/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
package jsr166y;
import jsr166y.forkjoin.*;
import java.util.concurrent.*;
import java.util.concurrent.atomic.*;
import java.util.concurrent.locks.LockSupport;
/**
* A reusable synchronization barrier, similar in functionality to a
* {@link java.util.concurrent.CyclicBarrier}, but supporting more
* flexible usage.
*
*
*
* - The number of parties synchronizing on the barrier may vary
* over time. A task may register to be a party in a barrier at any
* time, and may deregister upon arriving at the barrier. As is the
* case with most basic synchronization constructs, registration
* and deregistration affect only internal counts; they do not
* establish any further internal bookkeeping, so tasks cannot query
* whether they are registered.
*
*
- Each generation has an associated phase value, starting at
* zero, and advancing when all parties reach the barrier (wrapping
* around to zero after reaching Integer.MAX_VALUE).
*
*
- Like a CyclicBarrier, a Phaser may be repeatedly awaited.
* Method arriveAndAwaitAdvance has effect analogous to
* CyclicBarrier.await. However, Phasers separate two
* aspects of coordination, that may be invoked independently:
*
*
*
* - Arriving at a barrier. Methods arrive and
* arriveAndDeregister do not block, but return
* the phase value on entry to the method.
*
*
- Awaiting others. Method awaitAdvance requires an
* argument indicating the entry phase, and returns when the
* barrier advances to a new phase.
*
*
*
* - Barrier actions, performed by the task triggering a phase
* advance while others may be waiting, are arranged by overriding
* method onAdvance, that also controls termination.
*
*
- Phasers may enter a termination state in which all
* await actions immediately return, indicating (via a negative phase
* value) that execution is complete. Termination is triggered by
* executing the overridable onAdvance method that is invoked
* each time the barrier is tripped. When a Phaser is controlling an
* action with a fixed number of iterations, it is often convenient to
* override this method to cause termination when the current phase
* number reaches a threshold. Method forceTermination is
* also available to assist recovery actions upon failure.
*
*
- Unlike most synchronizers, a Phaser may also be used with
* ForkJoinTasks (as well as plain threads).
*
*
- By default, awaitAdvance continues to wait even if
* the current thread is interrupted. And unlike the case in
* CyclicBarriers, exceptions encountered while tasks wait
* interruptibly or with timeout do not change the state of the
* barrier. If necessary, you can perform any associated recovery
* within handlers of those exceptions.
*
*
*
* Sample usage:
*
*
[todo: non-FJ example]
*
*
A Phaser may be used to support a style of programming in
* which a task waits for others to complete, without otherwise
* needing to keep track of which tasks it is waiting for. This is
* similar to the "sync" construct in Cilk and "clocks" in X10.
* Special constructions based on such barriers are available using
* the LinkedAsyncAction and CyclicAction classes,
* but they can be useful in other contexts as well. For a simple
* (but not very useful) example, here is a variant of Fibonacci:
*
*
* class BarrierFibonacci extends RecursiveAction {
* int argument, result;
* final Phaser parentBarrier;
* BarrierFibonacci(int n, Phaser parentBarrier) {
* this.argument = n;
* this.parentBarrier = parentBarrier;
* parentBarrier.register();
* }
* protected void compute() {
* int n = argument;
* if (n <= 1)
* result = n;
* else {
* Phaser childBarrier = new Phaser(1);
* BarrierFibonacci f1 = new BarrierFibonacci(n - 1, childBarrier);
* BarrierFibonacci f2 = new BarrierFibonacci(n - 2, childBarrier);
* f1.fork();
* f2.fork();
* childBarrier.arriveAndAwait();
* result = f1.result + f2.result;
* }
* parentBarrier.arriveAndDeregister();
* }
* }
*
*
* Implementation notes: This implementation restricts the
* maximum number of parties to 65535. Attempts to register
* additional parties result in IllegalStateExceptions.
*/
public class Phaser {
/*
* This class implements an extension of X10 "clocks". Thanks to
* Vijay Saraswat for the idea of applying it to ForkJoinTasks,
* and to Vivek Sarkar for enhancements to extend functionality.
*/
/**
* Barrier state representation. Conceptually, a barrier contains
* four values:
*
* * parties -- the number of parties to wait (16 bits)
* * unarrived -- the number of parties yet to hit barrier (16 bits)
* * phase -- the generation of the barrier (31 bits)
* * terminated -- set if barrier is terminated (1 bit)
*
* However, to efficiently maintain atomicity, these values are
* packed into a single AtomicLong. Termination uses the sign bit
* of 32 bit representation of phase, so phase is set to -1 on
* termination.
*/
private final AtomicLong state;
/**
* Head of Treiber stack for waiting nonFJ threads.
*/
private final AtomicReference head = new AtomicReference();
private static final int ushortBits = 16;
private static final int ushortMask = (1 << ushortBits) - 1;
private static final int phaseMask = 0x7fffffff;
private static int unarrivedOf(long s) {
return (int)(s & ushortMask);
}
private static int partiesOf(long s) {
return (int)(s & (ushortMask << 16)) >>> 16;
}
private static int phaseOf(long s) {
return (int)(s >>> 32);
}
private static int arrivedOf(long s) {
return partiesOf(s) - unarrivedOf(s);
}
private static long stateFor(int phase, int parties, int unarrived) {
return (((long)phase) << 32) | ((parties << 16) | unarrived);
}
private static IllegalStateException badBounds(int parties, int unarrived) {
return new IllegalStateException("Attempt to set " + unarrived +
" unarrived of " + parties + " parties");
}
/**
* Creates a new Phaser without any initially registered parties,
* and initial phase number 0.
*/
public Phaser() {
state = new AtomicLong(stateFor(0, 0, 0));
}
/**
* Creates a new Phaser with the given numbers of registered
* unarrived parties and initial phase number 0.
* @param parties the number of parties required to trip barrier.
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported.
*/
public Phaser(int parties) {
if (parties < 0 || parties > ushortMask)
throw new IllegalArgumentException("Illegal number of parties");
state = new AtomicLong(stateFor(0, parties, parties));
}
/**
* Adds a new unarrived party to this phaser.
* @return the current barrier phase number upon registration
* @throws IllegalStateException if attempting to register more
* than the maximum supported number of parties.
*/
public int register() { // increment both parties and unarrived
final AtomicLong state = this.state;
for (;;) {
long s = state.get();
int phase = phaseOf(s);
int parties = partiesOf(s) + 1;
int unarrived = unarrivedOf(s) + 1;
if (parties > ushortMask || unarrived > ushortMask)
throw badBounds(parties, unarrived);
if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
return phase;
}
}
/**
* Arrives at the barrier, but does not wait for others. (You can
* in turn wait for others via {@link #awaitAdvance}).
*
* @return the current barrier phase number upon entry to
* this method, or a negative value if terminated;
* @throws IllegalStateException if the number of unarrived
* parties would become negative.
*/
public int arrive() { // decrement unarrived. If zero, trip
final AtomicLong state = this.state;
for (;;) {
long s = state.get();
int phase = phaseOf(s);
int parties = partiesOf(s);
int unarrived = unarrivedOf(s) - 1;
if (unarrived < 0)
throw badBounds(parties, unarrived);
if (unarrived == 0 && phase >= 0) {
trip(phase, parties);
return phase;
}
if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
return phase;
}
}
/**
* Arrives at the barrier, and deregisters from it, without
* waiting for others.
*
* @return the current barrier phase number upon entry to
* this method, or a negative value if terminated;
* @throws IllegalStateException if the number of registered or
* unarrived parties would become negative.
*/
public int arriveAndDeregister() { // Same as arrive, plus decrement parties
final AtomicLong state = this.state;
for (;;) {
long s = state.get();
int phase = phaseOf(s);
int parties = partiesOf(s) - 1;
int unarrived = unarrivedOf(s) - 1;
if (parties < 0 || unarrived < 0)
throw badBounds(parties, unarrived);
if (unarrived == 0 && phase >= 0) {
trip(phase, parties);
return phase;
}
if (state.compareAndSet(s, stateFor(phase, parties, unarrived)))
return phase;
}
}
/**
* Arrives at the barrier and awaits others. Unlike other arrival
* methods, this method returns the arrival index of the
* caller. The caller tripping the barrier returns zero, the
* previous caller 1, and so on.
* @return the arrival index
* @throws IllegalStateException if the number of unarrived
* parties would become negative.
*/
public int arriveAndAwaitAdvance() {
final AtomicLong state = this.state;
for (;;) {
long s = state.get();
int phase = phaseOf(s);
int parties = partiesOf(s);
int unarrived = unarrivedOf(s) - 1;
if (unarrived < 0)
throw badBounds(parties, unarrived);
if (unarrived == 0 && phase >= 0) {
trip(phase, parties);
return 0;
}
if (state.compareAndSet(s, stateFor(phase, parties, unarrived))) {
awaitAdvance(phase);
return unarrived;
}
}
}
/**
* Awaits the phase of the barrier to advance from the given
* value, or returns immediately if this barrier is terminated
* @param phase the phase on entry to this method
* @return the phase on exit from this method
*/
public int awaitAdvance(int phase) {
if (phase < 0)
return phase;
Thread current = Thread.currentThread();
if (current instanceof ForkJoinWorkerThread)
return helpingWait(phase);
if (untimedWait(current, phase, false))
current.interrupt();
return phaseOf(state.get());
}
/**
* Awaits the phase of the barrier to advance from the given
* value, or returns immediately if this barrier is terminated, or
* throws InterruptedException if interrupted while waiting.
* @param phase the phase on entry to this method
* @return the phase on exit from this method
* @throws InterruptedException if thread interrupted while waiting
*/
public int awaitAdvanceInterruptibly(int phase) throws InterruptedException {
if (phase < 0)
return phase;
Thread current = Thread.currentThread();
if (current instanceof ForkJoinWorkerThread)
return helpingWait(phase);
else if (Thread.interrupted() || untimedWait(current, phase, true))
throw new InterruptedException();
else
return phaseOf(state.get());
}
/**
* Awaits the phase of the barrier to advance from the given value
* or the given timeout elapses, or returns immediately if this
* barrier is terminated
* @param phase the phase on entry to this method
* @return the phase on exit from this method
* @throws InterruptedException if thread interrupted while waiting
* @throws TimeoutException if timed out while waiting
*/
public int awaitAdvanceInterruptibly(int phase, long timeout, TimeUnit unit)
throws InterruptedException, TimeoutException {
if (phase < 0)
return phase;
long nanos = unit.toNanos(timeout);
Thread current = Thread.currentThread();
if (current instanceof ForkJoinWorkerThread)
return timedHelpingWait(phase, nanos);
timedWait(current, phase, nanos);
return phaseOf(state.get());
}
/**
* Forces this barrier to enter termination state. Counts of
* arrived and registered parties are unaffected. This method may
* be useful for coordinating recovery after one or more tasks
* encounter unexpected exceptions.
*/
public void forceTermination() {
final AtomicLong state = this.state;
for (;;) {
long s = state.get();
int phase = phaseOf(s);
int parties = partiesOf(s);
int unarrived = unarrivedOf(s);
if (phase < 0 ||
state.compareAndSet(s, stateFor(-1, parties, unarrived))) {
if (head.get() != null)
releaseWaiters(-1);
return;
}
}
}
/**
* Resets the barrier with the given numbers of registered unarrived
* parties and phase number 0. This method allows repeated reuse
* of this barrier, but only if it is somehow known not to be in
* use for other purposes.
* @param parties the number of parties required to trip barrier.
* @throws IllegalArgumentException if parties less than zero
* or greater than the maximum number of parties supported.
*/
public void reset(int parties) {
if (parties < 0 || parties > ushortMask)
throw new IllegalArgumentException("Illegal number of parties");
state.set(stateFor(0, parties, parties));
if (head.get() != null)
releaseWaiters(0);
}
/**
* Returns the current phase number. The maximum phase number is
* Integer.MAX_VALUE, after which it restarts at
* zero. Upon termination, the phase number is negative.
* @return the phase number, or a negative value if terminated
*/
public int getPhase() {
return phaseOf(state.get());
}
/**
* Returns the number of parties registered at this barrier.
* @return the number of parties
*/
public int getRegisteredParties() {
return partiesOf(state.get());
}
/**
* Returns the number of parties that have arrived at the current
* phase of this barrier.
* @return the number of arrived parties
*/
public int getArrivedParties() {
return arrivedOf(state.get());
}
/**
* Returns the number of registered parties that have not yet
* arrived at the current phase of this barrier.
* @return the number of unarrived parties
*/
public int getUnarrivedParties() {
return unarrivedOf(state.get());
}
/**
* Returns true if this barrier has been terminated
* @return true if this barrier has been terminated
*/
public boolean isTerminated() {
return phaseOf(state.get()) < 0;
}
/**
* Overridable method to perform an action upon phase advance, and
* to control termination. This method is invoked whenever the
* barrier is tripped (and thus all other waiting parties are
* dormant). If it returns true, then, rather than advance the
* phase number, this barrier will be set to a final termination
* state, and subsequent calls to isTerminated will
* return true.
*
* The default version returns true when the number of
* registered parties is zero. Normally, overrides that arrange
* termination for other reasons should also preserve this
* property.
*
* @param phase the phase number on entering the barrier
* @param registeredParties the current number of registered
* parties.
* @return true if this barrier should terminate
*/
protected boolean onAdvance(int phase, int registeredParties) {
return registeredParties <= 0;
}
/**
* Returns a string identifying this barrier, as well as its
* state. The state, in brackets, includes the String {@code
* "phase ="} followed by the phase number, {@code "parties ="}
* followed by the number of registered parties, and {@code
* "arrived ="} followed by the number of arrived parties
*
* @return a string identifying this barrier, as well as its state
*/
public String toString() {
long s = state.get();
return super.toString() + "[phase = " + phaseOf(s) + " parties = " + partiesOf(s) + " arrived = " + arrivedOf(s) + "]";
}
// methods for tripping and waiting
/**
* Advance the current phase (or terminate)
*/
private void trip(int phase, int parties) {
int next = onAdvance(phase, parties)? -1 : ((phase + 1) & phaseMask);
state.set(stateFor(next, parties, parties));
if (head.get() != null)
releaseWaiters(next);
}
private int helpingWait(int phase) {
final AtomicLong state = this.state;
int p;
while ((p = phaseOf(state.get())) == phase) {
ForkJoinTask> t = ForkJoinWorkerThread.pollTask();
if (t != null) {
if ((p = phaseOf(state.get())) == phase)
t.exec();
else { // push task and exit if barrier advanced
t.fork();
break;
}
}
}
return p;
}
private int timedHelpingWait(int phase, long nanos) throws TimeoutException {
final AtomicLong state = this.state;
long lastTime = System.nanoTime();
int p;
while ((p = phaseOf(state.get())) == phase) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
if ((p = phaseOf(state.get())) == phase)
throw new TimeoutException();
else
break;
}
ForkJoinTask> t = ForkJoinWorkerThread.pollTask();
if (t != null) {
if ((p = phaseOf(state.get())) == phase)
t.exec();
else { // push task and exit if barrier advanced
t.fork();
break;
}
}
}
return p;
}
/**
* Wait nodes for Treiber stack representing wait queue for non-FJ
* tasks. The waiting scheme is an adaptation of the one used in
* forkjoin.PoolBarrier.
*/
static final class QNode {
QNode next;
volatile Thread thread; // nulled to cancel wait
final int phase;
QNode(Thread t, int c) {
thread = t;
phase = c;
}
}
private void releaseWaiters(int currentPhase) {
final AtomicReference head = this.head;
QNode p;
while ((p = head.get()) != null && p.phase != currentPhase) {
if (head.compareAndSet(p, null)) {
do {
Thread t = p.thread;
if (t != null) {
p.thread = null;
LockSupport.unpark(t);
}
} while ((p = p.next) != null);
}
}
}
/** The number of CPUs, for spin control */
static final int NCPUS = Runtime.getRuntime().availableProcessors();
/**
* The number of times to spin before blocking in timed waits.
* The value is empirically derived
*/
static final int maxTimedSpins = (NCPUS < 2)? 0 : 32;
/**
* The number of times to spin before blocking in untimed waits.
* This is greater than timed value because untimed waits spin
* faster since they don't need to check times on each spin.
*/
static final int maxUntimedSpins = maxTimedSpins * 32;
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices.
*/
static final long spinForTimeoutThreshold = 1000L;
/**
* Enqueues node and waits unless aborted or signalled.
*/
private boolean untimedWait(Thread thread, int currentPhase,
boolean abortOnInterrupt) {
final AtomicReference head = this.head;
final AtomicLong state = this.state;
boolean wasInterrupted = false;
QNode node = null;
boolean queued = false;
int spins = maxUntimedSpins;
while (phaseOf(state.get()) == currentPhase) {
QNode h;
if (node != null && queued) {
if (node.thread != null) {
LockSupport.park();
if (Thread.interrupted()) {
wasInterrupted = true;
if (abortOnInterrupt)
break;
}
}
}
else if ((h = head.get()) != null && h.phase != currentPhase) {
if (phaseOf(state.get()) == currentPhase) { // must recheck
if (head.compareAndSet(h, h.next)) {
Thread t = h.thread; // help clear out old waiters
if (t != null) {
h.thread = null;
LockSupport.unpark(t);
}
}
}
else
break;
}
else if (node != null)
queued = head.compareAndSet(node.next = h, node);
else if (spins <= 0)
node = new QNode(thread, currentPhase);
else
--spins;
}
if (node != null)
node.thread = null;
return wasInterrupted;
}
/**
* Messier timeout version
*/
private void timedWait(Thread thread, int currentPhase, long nanos)
throws InterruptedException, TimeoutException {
final AtomicReference head = this.head;
final AtomicLong state = this.state;
long lastTime = System.nanoTime();
QNode node = null;
boolean queued = false;
int spins = maxTimedSpins;
while (phaseOf(state.get()) == currentPhase) {
QNode h;
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
if (node != null)
node.thread = null;
if (phaseOf(state.get()) == currentPhase)
throw new TimeoutException();
else
break;
}
else if (node != null && queued) {
if (node.thread != null &&
nanos > spinForTimeoutThreshold) {
// LockSupport.parkNanos(this, nanos);
LockSupport.parkNanos(nanos);
if (Thread.interrupted()) {
node.thread = null;
throw new InterruptedException();
}
}
}
else if ((h = head.get()) != null && h.phase != currentPhase) {
if (phaseOf(state.get()) == currentPhase) { // must recheck
if (head.compareAndSet(h, h.next)) {
Thread t = h.thread; // help clear out old waiters
if (t != null) {
h.thread = null;
LockSupport.unpark(t);
}
}
}
else
break;
}
else if (node != null)
queued = head.compareAndSet(node.next = h, node);
else if (spins <= 0)
node = new QNode(thread, currentPhase);
else
--spins;
}
if (node != null)
node.thread = null;
}
}