }
+
+ /**
+ * The sneaky part of sneaky throw, relying on generics
+ * limitations to evade compiler complaints about rethrowing
+ * unchecked exceptions
+ */
+ @SuppressWarnings("unchecked") static <T extends Throwable>
+ T uncheckedThrowable(final Throwable t, final Class<T> c) {
+ return (T)t; // rely on vacuous cast
+ }
+
+ /**
+ * Throws exception, if any, associated with the given status.
+ */
+ private void reportException(int s) {
+ if (s == CANCELLED)
throw new CancellationException();
- if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
- UNSAFE.throwException(ex);
- return getRawResult();
+ if (s == EXCEPTIONAL)
+ rethrow(getThrowableException());
}
// public methods
/**
- * Arranges to asynchronously execute this task. While it is not
- * necessarily enforced, it is a usage error to fork a task more
- * than once unless it has completed and been reinitialized.
- * Subsequent modifications to the state of this task or any data
- * it operates on are not necessarily consistently observable by
- * any thread other than the one executing it unless preceded by a
- * call to {@link #join} or related methods, or a call to {@link
- * #isDone} returning {@code true}.
- *
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
+ * Arranges to asynchronously execute this task in the pool the
+ * current task is running in, if applicable, or using the {@link
+ * ForkJoinPool#commonPool()} if not {@link #inForkJoinPool}. While
+ * it is not necessarily enforced, it is a usage error to fork a
+ * task more than once unless it has completed and been
+ * reinitialized. Subsequent modifications to the state of this
+ * task or any data it operates on are not necessarily
+ * consistently observable by any thread other than the one
+ * executing it unless preceded by a call to {@link #join} or
+ * related methods, or a call to {@link #isDone} returning {@code
+ * true}.
*
* @return {@code this}, to simplify usage
*/
public final ForkJoinTask<V> fork() {
- ((ForkJoinWorkerThread) Thread.currentThread())
- .pushTask(this);
+ Thread t;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+ ((ForkJoinWorkerThread)t).workQueue.push(this);
+ else
+ ForkJoinPool.commonPool.externalPush(this);
return this;
}
@@ -636,10 +697,10 @@
* @return the computed result
*/
public final V join() {
- if (doJoin() != NORMAL)
- return reportResult();
- else
- return getRawResult();
+ int s;
+ if ((s = doJoin() & DONE_MASK) != NORMAL)
+ reportException(s);
+ return getRawResult();
}
/**
@@ -651,10 +712,10 @@
* @return the computed result
*/
public final V invoke() {
- if (doInvoke() != NORMAL)
- return reportResult();
- else
- return getRawResult();
+ int s;
+ if ((s = doInvoke() & DONE_MASK) != NORMAL)
+ reportException(s);
+ return getRawResult();
}
/**
@@ -670,20 +731,17 @@
* cancelled, completed normally or exceptionally, or left
* unprocessed.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @param t1 the first task
* @param t2 the second task
* @throws NullPointerException if any task is null
*/
public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
+ int s1, s2;
t2.fork();
- t1.invoke();
- t2.join();
+ if ((s1 = t1.doInvoke() & DONE_MASK) != NORMAL)
+ t1.reportException(s1);
+ if ((s2 = t2.doJoin() & DONE_MASK) != NORMAL)
+ t2.reportException(s2);
}
/**
@@ -698,12 +756,6 @@
* related methods to check if they have been cancelled, completed
* normally or exceptionally, or left unprocessed.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @param tasks the tasks
* @throws NullPointerException if any task is null
*/
@@ -726,12 +778,12 @@
if (t != null) {
if (ex != null)
t.cancel(false);
- else if (t.doJoin() < NORMAL && ex == null)
+ else if (t.doJoin() < NORMAL)
ex = t.getException();
}
}
if (ex != null)
- UNSAFE.throwException(ex);
+ rethrow(ex);
}
/**
@@ -747,12 +799,6 @@
* cancelled, completed normally or exceptionally, or left
* unprocessed.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @param tasks the collection of tasks
* @return the tasks argument, to simplify usage
* @throws NullPointerException if tasks or any element are null
@@ -783,12 +829,12 @@
if (t != null) {
if (ex != null)
t.cancel(false);
- else if (t.doJoin() < NORMAL && ex == null)
+ else if (t.doJoin() < NORMAL)
ex = t.getException();
}
}
if (ex != null)
- UNSAFE.throwException(ex);
+ rethrow(ex);
return tasks;
}
@@ -820,20 +866,7 @@
* @return {@code true} if this task is now cancelled
*/
public boolean cancel(boolean mayInterruptIfRunning) {
- return setCompletion(CANCELLED) == CANCELLED;
- }
-
- /**
- * Cancels, ignoring any exceptions thrown by cancel. Used during
- * worker and pool shutdown. Cancel is spec'ed not to throw any
- * exceptions, but if it does anyway, we have no recourse during
- * shutdown, so guard against this case.
- */
- final void cancelIgnoringExceptions() {
- try {
- cancel(false);
- } catch (Throwable ignore) {
- }
+ return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED;
}
public final boolean isDone() {
@@ -841,7 +874,7 @@
}
public final boolean isCancelled() {
- return status == CANCELLED;
+ return (status & DONE_MASK) == CANCELLED;
}
/**
@@ -861,7 +894,7 @@
* exception and was not cancelled
*/
public final boolean isCompletedNormally() {
- return status == NORMAL;
+ return (status & DONE_MASK) == NORMAL;
}
/**
@@ -872,7 +905,7 @@
* @return the exception, or {@code null} if none
*/
public final Throwable getException() {
- int s = status;
+ int s = status & DONE_MASK;
return ((s >= NORMAL) ? null :
(s == CANCELLED) ? new CancellationException() :
getThrowableException());
@@ -922,6 +955,18 @@
}
/**
+ * Completes this task normally without setting a value. The most
+ * recent value established by {@link #setRawResult} (or {@code
+ * null} by default) will be returned as the result of subsequent
+ * invocations of {@code join} and related operations.
+ *
+ * @since 1.8
+ */
+ public final void quietlyComplete() {
+ setCompletion(NORMAL);
+ }
+
+ /**
* Waits if necessary for the computation to complete, and then
* retrieves its result.
*
@@ -934,9 +979,9 @@
*/
public final V get() throws InterruptedException, ExecutionException {
int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ?
- doJoin() : externalInterruptibleAwaitDone(0L);
+ doJoin() : externalInterruptibleAwaitDone();
Throwable ex;
- if (s == CANCELLED)
+ if ((s &= DONE_MASK) == CANCELLED)
throw new CancellationException();
if (s == EXCEPTIONAL && (ex = getThrowableException()) != null)
throw new ExecutionException(ex);
@@ -959,32 +1004,62 @@
*/
public final V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException {
- Thread t = Thread.currentThread();
- if (t instanceof ForkJoinWorkerThread) {
- ForkJoinWorkerThread w = (ForkJoinWorkerThread) t;
- long nanos = unit.toNanos(timeout);
- if (status >= 0) {
- boolean completed = false;
- if (w.unpushTask(this)) {
- try {
- completed = exec();
- } catch (Throwable rex) {
- setExceptionalCompletion(rex);
+ if (Thread.interrupted())
+ throw new InterruptedException();
+ // Messy in part because we measure in nanosecs, but wait in millisecs
+ int s; long ns, ms;
+ if ((s = status) >= 0 && (ns = unit.toNanos(timeout)) > 0L) {
+ long deadline = System.nanoTime() + ns;
+ ForkJoinPool p = null;
+ ForkJoinPool.WorkQueue w = null;
+ Thread t = Thread.currentThread();
+ if (t instanceof ForkJoinWorkerThread) {
+ ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+ p = wt.pool;
+ w = wt.workQueue;
+ p.helpJoinOnce(w, this); // no retries on failure
+ }
+ else
+ ForkJoinPool.externalHelpJoin(this);
+ boolean canBlock = false;
+ boolean interrupted = false;
+ try {
+ while ((s = status) >= 0) {
+ if (w != null && w.qlock < 0)
+ cancelIgnoringExceptions(this);
+ else if (!canBlock) {
+ if (p == null || p.tryCompensate())
+ canBlock = true;
+ }
+ else {
+ if ((ms = TimeUnit.NANOSECONDS.toMillis(ns)) > 0L &&
+ U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) {
+ synchronized (this) {
+ if (status >= 0) {
+ try {
+ wait(ms);
+ } catch (InterruptedException ie) {
+ if (p == null)
+ interrupted = true;
+ }
+ }
+ else
+ notifyAll();
+ }
+ }
+ if ((s = status) < 0 || interrupted ||
+ (ns = deadline - System.nanoTime()) <= 0L)
+ break;
}
}
- if (completed)
- setCompletion(NORMAL);
- else if (status >= 0 && nanos > 0)
- w.pool.timedAwaitJoin(this, nanos);
+ } finally {
+ if (p != null && canBlock)
+ p.incrementActiveCount();
}
+ if (interrupted)
+ throw new InterruptedException();
}
- else {
- long millis = unit.toMillis(timeout);
- if (millis > 0)
- externalInterruptibleAwaitDone(millis);
- }
- int s = status;
- if (s != NORMAL) {
+ if ((s &= DONE_MASK) != NORMAL) {
Throwable ex;
if (s == CANCELLED)
throw new CancellationException();
@@ -1021,16 +1096,15 @@
* be of use in designs in which many tasks are forked, but none
* are explicitly joined, instead executing them until all are
* processed.
- *
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
*/
public static void helpQuiesce() {
- ((ForkJoinWorkerThread) Thread.currentThread())
- .helpQuiescePool();
+ Thread t;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) {
+ ForkJoinWorkerThread wt = (ForkJoinWorkerThread)t;
+ wt.pool.helpQuiescePool(wt.workQueue);
+ }
+ else
+ ForkJoinPool.externalHelpQuiescePool();
}
/**
@@ -1050,7 +1124,7 @@
* setRawResult(null)}.
*/
public void reinitialize() {
- if (status == EXCEPTIONAL)
+ if ((status & DONE_MASK) == EXCEPTIONAL)
clearExceptionalCompletion();
else
status = 0;
@@ -1083,23 +1157,19 @@
/**
* Tries to unschedule this task for execution. This method will
- * typically succeed if this task is the most recently forked task
- * by the current thread, and has not commenced executing in
- * another thread. This method may be useful when arranging
- * alternative local processing of tasks that could have been, but
- * were not, stolen.
- *
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
+ * typically (but is not guaranteed to) succeed if this task is
+ * the most recently forked task by the current thread, and has
+ * not commenced executing in another thread. This method may be
+ * useful when arranging alternative local processing of tasks
+ * that could have been, but were not, stolen.
*
* @return {@code true} if unforked
*/
public boolean tryUnfork() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .unpushTask(this);
+ Thread t;
+ return (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+ ((ForkJoinWorkerThread)t).workQueue.tryUnpush(this) :
+ ForkJoinPool.tryExternalUnpush(this));
}
/**
@@ -1108,40 +1178,32 @@
* value may be useful for heuristic decisions about whether to
* fork other tasks.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @return the number of tasks
*/
public static int getQueuedTaskCount() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .getQueueSize();
+ Thread t; ForkJoinPool.WorkQueue q;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+ q = ((ForkJoinWorkerThread)t).workQueue;
+ else
+ q = ForkJoinPool.commonSubmitterQueue();
+ return (q == null) ? 0 : q.queueSize();
}
/**
* Returns an estimate of how many more locally queued tasks are
* held by the current worker thread than there are other worker
- * threads that might steal them. This value may be useful for
+ * threads that might steal them, or zero if this thread is not
+ * operating in a ForkJoinPool. This value may be useful for
* heuristic decisions about whether to fork other tasks. In many
* usages of ForkJoinTasks, at steady state, each worker should
* aim to maintain a small constant surplus (for example, 3) of
* tasks, and to process computations locally if this threshold is
* exceeded.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @return the surplus number of tasks, which may be negative
*/
public static int getSurplusQueuedTaskCount() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .getEstimatedSurplusTaskCount();
+ return ForkJoinPool.getSurplusQueuedTaskCount();
}
// Extension methods
@@ -1167,15 +1229,18 @@
protected abstract void setRawResult(V value);
/**
- * Immediately performs the base action of this task. This method
- * is designed to support extensions, and should not in general be
- * called otherwise. The return value controls whether this task
- * is considered to be done normally. It may return false in
+ * Immediately performs the base action of this task and returns
+ * true if, upon return from this method, this task is guaranteed
+ * to have completed normally. This method may return false
+ * otherwise, to indicate that this task is not necessarily
+ * complete (or is not known to be complete), for example in
* asynchronous actions that require explicit invocations of
- * {@link #complete} to become joinable. It may also throw an
- * (unchecked) exception to indicate abnormal exit.
+ * completion methods. This method may also throw an (unchecked)
+ * exception to indicate abnormal exit. This method is designed to
+ * support extensions, and should not in general be called
+ * otherwise.
*
- * @return {@code true} if completed normally
+ * @return {@code true} if this task is known to have completed normally
*/
protected abstract boolean exec();
@@ -1189,59 +1254,102 @@
* primarily to support extensions, and is unlikely to be useful
* otherwise.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask<?> peekNextLocalTask() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .peekTask();
+ Thread t; ForkJoinPool.WorkQueue q;
+ if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)
+ q = ((ForkJoinWorkerThread)t).workQueue;
+ else
+ q = ForkJoinPool.commonSubmitterQueue();
+ return (q == null) ? null : q.peek();
}
/**
* Unschedules and returns, without executing, the next task
- * queued by the current thread but not yet executed. This method
- * is designed primarily to support extensions, and is unlikely to
- * be useful otherwise.
- *
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
+ * queued by the current thread but not yet executed, if the
+ * current thread is operating in a ForkJoinPool. This method is
+ * designed primarily to support extensions, and is unlikely to be
+ * useful otherwise.
*
* @return the next task, or {@code null} if none are available
*/
protected static ForkJoinTask<?> pollNextLocalTask() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .pollLocalTask();
+ Thread t;
+ return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+ ((ForkJoinWorkerThread)t).workQueue.nextLocalTask() :
+ null;
}
/**
- * Unschedules and returns, without executing, the next task
+ * If the current thread is operating in a ForkJoinPool,
+ * unschedules and returns, without executing, the next task
* queued by the current thread but not yet executed, if one is
* available, or if not available, a task that was forked by some
* other thread, if available. Availability may be transient, so a
- * {@code null} result does not necessarily imply quiescence
- * of the pool this task is operating in. This method is designed
+ * {@code null} result does not necessarily imply quiescence of
+ * the pool this task is operating in. This method is designed
* primarily to support extensions, and is unlikely to be useful
* otherwise.
*
- * <p>This method may be invoked only from within {@code
- * ForkJoinPool} computations (as may be determined using method
- * {@link #inForkJoinPool}). Attempts to invoke in other contexts
- * result in exceptions or errors, possibly including {@code
- * ClassCastException}.
- *
* @return a task, or {@code null} if none are available
*/
protected static ForkJoinTask<?> pollTask() {
- return ((ForkJoinWorkerThread) Thread.currentThread())
- .pollTask();
+ Thread t; ForkJoinWorkerThread wt;
+ return ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) ?
+ (wt = (ForkJoinWorkerThread)t).pool.nextTaskFor(wt.workQueue) :
+ null;
+ }
+
+ // tag operations
+
+ /**
+ * Returns the tag for this task.
+ *
+ * @return the tag for this task
+ * @since 1.8
+ */
+ public final short getForkJoinTaskTag() {
+ return (short)status;
+ }
+
+ /**
+ * Atomically sets the tag value for this task.
+ *
+ * @param tag the tag value
+ * @return the previous value of the tag
+ * @since 1.8
+ */
+ public final short setForkJoinTaskTag(short tag) {
+ for (int s;;) {
+ if (U.compareAndSwapInt(this, STATUS, s = status,
+ (s & ~SMASK) | (tag & SMASK)))
+ return (short)s;
+ }
+ }
+
+ /**
+ * Atomically conditionally sets the tag value for this task.
+ * Among other applications, tags can be used as visit markers
+ * in tasks operating on graphs, as in methods that check: {@code
+ * if (task.compareAndSetForkJoinTaskTag((short)0, (short)1))}
+ * before processing, otherwise exiting because the node has
+ * already been visited.
+ *
+ * @param e the expected tag value
+ * @param tag the new tag value
+ * @return true if successful; i.e., the current value was
+ * equal to e and is now tag.
+ * @since 1.8
+ */
+ public final boolean compareAndSetForkJoinTaskTag(short e, short tag) {
+ for (int s;;) {
+ if ((short)(s = status) != e)
+ return false;
+ if (U.compareAndSwapInt(this, STATUS, s,
+ (s & ~SMASK) | (tag & SMASK)))
+ return true;
+ }
}
/**
@@ -1252,21 +1360,33 @@
static final class AdaptedRunnable<T> extends ForkJoinTask<T>
implements RunnableFuture<T> {
final Runnable runnable;
- final T resultOnCompletion;
T result;
AdaptedRunnable(Runnable runnable, T result) {
if (runnable == null) throw new NullPointerException();
this.runnable = runnable;
- this.resultOnCompletion = result;
+ this.result = result; // OK to set this even before completion
}
- public T getRawResult() { return result; }
- public void setRawResult(T v) { result = v; }
- public boolean exec() {
- runnable.run();
- result = resultOnCompletion;
- return true;
+ public final T getRawResult() { return result; }
+ public final void setRawResult(T v) { result = v; }
+ public final boolean exec() { runnable.run(); return true; }
+ public final void run() { invoke(); }
+ private static final long serialVersionUID = 5232453952276885070L;
+ }
+
+ /**
+ * Adaptor for Runnables without results
+ */
+ static final class AdaptedRunnableAction extends ForkJoinTask<Void>
+ implements RunnableFuture<Void> {
+ final Runnable runnable;
+ AdaptedRunnableAction(Runnable runnable) {
+ if (runnable == null) throw new NullPointerException();
+ this.runnable = runnable;
}
- public void run() { invoke(); }
+ public final Void getRawResult() { return null; }
+ public final void setRawResult(Void v) { }
+ public final boolean exec() { runnable.run(); return true; }
+ public final void run() { invoke(); }
private static final long serialVersionUID = 5232453952276885070L;
}
@@ -1281,9 +1401,9 @@
if (callable == null) throw new NullPointerException();
this.callable = callable;
}
- public T getRawResult() { return result; }
- public void setRawResult(T v) { result = v; }
- public boolean exec() {
+ public final T getRawResult() { return result; }
+ public final void setRawResult(T v) { result = v; }
+ public final boolean exec() {
try {
result = callable.call();
return true;
@@ -1295,7 +1415,7 @@
throw new RuntimeException(ex);
}
}
- public void run() { invoke(); }
+ public final void run() { invoke(); }
private static final long serialVersionUID = 2838392045355241008L;
}
@@ -1308,7 +1428,7 @@
* @return the task
*/
public static ForkJoinTask<?> adapt(Runnable runnable) {
- return new AdaptedRunnable<Void>(runnable, null);
+ return new AdaptedRunnableAction(runnable);
}
/**
@@ -1342,11 +1462,10 @@
private static final long serialVersionUID = -7721805057305804111L;
/**
- * Saves the state to a stream (that is, serializes it).
+ * Saves this task to a stream (that is, serializes it).
*
* @serialData the current run status and the exception thrown
* during execution, or {@code null} if none
- * @param s the stream
*/
private void writeObject(java.io.ObjectOutputStream s)
throws java.io.IOException {
@@ -1355,9 +1474,7 @@
}
/**
- * Reconstitutes the instance from a stream (that is, deserializes it).
- *
- * @param s the stream
+ * Reconstitutes this task from a stream (that is, deserializes it).
*/
private void readObject(java.io.ObjectInputStream s)
throws java.io.IOException, ClassNotFoundException {
@@ -1368,16 +1485,18 @@
}
// Unsafe mechanics
- private static final sun.misc.Unsafe UNSAFE;
- private static final long statusOffset;
+ private static final sun.misc.Unsafe U;
+ private static final long STATUS;
+
static {
exceptionTableLock = new ReentrantLock();
exceptionTableRefQueue = new ReferenceQueue<Object>();
exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY];
try {
- UNSAFE = sun.misc.Unsafe.getUnsafe();
- statusOffset = UNSAFE.objectFieldOffset
- (ForkJoinTask.class.getDeclaredField("status"));
+ U = sun.misc.Unsafe.getUnsafe();
+ Class<?> k = ForkJoinTask.class;
+ STATUS = U.objectFieldOffset
+ (k.getDeclaredField("status"));
} catch (Exception e) {
throw new Error(e);
}
--- a/jdk/src/share/classes/java/util/concurrent/ForkJoinWorkerThread.java Thu Dec 20 17:24:56 2012 +0400
+++ b/jdk/src/share/classes/java/util/concurrent/ForkJoinWorkerThread.java Thu Dec 20 13:44:06 2012 +0000
@@ -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 @@
*/
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:
- *
- * 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.
+ * ForkJoinTasks. For explanation, see the internal documentation
+ * of class ForkJoinPool.
*
- * 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 @@
* @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 @@
* @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 @@
* 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 @@
* 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 @@
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);
- }
-
}