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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package jsr166y; |
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import jsr166y.forkjoin.*; |
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import java.util.concurrent.*; |
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import java.util.concurrent.atomic.*; |
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import java.util.concurrent.locks.LockSupport; |
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|
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/** |
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* A reusable synchronization barrier, similar in functionality to a |
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* {@link java.util.concurrent.CyclicBarrier}, but supporting more |
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* flexible usage. |
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* |
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* <ul> |
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* |
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* <li> The number of parties synchronizing on the barrier may vary |
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* over time. A task may register to be a party in a barrier at any |
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* time, and may deregister upon arriving at the barrier. As is the |
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* case with most basic synchronization constructs, registration |
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* and deregistration affect only internal counts; they do not |
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* establish any further internal bookkeeping, so tasks cannot query |
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* whether they are registered. |
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* |
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* <li> Each generation has an associated phase value, starting at |
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* zero, and advancing when all parties reach the barrier (wrapping |
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* around to zero after reaching <tt>Integer.MAX_VALUE</tt>). |
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* |
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* <li> Like a CyclicBarrier, a Phaser may be repeatedly awaited. |
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* Method <tt>arriveAndAwaitAdvance</tt> has effect analogous to |
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* <tt>CyclicBarrier.await</tt>. However, Phasers separate two |
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* aspects of coordination, that may be invoked independently: |
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* |
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* <ul> |
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* |
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* <li> Arriving at a barrier. Methods <tt>arrive</tt> and |
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* <tt>arriveAndDeregister</tt> do not block, but return |
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* the phase value on entry to the method. |
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* |
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* <li> Awaiting others. Method <tt>awaitAdvance</tt> requires an |
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* argument indicating the entry phase, and returns when the |
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* barrier advances to a new phase. |
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* </ul> |
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* |
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* |
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* <li> Barrier actions, performed by the task triggering a phase |
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* advance while others may be waiting, are arranged by overriding |
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* method <tt>onAdvance</tt>, that also controls termination. |
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* |
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* <li> Phasers may enter a <em>termination</em> state in which all |
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* await actions immediately return, indicating (via a negative phase |
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* value) that execution is complete. Termination is triggered by |
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* executing the overridable <tt>onAdvance</tt> method that is invoked |
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* each time the barrier is tripped. When a Phaser is controlling an |
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* action with a fixed number of iterations, it is often convenient to |
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* override this method to cause termination when the current phase |
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* number reaches a threshold. Method <tt>forceTermination</tt> is |
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* also available to assist recovery actions upon failure. |
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* |
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* <li> Unlike most synchronizers, a Phaser may also be used with |
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* ForkJoinTasks (as well as plain threads). |
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* |
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* <li> By default, <tt>awaitAdvance</tt> continues to wait even if |
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* the current thread is interrupted. And unlike the case in |
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* CyclicBarriers, exceptions encountered while tasks wait |
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* interruptibly or with timeout do not change the state of the |
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* barrier. If necessary, you can perform any associated recovery |
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* within handlers of those exceptions. |
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* |
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* </ul> |
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* |
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* <p><b>Sample usage:</b> |
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* |
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* <p>[todo: non-FJ example] |
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* |
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* <p> A Phaser may be used to support a style of programming in |
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* which a task waits for others to complete, without otherwise |
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* needing to keep track of which tasks it is waiting for. This is |
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* similar to the "sync" construct in Cilk and "clocks" in X10. |
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* Special constructions based on such barriers are available using |
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* the <tt>LinkedAsyncAction</tt> and <tt>CyclicAction</tt> classes, |
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* but they can be useful in other contexts as well. For a simple |
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* (but not very useful) example, here is a variant of Fibonacci: |
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* |
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* <pre> |
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* class BarrierFibonacci extends RecursiveAction { |
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* int argument, result; |
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* final Phaser parentBarrier; |
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* BarrierFibonacci(int n, Phaser parentBarrier) { |
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* this.argument = n; |
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* this.parentBarrier = parentBarrier; |
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* parentBarrier.register(); |
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* } |
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* protected void compute() { |
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* int n = argument; |
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* if (n <= 1) |
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* result = n; |
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* else { |
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* Phaser childBarrier = new Phaser(1); |
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* BarrierFibonacci f1 = new BarrierFibonacci(n - 1, childBarrier); |
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* BarrierFibonacci f2 = new BarrierFibonacci(n - 2, childBarrier); |
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* f1.fork(); |
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* f2.fork(); |
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* childBarrier.arriveAndAwait(); |
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* result = f1.result + f2.result; |
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* } |
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* parentBarrier.arriveAndDeregister(); |
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* } |
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* } |
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* </pre> |
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* |
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* <p><b>Implementation notes</b>: This implementation restricts the |
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* maximum number of parties to 65535. Attempts to register |
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* additional parties result in IllegalStateExceptions. |
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*/ |
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public class Phaser { |
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/* |
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* This class implements an extension of X10 "clocks". Thanks to |
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* Vijay Saraswat for the idea of applying it to ForkJoinTasks, |
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* and to Vivek Sarkar for enhancements to extend functionality. |
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*/ |
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|
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/** |
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* Barrier state representation. Conceptually, a barrier contains |
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* four values: |
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* |
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* * parties -- the number of parties to wait (16 bits) |
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* * unarrived -- the number of parties yet to hit barrier (16 bits) |
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* * phase -- the generation of the barrier (31 bits) |
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* * terminated -- set if barrier is terminated (1 bit) |
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* |
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* However, to efficiently maintain atomicity, these values are |
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* packed into a single AtomicLong. Termination uses the sign bit |
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* of 32 bit representation of phase, so phase is set to -1 on |
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* termination. |
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*/ |
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private final AtomicLong state; |
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|
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/** |
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* Head of Treiber stack for waiting nonFJ threads. |
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*/ |
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private final AtomicReference<QNode> head = new AtomicReference<QNode>(); |
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|
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private static final int ushortBits = 16; |
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private static final int ushortMask = (1 << ushortBits) - 1; |
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private static final int phaseMask = 0x7fffffff; |
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|
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private static int unarrivedOf(long s) { |
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return (int)(s & ushortMask); |
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} |
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|
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private static int partiesOf(long s) { |
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return (int)(s & (ushortMask << 16)) >>> 16; |
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} |
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|
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private static int phaseOf(long s) { |
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return (int)(s >>> 32); |
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} |
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|
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private static int arrivedOf(long s) { |
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return partiesOf(s) - unarrivedOf(s); |
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} |
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|
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private static long stateFor(int phase, int parties, int unarrived) { |
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return (((long)phase) << 32) | ((parties << 16) | unarrived); |
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} |
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|
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private static IllegalStateException badBounds(int parties, int unarrived) { |
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return new IllegalStateException("Attempt to set " + unarrived + |
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" unarrived of " + parties + " parties"); |
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} |
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|
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/** |
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* Creates a new Phaser without any initially registered parties, |
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* and initial phase number 0. |
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*/ |
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public Phaser() { |
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state = new AtomicLong(stateFor(0, 0, 0)); |
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} |
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|
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/** |
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* Creates a new Phaser with the given numbers of registered |
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* unarrived parties and initial phase number 0. |
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* @param parties the number of parties required to trip barrier. |
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* @throws IllegalArgumentException if parties less than zero |
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* or greater than the maximum number of parties supported. |
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*/ |
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public Phaser(int parties) { |
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if (parties < 0 || parties > ushortMask) |
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throw new IllegalArgumentException("Illegal number of parties"); |
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state = new AtomicLong(stateFor(0, parties, parties)); |
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} |
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|
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/** |
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* Adds a new unarrived party to this phaser. |
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* @return the current barrier phase number upon registration |
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* @throws IllegalStateException if attempting to register more |
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* than the maximum supported number of parties. |
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*/ |
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public int register() { // increment both parties and unarrived |
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final AtomicLong state = this.state; |
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for (;;) { |
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long s = state.get(); |
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int phase = phaseOf(s); |
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int parties = partiesOf(s) + 1; |
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int unarrived = unarrivedOf(s) + 1; |
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if (parties > ushortMask || unarrived > ushortMask) |
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throw badBounds(parties, unarrived); |
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if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) |
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return phase; |
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} |
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} |
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|
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/** |
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* Arrives at the barrier, but does not wait for others. (You can |
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* in turn wait for others via {@link #awaitAdvance}). |
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* |
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* @return the current barrier phase number upon entry to |
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* this method, or a negative value if terminated; |
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* @throws IllegalStateException if the number of unarrived |
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* parties would become negative. |
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*/ |
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public int arrive() { // decrement unarrived. If zero, trip |
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final AtomicLong state = this.state; |
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for (;;) { |
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long s = state.get(); |
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int phase = phaseOf(s); |
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int parties = partiesOf(s); |
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int unarrived = unarrivedOf(s) - 1; |
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if (unarrived < 0) |
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throw badBounds(parties, unarrived); |
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if (unarrived == 0 && phase >= 0) { |
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trip(phase, parties); |
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return phase; |
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} |
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if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) |
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return phase; |
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} |
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} |
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|
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/** |
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* Arrives at the barrier, and deregisters from it, without |
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* waiting for others. |
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* |
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* @return the current barrier phase number upon entry to |
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* this method, or a negative value if terminated; |
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* @throws IllegalStateException if the number of registered or |
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* unarrived parties would become negative. |
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*/ |
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public int arriveAndDeregister() { // Same as arrive, plus decrement parties |
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final AtomicLong state = this.state; |
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for (;;) { |
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long s = state.get(); |
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int phase = phaseOf(s); |
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int parties = partiesOf(s) - 1; |
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int unarrived = unarrivedOf(s) - 1; |
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if (parties < 0 || unarrived < 0) |
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throw badBounds(parties, unarrived); |
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if (unarrived == 0 && phase >= 0) { |
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trip(phase, parties); |
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return phase; |
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} |
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if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) |
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return phase; |
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} |
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} |
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|
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/** |
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* Arrives at the barrier and awaits others. Unlike other arrival |
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* methods, this method returns the arrival index of the |
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* caller. The caller tripping the barrier returns zero, the |
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* previous caller 1, and so on. |
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* @return the arrival index |
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* @throws IllegalStateException if the number of unarrived |
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* parties would become negative. |
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*/ |
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public int arriveAndAwaitAdvance() { |
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final AtomicLong state = this.state; |
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for (;;) { |
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long s = state.get(); |
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int phase = phaseOf(s); |
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int parties = partiesOf(s); |
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int unarrived = unarrivedOf(s) - 1; |
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if (unarrived < 0) |
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throw badBounds(parties, unarrived); |
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if (unarrived == 0 && phase >= 0) { |
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trip(phase, parties); |
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return 0; |
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} |
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if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) { |
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awaitAdvance(phase); |
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return unarrived; |
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} |
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} |
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} |
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|
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/** |
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* Awaits the phase of the barrier to advance from the given |
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* value, or returns immediately if this barrier is terminated |
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* @param phase the phase on entry to this method |
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* @return the phase on exit from this method |
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*/ |
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public int awaitAdvance(int phase) { |
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if (phase < 0) |
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return phase; |
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Thread current = Thread.currentThread(); |
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if (current instanceof ForkJoinWorkerThread) |
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return helpingWait(phase); |
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if (untimedWait(current, phase, false)) |
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current.interrupt(); |
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return phaseOf(state.get()); |
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} |
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|
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/** |
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* Awaits the phase of the barrier to advance from the given |
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* value, or returns immediately if this barrier is terminated, or |
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* throws InterruptedException if interrupted while waiting. |
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* @param phase the phase on entry to this method |
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* @return the phase on exit from this method |
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* @throws InterruptedException if thread interrupted while waiting |
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*/ |
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public int awaitAdvanceInterruptibly(int phase) throws InterruptedException { |
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if (phase < 0) |
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return phase; |
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Thread current = Thread.currentThread(); |
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if (current instanceof ForkJoinWorkerThread) |
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return helpingWait(phase); |
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else if (Thread.interrupted() || untimedWait(current, phase, true)) |
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throw new InterruptedException(); |
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else |
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return phaseOf(state.get()); |
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} |
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|
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/** |
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* Awaits the phase of the barrier to advance from the given value |
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* or the given timeout elapses, or returns immediately if this |
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* barrier is terminated |
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* @param phase the phase on entry to this method |
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* @return the phase on exit from this method |
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* @throws InterruptedException if thread interrupted while waiting |
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* @throws TimeoutException if timed out while waiting |
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*/ |
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public int awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit) |
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throws InterruptedException, TimeoutException { |
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if (phase < 0) |
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return phase; |
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long nanos = unit.toNanos(timeout); |
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Thread current = Thread.currentThread(); |
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if (current instanceof ForkJoinWorkerThread) |
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return timedHelpingWait(phase, nanos); |
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timedWait(current, phase, nanos); |
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return phaseOf(state.get()); |
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} |
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|
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/** |
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* Forces this barrier to enter termination state. Counts of |
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* arrived and registered parties are unaffected. This method may |
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* be useful for coordinating recovery after one or more tasks |
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* encounter unexpected exceptions. |
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*/ |
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public void forceTermination() { |
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final AtomicLong state = this.state; |
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for (;;) { |
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long s = state.get(); |
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int phase = phaseOf(s); |
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int parties = partiesOf(s); |
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int unarrived = unarrivedOf(s); |
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if (phase < 0 || |
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state.compareAndSet(s, stateFor(-1, parties, unarrived))) { |
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if (head.get() != null) |
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releaseWaiters(-1); |
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return; |
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} |
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} |
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} |
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|
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/** |
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* Resets the barrier with the given numbers of registered unarrived |
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* parties and phase number 0. This method allows repeated reuse |
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* of this barrier, but only if it is somehow known not to be in |
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* use for other purposes. |
385 |
* @param parties the number of parties required to trip barrier. |
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* @throws IllegalArgumentException if parties less than zero |
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* or greater than the maximum number of parties supported. |
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*/ |
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public void reset(int parties) { |
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if (parties < 0 || parties > ushortMask) |
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throw new IllegalArgumentException("Illegal number of parties"); |
392 |
state.set(stateFor(0, parties, parties)); |
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if (head.get() != null) |
394 |
releaseWaiters(0); |
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} |
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|
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/** |
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* Returns the current phase number. The maximum phase number is |
399 |
* <tt>Integer.MAX_VALUE</tt>, after which it restarts at |
400 |
* zero. Upon termination, the phase number is negative. |
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* @return the phase number, or a negative value if terminated |
402 |
*/ |
403 |
public int getPhase() { |
404 |
return phaseOf(state.get()); |
405 |
} |
406 |
|
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/** |
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* Returns the number of parties registered at this barrier. |
409 |
* @return the number of parties |
410 |
*/ |
411 |
public int getRegisteredParties() { |
412 |
return partiesOf(state.get()); |
413 |
} |
414 |
|
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/** |
416 |
* Returns the number of parties that have arrived at the current |
417 |
* phase of this barrier. |
418 |
* @return the number of arrived parties |
419 |
*/ |
420 |
public int getArrivedParties() { |
421 |
return arrivedOf(state.get()); |
422 |
} |
423 |
|
424 |
/** |
425 |
* Returns the number of registered parties that have not yet |
426 |
* arrived at the current phase of this barrier. |
427 |
* @return the number of unarrived parties |
428 |
*/ |
429 |
public int getUnarrivedParties() { |
430 |
return unarrivedOf(state.get()); |
431 |
} |
432 |
|
433 |
/** |
434 |
* Returns true if this barrier has been terminated |
435 |
* @return true if this barrier has been terminated |
436 |
*/ |
437 |
public boolean isTerminated() { |
438 |
return phaseOf(state.get()) < 0; |
439 |
} |
440 |
|
441 |
/** |
442 |
* Overridable method to perform an action upon phase advance, and |
443 |
* to control termination. This method is invoked whenever the |
444 |
* barrier is tripped (and thus all other waiting parties are |
445 |
* dormant). If it returns true, then, rather than advance the |
446 |
* phase number, this barrier will be set to a final termination |
447 |
* state, and subsequent calls to <tt>isTerminated</tt> will |
448 |
* return true. |
449 |
* |
450 |
* <p> The default version returns true when the number of |
451 |
* registered parties is zero. Normally, overrides that arrange |
452 |
* termination for other reasons should also preserve this |
453 |
* property. |
454 |
* |
455 |
* @param phase the phase number on entering the barrier |
456 |
* @param registeredParties the current number of registered |
457 |
* parties. |
458 |
* @return true if this barrier should terminate |
459 |
*/ |
460 |
protected boolean onAdvance(int phase, int registeredParties) { |
461 |
return registeredParties <= 0; |
462 |
} |
463 |
|
464 |
/** |
465 |
* Returns a string identifying this barrier, as well as its |
466 |
* state. The state, in brackets, includes the String {@code |
467 |
* "phase ="} followed by the phase number, {@code "parties ="} |
468 |
* followed by the number of registered parties, and {@code |
469 |
* "arrived ="} followed by the number of arrived parties |
470 |
* |
471 |
* @return a string identifying this barrier, as well as its state |
472 |
*/ |
473 |
public String toString() { |
474 |
long s = state.get(); |
475 |
return super.toString() + "[phase = " + phaseOf(s) + " parties = " + partiesOf(s) + " arrived = " + arrivedOf(s) + "]"; |
476 |
} |
477 |
|
478 |
// methods for tripping and waiting |
479 |
|
480 |
/** |
481 |
* Advance the current phase (or terminate) |
482 |
*/ |
483 |
private void trip(int phase, int parties) { |
484 |
int next = onAdvance(phase, parties)? -1 : ((phase + 1) & phaseMask); |
485 |
state.set(stateFor(next, parties, parties)); |
486 |
if (head.get() != null) |
487 |
releaseWaiters(next); |
488 |
} |
489 |
|
490 |
private int helpingWait(int phase) { |
491 |
final AtomicLong state = this.state; |
492 |
int p; |
493 |
while ((p = phaseOf(state.get())) == phase) { |
494 |
ForkJoinTask<?> t = ForkJoinWorkerThread.pollTask(); |
495 |
if (t != null) { |
496 |
if ((p = phaseOf(state.get())) == phase) |
497 |
t.exec(); |
498 |
else { // push task and exit if barrier advanced |
499 |
t.fork(); |
500 |
break; |
501 |
} |
502 |
} |
503 |
} |
504 |
return p; |
505 |
} |
506 |
|
507 |
private int timedHelpingWait(int phase, long nanos) throws TimeoutException { |
508 |
final AtomicLong state = this.state; |
509 |
long lastTime = System.nanoTime(); |
510 |
int p; |
511 |
while ((p = phaseOf(state.get())) == phase) { |
512 |
long now = System.nanoTime(); |
513 |
nanos -= now - lastTime; |
514 |
lastTime = now; |
515 |
if (nanos <= 0) { |
516 |
if ((p = phaseOf(state.get())) == phase) |
517 |
throw new TimeoutException(); |
518 |
else |
519 |
break; |
520 |
} |
521 |
ForkJoinTask<?> t = ForkJoinWorkerThread.pollTask(); |
522 |
if (t != null) { |
523 |
if ((p = phaseOf(state.get())) == phase) |
524 |
t.exec(); |
525 |
else { // push task and exit if barrier advanced |
526 |
t.fork(); |
527 |
break; |
528 |
} |
529 |
} |
530 |
} |
531 |
return p; |
532 |
} |
533 |
|
534 |
/** |
535 |
* Wait nodes for Treiber stack representing wait queue for non-FJ |
536 |
* tasks. The waiting scheme is an adaptation of the one used in |
537 |
* forkjoin.PoolBarrier. |
538 |
*/ |
539 |
static final class QNode { |
540 |
QNode next; |
541 |
volatile Thread thread; // nulled to cancel wait |
542 |
final int phase; |
543 |
QNode(Thread t, int c) { |
544 |
thread = t; |
545 |
phase = c; |
546 |
} |
547 |
} |
548 |
|
549 |
private void releaseWaiters(int currentPhase) { |
550 |
final AtomicReference<QNode> head = this.head; |
551 |
QNode p; |
552 |
while ((p = head.get()) != null && p.phase != currentPhase) { |
553 |
if (head.compareAndSet(p, null)) { |
554 |
do { |
555 |
Thread t = p.thread; |
556 |
if (t != null) { |
557 |
p.thread = null; |
558 |
LockSupport.unpark(t); |
559 |
} |
560 |
} while ((p = p.next) != null); |
561 |
} |
562 |
} |
563 |
} |
564 |
|
565 |
/** The number of CPUs, for spin control */ |
566 |
static final int NCPUS = Runtime.getRuntime().availableProcessors(); |
567 |
|
568 |
/** |
569 |
* The number of times to spin before blocking in timed waits. |
570 |
* The value is empirically derived |
571 |
*/ |
572 |
static final int maxTimedSpins = (NCPUS < 2)? 0 : 32; |
573 |
|
574 |
/** |
575 |
* The number of times to spin before blocking in untimed waits. |
576 |
* This is greater than timed value because untimed waits spin |
577 |
* faster since they don't need to check times on each spin. |
578 |
*/ |
579 |
static final int maxUntimedSpins = maxTimedSpins * 32; |
580 |
|
581 |
/** |
582 |
* The number of nanoseconds for which it is faster to spin |
583 |
* rather than to use timed park. A rough estimate suffices. |
584 |
*/ |
585 |
static final long spinForTimeoutThreshold = 1000L; |
586 |
|
587 |
/** |
588 |
* Enqueues node and waits unless aborted or signalled. |
589 |
*/ |
590 |
private boolean untimedWait(Thread thread, int currentPhase, |
591 |
boolean abortOnInterrupt) { |
592 |
final AtomicReference<QNode> head = this.head; |
593 |
final AtomicLong state = this.state; |
594 |
boolean wasInterrupted = false; |
595 |
QNode node = null; |
596 |
boolean queued = false; |
597 |
int spins = maxUntimedSpins; |
598 |
while (phaseOf(state.get()) == currentPhase) { |
599 |
QNode h; |
600 |
if (node != null && queued) { |
601 |
if (node.thread != null) { |
602 |
LockSupport.park(); |
603 |
if (Thread.interrupted()) { |
604 |
wasInterrupted = true; |
605 |
if (abortOnInterrupt) |
606 |
break; |
607 |
} |
608 |
} |
609 |
} |
610 |
else if ((h = head.get()) != null && h.phase != currentPhase) { |
611 |
if (phaseOf(state.get()) == currentPhase) { // must recheck |
612 |
if (head.compareAndSet(h, h.next)) { |
613 |
Thread t = h.thread; // help clear out old waiters |
614 |
if (t != null) { |
615 |
h.thread = null; |
616 |
LockSupport.unpark(t); |
617 |
} |
618 |
} |
619 |
} |
620 |
else |
621 |
break; |
622 |
} |
623 |
else if (node != null) |
624 |
queued = head.compareAndSet(node.next = h, node); |
625 |
else if (spins <= 0) |
626 |
node = new QNode(thread, currentPhase); |
627 |
else |
628 |
--spins; |
629 |
} |
630 |
if (node != null) |
631 |
node.thread = null; |
632 |
return wasInterrupted; |
633 |
} |
634 |
|
635 |
/** |
636 |
* Messier timeout version |
637 |
*/ |
638 |
private void timedWait(Thread thread, int currentPhase, long nanos) |
639 |
throws InterruptedException, TimeoutException { |
640 |
final AtomicReference<QNode> head = this.head; |
641 |
final AtomicLong state = this.state; |
642 |
long lastTime = System.nanoTime(); |
643 |
QNode node = null; |
644 |
boolean queued = false; |
645 |
int spins = maxTimedSpins; |
646 |
while (phaseOf(state.get()) == currentPhase) { |
647 |
QNode h; |
648 |
long now = System.nanoTime(); |
649 |
nanos -= now - lastTime; |
650 |
lastTime = now; |
651 |
if (nanos <= 0) { |
652 |
if (node != null) |
653 |
node.thread = null; |
654 |
if (phaseOf(state.get()) == currentPhase) |
655 |
throw new TimeoutException(); |
656 |
else |
657 |
break; |
658 |
} |
659 |
else if (node != null && queued) { |
660 |
if (node.thread != null && |
661 |
nanos > spinForTimeoutThreshold) { |
662 |
// LockSupport.parkNanos(this, nanos); |
663 |
LockSupport.parkNanos(nanos); |
664 |
if (Thread.interrupted()) { |
665 |
node.thread = null; |
666 |
throw new InterruptedException(); |
667 |
} |
668 |
} |
669 |
} |
670 |
else if ((h = head.get()) != null && h.phase != currentPhase) { |
671 |
if (phaseOf(state.get()) == currentPhase) { // must recheck |
672 |
if (head.compareAndSet(h, h.next)) { |
673 |
Thread t = h.thread; // help clear out old waiters |
674 |
if (t != null) { |
675 |
h.thread = null; |
676 |
LockSupport.unpark(t); |
677 |
} |
678 |
} |
679 |
} |
680 |
else |
681 |
break; |
682 |
} |
683 |
else if (node != null) |
684 |
queued = head.compareAndSet(node.next = h, node); |
685 |
else if (spins <= 0) |
686 |
node = new QNode(thread, currentPhase); |
687 |
else |
688 |
--spins; |
689 |
} |
690 |
if (node != null) |
691 |
node.thread = null; |
692 |
} |
693 |
|
694 |
} |
695 |
|