--- jsr166/src/jsr166y/ForkJoinTask.java 2010/09/06 11:55:39 1.58 +++ jsr166/src/jsr166y/ForkJoinTask.java 2012/02/20 18:20:06 1.86 @@ -1,20 +1,26 @@ /* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at - * http://creativecommons.org/licenses/publicdomain + * http://creativecommons.org/publicdomain/zero/1.0/ */ package jsr166y; - -import java.util.concurrent.*; - 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.util.WeakHashMap; +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.Future; +import java.util.concurrent.RejectedExecutionException; +import java.util.concurrent.RunnableFuture; +import java.util.concurrent.TimeUnit; +import java.util.concurrent.TimeoutException; +import java.util.concurrent.locks.ReentrantLock; +import java.lang.reflect.Constructor; /** * Abstract base class for tasks that run within a {@link ForkJoinPool}. @@ -28,37 +34,50 @@ import java.util.WeakHashMap; * 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}. 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. + * #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. * *

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. + * 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 IO, 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. + * + *

It is possible to define and use ForkJoinTasks that may block, + * but doing do requires three further considerations: (1) Completion + * of few if any other tasks should be dependent on a task + * that blocks on external synchronization or IO. Event-style async + * tasks that are never joined 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. * *

The primary method for awaiting completion and extracting * results of a task is {@link #join}, but there are several variants: @@ -74,6 +93,13 @@ import java.util.WeakHashMap; * performs the most common form of parallel invocation: forking a set * of tasks and joining them all. * + *

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}. + * *

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); @@ -102,6 +128,27 @@ import java.util.WeakHashMap; * result in exceptions or errors, possibly including * {@code ClassCastException}. * + *

Method {@link #join} and its variants are appropriate for use + * only when completion dependencies are acyclic; that is, the + * parallel computation can be described as a directed acyclic graph + * (DAG). Otherwise, executions may encounter a form of deadlock as + * tasks cyclically wait for each other. However, this framework + * 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. To support such usages a + * ForkJoinTask may be atomically marked using {@link + * #markForkJoinTask} and checked for marking using {@link + * #isMarkedForkJoinTask}. The ForkJoinTask implementation does not + * use these {@code protected} methods or marks 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. + * Also, completion based designs can use them to record that one + * subtask has completed. (Method names for marking are bulky in part + * to encourage definition of methods that reflect their usage + * patterns.) + * *

Most base support methods are {@code final}, to prevent * overriding of implementations that are intrinsically tied to the * underlying lightweight task scheduling framework. Developers @@ -116,9 +163,10 @@ import java.util.WeakHashMap; * computation. Large tasks should be split into smaller subtasks, * usually via recursive decomposition. As a very rough rule of thumb, * a task should perform more than 100 and less than 10000 basic - * computational steps. If tasks are too big, then parallelism cannot - * improve throughput. If too small, then memory and internal task - * maintenance overhead may overwhelm processing. + * computational steps, and should avoid indefinite looping. If tasks + * are too big, then parallelism cannot improve throughput. If too + * small, then memory and internal task maintenance overhead may + * overwhelm processing. * *

This class provides {@code adapt} methods for {@link Runnable} * and {@link Callable}, that may be of use when mixing execution of @@ -139,160 +187,411 @@ public abstract class ForkJoinTask im * 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. In particular, most - * join mechanics are in method quietlyJoin, below. + * 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. */ /** 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 = 0x00000001; + static final int MARKED = 0x00000002; /** - * Table of exceptions thrown by tasks, to enable reporting by - * callers. Because exceptions are rare, we don't directly keep - * them with task objects, but instead use a weak ref table. Note - * that cancellation exceptions don't appear in the table, but are - * instead recorded as status values. - * TODO: Use ConcurrentReferenceHashMap + * Marks completion and wakes up threads waiting to join this + * task. A specialization for NORMAL completion is in method + * doExec. + * + * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL + * @return completion status on exit */ - static final Map, Throwable> exceptionMap = - Collections.synchronizedMap - (new WeakHashMap, Throwable>()); + private int setCompletion(int completion) { + for (int s;;) { + if ((s = status) < 0) + return s; + if (U.compareAndSwapInt(this, STATUS, s, s | completion)) { + if ((s & SIGNAL) != 0) + synchronized (this) { notifyAll(); } + return completion; + } + } + } - // Maintaining completion status + /** + * Primary execution method for stolen tasks. Unless done, calls + * exec and records status if completed, but doesn't wait for + * completion otherwise. + * + * @return status on exit from this method + */ + final int doExec() { + int s; boolean completed; + if ((s = status) >= 0) { + try { + completed = exec(); + } catch (Throwable rex) { + return setExceptionalCompletion(rex); + } + while ((s = status) >= 0 && completed) { + if (U.compareAndSwapInt(this, STATUS, s, s | NORMAL)) { + if ((s & SIGNAL) != 0) + synchronized (this) { notifyAll(); } + return NORMAL; + } + } + } + return s; + } /** - * Marks completion and wakes up threads waiting to join this task, - * also clearing signal request bits. + * Tries to set SIGNAL status. Used by ForkJoinPool. Other + * variants are directly incorporated into externalAwaitDone etc. * - * @param completion one of NORMAL, CANCELLED, EXCEPTIONAL + * @return true if successful */ - private void setCompletion(int completion) { + final boolean trySetSignal() { + int s; + return U.compareAndSwapInt(this, STATUS, s = status, s | SIGNAL); + } + + /** + * Blocks a non-worker-thread until completion. + * @return status upon completion + */ + private int externalAwaitDone() { + boolean interrupted = false; int s; while ((s = status) >= 0) { - if (UNSAFE.compareAndSwapInt(this, statusOffset, s, completion)) { - if (s != 0) - synchronized (this) { notifyAll(); } - break; + 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(); + return s; + } + + /** + * Blocks a non-worker-thread until completion or interruption. + */ + private int externalInterruptibleAwaitDone() throws InterruptedException { + int s; + if (Thread.interrupted()) + throw new InterruptedException(); + while ((s = status) >= 0) { + if (U.compareAndSwapInt(this, STATUS, s, s | SIGNAL)) { + synchronized (this) { + if (status >= 0) + wait(); + else + notifyAll(); + } + } + } + return s; + } + + + /** + * 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() { + int s; Thread t; ForkJoinWorkerThread wt; ForkJoinPool.WorkQueue w; + if ((s = status) >= 0) { + if (((t = Thread.currentThread()) instanceof ForkJoinWorkerThread)) { + if (!(w = (wt = (ForkJoinWorkerThread)t).workQueue). + tryUnpush(this) || (s = doExec()) >= 0) + s = wt.pool.awaitJoin(w, this); } + else + s = externalAwaitDone(); + } + return s; + } + + /** + * Implementation for invoke, quietlyInvoke. + * + * @return status upon completion + */ + private int doInvoke() { + int s; Thread t; ForkJoinWorkerThread wt; + if ((s = doExec()) >= 0) { + if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) + s = (wt = (ForkJoinWorkerThread)t).pool.awaitJoin(wt.workQueue, + this); + else + s = externalAwaitDone(); + } + return s; + } + + // Exception table support + + /** + * Table of exceptions thrown by tasks, to enable reporting by + * callers. Because exceptions are rare, we don't directly keep + * them with task objects, but instead use a weak ref table. Note + * that cancellation exceptions don't appear in the table, but are + * instead recorded as status values. + * + * Note: These statics are initialized below in static block. + */ + private static final ExceptionNode[] exceptionTable; + private static final ReentrantLock exceptionTableLock; + private static final ReferenceQueue exceptionTableRefQueue; + + /** + * Fixed capacity for exceptionTable. + */ + private static final int EXCEPTION_MAP_CAPACITY = 32; + + /** + * Key-value nodes for exception table. The chained hash table + * uses identity comparisons, full locking, and weak references + * for keys. The table has a fixed capacity because it only + * maintains task exceptions long enough for joiners to access + * them, so should never become very large for sustained + * periods. However, since we do not know when the last joiner + * completes, we must use weak references and expunge them. We do + * so on each operation (hence full locking). Also, some thread in + * any ForkJoinPool will call helpExpungeStaleExceptions when its + * pool becomes isQuiescent. + */ + static final class ExceptionNode extends WeakReference> { + final Throwable ex; + ExceptionNode next; + final long thrower; // use id not ref to avoid weak cycles + ExceptionNode(ForkJoinTask task, Throwable ex, ExceptionNode next) { + super(task, exceptionTableRefQueue); + this.ex = ex; + this.next = next; + this.thrower = Thread.currentThread().getId(); } } /** * Records exception and sets exceptional completion. - * + * * @return status on exit */ - private void setExceptionalCompletion(Throwable rex) { - exceptionMap.put(this, rex); - setCompletion(EXCEPTIONAL); + 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; + } + } finally { + lock.unlock(); + } + return setCompletion(EXCEPTIONAL); } /** - * Blocks a worker thread until completion. Called only by - * pool. Currently unused -- pool-based waits use timeout - * version below. + * 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 internalAwaitDone() { - int s; // the odd construction reduces lock bias effects - while ((s = status) >= 0) { + static final void cancelIgnoringExceptions(ForkJoinTask t) { + if (t != null && t.status >= 0) { try { - synchronized(this) { - if (UNSAFE.compareAndSwapInt(this, statusOffset, s,SIGNAL)) - wait(); - } - } catch (InterruptedException ie) { - cancelIfTerminating(); + t.cancel(false); + } catch (Throwable ignore) { } } } /** - * Blocks a worker thread until completed or timed out. Called - * only by pool. - * - * @return status on exit + * Removes exception node and clears status */ - final int internalAwaitDone(long millis) { - int s; - if ((s = status) >= 0) { + private void clearExceptionalCompletion() { + int h = System.identityHashCode(this); + final ReentrantLock lock = exceptionTableLock; + lock.lock(); + try { + ExceptionNode[] t = exceptionTable; + int i = h & (t.length - 1); + ExceptionNode e = t[i]; + ExceptionNode pred = null; + while (e != null) { + ExceptionNode next = e.next; + if (e.get() == this) { + if (pred == null) + t[i] = next; + else + pred.next = next; + break; + } + pred = e; + e = next; + } + expungeStaleExceptions(); + status = 0; + } finally { + lock.unlock(); + } + } + + /** + * Returns a rethrowable exception for the given task, if + * available. To provide accurate stack traces, if the exception + * was not thrown by the current thread, we try to create a new + * exception of the same type as the one thrown, but with the + * recorded exception as its cause. If there is no such + * constructor, we instead try to use a no-arg constructor, + * followed by initCause, to the same effect. If none of these + * apply, or any fail due to other exceptions, we return the + * recorded exception, which is still correct, although it may + * contain a misleading stack trace. + * + * @return the exception, or null if none + */ + private Throwable getThrowableException() { + if ((status & DONE_MASK) != EXCEPTIONAL) + return null; + int h = System.identityHashCode(this); + ExceptionNode e; + final ReentrantLock lock = exceptionTableLock; + lock.lock(); + try { + expungeStaleExceptions(); + ExceptionNode[] t = exceptionTable; + e = t[h & (t.length - 1)]; + while (e != null && e.get() != this) + e = e.next; + } finally { + lock.unlock(); + } + Throwable ex; + if (e == null || (ex = e.ex) == null) + return null; + if (e.thrower != Thread.currentThread().getId()) { + Class ec = ex.getClass(); try { - synchronized(this) { - if (UNSAFE.compareAndSwapInt(this, statusOffset, s,SIGNAL)) - wait(millis, 0); + Constructor noArgCtor = null; + Constructor[] cs = ec.getConstructors();// public ctors only + for (int i = 0; i < cs.length; ++i) { + Constructor c = cs[i]; + Class[] ps = c.getParameterTypes(); + if (ps.length == 0) + noArgCtor = c; + else if (ps.length == 1 && ps[0] == Throwable.class) + return (Throwable)(c.newInstance(ex)); + } + if (noArgCtor != null) { + Throwable wx = (Throwable)(noArgCtor.newInstance()); + wx.initCause(ex); + return wx; } - } catch (InterruptedException ie) { - cancelIfTerminating(); + } catch (Exception ignore) { } - s = status; } - return s; + return ex; } /** - * Blocks a non-worker-thread until completion. + * Poll stale refs and remove them. Call only while holding lock. */ - private void externalAwaitDone() { - int s; - while ((s = status) >= 0) { - synchronized(this) { - if (UNSAFE.compareAndSwapInt(this, statusOffset, s, SIGNAL)){ - boolean interrupted = false; - while (status >= 0) { - try { - wait(); - } catch (InterruptedException ie) { - interrupted = true; - } + private static void expungeStaleExceptions() { + for (Object x; (x = exceptionTableRefQueue.poll()) != null;) { + if (x instanceof ExceptionNode) { + ForkJoinTask key = ((ExceptionNode)x).get(); + ExceptionNode[] t = exceptionTable; + int i = System.identityHashCode(key) & (t.length - 1); + ExceptionNode e = t[i]; + ExceptionNode pred = null; + while (e != null) { + ExceptionNode next = e.next; + if (e == x) { + if (pred == null) + t[i] = next; + else + pred.next = next; + break; } - if (interrupted) - Thread.currentThread().interrupt(); - break; + pred = e; + e = next; } } } } /** - * Unless done, calls exec and records status if completed, but - * doesn't wait for completion otherwise. Primary execution method - * for ForkJoinWorkerThread. + * If lock is available, poll stale refs and remove them. + * Called from ForkJoinPool when pools become quiescent. */ - final void quietlyExec() { - try { - if (status < 0 || !exec()) - return; - } catch (Throwable rex) { - setExceptionalCompletion(rex); - return; + static final void helpExpungeStaleExceptions() { + final ReentrantLock lock = exceptionTableLock; + if (lock.tryLock()) { + try { + expungeStaleExceptions(); + } finally { + lock.unlock(); + } } - setCompletion(NORMAL); // must be outside try block + } + + /** + * Throws exception, if any, associated with the given status. + */ + private void reportException(int s) { + Throwable ex = ((s == CANCELLED) ? new CancellationException() : + (s == EXCEPTIONAL) ? getThrowableException() : + null); + if (ex != null) + U.throwException(ex); } // public methods @@ -308,7 +607,7 @@ public abstract class ForkJoinTask im * #isDone} returning {@code true}. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -316,24 +615,25 @@ public abstract class ForkJoinTask im * @return {@code this}, to simplify usage */ public final ForkJoinTask fork() { - ((ForkJoinWorkerThread) Thread.currentThread()) - .pushTask(this); + ((ForkJoinWorkerThread)Thread.currentThread()).workQueue.push(this); return this; } /** - * Returns the result of the computation when it {@link #isDone is done}. - * This method differs from {@link #get()} in that + * Returns the result of the computation when it {@link #isDone is + * done}. This method differs from {@link #get()} in that * abnormal completion results in {@code RuntimeException} or - * {@code Error}, not {@code ExecutionException}. + * {@code Error}, not {@code ExecutionException}, and that + * interrupts of the calling thread do not cause the + * method to abruptly return by throwing {@code + * InterruptedException}. * * @return the computed result */ public final V join() { - quietlyJoin(); - Throwable ex; - if (status < NORMAL && (ex = getException()) != null) - UNSAFE.throwException(ex); + int s; + if ((s = doJoin() & DONE_MASK) != NORMAL) + reportException(s); return getRawResult(); } @@ -346,10 +646,9 @@ public abstract class ForkJoinTask im * @return the computed result */ public final V invoke() { - quietlyInvoke(); - Throwable ex; - if (status < NORMAL && (ex = getException()) != null) - UNSAFE.throwException(ex); + int s; + if ((s = doInvoke() & DONE_MASK) != NORMAL) + reportException(s); return getRawResult(); } @@ -367,7 +666,7 @@ public abstract class ForkJoinTask im * unprocessed. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -377,9 +676,12 @@ public abstract class ForkJoinTask im * @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); } /** @@ -395,7 +697,7 @@ public abstract class ForkJoinTask im * normally or exceptionally, or left unprocessed. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -414,26 +716,20 @@ public abstract class ForkJoinTask im } else if (i != 0) t.fork(); - else { - t.quietlyInvoke(); - if (ex == null && t.status < NORMAL) - ex = t.getException(); - } + else if (t.doInvoke() < NORMAL && ex == null) + ex = t.getException(); } for (int i = 1; i <= last; ++i) { ForkJoinTask t = tasks[i]; if (t != null) { if (ex != null) t.cancel(false); - else { - t.quietlyJoin(); - if (ex == null && t.status < NORMAL) - ex = t.getException(); - } + else if (t.doJoin() < NORMAL) + ex = t.getException(); } } if (ex != null) - UNSAFE.throwException(ex); + U.throwException(ex); } /** @@ -450,7 +746,7 @@ public abstract class ForkJoinTask im * unprocessed. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -477,84 +773,52 @@ public abstract class ForkJoinTask im } else if (i != 0) t.fork(); - else { - t.quietlyInvoke(); - if (ex == null && t.status < NORMAL) - ex = t.getException(); - } + else if (t.doInvoke() < NORMAL && ex == null) + ex = t.getException(); } for (int i = 1; i <= last; ++i) { ForkJoinTask t = ts.get(i); if (t != null) { if (ex != null) t.cancel(false); - else { - t.quietlyJoin(); - if (ex == null && t.status < NORMAL) - ex = t.getException(); - } + else if (t.doJoin() < NORMAL) + ex = t.getException(); } } if (ex != null) - UNSAFE.throwException(ex); + U.throwException(ex); return tasks; } /** * Attempts to cancel execution of this task. This attempt will - * fail if the task has already completed, has already been - * cancelled, or could not be cancelled for some other reason. If - * successful, and this task has not started when cancel is - * called, execution of this task is suppressed, {@link - * #isCancelled} will report true, and {@link #join} will result - * in a {@code CancellationException} being thrown. + * fail if the task has already completed or could not be + * cancelled for some other reason. If successful, and this task + * has not started when {@code cancel} is called, execution of + * this task is suppressed. After this method returns + * successfully, unless there is an intervening call to {@link + * #reinitialize}, subsequent calls to {@link #isCancelled}, + * {@link #isDone}, and {@code cancel} will return {@code true} + * and calls to {@link #join} and related methods will result in + * {@code CancellationException}. * *

This method may be overridden in subclasses, but if so, must - * still ensure that these minimal properties hold. In particular, - * the {@code cancel} method itself must not throw exceptions. + * still ensure that these properties hold. In particular, the + * {@code cancel} method itself must not throw exceptions. * *

This method is designed to be invoked by other * tasks. To terminate the current task, you can just return or * throw an unchecked exception from its computation method, or * invoke {@link #completeExceptionally}. * - * @param mayInterruptIfRunning this value is ignored in the - * default implementation because tasks are not - * cancelled via interruption + * @param mayInterruptIfRunning this value has no effect in the + * default implementation because interrupts are not used to + * control cancellation. * * @return {@code true} if this task is now cancelled */ public boolean cancel(boolean mayInterruptIfRunning) { - setCompletion(CANCELLED); - return status == 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) { - } - } - - /** - * Cancels if current thread is a terminating worker thread, - * ignoring any exceptions thrown by cancel. - */ - final void cancelIfTerminating() { - Thread t = Thread.currentThread(); - if ((t instanceof ForkJoinWorkerThread) && - ((ForkJoinWorkerThread) t).isTerminating()) { - try { - cancel(false); - } catch (Throwable ignore) { - } - } + return (setCompletion(CANCELLED) & DONE_MASK) == CANCELLED; } public final boolean isDone() { @@ -562,7 +826,7 @@ public abstract class ForkJoinTask im } public final boolean isCancelled() { - return status == CANCELLED; + return (status & DONE_MASK) == CANCELLED; } /** @@ -582,7 +846,7 @@ public abstract class ForkJoinTask im * exception and was not cancelled */ public final boolean isCompletedNormally() { - return status == NORMAL; + return (status & DONE_MASK) == NORMAL; } /** @@ -593,10 +857,10 @@ public abstract class ForkJoinTask im * @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() : - exceptionMap.get(this)); + getThrowableException()); } /** @@ -642,91 +906,105 @@ public abstract class ForkJoinTask im setCompletion(NORMAL); } + /** + * Waits if necessary for the computation to complete, and then + * retrieves its result. + * + * @return the computed result + * @throws CancellationException if the computation was cancelled + * @throws ExecutionException if the computation threw an + * exception + * @throws InterruptedException if the current thread is not a + * member of a ForkJoinPool and was interrupted while waiting + */ public final V get() throws InterruptedException, ExecutionException { - quietlyJoin(); - if (Thread.interrupted()) - throw new InterruptedException(); - int s = status; - if (s < NORMAL) { - Throwable ex; - if (s == CANCELLED) - throw new CancellationException(); - if (s == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) - throw new ExecutionException(ex); - } + int s = (Thread.currentThread() instanceof ForkJoinWorkerThread) ? + doJoin() : externalInterruptibleAwaitDone(); + Throwable ex; + if ((s &= DONE_MASK) == CANCELLED) + throw new CancellationException(); + if (s == EXCEPTIONAL && (ex = getThrowableException()) != null) + throw new ExecutionException(ex); return getRawResult(); } + /** + * Waits if necessary for at most the given time for the computation + * to complete, and then retrieves its result, if available. + * + * @param timeout the maximum time to wait + * @param unit the time unit of the timeout argument + * @return the computed result + * @throws CancellationException if the computation was cancelled + * @throws ExecutionException if the computation threw an + * exception + * @throws InterruptedException if the current thread is not a + * member of a ForkJoinPool and was interrupted while waiting + * @throws TimeoutException if the wait timed out + */ public final V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { - Thread t = Thread.currentThread(); - ForkJoinPool pool; - if (t instanceof ForkJoinWorkerThread) { - ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; - if (status >= 0 && w.unpushTask(this)) - quietlyExec(); - pool = w.pool; - } - else - pool = null; - /* - * Timed wait loop intermixes cases for FJ (pool != null) and - * non FJ threads. For FJ, decrement pool count but don't try - * for replacement; increment count on completion. For non-FJ, - * deal with interrupts. This is messy, but a little less so - * than is splitting the FJ and nonFJ cases. - */ - boolean interrupted = false; - boolean dec = false; // true if pool count decremented - long nanos = unit.toNanos(timeout); - for (;;) { - if (pool == null && Thread.interrupted()) { - interrupted = true; - break; + 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; + s = p.helpJoinOnce(w, this); // no retries on failure } - int s = status; - if (s < 0) - break; - if (UNSAFE.compareAndSwapInt(this, statusOffset, s, SIGNAL)) { - long startTime = System.nanoTime(); - long nt; // wait time - while (status >= 0 && - (nt = nanos - (System.nanoTime() - startTime)) > 0) { - if (pool != null && !dec) - dec = pool.tryDecrementRunningCount(); + boolean canBlock = false; + boolean interrupted = false; + try { + while ((s = status) >= 0) { + if (w != null && w.runState < 0) + cancelIgnoringExceptions(this); + else if (!canBlock) { + if (p == null || p.tryCompensate(this, null)) + canBlock = true; + } else { - long ms = nt / 1000000; - int ns = (int) (nt % 1000000); - try { - synchronized(this) { - if (status >= 0) - wait(ms, ns); - } - } catch (InterruptedException ie) { - if (pool != null) - cancelIfTerminating(); - else { - interrupted = true; - break; + 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; } } - break; + } finally { + if (p != null && canBlock) + p.incrementActiveCount(); } + if (interrupted) + throw new InterruptedException(); } - if (pool != null && dec) - pool.incrementRunningCount(); - if (interrupted) - throw new InterruptedException(); - int es = status; - if (es != NORMAL) { + if ((s &= DONE_MASK) != NORMAL) { Throwable ex; - if (es == CANCELLED) + if (s == CANCELLED) throw new CancellationException(); - if (es == EXCEPTIONAL && (ex = exceptionMap.get(this)) != null) + if (s != EXCEPTIONAL) + throw new TimeoutException(); + if ((ex = getThrowableException()) != null) throw new ExecutionException(ex); - throw new TimeoutException(); } return getRawResult(); } @@ -738,28 +1016,7 @@ public abstract class ForkJoinTask im * known to have aborted. */ public final void quietlyJoin() { - Thread t; - if ((t = Thread.currentThread()) instanceof ForkJoinWorkerThread) { - ForkJoinWorkerThread w = (ForkJoinWorkerThread) t; - if (status >= 0) { - if (w.unpushTask(this)) { - boolean completed; - try { - completed = exec(); - } catch (Throwable rex) { - setExceptionalCompletion(rex); - return; - } - if (completed) { - setCompletion(NORMAL); - return; - } - } - w.joinTask(this); - } - } - else - externalAwaitDone(); + doJoin(); } /** @@ -768,19 +1025,7 @@ public abstract class ForkJoinTask im * exception. */ public final void quietlyInvoke() { - if (status >= 0) { - boolean completed; - try { - completed = exec(); - } catch (Throwable rex) { - setExceptionalCompletion(rex); - return; - } - if (completed) - setCompletion(NORMAL); - else - quietlyJoin(); - } + doInvoke(); } /** @@ -791,14 +1036,15 @@ public abstract class ForkJoinTask im * processed. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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(); + ForkJoinWorkerThread wt = + (ForkJoinWorkerThread)Thread.currentThread(); + wt.pool.helpQuiescePool(wt.workQueue); } /** @@ -810,11 +1056,18 @@ public abstract class ForkJoinTask im * under any other usage conditions are not guaranteed. * This method may be useful when executing * pre-constructed trees of subtasks in loops. + * + *

Upon completion of this method, {@code isDone()} reports + * {@code false}, and {@code getException()} reports {@code + * null}. However, the value returned by {@code getRawResult} is + * unaffected. To clear this value, you can invoke {@code + * setRawResult(null)}. */ public void reinitialize() { - if (status == EXCEPTIONAL) - exceptionMap.remove(this); - status = 0; + if ((status & DONE_MASK) == EXCEPTIONAL) + clearExceptionalCompletion(); + else + status = 0; } /** @@ -831,11 +1084,12 @@ public abstract class ForkJoinTask im } /** - * Returns {@code true} if the current thread is executing as a - * ForkJoinPool computation. + * Returns {@code true} if the current thread is a {@link + * ForkJoinWorkerThread} executing as a ForkJoinPool computation. * - * @return {@code true} if the current thread is executing as a - * ForkJoinPool computation, or false otherwise + * @return {@code true} if the current thread is a {@link + * ForkJoinWorkerThread} executing as a ForkJoinPool computation, + * or {@code false} otherwise */ public static boolean inForkJoinPool() { return Thread.currentThread() instanceof ForkJoinWorkerThread; @@ -850,7 +1104,7 @@ public abstract class ForkJoinTask im * were not, stolen. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -858,8 +1112,8 @@ public abstract class ForkJoinTask im * @return {@code true} if unforked */ public boolean tryUnfork() { - return ((ForkJoinWorkerThread) Thread.currentThread()) - .unpushTask(this); + return ((ForkJoinWorkerThread)Thread.currentThread()) + .workQueue.tryUnpush(this); } /** @@ -869,7 +1123,7 @@ public abstract class ForkJoinTask im * fork other tasks. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -878,7 +1132,7 @@ public abstract class ForkJoinTask im */ public static int getQueuedTaskCount() { return ((ForkJoinWorkerThread) Thread.currentThread()) - .getQueueSize(); + .workQueue.queueSize(); } /** @@ -892,7 +1146,7 @@ public abstract class ForkJoinTask im * exceeded. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -900,8 +1154,52 @@ public abstract class ForkJoinTask im * @return the surplus number of tasks, which may be negative */ public static int getSurplusQueuedTaskCount() { - return ((ForkJoinWorkerThread) Thread.currentThread()) - .getEstimatedSurplusTaskCount(); + /* + * The aim of this method is to return 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. + */ + ForkJoinWorkerThread wt = + (ForkJoinWorkerThread)Thread.currentThread(); + return wt.workQueue.queueSize() - wt.pool.idlePerActive(); } // Extension methods @@ -950,7 +1248,7 @@ public abstract class ForkJoinTask im * otherwise. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -958,8 +1256,7 @@ public abstract class ForkJoinTask im * @return the next task, or {@code null} if none are available */ protected static ForkJoinTask peekNextLocalTask() { - return ((ForkJoinWorkerThread) Thread.currentThread()) - .peekTask(); + return ((ForkJoinWorkerThread) Thread.currentThread()).workQueue.peek(); } /** @@ -969,7 +1266,7 @@ public abstract class ForkJoinTask im * be useful otherwise. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -978,7 +1275,7 @@ public abstract class ForkJoinTask im */ protected static ForkJoinTask pollNextLocalTask() { return ((ForkJoinWorkerThread) Thread.currentThread()) - .pollLocalTask(); + .workQueue.nextLocalTask(); } /** @@ -992,7 +1289,7 @@ public abstract class ForkJoinTask im * otherwise. * *

This method may be invoked only from within {@code - * ForkJoinTask} computations (as may be determined using method + * 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}. @@ -1000,8 +1297,51 @@ public abstract class ForkJoinTask im * @return a task, or {@code null} if none are available */ protected static ForkJoinTask pollTask() { - return ((ForkJoinWorkerThread) Thread.currentThread()) - .pollTask(); + ForkJoinWorkerThread wt = + (ForkJoinWorkerThread)Thread.currentThread(); + return wt.pool.nextTaskFor(wt.workQueue); + } + + // Mark-bit operations + + /** + * Returns true if this task is marked. + * + * @return true if this task is marked + * @since 1.8 + */ + public final boolean isMarkedForkJoinTask() { + return (status & MARKED) != 0; + } + + /** + * Atomically sets the mark on this task. + * + * @return true if this task was previously unmarked + * @since 1.8 + */ + public final boolean markForkJoinTask() { + for (int s;;) { + if (((s = status) & MARKED) != 0) + return false; + if (U.compareAndSwapInt(this, STATUS, s, s | MARKED)) + return true; + } + } + + /** + * Atomically clears the mark on this task. + * + * @return true if this task was previously marked + * @since 1.8 + */ + public final boolean unmarkForkJoinTask() { + for (int s;;) { + if (((s = status) & MARKED) == 0) + return false; + if (U.compareAndSwapInt(this, STATUS, s, s & ~MARKED)) + return true; + } } /** @@ -1012,21 +1352,33 @@ public abstract class ForkJoinTask im static final class AdaptedRunnable extends ForkJoinTask implements RunnableFuture { 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 + implements RunnableFuture { + 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; } @@ -1041,9 +1393,9 @@ public abstract class ForkJoinTask im 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; @@ -1055,7 +1407,7 @@ public abstract class ForkJoinTask im throw new RuntimeException(ex); } } - public void run() { invoke(); } + public final void run() { invoke(); } private static final long serialVersionUID = 2838392045355241008L; } @@ -1068,7 +1420,7 @@ public abstract class ForkJoinTask im * @return the task */ public static ForkJoinTask adapt(Runnable runnable) { - return new AdaptedRunnable(runnable, null); + return new AdaptedRunnableAction(runnable); } /** @@ -1102,11 +1454,10 @@ public abstract class ForkJoinTask im 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 { @@ -1115,32 +1466,29 @@ public abstract class ForkJoinTask im } /** - * 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 { s.defaultReadObject(); Object ex = s.readObject(); if (ex != null) - setExceptionalCompletion((Throwable) ex); + setExceptionalCompletion((Throwable)ex); } // Unsafe mechanics - - private static final sun.misc.Unsafe UNSAFE = getUnsafe(); - private static final long statusOffset = - objectFieldOffset("status", ForkJoinTask.class); - - private static long objectFieldOffset(String field, Class klazz) { + private static final sun.misc.Unsafe U; + private static final long STATUS; + static { + exceptionTableLock = new ReentrantLock(); + exceptionTableRefQueue = new ReferenceQueue(); + exceptionTable = new ExceptionNode[EXCEPTION_MAP_CAPACITY]; try { - return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); - } catch (NoSuchFieldException e) { - // Convert Exception to corresponding Error - NoSuchFieldError error = new NoSuchFieldError(field); - error.initCause(e); - throw error; + U = getUnsafe(); + STATUS = U.objectFieldOffset + (ForkJoinTask.class.getDeclaredField("status")); + } catch (Exception e) { + throw new Error(e); } }