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 {@code Phaser} that - * it registers with upon construction: + *
To create a set of {@code n} tasks using a tree of phasers, you + * could use code of the following form, assuming a Task class with a + * constructor accepting a {@code Phaser} that it registers with upon + * construction. After invocation of {@code build(new Task[n], 0, n, + * new Phaser())}, these tasks could then be started, for example by + * submitting to a pool: * *
{@code
- * void build(Task[] actions, int lo, int hi, Phaser ph) {
+ * void build(Task[] tasks, 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));
+ * build(tasks, i, j, new Phaser(ph));
* }
* } else {
* for (int i = lo; i < hi; ++i)
- * actions[i] = new Task(ph);
+ * tasks[i] = new Task(ph);
* // assumes new Task(ph) performs ph.register()
* }
- * }
- * // .. initially called, for n tasks via
- * build(new Task[n], 0, n, new Phaser());}
+ * }}
*
* The best value of {@code TASKS_PER_PHASER} depends mainly on
* expected synchronization rates. A value as low as four may
@@ -233,11 +233,26 @@ public class Phaser {
* * 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.
+ * Except that a phaser with no registered parties is
+ * distinguished with the otherwise illegal state of having zero
+ * parties and one unarrived parties (encoded as EMPTY below).
+ *
+ * To efficiently maintain atomicity, these values are packed into
+ * a single (atomic) long. Good performance relies on keeping
+ * state decoding and encoding simple, and keeping race windows
+ * short.
+ *
+ * All state updates are performed via CAS except initial
+ * registration of a sub-phaser (i.e., one with a non-null
+ * parent). In this (relatively rare) case, we use built-in
+ * synchronization to lock while first registering with its
+ * parent.
+ *
+ * The phase of a subphaser is allowed to lag that of its
+ * ancestors until it is actually accessed. Method reconcileState
+ * is usually attempted only only when the number of unarrived
+ * parties appears to be zero, which indicates a potential lag in
+ * updating phase after the root advanced.
*/
private volatile long state;
@@ -247,18 +262,23 @@ public class Phaser {
private static final int PHASE_SHIFT = 32;
private static final int UNARRIVED_MASK = 0xffff; // to mask ints
private static final long PARTIES_MASK = 0xffff0000L; // to mask longs
- private static final long ONE_ARRIVAL = 1L;
- private static final long ONE_PARTY = 1L << PARTIES_SHIFT;
private static final long TERMINATION_BIT = 1L << 63;
+ // some special values
+ private static final int ONE_ARRIVAL = 1;
+ private static final int ONE_PARTY = 1 << PARTIES_SHIFT;
+ private static final int EMPTY = 1;
+
// The following unpacking methods are usually manually inlined
private static int unarrivedOf(long s) {
- return (int)s & UNARRIVED_MASK;
+ int counts = (int)s;
+ return (counts == EMPTY)? 0 : counts & UNARRIVED_MASK;
}
private static int partiesOf(long s) {
- return (int)s >>> PARTIES_SHIFT;
+ int counts = (int)s;
+ return (counts == EMPTY)? 0 : counts >>> PARTIES_SHIFT;
}
private static int phaseOf(long s) {
@@ -266,7 +286,9 @@ public class Phaser {
}
private static int arrivedOf(long s) {
- return partiesOf(s) - unarrivedOf(s);
+ int counts = (int)s;
+ return (counts == EMPTY)? 0 :
+ (counts >>> PARTIES_SHIFT) - (counts & UNARRIVED_MASK);
}
/**
@@ -275,8 +297,7 @@ public class Phaser {
private final Phaser parent;
/**
- * The root of phaser tree. Equals this if not in a tree. Used to
- * support faster state push-down.
+ * The root of phaser tree. Equals this if not in a tree.
*/
private final Phaser root;
@@ -314,48 +335,45 @@ public class Phaser {
* 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)
+ * @param deregister false for arrive, true for arriveAndDeregister
*/
- private int doArrive(long adj) {
- for (;;) {
- long s = state;
- int unarrived = (int)s & UNARRIVED_MASK;
- int phase = (int)(s >>> PHASE_SHIFT);
- if (phase < 0)
- return phase;
- else if (unarrived == 0) {
- if (reconcileState() == s) // recheck
+ private int doArrive(boolean deregister) {
+ int adj = deregister ? ONE_ARRIVAL|ONE_PARTY : ONE_ARRIVAL;
+ long s;
+ int phase;
+ while ((phase = (int)((s = state) >>> PHASE_SHIFT)) >= 0) {
+ int counts = (int)s;
+ int unarrived = counts & UNARRIVED_MASK;
+ if (counts == EMPTY || unarrived == 0) {
+ if (reconcileState() == s)
throw new IllegalStateException(badArrive(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_BIT;
- UNSAFE.compareAndSwapLong(this, stateOffset, s, next);
- releaseWaiters(phase);
+ long n = s & PARTIES_MASK; // unshifted parties field
+ int u = ((int)n) >>> PARTIES_SHIFT;
+ Phaser par = parent;
+ if (par != null) {
+ par.doArrive(u == 0);
+ reconcileState();
}
else {
- parent.doArrive((u == 0) ?
- ONE_ARRIVAL|ONE_PARTY : ONE_ARRIVAL);
- if ((int)(parent.state >>> PHASE_SHIFT) != nextPhase)
- reconcileState();
- else if (state == s)
- UNSAFE.compareAndSwapLong(this, stateOffset, s,
- next);
+ n |= (((long)((phase+1) & MAX_PHASE)) << PHASE_SHIFT);
+ if (onAdvance(phase, u))
+ n |= TERMINATION_BIT;
+ else if (u == 0)
+ n |= EMPTY; // reset to unregistered
+ else
+ n |= (long)u; // reset unarr to parties
+ // assert state == s || isTerminated();
+ UNSAFE.compareAndSwapLong(this, stateOffset, s, n);
+ releaseWaiters(phase);
}
}
- return phase;
+ break;
}
}
+ return phase;
}
/**
@@ -367,60 +385,73 @@ public class Phaser {
private int doRegister(int registrations) {
// adjustment to state
long adj = ((long)registrations << PARTIES_SHIFT) | registrations;
- final Phaser parent = this.parent;
+ Phaser par = parent;
+ int phase;
for (;;) {
- long s = (parent == null) ? state : reconcileState();
- int parties = (int)s >>> PARTIES_SHIFT;
- int phase = (int)(s >>> PHASE_SHIFT);
- if (phase < 0)
- return phase;
- else if (registrations > MAX_PARTIES - parties)
+ long s = state;
+ int counts = (int)s;
+ int parties = counts >>> PARTIES_SHIFT;
+ int unarrived = counts & UNARRIVED_MASK;
+ if (registrations > MAX_PARTIES - parties)
throw new IllegalStateException(badRegister(s));
- else if ((parties == 0 && parent == null) || // first reg of root
- ((int)s & UNARRIVED_MASK) != 0) { // not advancing
- if (UNSAFE.compareAndSwapLong(this, stateOffset, s, s + adj))
- return phase;
- }
- else if (parties != 0) // wait for onAdvance
- root.internalAwaitAdvance(phase, null);
- else { // 1st registration of child
- synchronized (this) { // register parent first
- if (reconcileState() == s) { // recheck under lock
- parent.doRegister(1); // OK if throws IllegalState
- for (;;) { // simpler form of outer loop
- s = reconcileState();
- phase = (int)(s >>> PHASE_SHIFT);
- if (phase < 0 ||
- UNSAFE.compareAndSwapLong(this, stateOffset,
- s, s + adj))
- return phase;
- }
+ else if ((phase = (int)(s >>> PHASE_SHIFT)) < 0)
+ break;
+ else if (counts != EMPTY) { // not 1st registration
+ if (par == null || reconcileState() == s) {
+ if (unarrived == 0) // wait out advance
+ root.internalAwaitAdvance(phase, null);
+ else if (UNSAFE.compareAndSwapLong(this, stateOffset,
+ s, s + adj))
+ break;
+ }
+ }
+ else if (par == null) { // 1st root registration
+ long next = (((long) phase) << PHASE_SHIFT) | adj;
+ if (UNSAFE.compareAndSwapLong(this, stateOffset, s, next))
+ break;
+ }
+ else {
+ synchronized(this) { // 1st sub registration
+ if (state == s) { // recheck under lock
+ par.doRegister(1);
+ do { // force current phase
+ phase = (int)(root.state >>> PHASE_SHIFT);
+ // assert phase < 0 || (int)state == EMPTY;
+ } while (!UNSAFE.compareAndSwapLong
+ (this, stateOffset, state,
+ (((long) phase) << PHASE_SHIFT) | adj));
+ break;
}
}
}
}
+ return phase;
}
/**
- * Recursively resolves lagged phase propagation from root if necessary.
+ * Resolves lagged phase propagation from root if necessary.
*/
private long reconcileState() {
- Phaser par = parent;
+ Phaser rt = root;
long s = state;
- if (par != null) {
- Phaser rt = root;
- int phase, rPhase;
- while ((phase = (int)(s >>> PHASE_SHIFT)) >= 0 &&
- (rPhase = (int)(rt.state >>> PHASE_SHIFT)) != phase) {
- if (par != rt && (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));
- UNSAFE.compareAndSwapLong(this, stateOffset, s, next);
+ if (rt != this) {
+ int phase;
+ while ((phase = (int)(rt.state >>> PHASE_SHIFT)) !=
+ (int)(s >>> PHASE_SHIFT)) {
+ // assert phase < 0 || unarrivedOf(s) == 0
+ long t; // to reread s
+ long p = s & PARTIES_MASK; // unshifted parties field
+ long n = (((long) phase) << PHASE_SHIFT) | p;
+ if (phase >= 0) {
+ if (p == 0L)
+ n |= EMPTY; // reset to empty
+ else
+ n |= p >>> PARTIES_SHIFT; // set unarr to parties
}
- s = state;
+ if ((t = state) == s &&
+ UNSAFE.compareAndSwapLong(this, stateOffset, s, s = n))
+ break;
+ s = t;
}
}
return s;
@@ -478,7 +509,7 @@ public class Phaser {
public Phaser(Phaser parent, int parties) {
if (parties >>> PARTIES_SHIFT != 0)
throw new IllegalArgumentException("Illegal number of parties");
- long s = ((long) parties) | (((long) parties) << PARTIES_SHIFT);
+ int phase = 0;
this.parent = parent;
if (parent != null) {
Phaser r = parent.root;
@@ -486,14 +517,17 @@ public class Phaser {
this.evenQ = r.evenQ;
this.oddQ = r.oddQ;
if (parties != 0)
- s |= ((long)(parent.doRegister(1))) << PHASE_SHIFT;
+ phase = parent.doRegister(1);
}
else {
this.root = this;
this.evenQ = new AtomicReferenceThe arguments to this method provide the state of the phaser
* prevailing for the current transition. The effects of invoking
@@ -856,10 +893,10 @@ public class Phaser {
QNode q; // first element of queue
int p; // its phase
Thread t; // its thread
+ // assert phase != phaseOf(root.state);
AtomicReference