Registration. Unlike the case for other barriers, the + * number of parties registered to synchronize on a phaser + * may vary over time. Tasks may be registered at any time (using + * methods {@link #register}, {@link #bulkRegister}, or forms of + * constructors establishing initial numbers of parties), and + * optionally deregistered upon any arrival (using {@link + * #arriveAndDeregister}). 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. + * (However, you can introduce such bookkeeping by subclassing this + * class.) + * + *
Synchronization. Like a {@code CyclicBarrier}, a {@code + * Phaser} may be repeatedly awaited. Method {@link + * #arriveAndAwaitAdvance} has effect analogous to {@link + * java.util.concurrent.CyclicBarrier#await CyclicBarrier.await}. Each + * generation of a {@code Phaser} has an associated phase number. The + * phase number starts at zero, and advances when all parties arrive + * at the barrier, wrapping around to zero after reaching {@code + * Integer.MAX_VALUE}. The use of phase numbers enables independent + * control of actions upon arrival at a barrier and upon awaiting + * others, via two kinds of methods that may be invoked by any + * registered party: * *
Termination. A {@code Phaser} may enter a + * termination state in which all synchronization methods + * immediately return without updating phaser state or waiting for + * advance, and indicating (via a negative phase value) that execution + * is complete. Termination is triggered when an invocation of {@code + * onAdvance} returns {@code true}. As illustrated below, when + * phasers control actions 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 {@link * #forceTermination} is also available to abruptly release waiting * threads and allow them to terminate. * - *
Tiering. Phasers may be tiered (i.e., arranged + * in tree structures) to reduce contention. Phasers with large * numbers of parties that would otherwise experience heavy - * synchronization contention costs may instead be arranged in trees. - * This will typically greatly increase throughput even though it - * incurs somewhat greater per-operation overhead. - * - *
Monitoring. While synchronization methods may be invoked + * only by registered parties, the current state of a phaser may be + * monitored by any caller. At any given moment there are {@link + * #getRegisteredParties} parties in total, of which {@link + * #getArrivedParties} have arrived at the current phase ({@link + * #getPhase}). When the remaining ({@link #getUnarrivedParties}) + * parties arrive, the phase advances. The values returned by these + * methods may reflect transient states and so are not in general + * useful for synchronization control. Method {@link #toString} + * returns snapshots of these state queries in a form convenient for + * informal monitoring. * *
Sample usages: * *
A {@code Phaser} may be used instead of a {@code CountDownLatch} - * to control a one-shot action serving a variable number of - * parties. The typical idiom is for the method setting this up to - * first register, then start the actions, then deregister, as in: + * to control a one-shot action serving a variable number of parties. + * The typical idiom is for the method setting this up to first + * register, then start the actions, then deregister, as in: * *
{@code
* void runTasks(List tasks) {
@@ -118,43 +137,66 @@ import java.util.concurrent.locks.LockSu
* {@code
* void startTasks(List tasks, final int iterations) {
* final Phaser phaser = new Phaser() {
- * public boolean onAdvance(int phase, int registeredParties) {
+ * protected boolean onAdvance(int phase, int registeredParties) {
* return phase >= iterations || registeredParties == 0;
* }
* };
* phaser.register();
- * for (Runnable task : tasks) {
+ * for (final Runnable task : tasks) {
* phaser.register();
* new Thread() {
* public void run() {
* do {
* task.run();
* phaser.arriveAndAwaitAdvance();
- * } while(!phaser.isTerminated();
+ * } while (!phaser.isTerminated());
* }
* }.start();
* }
* phaser.arriveAndDeregister(); // deregister self, don't wait
* }}
*
+ * If the main task must later await termination, it
+ * may re-register and then execute a similar loop:
+ * {@code
+ * // ...
+ * phaser.register();
+ * while (!phaser.isTerminated())
+ * phaser.arriveAndAwaitAdvance();}
+ *
+ * Related constructions may be used to await particular phase numbers
+ * in contexts where you are sure that the phase will never wrap around
+ * {@code Integer.MAX_VALUE}. For example:
+ *
+ *
{@code
+ * void awaitPhase(Phaser phaser, int phase) {
+ * int p = phaser.register(); // assumes caller not already registered
+ * while (p < phase) {
+ * if (phaser.isTerminated())
+ * // ... deal with unexpected termination
+ * else
+ * p = phaser.arriveAndAwaitAdvance();
+ * }
+ * phaser.arriveAndDeregister();
+ * }}
+ *
+ *
* To create a set of tasks using a tree of phasers,
* you could use code of the following form, assuming a
* Task class with a constructor accepting a phaser that
- * it registers for upon construction:
+ * it registers with upon construction:
+ *
*
{@code
- * void build(Task[] actions, int lo, int hi, Phaser b) {
- * int step = (hi - lo) / TASKS_PER_PHASER;
- * if (step > 1) {
- * int i = lo;
- * while (i < hi) {
- * int r = Math.min(i + step, hi);
- * build(actions, i, r, new Phaser(b));
- * i = r;
+ * void build(Task[] actions, int lo, int hi, Phaser ph) {
+ * if (hi - lo > TASKS_PER_PHASER) {
+ * for (int i = lo; i < hi; i += TASKS_PER_PHASER) {
+ * int j = Math.min(i + TASKS_PER_PHASER, hi);
+ * build(actions, i, j, new Phaser(ph));
* }
* } else {
* for (int i = lo; i < hi; ++i)
- * actions[i] = new Task(b);
- * // assumes new Task(b) performs b.register()
+ * actions[i] = new Task(ph);
+ * // assumes new Task(ph) performs ph.register()
* }
* }
* // .. initially called, for n tasks via
@@ -165,8 +207,6 @@ import java.util.concurrent.locks.LockSu
* be appropriate for extremely small per-barrier task bodies (thus
* high rates), or up to hundreds for extremely large ones.
*
- *
- *
* Implementation notes: This implementation restricts the
* maximum number of parties to 65535. Attempts to register additional
* parties result in {@code IllegalStateException}. However, you can and
@@ -187,60 +227,47 @@ public class Phaser {
* 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)
+ * * unarrived -- the number of parties yet to hit barrier (bits 0-15)
+ * * parties -- the number of parties to wait (bits 16-31)
+ * * phase -- the generation of the barrier (bits 32-62)
+ * * terminated -- set if barrier is terminated (bit 63 / sign)
*
* However, to efficiently maintain atomicity, these values are
* packed into a single (atomic) long. Termination uses the sign
* bit of 32 bit representation of phase, so phase is set to -1 on
* termination. Good performance relies on keeping state decoding
* and encoding simple, and keeping race windows short.
- *
- * Note: there are some cheats in arrive() that rely on unarrived
- * count being lowest 16 bits.
*/
private volatile long state;
- private static final int ushortBits = 16;
- private static final int ushortMask = 0xffff;
- private static final int phaseMask = 0x7fffffff;
+ private static final int MAX_PARTIES = 0xffff;
+ private static final int MAX_PHASE = 0x7fffffff;
+ private static final int PARTIES_SHIFT = 16;
+ private static final int PHASE_SHIFT = 32;
+ private static final int UNARRIVED_MASK = 0xffff;
+ private static final long PARTIES_MASK = 0xffff0000L; // for masking long
+ private static final long ONE_ARRIVAL = 1L;
+ private static final long ONE_PARTY = 1L << PARTIES_SHIFT;
+ private static final long TERMINATION_PHASE = -1L << PHASE_SHIFT;
+
+ // The following unpacking methods are usually manually inlined
private static int unarrivedOf(long s) {
- return (int) (s & ushortMask);
+ return (int)s & UNARRIVED_MASK;
}
private static int partiesOf(long s) {
- return ((int) s) >>> 16;
+ return (int)s >>> PARTIES_SHIFT;
}
private static int phaseOf(long s) {
- return (int) (s >>> 32);
+ return (int) (s >>> PHASE_SHIFT);
}
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) | (((long) parties) << 16) |
- (long) unarrived);
- }
-
- private static long trippedStateFor(int phase, int parties) {
- long lp = (long) parties;
- return (((long) phase) << 32) | (lp << 16) | lp;
- }
-
- /**
- * Returns message string for bad bounds exceptions.
- */
- private static String badBounds(int parties, int unarrived) {
- return ("Attempt to set " + unarrived +
- " unarrived of " + parties + " parties");
- }
-
/**
* The parent of this phaser, or null if none
*/
@@ -252,46 +279,133 @@ public class Phaser {
*/
private final Phaser root;
- // Wait queues
-
/**
* Heads of Treiber stacks for waiting threads. To eliminate
- * contention while releasing some threads while adding others, we
+ * contention when releasing some threads while adding others, we
* use two of them, alternating across even and odd phases.
+ * Subphasers share queues with root to speed up releases.
*/
- private final AtomicReference evenQ = new AtomicReference();
- private final AtomicReference oddQ = new AtomicReference();
+ private final AtomicReference evenQ;
+ private final AtomicReference oddQ;
private AtomicReference queueFor(int phase) {
return ((phase & 1) == 0) ? evenQ : oddQ;
}
/**
- * Returns current state, first resolving lagged propagation from
- * root if necessary.
+ * Main implementation for methods arrive and arriveAndDeregister.
+ * Manually tuned to speed up and minimize race windows for the
+ * common case of just decrementing unarrived field.
+ *
+ * @param adj - adjustment to apply to state -- either
+ * ONE_ARRIVAL (for arrive) or
+ * ONE_ARRIVAL|ONE_PARTY (for arriveAndDeregister)
+ */
+ private int doArrive(long adj) {
+ for (;;) {
+ long s = state;
+ int phase = (int)(s >>> PHASE_SHIFT);
+ if (phase < 0)
+ return phase;
+ int unarrived = (int)s & UNARRIVED_MASK;
+ if (unarrived == 0)
+ checkBadArrive(s);
+ else if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s-=adj)) {
+ if (unarrived == 1) {
+ long p = s & PARTIES_MASK; // unshifted parties field
+ long lu = p >>> PARTIES_SHIFT;
+ int u = (int)lu;
+ int nextPhase = (phase + 1) & MAX_PHASE;
+ long next = ((long)nextPhase << PHASE_SHIFT) | p | lu;
+ final Phaser parent = this.parent;
+ if (parent == null) {
+ if (onAdvance(phase, u))
+ next |= TERMINATION_PHASE; // obliterate phase
+ UNSAFE.compareAndSwapLong(this, stateOffset, s, next);
+ releaseWaiters(phase);
+ }
+ else {
+ parent.doArrive((u == 0) ?
+ ONE_ARRIVAL|ONE_PARTY : ONE_ARRIVAL);
+ if ((int)(parent.state >>> PHASE_SHIFT) != nextPhase ||
+ ((int)(state >>> PHASE_SHIFT) != nextPhase &&
+ !UNSAFE.compareAndSwapLong(this, stateOffset,
+ s, next)))
+ reconcileState();
+ }
+ }
+ return phase;
+ }
+ }
+ }
+
+ /**
+ * Rechecks state and throws bounds exceptions on arrival -- called
+ * only if unarrived is apparently zero.
+ */
+ private void checkBadArrive(long s) {
+ if (reconcileState() == s)
+ throw new IllegalStateException
+ ("Attempted arrival of unregistered party for " +
+ stateToString(s));
+ }
+
+ /**
+ * Implementation of register, bulkRegister
+ *
+ * @param registrations number to add to both parties and unarrived fields
+ */
+ private int doRegister(int registrations) {
+ // assert registrations > 0;
+ // adjustment to state
+ long adj = ((long)registrations << PARTIES_SHIFT) | registrations;
+ final Phaser parent = this.parent;
+ for (;;) {
+ long s = (parent == null) ? state : reconcileState();
+ int phase = (int)(s >>> PHASE_SHIFT);
+ if (phase < 0)
+ return phase;
+ int parties = (int)s >>> PARTIES_SHIFT;
+ if (parties != 0 && ((int)s & UNARRIVED_MASK) == 0)
+ internalAwaitAdvance(phase, null); // wait for onAdvance
+ else if (registrations > MAX_PARTIES - parties)
+ throw new IllegalStateException(badRegister(s));
+ else if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s + adj))
+ return phase;
+ }
+ }
+
+ /**
+ * Returns message string for out of bounds exceptions on registration.
*/
- private long getReconciledState() {
- return (parent == null) ? state : reconcileState();
+ private String badRegister(long s) {
+ return "Attempt to register more than " +
+ MAX_PARTIES + " parties for " + stateToString(s);
}
/**
- * Recursively resolves state.
+ * Recursively resolves lagged phase propagation from root if necessary.
*/
private long reconcileState() {
- Phaser p = parent;
+ Phaser par = parent;
long s = state;
- if (p != null) {
- while (unarrivedOf(s) == 0 && phaseOf(s) != phaseOf(root.state)) {
- long parentState = p.getReconciledState();
- int parentPhase = phaseOf(parentState);
- int phase = phaseOf(s = state);
- if (phase != parentPhase) {
- long next = trippedStateFor(parentPhase, partiesOf(s));
- if (casState(s, next)) {
- releaseWaiters(phase);
- s = next;
- }
+ if (par != null) {
+ Phaser rt = root;
+ int phase, rPhase;
+ while ((phase = (int)(s >>> PHASE_SHIFT)) >= 0 &&
+ (rPhase = (int)(rt.state >>> PHASE_SHIFT)) != phase) {
+ if ((int)(par.state >>> PHASE_SHIFT) != rPhase)
+ par.reconcileState();
+ else if (rPhase < 0 || ((int)s & UNARRIVED_MASK) == 0) {
+ long u = s & PARTIES_MASK; // reset unarrived to parties
+ long next = ((((long) rPhase) << PHASE_SHIFT) | u |
+ (u >>> PARTIES_SHIFT));
+ if (state == s &&
+ UNSAFE.compareAndSwapLong(this, stateOffset,
+ s, s = next))
+ break;
}
+ s = state;
}
}
return s;
@@ -303,11 +417,11 @@ public class Phaser {
* phaser will need to first register for it.
*/
public Phaser() {
- this(null);
+ this(null, 0);
}
/**
- * Creates a new phaser with the given numbers of registered
+ * Creates a new phaser with the given number of registered
* unarrived parties, initial phase number 0, and no parent.
*
* @param parties the number of parties required to trip barrier
@@ -327,19 +441,11 @@ public class Phaser {
* @param parent the parent phaser
*/
public Phaser(Phaser parent) {
- int phase = 0;
- this.parent = parent;
- if (parent != null) {
- this.root = parent.root;
- phase = parent.register();
- }
- else
- this.root = this;
- this.state = trippedStateFor(phase, 0);
+ this(parent, 0);
}
/**
- * Creates a new phaser with the given parent and numbers of
+ * Creates a new phaser with the given parent and number of
* registered unarrived parties. If parent is non-null, this phaser
* is registered with the parent and its initial phase number is
* the same as that of parent phaser.
@@ -350,23 +456,33 @@ public class Phaser {
* or greater than the maximum number of parties supported
*/
public Phaser(Phaser parent, int parties) {
- if (parties < 0 || parties > ushortMask)
+ if (parties >>> PARTIES_SHIFT != 0)
throw new IllegalArgumentException("Illegal number of parties");
- int phase = 0;
+ int phase;
this.parent = parent;
if (parent != null) {
- this.root = parent.root;
+ Phaser r = parent.root;
+ this.root = r;
+ this.evenQ = r.evenQ;
+ this.oddQ = r.oddQ;
phase = parent.register();
}
- else
+ else {
this.root = this;
- this.state = trippedStateFor(phase, parties);
+ this.evenQ = new AtomicReference();
+ this.oddQ = new AtomicReference();
+ phase = 0;
+ }
+ long p = (long)parties;
+ this.state = (((long)phase) << PHASE_SHIFT) | p | (p << PARTIES_SHIFT);
}
/**
* Adds a new unarrived party to this phaser.
+ * If an ongoing invocation of {@link #onAdvance} is in progress,
+ * this method may wait until its completion before registering.
*
- * @return the current barrier phase number upon registration
+ * @return the arrival phase number to which this registration applied
* @throws IllegalStateException if attempting to register more
* than the maximum supported number of parties
*/
@@ -376,11 +492,14 @@ public class Phaser {
/**
* Adds the given number of new unarrived parties to this phaser.
+ * If an ongoing invocation of {@link #onAdvance} is in progress,
+ * this method may wait until its completion before registering.
*
- * @param parties the number of parties required to trip barrier
- * @return the current barrier phase number upon registration
+ * @param parties the number of additional parties required to trip barrier
+ * @return the arrival phase number to which this registration applied
* @throws IllegalStateException if attempting to register more
* than the maximum supported number of parties
+ * @throws IllegalArgumentException if {@code parties < 0}
*/
public int bulkRegister(int parties) {
if (parties < 0)
@@ -391,73 +510,17 @@ public class Phaser {
}
/**
- * Shared code for register, bulkRegister
- */
- private int doRegister(int registrations) {
- int phase;
- for (;;) {
- long s = getReconciledState();
- phase = phaseOf(s);
- int unarrived = unarrivedOf(s) + registrations;
- int parties = partiesOf(s) + registrations;
- if (phase < 0)
- break;
- if (parties > ushortMask || unarrived > ushortMask)
- throw new IllegalStateException(badBounds(parties, unarrived));
- if (phase == phaseOf(root.state) &&
- casState(s, stateFor(phase, parties, unarrived)))
- break;
- }
- return phase;
- }
-
- /**
* Arrives at the barrier, but does not wait for others. (You can
- * in turn wait for others via {@link #awaitAdvance}).
+ * in turn wait for others via {@link #awaitAdvance}). It is an
+ * unenforced usage error for an unregistered party to invoke this
+ * method.
*
- * @return the barrier phase number upon entry to this method, or a
- * negative value if terminated
+ * @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
* of unarrived parties would become negative
*/
public int arrive() {
- int phase;
- for (;;) {
- long s = state;
- phase = phaseOf(s);
- if (phase < 0)
- break;
- int parties = partiesOf(s);
- int unarrived = unarrivedOf(s) - 1;
- if (unarrived > 0) { // Not the last arrival
- if (casState(s, s - 1)) // s-1 adds one arrival
- break;
- }
- else if (unarrived == 0) { // the last arrival
- Phaser par = parent;
- if (par == null) { // directly trip
- if (casState
- (s,
- trippedStateFor(onAdvance(phase, parties) ? -1 :
- ((phase + 1) & phaseMask), parties))) {
- releaseWaiters(phase);
- break;
- }
- }
- else { // cascade to parent
- if (casState(s, s - 1)) { // zeroes unarrived
- par.arrive();
- reconcileState();
- break;
- }
- }
- }
- else if (phase != phaseOf(root.state)) // or if unreconciled
- reconcileState();
- else
- throw new IllegalStateException(badBounds(parties, unarrived));
- }
- return phase;
+ return doArrive(ONE_ARRIVAL);
}
/**
@@ -466,66 +529,27 @@ public class Phaser {
* required to trip the barrier in future phases. If this phaser
* has a parent, and deregistration causes this phaser to have
* zero parties, this phaser also arrives at and is deregistered
- * from its parent.
+ * from its parent. It is an unenforced usage error for an
+ * unregistered party to invoke this method.
*
- * @return the current barrier phase number upon entry to
- * this method, or a negative value if terminated
+ * @return the arrival phase number, or a negative value if terminated
* @throws IllegalStateException if not terminated and the number
* of registered or unarrived parties would become negative
*/
public int arriveAndDeregister() {
- // similar code to arrive, but too different to merge
- Phaser par = parent;
- int phase;
- for (;;) {
- long s = state;
- phase = phaseOf(s);
- if (phase < 0)
- break;
- int parties = partiesOf(s) - 1;
- int unarrived = unarrivedOf(s) - 1;
- if (parties >= 0) {
- if (unarrived > 0 || (unarrived == 0 && par != null)) {
- if (casState
- (s,
- stateFor(phase, parties, unarrived))) {
- if (unarrived == 0) {
- par.arriveAndDeregister();
- reconcileState();
- }
- break;
- }
- continue;
- }
- if (unarrived == 0) {
- if (casState
- (s,
- trippedStateFor(onAdvance(phase, parties) ? -1 :
- ((phase + 1) & phaseMask), parties))) {
- releaseWaiters(phase);
- break;
- }
- continue;
- }
- if (par != null && phase != phaseOf(root.state)) {
- reconcileState();
- continue;
- }
- }
- throw new IllegalStateException(badBounds(parties, unarrived));
- }
- return phase;
+ return doArrive(ONE_ARRIVAL|ONE_PARTY);
}
/**
* Arrives at the barrier and awaits others. Equivalent in effect
* to {@code awaitAdvance(arrive())}. If you need to await with
* interruption or timeout, you can arrange this with an analogous
- * construction using one of the other forms of the awaitAdvance
- * method. If instead you need to deregister upon arrival use
- * {@code arriveAndDeregister}.
+ * construction using one of the other forms of the {@code
+ * awaitAdvance} method. If instead you need to deregister upon
+ * arrival, use {@link #arriveAndDeregister}. It is an unenforced
+ * usage error for an unregistered party to invoke this method.
*
- * @return the phase on entry to this method
+ * @return the arrival phase number, or a negative number if terminated
* @throws IllegalStateException if not terminated and the number
* of unarrived parties would become negative
*/
@@ -539,65 +563,66 @@ public class Phaser {
* barrier is not equal to the given phase value or this barrier
* is terminated.
*
- * @param phase the phase on entry to this method
- * @return the current barrier phase number upon exit of
- * this method, or a negative value if terminated or
- * argument is negative
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or its variants
+ * @return the next arrival phase number, or a negative value
+ * if terminated or argument is negative
*/
public int awaitAdvance(int phase) {
if (phase < 0)
return phase;
- long s = getReconciledState();
- int p = phaseOf(s);
- if (p != phase)
- return p;
- if (unarrivedOf(s) == 0 && parent != null)
- parent.awaitAdvance(phase);
- // Fall here even if parent waited, to reconcile and help release
- return untimedWait(phase);
+ long s = (parent == null) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ return (p != phase) ? p : internalAwaitAdvance(phase, null);
}
/**
* Awaits the phase of the barrier to advance from the given phase
- * value, throwing {@code InterruptedException} if interrupted while
- * waiting, or returning immediately if the current phase of the
- * barrier is not equal to the given phase value or this barrier
- * is terminated.
- *
- * @param phase the phase on entry to this method
- * @return the current barrier phase number upon exit of
- * this method, or a negative value if terminated or
- * argument is negative
+ * value, throwing {@code InterruptedException} if interrupted
+ * while waiting, or returning immediately if the current phase of
+ * the barrier is not equal to the given phase value or this
+ * barrier is terminated.
+ *
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or its variants
+ * @return the next arrival phase number, or a negative value
+ * if terminated or argument is negative
* @throws InterruptedException if thread interrupted while waiting
*/
public int awaitAdvanceInterruptibly(int phase)
throws InterruptedException {
if (phase < 0)
return phase;
- long s = getReconciledState();
- int p = phaseOf(s);
- if (p != phase)
- return p;
- if (unarrivedOf(s) == 0 && parent != null)
- parent.awaitAdvanceInterruptibly(phase);
- return interruptibleWait(phase);
+ long s = (parent == null) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ if (p == phase) {
+ QNode node = new QNode(this, phase, true, false, 0L);
+ p = internalAwaitAdvance(phase, node);
+ if (node.wasInterrupted)
+ throw new InterruptedException();
+ }
+ return p;
}
/**
* Awaits the phase of the barrier to advance from the given phase
- * value or the given timeout to elapse, throwing
- * {@code InterruptedException} if interrupted while waiting, or
- * returning immediately if the current phase of the barrier is not
- * equal to the given phase value or this barrier is terminated.
- *
- * @param phase the phase on entry to this method
+ * value or the given timeout to elapse, throwing {@code
+ * InterruptedException} if interrupted while waiting, or
+ * returning immediately if the current phase of the barrier is
+ * not equal to the given phase value or this barrier is
+ * terminated.
+ *
+ * @param phase an arrival phase number, or negative value if
+ * terminated; this argument is normally the value returned by a
+ * previous call to {@code arrive} or its variants
* @param timeout how long to wait before giving up, in units of
* {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
- * @return the current barrier phase number upon exit of
- * this method, or a negative value if terminated or
- * argument is negative
+ * @return the next arrival phase number, or a negative value
+ * if terminated or argument is negative
* @throws InterruptedException if thread interrupted while waiting
* @throws TimeoutException if timed out while waiting
*/
@@ -606,34 +631,38 @@ public class Phaser {
throws InterruptedException, TimeoutException {
if (phase < 0)
return phase;
- long s = getReconciledState();
- int p = phaseOf(s);
- if (p != phase)
- return p;
- if (unarrivedOf(s) == 0 && parent != null)
- parent.awaitAdvanceInterruptibly(phase, timeout, unit);
- return timedWait(phase, unit.toNanos(timeout));
+ long s = (parent == null) ? state : reconcileState();
+ int p = (int)(s >>> PHASE_SHIFT);
+ if (p == phase) {
+ long nanos = unit.toNanos(timeout);
+ QNode node = new QNode(this, phase, true, true, nanos);
+ p = internalAwaitAdvance(phase, node);
+ if (node.wasInterrupted)
+ throw new InterruptedException();
+ else if (p == phase)
+ throw new TimeoutException();
+ }
+ return p;
}
/**
- * Forces this barrier to enter termination state. Counts of
- * arrived and registered parties are unaffected. If this phaser
- * has a parent, it too is terminated. This method may be useful
- * for coordinating recovery after one or more tasks encounter
- * unexpected exceptions.
+ * Forces this barrier to enter termination state. Counts of
+ * arrived and registered parties are unaffected. If this phaser
+ * is a member of a tiered set of phasers, then all of the phasers
+ * in the set are terminated. If this phaser is already
+ * terminated, this method has no effect. This method may be
+ * useful for coordinating recovery after one or more tasks
+ * encounter unexpected exceptions.
*/
public void forceTermination() {
- for (;;) {
- long s = getReconciledState();
- int phase = phaseOf(s);
- int parties = partiesOf(s);
- int unarrived = unarrivedOf(s);
- if (phase < 0 ||
- casState(s, stateFor(-1, parties, unarrived))) {
- releaseWaiters(0);
+ // Only need to change root state
+ final Phaser root = this.root;
+ long s;
+ while ((s = root.state) >= 0) {
+ if (UNSAFE.compareAndSwapLong(root, stateOffset,
+ s, s | TERMINATION_PHASE)) {
+ releaseWaiters(0); // signal all threads
releaseWaiters(1);
- if (parent != null)
- parent.forceTermination();
return;
}
}
@@ -647,7 +676,7 @@ public class Phaser {
* @return the phase number, or a negative value if terminated
*/
public final int getPhase() {
- return phaseOf(getReconciledState());
+ return (int)(root.state >>> PHASE_SHIFT);
}
/**
@@ -660,13 +689,13 @@ public class Phaser {
}
/**
- * Returns the number of parties that have arrived at the current
- * phase of this barrier.
+ * Returns the number of registered parties that have arrived at
+ * the current phase of this barrier.
*
* @return the number of arrived parties
*/
public int getArrivedParties() {
- return arrivedOf(state);
+ return arrivedOf(parent==null? state : reconcileState());
}
/**
@@ -676,7 +705,7 @@ public class Phaser {
* @return the number of unarrived parties
*/
public int getUnarrivedParties() {
- return unarrivedOf(state);
+ return unarrivedOf(parent==null? state : reconcileState());
}
/**
@@ -704,30 +733,36 @@ public class Phaser {
* @return {@code true} if this barrier has been terminated
*/
public boolean isTerminated() {
- return getPhase() < 0;
+ return root.state < 0L;
}
/**
- * 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 {@code true}, then, rather than advance
- * the phase number, this barrier will be set to a final
- * termination state, and subsequent calls to {@link #isTerminated}
- * will return true.
+ * Overridable method to perform an action upon impending phase
+ * advance, and to control termination. This method is invoked
+ * upon arrival of the party tripping the barrier (when all other
+ * waiting parties are dormant). If this method returns {@code
+ * true}, then, rather than advance the phase number, this barrier
+ * will be set to a final termination state, and subsequent calls
+ * to {@link #isTerminated} will return true. Any (unchecked)
+ * Exception or Error thrown by an invocation of this method is
+ * propagated to the party attempting to trip the barrier, in
+ * which case no advance occurs.
+ *
+ * The arguments to this method provide the state of the phaser
+ * prevailing for the current transition. The effects of invoking
+ * arrival, registration, and waiting methods on this Phaser from
+ * within {@code onAdvance} are unspecified and should not be
+ * relied on.
+ *
+ *
If this Phaser is a member of a tiered set of Phasers, then
+ * {@code onAdvance} is invoked only for its root Phaser on each
+ * advance.
*
*
The default version returns {@code true} when the number of
* registered parties is zero. Normally, overrides that arrange
* termination for other reasons should also preserve this
* property.
*
- *
You may override this method to perform an action with side
- * effects visible to participating tasks, but it is in general
- * only sensible to do so in designs where all parties register
- * before any arrive, and all {@link #awaitAdvance} at each phase.
- * Otherwise, you cannot ensure lack of interference from other
- * parties during the invocation of this method.
- *
* @param phase the phase number on entering the barrier
* @param registeredParties the current number of registered parties
* @return {@code true} if this barrier should terminate
@@ -746,14 +781,119 @@ public class Phaser {
* @return a string identifying this barrier, as well as its state
*/
public String toString() {
- long s = getReconciledState();
+ return stateToString(reconcileState());
+ }
+
+ /**
+ * Implementation of toString and string-based error messages
+ */
+ private String stateToString(long s) {
return super.toString() +
"[phase = " + phaseOf(s) +
" parties = " + partiesOf(s) +
" arrived = " + arrivedOf(s) + "]";
}
- // methods for waiting
+ // Waiting mechanics
+
+ /**
+ * Removes and signals threads from queue for phase.
+ */
+ private void releaseWaiters(int phase) {
+ AtomicReference head = queueFor(phase);
+ QNode q;
+ int p;
+ while ((q = head.get()) != null &&
+ ((p = q.phase) == phase ||
+ (int)(root.state >>> PHASE_SHIFT) != p)) {
+ if (head.compareAndSet(q, q.next))
+ q.signal();
+ }
+ }
+
+ /** The number of CPUs, for spin control */
+ private static final int NCPU = Runtime.getRuntime().availableProcessors();
+
+ /**
+ * The number of times to spin before blocking while waiting for
+ * advance, per arrival while waiting. On multiprocessors, fully
+ * blocking and waking up a large number of threads all at once is
+ * usually a very slow process, so we use rechargeable spins to
+ * avoid it when threads regularly arrive: When a thread in
+ * internalAwaitAdvance notices another arrival before blocking,
+ * and there appear to be enough CPUs available, it spins
+ * SPINS_PER_ARRIVAL more times before blocking. Plus, even on
+ * uniprocessors, there is at least one intervening Thread.yield
+ * before blocking. The value trades off good-citizenship vs big
+ * unnecessary slowdowns.
+ */
+ static final int SPINS_PER_ARRIVAL = (NCPU < 2) ? 1 : 1 << 8;
+
+ /**
+ * Possibly blocks and waits for phase to advance unless aborted.
+ *
+ * @param phase current phase
+ * @param node if non-null, the wait node to track interrupt and timeout;
+ * if null, denotes noninterruptible wait
+ * @return current phase
+ */
+ private int internalAwaitAdvance(int phase, QNode node) {
+ Phaser current = this; // to eventually wait at root if tiered
+ boolean queued = false; // true when node is enqueued
+ int lastUnarrived = -1; // to increase spins upon change
+ int spins = SPINS_PER_ARRIVAL;
+ long s;
+ int p;
+ while ((p = (int)((s = current.state) >>> PHASE_SHIFT)) == phase) {
+ Phaser par;
+ int unarrived = (int)s & UNARRIVED_MASK;
+ if (unarrived != lastUnarrived) {
+ if (lastUnarrived == -1) // ensure old queue clean
+ releaseWaiters(phase-1);
+ if ((lastUnarrived = unarrived) < NCPU)
+ spins += SPINS_PER_ARRIVAL;
+ }
+ else if (unarrived == 0 && (par = current.parent) != null) {
+ current = par; // if all arrived, use parent
+ par = par.parent;
+ lastUnarrived = -1;
+ }
+ else if (spins > 0) {
+ if (--spins == (SPINS_PER_ARRIVAL >>> 1))
+ Thread.yield(); // yield midway through spin
+ }
+ else if (node == null) // must be noninterruptible
+ node = new QNode(this, phase, false, false, 0L);
+ else if (node.isReleasable()) {
+ if ((p = (int)(root.state >>> PHASE_SHIFT)) != phase)
+ break;
+ else
+ return phase; // aborted
+ }
+ else if (!queued) { // push onto queue
+ AtomicReference head = queueFor(phase);
+ QNode q = head.get();
+ if (q == null || q.phase == phase) {
+ node.next = q;
+ if ((p = (int)(root.state >>> PHASE_SHIFT)) != phase)
+ break; // recheck to avoid stale enqueue
+ else
+ queued = head.compareAndSet(q, node);
+ }
+ }
+ else {
+ try {
+ ForkJoinPool.managedBlock(node);
+ } catch (InterruptedException ie) {
+ node.wasInterrupted = true;
+ }
+ }
+ }
+ releaseWaiters(phase);
+ if (node != null)
+ node.onRelease();
+ return p;
+ }
/**
* Wait nodes for Treiber stack representing wait queue
@@ -761,45 +901,62 @@ public class Phaser {
static final class QNode implements ForkJoinPool.ManagedBlocker {
final Phaser phaser;
final int phase;
- final long startTime;
- final long nanos;
- final boolean timed;
final boolean interruptible;
- volatile boolean wasInterrupted = false;
+ final boolean timed;
+ boolean wasInterrupted;
+ long nanos;
+ long lastTime;
volatile Thread thread; // nulled to cancel wait
QNode next;
+
QNode(Phaser phaser, int phase, boolean interruptible,
- boolean timed, long startTime, long nanos) {
+ boolean timed, long nanos) {
this.phaser = phaser;
this.phase = phase;
- this.timed = timed;
this.interruptible = interruptible;
- this.startTime = startTime;
this.nanos = nanos;
+ this.timed = timed;
+ this.lastTime = timed? System.nanoTime() : 0L;
thread = Thread.currentThread();
}
+
public boolean isReleasable() {
- return (thread == null ||
- phaser.getPhase() != phase ||
- (interruptible && wasInterrupted) ||
- (timed && (nanos - (System.nanoTime() - startTime)) <= 0));
+ Thread t = thread;
+ if (t != null) {
+ if (phaser.getPhase() != phase)
+ t = null;
+ else {
+ if (Thread.interrupted())
+ wasInterrupted = true;
+ if (interruptible && wasInterrupted)
+ t = null;
+ else if (timed) {
+ if (nanos > 0) {
+ long now = System.nanoTime();
+ nanos -= now - lastTime;
+ lastTime = now;
+ }
+ if (nanos <= 0)
+ t = null;
+ }
+ }
+ if (t != null)
+ return false;
+ thread = null;
+ }
+ return true;
}
+
public boolean block() {
- if (Thread.interrupted()) {
- wasInterrupted = true;
- if (interruptible)
- return true;
- }
- if (!timed)
+ if (isReleasable())
+ return true;
+ else if (!timed)
LockSupport.park(this);
- else {
- long waitTime = nanos - (System.nanoTime() - startTime);
- if (waitTime <= 0)
- return true;
- LockSupport.parkNanos(this, waitTime);
- }
+ else if (nanos > 0)
+ LockSupport.parkNanos(this, nanos);
return isReleasable();
}
+
void signal() {
Thread t = thread;
if (t != null) {
@@ -807,128 +964,14 @@ public class Phaser {
LockSupport.unpark(t);
}
}
- boolean doWait() {
- if (thread != null) {
- try {
- ForkJoinPool.managedBlock(this, false);
- } catch (InterruptedException ie) {
- }
- }
- return wasInterrupted;
- }
- }
-
- /**
- * Removes and signals waiting threads from wait queue.
- */
- private void releaseWaiters(int phase) {
- AtomicReference head = queueFor(phase);
- QNode q;
- while ((q = head.get()) != null) {
- if (head.compareAndSet(q, q.next))
- q.signal();
- }
- }
-
- /**
- * Tries to enqueue given node in the appropriate wait queue.
- *
- * @return true if successful
- */
- private boolean tryEnqueue(QNode node) {
- AtomicReference head = queueFor(node.phase);
- return head.compareAndSet(node.next = head.get(), node);
- }
-
- /**
- * Enqueues node and waits unless aborted or signalled.
- *
- * @return current phase
- */
- private int untimedWait(int phase) {
- QNode node = null;
- boolean queued = false;
- boolean interrupted = false;
- int p;
- while ((p = getPhase()) == phase) {
- if (Thread.interrupted())
- interrupted = true;
- else if (node == null)
- node = new QNode(this, phase, false, false, 0, 0);
- else if (!queued)
- queued = tryEnqueue(node);
- else
- interrupted = node.doWait();
- }
- if (node != null)
- node.thread = null;
- releaseWaiters(phase);
- if (interrupted)
- Thread.currentThread().interrupt();
- return p;
- }
-
- /**
- * Interruptible version
- * @return current phase
- */
- private int interruptibleWait(int phase) throws InterruptedException {
- QNode node = null;
- boolean queued = false;
- boolean interrupted = false;
- int p;
- while ((p = getPhase()) == phase && !interrupted) {
- if (Thread.interrupted())
- interrupted = true;
- else if (node == null)
- node = new QNode(this, phase, true, false, 0, 0);
- else if (!queued)
- queued = tryEnqueue(node);
- else
- interrupted = node.doWait();
+ void onRelease() { // actions upon return from internalAwaitAdvance
+ if (!interruptible && wasInterrupted)
+ Thread.currentThread().interrupt();
+ if (thread != null)
+ thread = null;
}
- if (node != null)
- node.thread = null;
- if (p != phase || (p = getPhase()) != phase)
- releaseWaiters(phase);
- if (interrupted)
- throw new InterruptedException();
- return p;
- }
- /**
- * Timeout version.
- * @return current phase
- */
- private int timedWait(int phase, long nanos)
- throws InterruptedException, TimeoutException {
- long startTime = System.nanoTime();
- QNode node = null;
- boolean queued = false;
- boolean interrupted = false;
- int p;
- while ((p = getPhase()) == phase && !interrupted) {
- if (Thread.interrupted())
- interrupted = true;
- else if (nanos - (System.nanoTime() - startTime) <= 0)
- break;
- else if (node == null)
- node = new QNode(this, phase, true, true, startTime, nanos);
- else if (!queued)
- queued = tryEnqueue(node);
- else
- interrupted = node.doWait();
- }
- if (node != null)
- node.thread = null;
- if (p != phase || (p = getPhase()) != phase)
- releaseWaiters(phase);
- if (interrupted)
- throw new InterruptedException();
- if (p == phase)
- throw new TimeoutException();
- return p;
}
// Unsafe mechanics
@@ -937,10 +980,6 @@ public class Phaser {
private static final long stateOffset =
objectFieldOffset("state", Phaser.class);
- private final boolean casState(long cmp, long val) {
- return UNSAFE.compareAndSwapLong(this, stateOffset, cmp, val);
- }
-
private static long objectFieldOffset(String field, Class klazz) {
try {
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));