1 |
/* |
2 |
* Written by Doug Lea with assistance from members of JCP JSR-166 |
3 |
* Expert Group and released to the public domain, as explained at |
4 |
* http://creativecommons.org/publicdomain/zero/1.0/ |
5 |
*/ |
6 |
|
7 |
package jsr166y; |
8 |
|
9 |
/** |
10 |
* A {@link ForkJoinTask} with a completion action |
11 |
* performed when triggered and there are no remaining pending |
12 |
* actions. Uses of CountedCompleter are similar to those of other |
13 |
* completion based components (such as {@link |
14 |
* java.nio.channels.CompletionHandler}) except that multiple |
15 |
* <em>pending</em> completions may be necessary to trigger the {@link |
16 |
* #onCompletion} action, not just one. Unless initialized otherwise, |
17 |
* the {@link #getPendingCount pending count} starts at zero, but may |
18 |
* be (atomically) changed using methods {@link #setPendingCount}, |
19 |
* {@link #addToPendingCount}, and {@link |
20 |
* #compareAndSetPendingCount}. Upon invocation of {@link |
21 |
* #tryComplete}, if the pending action count is nonzero, it is |
22 |
* decremented; otherwise, the completion action is performed, and if |
23 |
* this completer itself has a completer, the process is continued |
24 |
* with its completer. As is the case with related synchronization |
25 |
* components such as {@link java.util.concurrent.Phaser Phaser} and |
26 |
* {@link java.util.concurrent.Semaphore Semaphore}, these methods |
27 |
* affect only internal counts; they do not establish any further |
28 |
* internal bookkeeping. In particular, the identities of pending |
29 |
* tasks are not maintained. As illustrated below, you can create |
30 |
* subclasses that do record some or all pending tasks or their |
31 |
* results when needed. |
32 |
* |
33 |
* <p>A concrete CountedCompleter class must define method {@link |
34 |
* #compute}, that should, in almost all use cases, invoke {@code |
35 |
* tryComplete()} once before returning. The class may also optionally |
36 |
* override method {@link #onCompletion} to perform an action upon |
37 |
* normal completion, and method {@link #onExceptionalCompletion} to |
38 |
* perform an action upon any exception. |
39 |
* |
40 |
* <p>CountedCompleters most often do not bear results, in which case |
41 |
* they are normally declared as {@code CountedCompleter<Void>}, and |
42 |
* will always return {@code null} as a result value. In other cases, |
43 |
* you should override method {@link #getRawResult} to provide a |
44 |
* result from {@code join(), invoke()}, and related methods. (Method |
45 |
* {@link #setRawResult} by default plays no role in CountedCompleters |
46 |
* but may be overridden for example to maintain fields holding result |
47 |
* data.) |
48 |
* |
49 |
* <p>A CountedCompleter that does not itself have a completer (i.e., |
50 |
* one for which {@link #getCompleter} returns {@code null}) can be |
51 |
* used as a regular ForkJoinTask with this added functionality. |
52 |
* However, any completer that in turn has another completer serves |
53 |
* only as an internal helper for other computations, so its own task |
54 |
* status (as reported in methods such as {@link ForkJoinTask#isDone}) |
55 |
* is arbitrary; this status changes only upon explicit invocations of |
56 |
* {@link #complete}, {@link ForkJoinTask#cancel}, {@link |
57 |
* ForkJoinTask#completeExceptionally} or upon exceptional completion |
58 |
* of method {@code compute}. Upon any exceptional completion, the |
59 |
* exception may be relayed to a task's completer (and its completer, |
60 |
* and so on), if one exists and it has not otherwise already |
61 |
* completed. |
62 |
* |
63 |
* <p><b>Sample Usages.</b> |
64 |
* |
65 |
* <p><b>Parallel recursive decomposition.</b> CountedCompleters may |
66 |
* be arranged in trees similar to those often used with {@link |
67 |
* RecursiveAction}s, although the constructions involved in setting |
68 |
* them up typically vary. Here, the completer of each task is its |
69 |
* parent in the computation tree. Even though they entail a bit more |
70 |
* bookkeeping, CountedCompleters may be better choices when applying |
71 |
* a possibly time-consuming operation (that cannot be further |
72 |
* subdivided) to each element of an array or collection; especially |
73 |
* when the operation takes a significantly different amount of time |
74 |
* to complete for some elements than others, either because of |
75 |
* intrinsic variation (for example IO) or auxiliary effects such as |
76 |
* garbage collection. Because CountedCompleters provide their own |
77 |
* continuations, other threads need not block waiting to perform |
78 |
* them. |
79 |
* |
80 |
* <p> For example, here is an initial version of a class that uses |
81 |
* divide-by-two recursive decomposition to divide work into single |
82 |
* pieces (leaf tasks). Even when work is split into individual calls, |
83 |
* tree-based techniques are usually preferable to directly forking |
84 |
* leaf tasks, because they reduce inter-thread communication and |
85 |
* improve load balancing. In the recursive case, the second of each |
86 |
* pair of subtasks to finish triggers completion of its parent |
87 |
* (because no result combination is performed, the default no-op |
88 |
* implementation of method {@code onCompletion} is not overridden). A |
89 |
* static utility method sets up the base task and invokes it: |
90 |
* |
91 |
* <pre> {@code |
92 |
* class MyOperation<E> { void apply(E e) { ... } } |
93 |
* |
94 |
* class ForEach<E> extends CountedCompleter<Void> { |
95 |
* |
96 |
* public static <E> void forEach(ForkJoinPool pool, E[] array, MyOperation<E> op) { |
97 |
* pool.invoke(new ForEach<E>(null, array, op, 0, array.length)); |
98 |
* } |
99 |
* |
100 |
* final E[] array; final MyOperation<E> op; final int lo, hi; |
101 |
* ForEach(CountedCompleter<?> p, E[] array, MyOperation<E> op, int lo, int hi) { |
102 |
* super(p); |
103 |
* this.array = array; this.op = op; this.lo = lo; this.hi = hi; |
104 |
* } |
105 |
* |
106 |
* public void compute() { // version 1 |
107 |
* if (hi - lo >= 2) { |
108 |
* int mid = (lo + hi) >>> 1; |
109 |
* setPendingCount(2); // must set pending count before fork |
110 |
* new ForEach(this, array, op, mid, hi).fork(); // right child |
111 |
* new ForEach(this, array, op, lo, mid).fork(); // left child |
112 |
* } |
113 |
* else if (hi > lo) |
114 |
* op.apply(array[lo]); |
115 |
* tryComplete(); |
116 |
* } |
117 |
* } }</pre> |
118 |
* |
119 |
* This design can be improved by noticing that in the recursive case, |
120 |
* the task has nothing to do after forking its right task, so can |
121 |
* directly invoke its left task before returning. (This is an analog |
122 |
* of tail recursion removal.) Also, because the task returns upon |
123 |
* executing its left task (rather than falling through to invoke |
124 |
* tryComplete) the pending count is set to one: |
125 |
* |
126 |
* <pre> {@code |
127 |
* class ForEach<E> ... |
128 |
* public void compute() { // version 2 |
129 |
* if (hi - lo >= 2) { |
130 |
* int mid = (lo + hi) >>> 1; |
131 |
* setPendingCount(1); // only one pending |
132 |
* new ForEach(this, array, op, mid, hi).fork(); // right child |
133 |
* new ForEach(this, array, op, lo, mid).compute(); // direct invoke |
134 |
* } |
135 |
* else { |
136 |
* if (hi > lo) |
137 |
* op.apply(array[lo]); |
138 |
* tryComplete(); |
139 |
* } |
140 |
* } |
141 |
* }</pre> |
142 |
* |
143 |
* As a further improvement, notice that the left task need not even |
144 |
* exist. Instead of creating a new one, we can iterate using the |
145 |
* original task, and add a pending count for each fork. Additionally, |
146 |
* this version uses {@code helpComplete} to streamline assistance in |
147 |
* the execution of forked tasks. |
148 |
* |
149 |
* <pre> {@code |
150 |
* class ForEach<E> ... |
151 |
* public void compute() { // version 3 |
152 |
* int l = lo, h = hi; |
153 |
* while (h - l >= 2) { |
154 |
* int mid = (l + h) >>> 1; |
155 |
* addToPendingCount(1); |
156 |
* new ForEach(this, array, op, mid, h).fork(); // right child |
157 |
* h = mid; |
158 |
* } |
159 |
* if (h > l) |
160 |
* op.apply(array[l]); |
161 |
* helpComplete(); |
162 |
* } |
163 |
* }</pre> |
164 |
* |
165 |
* Additional improvements of such classes might entail precomputing |
166 |
* pending counts so that they can be established in constructors, |
167 |
* specializing classes for leaf steps, subdividing by say, four, |
168 |
* instead of two per iteration, and using an adaptive threshold |
169 |
* instead of always subdividing down to single elements. |
170 |
* |
171 |
* <p><b>Recording subtasks.</b> CountedCompleter tasks that combine |
172 |
* results of multiple subtasks usually need to access these results |
173 |
* in method {@link #onCompletion}. As illustrated in the following |
174 |
* class (that performs a simplified form of map-reduce where mappings |
175 |
* and reductions are all of type {@code E}), one way to do this in |
176 |
* divide and conquer designs is to have each subtask record its |
177 |
* sibling, so that it can be accessed in method {@code onCompletion}. |
178 |
* This technique applies to reductions in which the order of |
179 |
* combining left and right results does not matter; ordered |
180 |
* reductions require explicit left/right designations. Variants of |
181 |
* other streamlinings seen in the above examples may also apply. |
182 |
* |
183 |
* <pre> {@code |
184 |
* class MyMapper<E> { E apply(E v) { ... } } |
185 |
* class MyReducer<E> { E apply(E x, E y) { ... } } |
186 |
* class MapReducer<E> extends CountedCompleter<E> { |
187 |
* final E[] array; final MyMapper<E> mapper; |
188 |
* final MyReducer<E> reducer; final int lo, hi; |
189 |
* MapReducer<E> sibling; |
190 |
* E result; |
191 |
* MapReducer(CountedCompleter p, E[] array, MyMapper<E> mapper, |
192 |
* MyReducer<E> reducer, int lo, int hi) { |
193 |
* super(p); |
194 |
* this.array = array; this.mapper = mapper; |
195 |
* this.reducer = reducer; this.lo = lo; this.hi = hi; |
196 |
* } |
197 |
* public void compute() { |
198 |
* if (hi - lo >= 2) { |
199 |
* int mid = (lo + hi) >>> 1; |
200 |
* MapReducer<E> left = new MapReducer(this, array, mapper, reducer, lo, mid); |
201 |
* MapReducer<E> right = new MapReducer(this, array, mapper, reducer, mid, hi); |
202 |
* left.sibling = right; |
203 |
* right.sibling = left; |
204 |
* setPendingCount(1); // only right is pending |
205 |
* right.fork(); |
206 |
* left.compute(); // directly execute left |
207 |
* } |
208 |
* else { |
209 |
* if (hi > lo) |
210 |
* result = mapper.apply(array[lo]); |
211 |
* tryComplete(); |
212 |
* } |
213 |
* } |
214 |
* public void onCompletion(CountedCompleter caller) { |
215 |
* if (caller != this) { |
216 |
* MapReducer<E> child = (MapReducer<E>)caller; |
217 |
* MapReducer<E> sib = child.sibling; |
218 |
* if (sib == null || sib.result == null) |
219 |
* result = child.result; |
220 |
* else |
221 |
* result = reducer.apply(child.result, sib.result); |
222 |
* } |
223 |
* } |
224 |
* public E getRawResult() { return result; } |
225 |
* |
226 |
* public static <E> E mapReduce(ForkJoinPool pool, E[] array, |
227 |
* MyMapper<E> mapper, MyReducer<E> reducer) { |
228 |
* return pool.invoke(new MapReducer<E>(null, array, mapper, |
229 |
* reducer, 0, array.length)); |
230 |
* } |
231 |
* } }</pre> |
232 |
* |
233 |
* <p><b>Triggers.</b> Some CountedCompleters are themselves never |
234 |
* forked, but instead serve as bits of plumbing in other designs; |
235 |
* including those in which the completion of one of more async tasks |
236 |
* triggers another async task. For example: |
237 |
* |
238 |
* <pre> {@code |
239 |
* class HeaderBuilder extends CountedCompleter<...> { ... } |
240 |
* class BodyBuilder extends CountedCompleter<...> { ... } |
241 |
* class PacketSender extends CountedCompleter<...> { |
242 |
* PacketSender(...) { super(null, 1); ... } // trigger on second completion |
243 |
* public void compute() { } // never called |
244 |
* public void onCompletion(CountedCompleter<?> caller) { sendPacket(); } |
245 |
* } |
246 |
* // sample use: |
247 |
* PacketSender p = new PacketSender(); |
248 |
* new HeaderBuilder(p, ...).fork(); |
249 |
* new BodyBuilder(p, ...).fork(); |
250 |
* }</pre> |
251 |
* |
252 |
* @since 1.8 |
253 |
* @author Doug Lea |
254 |
*/ |
255 |
public abstract class CountedCompleter<T> extends ForkJoinTask<T> { |
256 |
private static final long serialVersionUID = 5232453752276485070L; |
257 |
|
258 |
/** This task's completer, or null if none */ |
259 |
final CountedCompleter<?> completer; |
260 |
/** The number of pending tasks until completion */ |
261 |
volatile int pending; |
262 |
|
263 |
/** |
264 |
* Creates a new CountedCompleter with the given completer |
265 |
* and initial pending count. |
266 |
* |
267 |
* @param completer this tasks completer, or {@code null} if none |
268 |
* @param initialPendingCount the initial pending count |
269 |
*/ |
270 |
protected CountedCompleter(CountedCompleter<?> completer, |
271 |
int initialPendingCount) { |
272 |
this.completer = completer; |
273 |
this.pending = initialPendingCount; |
274 |
} |
275 |
|
276 |
/** |
277 |
* Creates a new CountedCompleter with the given completer |
278 |
* and an initial pending count of zero. |
279 |
* |
280 |
* @param completer this tasks completer, or {@code null} if none |
281 |
*/ |
282 |
protected CountedCompleter(CountedCompleter<?> completer) { |
283 |
this.completer = completer; |
284 |
} |
285 |
|
286 |
/** |
287 |
* Creates a new CountedCompleter with no completer |
288 |
* and an initial pending count of zero. |
289 |
*/ |
290 |
protected CountedCompleter() { |
291 |
this.completer = null; |
292 |
} |
293 |
|
294 |
/** |
295 |
* The main computation performed by this task. |
296 |
*/ |
297 |
public abstract void compute(); |
298 |
|
299 |
/** |
300 |
* Performs an action when method {@link #tryComplete} is invoked |
301 |
* and there are no pending counts, or when the unconditional |
302 |
* method {@link #complete} is invoked. By default, this method |
303 |
* does nothing. |
304 |
* |
305 |
* @param caller the task invoking this method (which may |
306 |
* be this task itself). |
307 |
*/ |
308 |
public void onCompletion(CountedCompleter<?> caller) { |
309 |
} |
310 |
|
311 |
/** |
312 |
* Performs an action when method {@link #completeExceptionally} |
313 |
* is invoked or method {@link #compute} throws an exception, and |
314 |
* this task has not otherwise already completed normally. On |
315 |
* entry to this method, this task {@link |
316 |
* ForkJoinTask#isCompletedAbnormally}. The return value of this |
317 |
* method controls further propagation: If {@code true} and this |
318 |
* task has a completer, then this completer is also completed |
319 |
* exceptionally. The default implementation of this method does |
320 |
* nothing except return {@code true}. |
321 |
* |
322 |
* @param ex the exception |
323 |
* @param caller the task invoking this method (which may |
324 |
* be this task itself). |
325 |
* @return true if this exception should be propagated to this |
326 |
* tasks completer, if one exists. |
327 |
*/ |
328 |
public boolean onExceptionalCompletion(Throwable ex, CountedCompleter<?> caller) { |
329 |
return true; |
330 |
} |
331 |
|
332 |
/** |
333 |
* Returns the completer established in this task's constructor, |
334 |
* or {@code null} if none. |
335 |
* |
336 |
* @return the completer |
337 |
*/ |
338 |
public final CountedCompleter<?> getCompleter() { |
339 |
return completer; |
340 |
} |
341 |
|
342 |
/** |
343 |
* Returns the current pending count. |
344 |
* |
345 |
* @return the current pending count |
346 |
*/ |
347 |
public final int getPendingCount() { |
348 |
return pending; |
349 |
} |
350 |
|
351 |
/** |
352 |
* Sets the pending count to the given value. |
353 |
* |
354 |
* @param count the count |
355 |
*/ |
356 |
public final void setPendingCount(int count) { |
357 |
pending = count; |
358 |
} |
359 |
|
360 |
/** |
361 |
* Adds (atomically) the given value to the pending count. |
362 |
* |
363 |
* @param delta the value to add |
364 |
*/ |
365 |
public final void addToPendingCount(int delta) { |
366 |
int c; // note: can replace with intrinsic in jdk8 |
367 |
do {} while (!U.compareAndSwapInt(this, PENDING, c = pending, c+delta)); |
368 |
} |
369 |
|
370 |
/** |
371 |
* Sets (atomically) the pending count to the given count only if |
372 |
* it currently holds the given expected value. |
373 |
* |
374 |
* @param expected the expected value |
375 |
* @param count the new value |
376 |
* @return true is successful |
377 |
*/ |
378 |
public final boolean compareAndSetPendingCount(int expected, int count) { |
379 |
return U.compareAndSwapInt(this, PENDING, expected, count); |
380 |
} |
381 |
|
382 |
/** |
383 |
* Returns the root of the current computation; i.e., this |
384 |
* task if it has no completer, else its completer's root. |
385 |
* |
386 |
* @return the root of the current computation |
387 |
*/ |
388 |
public final CountedCompleter<?> getRoot() { |
389 |
CountedCompleter<?> a = this, p; |
390 |
while ((p = a.completer) != null) |
391 |
a = p; |
392 |
return a; |
393 |
} |
394 |
|
395 |
/** |
396 |
* If the pending count is nonzero, decrements the count; |
397 |
* otherwise invokes {@link #onCompletion} and then similarly |
398 |
* tries to complete this task's completer, if one exists, |
399 |
* else marks this task as complete. |
400 |
*/ |
401 |
public final void tryComplete() { |
402 |
CountedCompleter<?> a = this, s = a; |
403 |
for (int c;;) { |
404 |
if ((c = a.pending) == 0) { |
405 |
a.onCompletion(s); |
406 |
if ((a = (s = a).completer) == null) { |
407 |
s.quietlyComplete(); |
408 |
return; |
409 |
} |
410 |
} |
411 |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) |
412 |
return; |
413 |
} |
414 |
} |
415 |
|
416 |
/** |
417 |
* Identical to {@link #tryComplete}, but may additionally execute |
418 |
* other tasks within the current computation (i.e., those |
419 |
* with the same {@link #getRoot}. |
420 |
*/ |
421 |
public final void helpComplete() { |
422 |
CountedCompleter<?> a = this, s = a; |
423 |
for (int c;;) { |
424 |
if ((c = a.pending) == 0) { |
425 |
a.onCompletion(s); |
426 |
if ((a = (s = a).completer) == null) { |
427 |
s.quietlyComplete(); |
428 |
return; |
429 |
} |
430 |
} |
431 |
else if (U.compareAndSwapInt(a, PENDING, c, c - 1)) { |
432 |
CountedCompleter<?> root = a.getRoot(); |
433 |
Thread thread = Thread.currentThread(); |
434 |
ForkJoinPool.WorkQueue wq = |
435 |
(thread instanceof ForkJoinWorkerThread) ? |
436 |
((ForkJoinWorkerThread)thread).workQueue : null; |
437 |
ForkJoinTask<?> t; |
438 |
while ((t = (wq != null) ? wq.popCC(root) : |
439 |
ForkJoinPool.popCCFromCommonPool(root)) != null) { |
440 |
t.doExec(); |
441 |
if (root.isDone()) |
442 |
break; |
443 |
} |
444 |
return; |
445 |
} |
446 |
} |
447 |
} |
448 |
|
449 |
/** |
450 |
* Regardless of pending count, invokes {@link #onCompletion}, |
451 |
* marks this task as complete and further triggers {@link |
452 |
* #tryComplete} on this task's completer, if one exists. This |
453 |
* method may be useful when forcing completion as soon as any one |
454 |
* (versus all) of several subtask results are obtained. The |
455 |
* given rawResult is used as an argument to {@link #setRawResult} |
456 |
* before marking this task as complete; its value is meaningful |
457 |
* only for classes overriding {@code setRawResult}. |
458 |
* |
459 |
* @param rawResult the raw result |
460 |
*/ |
461 |
public void complete(T rawResult) { |
462 |
CountedCompleter<?> p; |
463 |
onCompletion(this); |
464 |
setRawResult(rawResult); |
465 |
quietlyComplete(); |
466 |
if ((p = completer) != null) |
467 |
p.tryComplete(); |
468 |
} |
469 |
|
470 |
/** |
471 |
* Support for FJT exception propagation |
472 |
*/ |
473 |
void internalPropagateException(Throwable ex) { |
474 |
CountedCompleter<?> a = this, s = a; |
475 |
while (a.onExceptionalCompletion(ex, s) && |
476 |
(a = (s = a).completer) != null && a.status >= 0) |
477 |
a.recordExceptionalCompletion(ex); |
478 |
} |
479 |
|
480 |
/** |
481 |
* Implements execution conventions for CountedCompleters |
482 |
*/ |
483 |
protected final boolean exec() { |
484 |
compute(); |
485 |
return false; |
486 |
} |
487 |
|
488 |
/** |
489 |
* Returns the result of the computation. By default |
490 |
* returns {@code null}, which is appropriate for {@code Void} |
491 |
* actions, but in other cases should be overridden. |
492 |
* |
493 |
* @return the result of the computation |
494 |
*/ |
495 |
public T getRawResult() { return null; } |
496 |
|
497 |
/** |
498 |
* A method that result-bearing CountedCompleters may optionally |
499 |
* use to help maintain result data. By default, does nothing. |
500 |
*/ |
501 |
protected void setRawResult(T t) { } |
502 |
|
503 |
// Unsafe mechanics |
504 |
private static final sun.misc.Unsafe U; |
505 |
private static final long PENDING; |
506 |
static { |
507 |
try { |
508 |
U = getUnsafe(); |
509 |
PENDING = U.objectFieldOffset |
510 |
(CountedCompleter.class.getDeclaredField("pending")); |
511 |
} catch (Exception e) { |
512 |
throw new Error(e); |
513 |
} |
514 |
} |
515 |
|
516 |
/** |
517 |
* Returns a sun.misc.Unsafe. Suitable for use in a 3rd party package. |
518 |
* Replace with a simple call to Unsafe.getUnsafe when integrating |
519 |
* into a jdk. |
520 |
* |
521 |
* @return a sun.misc.Unsafe |
522 |
*/ |
523 |
private static sun.misc.Unsafe getUnsafe() { |
524 |
try { |
525 |
return sun.misc.Unsafe.getUnsafe(); |
526 |
} catch (SecurityException se) { |
527 |
try { |
528 |
return java.security.AccessController.doPrivileged |
529 |
(new java.security |
530 |
.PrivilegedExceptionAction<sun.misc.Unsafe>() { |
531 |
public sun.misc.Unsafe run() throws Exception { |
532 |
java.lang.reflect.Field f = sun.misc |
533 |
.Unsafe.class.getDeclaredField("theUnsafe"); |
534 |
f.setAccessible(true); |
535 |
return (sun.misc.Unsafe) f.get(null); |
536 |
}}); |
537 |
} catch (java.security.PrivilegedActionException e) { |
538 |
throw new RuntimeException("Could not initialize intrinsics", |
539 |
e.getCause()); |
540 |
} |
541 |
} |
542 |
} |
543 |
|
544 |
} |