15 |
|
import java.util.NoSuchElementException; |
16 |
|
import java.util.Queue; |
17 |
|
import java.util.concurrent.locks.LockSupport; |
18 |
+ |
|
19 |
|
/** |
20 |
|
* An unbounded {@link TransferQueue} based on linked nodes. |
21 |
|
* This queue orders elements FIFO (first-in-first-out) with respect |
207 |
|
* additional GC bookkeeping ("write barriers") that are sometimes |
208 |
|
* more costly than the writes themselves because of contention). |
209 |
|
* |
209 |
– |
* Removal of interior nodes (due to timed out or interrupted |
210 |
– |
* waits, or calls to remove(x) or Iterator.remove) can use a |
211 |
– |
* scheme roughly similar to that described in Scherer, Lea, and |
212 |
– |
* Scott's SynchronousQueue. Given a predecessor, we can unsplice |
213 |
– |
* any node except the (actual) tail of the queue. To avoid |
214 |
– |
* build-up of cancelled trailing nodes, upon a request to remove |
215 |
– |
* a trailing node, it is placed in field "cleanMe" to be |
216 |
– |
* unspliced upon the next call to unsplice any other node. |
217 |
– |
* Situations needing such mechanics are not common but do occur |
218 |
– |
* in practice; for example when an unbounded series of short |
219 |
– |
* timed calls to poll repeatedly time out but never otherwise |
220 |
– |
* fall off the list because of an untimed call to take at the |
221 |
– |
* front of the queue. Note that maintaining field cleanMe does |
222 |
– |
* not otherwise much impact garbage retention even if never |
223 |
– |
* cleared by some other call because the held node will |
224 |
– |
* eventually either directly or indirectly lead to a self-link |
225 |
– |
* once off the list. |
226 |
– |
* |
210 |
|
* *** Overview of implementation *** |
211 |
|
* |
212 |
|
* We use a threshold-based approach to updates, with a slack |
222 |
|
* per-thread one available, but even ThreadLocalRandom is too |
223 |
|
* heavy for these purposes. |
224 |
|
* |
225 |
< |
* With such a small slack threshold value, it is rarely |
226 |
< |
* worthwhile to augment this with path short-circuiting; i.e., |
227 |
< |
* unsplicing nodes between head and the first unmatched node, or |
228 |
< |
* similarly for tail, rather than advancing head or tail |
246 |
< |
* proper. However, it is used (in awaitMatch) immediately before |
247 |
< |
* a waiting thread starts to block, as a final bit of helping at |
248 |
< |
* a point when contention with others is extremely unlikely |
249 |
< |
* (since if other threads that could release it are operating, |
250 |
< |
* then the current thread wouldn't be blocking). |
225 |
> |
* With such a small slack threshold value, it is not worthwhile |
226 |
> |
* to augment this with path short-circuiting (i.e., unsplicing |
227 |
> |
* interior nodes) except in the case of cancellation/removal (see |
228 |
> |
* below). |
229 |
|
* |
230 |
|
* We allow both the head and tail fields to be null before any |
231 |
|
* nodes are enqueued; initializing upon first append. This |
307 |
|
* versa) compared to their predecessors receive additional |
308 |
|
* chained spins, reflecting longer paths typically required to |
309 |
|
* unblock threads during phase changes. |
310 |
+ |
* |
311 |
+ |
* |
312 |
+ |
* ** Unlinking removed interior nodes ** |
313 |
+ |
* |
314 |
+ |
* In addition to minimizing garbage retention via self-linking |
315 |
+ |
* described above, we also unlink removed interior nodes. These |
316 |
+ |
* may arise due to timed out or interrupted waits, or calls to |
317 |
+ |
* remove(x) or Iterator.remove. Normally, given a node that was |
318 |
+ |
* at one time known to be the predecessor of some node s that is |
319 |
+ |
* to be removed, we can unsplice s by CASing the next field of |
320 |
+ |
* its predecessor if it still points to s (otherwise s must |
321 |
+ |
* already have been removed or is now offlist). But there are two |
322 |
+ |
* situations in which we cannot guarantee to make node s |
323 |
+ |
* unreachable in this way: (1) If s is the trailing node of list |
324 |
+ |
* (i.e., with null next), then it is pinned as the target node |
325 |
+ |
* for appends, so can only be removed later after other nodes are |
326 |
+ |
* appended. (2) We cannot necessarily unlink s given a |
327 |
+ |
* predecessor node that is matched (including the case of being |
328 |
+ |
* cancelled): the predecessor may already be unspliced, in which |
329 |
+ |
* case some previous reachable node may still point to s. |
330 |
+ |
* (For further explanation see Herlihy & Shavit "The Art of |
331 |
+ |
* Multiprocessor Programming" chapter 9). Although, in both |
332 |
+ |
* cases, we can rule out the need for further action if either s |
333 |
+ |
* or its predecessor are (or can be made to be) at, or fall off |
334 |
+ |
* from, the head of list. |
335 |
+ |
* |
336 |
+ |
* Without taking these into account, it would be possible for an |
337 |
+ |
* unbounded number of supposedly removed nodes to remain |
338 |
+ |
* reachable. Situations leading to such buildup are uncommon but |
339 |
+ |
* can occur in practice; for example when a series of short timed |
340 |
+ |
* calls to poll repeatedly time out but never otherwise fall off |
341 |
+ |
* the list because of an untimed call to take at the front of the |
342 |
+ |
* queue. |
343 |
+ |
* |
344 |
+ |
* When these cases arise, rather than always retraversing the |
345 |
+ |
* entire list to find an actual predecessor to unlink (which |
346 |
+ |
* won't help for case (1) anyway), we record a conservative |
347 |
+ |
* estimate of possible unsplice failures (in "sweepVotes"). |
348 |
+ |
* We trigger a full sweep when the estimate exceeds a threshold |
349 |
+ |
* ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
350 |
+ |
* removal failures to tolerate before sweeping through, unlinking |
351 |
+ |
* cancelled nodes that were not unlinked upon initial removal. |
352 |
+ |
* We perform sweeps by the thread hitting threshold (rather than |
353 |
+ |
* background threads or by spreading work to other threads) |
354 |
+ |
* because in the main contexts in which removal occurs, the |
355 |
+ |
* caller is already timed-out, cancelled, or performing a |
356 |
+ |
* potentially O(n) operation (e.g. remove(x)), none of which are |
357 |
+ |
* time-critical enough to warrant the overhead that alternatives |
358 |
+ |
* would impose on other threads. |
359 |
+ |
* |
360 |
+ |
* Because the sweepVotes estimate is conservative, and because |
361 |
+ |
* nodes become unlinked "naturally" as they fall off the head of |
362 |
+ |
* the queue, and because we allow votes to accumulate even while |
363 |
+ |
* sweeps are in progress, there are typically significantly fewer |
364 |
+ |
* such nodes than estimated. Choice of a threshold value |
365 |
+ |
* balances the likelihood of wasted effort and contention, versus |
366 |
+ |
* providing a worst-case bound on retention of interior nodes in |
367 |
+ |
* quiescent queues. The value defined below was chosen |
368 |
+ |
* empirically to balance these under various timeout scenarios. |
369 |
+ |
* |
370 |
+ |
* Note that we cannot self-link unlinked interior nodes during |
371 |
+ |
* sweeps. However, the associated garbage chains terminate when |
372 |
+ |
* some successor ultimately falls off the head of the list and is |
373 |
+ |
* self-linked. |
374 |
|
*/ |
375 |
|
|
376 |
|
/** True if on multiprocessor */ |
397 |
|
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
398 |
|
|
399 |
|
/** |
400 |
+ |
* The maximum number of estimated removal failures (sweepVotes) |
401 |
+ |
* to tolerate before sweeping through the queue unlinking |
402 |
+ |
* cancelled nodes that were not unlinked upon initial |
403 |
+ |
* removal. See above for explanation. The value must be at least |
404 |
+ |
* two to avoid useless sweeps when removing trailing nodes. |
405 |
+ |
*/ |
406 |
+ |
static final int SWEEP_THRESHOLD = 32; |
407 |
+ |
|
408 |
+ |
/** |
409 |
|
* Queue nodes. Uses Object, not E, for items to allow forgetting |
410 |
|
* them after use. Relies heavily on Unsafe mechanics to minimize |
411 |
< |
* unnecessary ordering constraints: Writes that intrinsically |
412 |
< |
* precede or follow CASes use simple relaxed forms. Other |
362 |
< |
* cleanups use releasing/lazy writes. |
411 |
> |
* unnecessary ordering constraints: Writes that are intrinsically |
412 |
> |
* ordered wrt other accesses or CASes use simple relaxed forms. |
413 |
|
*/ |
414 |
|
static final class Node { |
415 |
|
final boolean isData; // false if this is a request node |
423 |
|
} |
424 |
|
|
425 |
|
final boolean casItem(Object cmp, Object val) { |
426 |
< |
assert cmp == null || cmp.getClass() != Node.class; |
426 |
> |
// assert cmp == null || cmp.getClass() != Node.class; |
427 |
|
return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
428 |
|
} |
429 |
|
|
430 |
|
/** |
431 |
< |
* Creates a new node. Uses relaxed write because item can only |
432 |
< |
* be seen if followed by CAS. |
431 |
> |
* Constructs a new node. Uses relaxed write because item can |
432 |
> |
* only be seen after publication via casNext. |
433 |
|
*/ |
434 |
|
Node(Object item, boolean isData) { |
435 |
|
UNSAFE.putObject(this, itemOffset, item); // relaxed write |
445 |
|
} |
446 |
|
|
447 |
|
/** |
448 |
< |
* Sets item to self (using a releasing/lazy write) and waiter |
449 |
< |
* to null, to avoid garbage retention after extracting or |
450 |
< |
* cancelling. |
448 |
> |
* Sets item to self and waiter to null, to avoid garbage |
449 |
> |
* retention after matching or cancelling. Uses relaxed writes |
450 |
> |
* because order is already constrained in the only calling |
451 |
> |
* contexts: item is forgotten only after volatile/atomic |
452 |
> |
* mechanics that extract items. Similarly, clearing waiter |
453 |
> |
* follows either CAS or return from park (if ever parked; |
454 |
> |
* else we don't care). |
455 |
|
*/ |
456 |
|
final void forgetContents() { |
457 |
< |
UNSAFE.putOrderedObject(this, itemOffset, this); |
458 |
< |
UNSAFE.putOrderedObject(this, waiterOffset, null); |
457 |
> |
UNSAFE.putObject(this, itemOffset, this); |
458 |
> |
UNSAFE.putObject(this, waiterOffset, null); |
459 |
|
} |
460 |
|
|
461 |
|
/** |
489 |
|
* Tries to artificially match a data node -- used by remove. |
490 |
|
*/ |
491 |
|
final boolean tryMatchData() { |
492 |
< |
assert isData; |
492 |
> |
// assert isData; |
493 |
|
Object x = item; |
494 |
|
if (x != null && x != this && casItem(x, null)) { |
495 |
|
LockSupport.unpark(waiter); |
513 |
|
/** head of the queue; null until first enqueue */ |
514 |
|
transient volatile Node head; |
515 |
|
|
462 |
– |
/** predecessor of dangling unspliceable node */ |
463 |
– |
private transient volatile Node cleanMe; // decl here reduces contention |
464 |
– |
|
516 |
|
/** tail of the queue; null until first append */ |
517 |
|
private transient volatile Node tail; |
518 |
|
|
519 |
+ |
/** The number of apparent failures to unsplice removed nodes */ |
520 |
+ |
private transient volatile int sweepVotes; |
521 |
+ |
|
522 |
|
// CAS methods for fields |
523 |
|
private boolean casTail(Node cmp, Node val) { |
524 |
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
528 |
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
529 |
|
} |
530 |
|
|
531 |
< |
private boolean casCleanMe(Node cmp, Node val) { |
532 |
< |
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
531 |
> |
private boolean casSweepVotes(int cmp, int val) { |
532 |
> |
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
533 |
|
} |
534 |
|
|
535 |
|
/* |
542 |
|
|
543 |
|
@SuppressWarnings("unchecked") |
544 |
|
static <E> E cast(Object item) { |
545 |
< |
assert item == null || item.getClass() != Node.class; |
545 |
> |
// assert item == null || item.getClass() != Node.class; |
546 |
|
return (E) item; |
547 |
|
} |
548 |
|
|
571 |
|
break; |
572 |
|
if (p.casItem(item, e)) { // match |
573 |
|
for (Node q = p; q != h;) { |
574 |
< |
Node n = q.next; // update head by 2 |
575 |
< |
if (n != null) // unless singleton |
522 |
< |
q = n; |
523 |
< |
if (head == h && casHead(h, q)) { |
574 |
> |
Node n = q.next; // update by 2 unless singleton |
575 |
> |
if (head == h && casHead(h, n == null? q : n)) { |
576 |
|
h.forgetNext(); |
577 |
|
break; |
578 |
|
} // advance and retry |
657 |
|
for (;;) { |
658 |
|
Object item = s.item; |
659 |
|
if (item != e) { // matched |
660 |
< |
assert item != s; |
660 |
> |
// assert item != s; |
661 |
|
s.forgetContents(); // avoid garbage |
662 |
|
return this.<E>cast(item); |
663 |
|
} |
664 |
|
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
665 |
< |
s.casItem(e, s)) { // cancel |
665 |
> |
s.casItem(e, s)) { // cancel |
666 |
|
unsplice(pred, s); |
667 |
|
return e; |
668 |
|
} |
672 |
|
randomYields = ThreadLocalRandom.current(); |
673 |
|
} |
674 |
|
else if (spins > 0) { // spin |
675 |
< |
if (--spins == 0) |
676 |
< |
shortenHeadPath(); // reduce slack before blocking |
625 |
< |
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
675 |
> |
--spins; |
676 |
> |
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
677 |
|
Thread.yield(); // occasionally yield |
678 |
|
} |
679 |
|
else if (s.waiter == null) { |
687 |
|
} |
688 |
|
else { |
689 |
|
LockSupport.park(this); |
639 |
– |
s.waiter = null; |
640 |
– |
spins = -1; // spin if front upon wakeup |
690 |
|
} |
691 |
|
} |
692 |
|
} |
707 |
|
return 0; |
708 |
|
} |
709 |
|
|
661 |
– |
/** |
662 |
– |
* Tries (once) to unsplice nodes between head and first unmatched |
663 |
– |
* or trailing node; failing on contention. |
664 |
– |
*/ |
665 |
– |
private void shortenHeadPath() { |
666 |
– |
Node h, hn, p, q; |
667 |
– |
if ((p = h = head) != null && h.isMatched() && |
668 |
– |
(q = hn = h.next) != null) { |
669 |
– |
Node n; |
670 |
– |
while ((n = q.next) != q) { |
671 |
– |
if (n == null || !q.isMatched()) { |
672 |
– |
if (hn != q && h.next == hn) |
673 |
– |
h.casNext(hn, q); |
674 |
– |
break; |
675 |
– |
} |
676 |
– |
p = q; |
677 |
– |
q = n; |
678 |
– |
} |
679 |
– |
} |
680 |
– |
} |
681 |
– |
|
710 |
|
/* -------------- Traversal methods -------------- */ |
711 |
|
|
712 |
|
/** |
819 |
|
public final void remove() { |
820 |
|
Node p = lastRet; |
821 |
|
if (p == null) throw new IllegalStateException(); |
822 |
< |
findAndRemoveDataNode(lastPred, p); |
822 |
> |
if (p.tryMatchData()) |
823 |
> |
unsplice(lastPred, p); |
824 |
|
} |
825 |
|
} |
826 |
|
|
830 |
|
* Unsplices (now or later) the given deleted/cancelled node with |
831 |
|
* the given predecessor. |
832 |
|
* |
833 |
< |
* @param pred predecessor of node to be unspliced |
833 |
> |
* @param pred a node that was at one time known to be the |
834 |
> |
* predecessor of s, or null or s itself if s is/was at head |
835 |
|
* @param s the node to be unspliced |
836 |
|
*/ |
837 |
< |
private void unsplice(Node pred, Node s) { |
838 |
< |
s.forgetContents(); // clear unneeded fields |
837 |
> |
final void unsplice(Node pred, Node s) { |
838 |
> |
s.forgetContents(); // forget unneeded fields |
839 |
|
/* |
840 |
< |
* At any given time, exactly one node on list cannot be |
841 |
< |
* unlinked -- the last inserted node. To accommodate this, if |
842 |
< |
* we cannot unlink s, we save its predecessor as "cleanMe", |
843 |
< |
* processing the previously saved version first. Because only |
844 |
< |
* one node in the list can have a null next, at least one of |
815 |
< |
* node s or the node previously saved can always be |
816 |
< |
* processed, so this always terminates. |
840 |
> |
* See above for rationale. Briefly: if pred still points to |
841 |
> |
* s, try to unlink s. If s cannot be unlinked, because it is |
842 |
> |
* trailing node or pred might be unlinked, and neither pred |
843 |
> |
* nor s are head or offlist, add to sweepVotes, and if enough |
844 |
> |
* votes have accumulated, sweep. |
845 |
|
*/ |
846 |
< |
if (pred != null && pred != s) { |
847 |
< |
while (pred.next == s) { |
848 |
< |
Node oldpred = (cleanMe == null) ? null : reclean(); |
849 |
< |
Node n = s.next; |
850 |
< |
if (n != null) { |
851 |
< |
if (n != s) |
852 |
< |
pred.casNext(s, n); |
853 |
< |
break; |
846 |
> |
if (pred != null && pred != s && pred.next == s) { |
847 |
> |
Node n = s.next; |
848 |
> |
if (n == null || |
849 |
> |
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
850 |
> |
for (;;) { // check if at, or could be, head |
851 |
> |
Node h = head; |
852 |
> |
if (h == pred || h == s || h == null) |
853 |
> |
return; // at head or list empty |
854 |
> |
if (!h.isMatched()) |
855 |
> |
break; |
856 |
> |
Node hn = h.next; |
857 |
> |
if (hn == null) |
858 |
> |
return; // now empty |
859 |
> |
if (hn != h && casHead(h, hn)) |
860 |
> |
h.forgetNext(); // advance head |
861 |
|
} |
862 |
< |
if (oldpred == pred || // Already saved |
863 |
< |
((oldpred == null || oldpred.next == s) && |
864 |
< |
casCleanMe(oldpred, pred))) { |
865 |
< |
break; |
862 |
> |
if (pred.next != pred && s.next != s) { // recheck if offlist |
863 |
> |
for (;;) { // sweep now if enough votes |
864 |
> |
int v = sweepVotes; |
865 |
> |
if (v < SWEEP_THRESHOLD) { |
866 |
> |
if (casSweepVotes(v, v + 1)) |
867 |
> |
break; |
868 |
> |
} |
869 |
> |
else if (casSweepVotes(v, 0)) { |
870 |
> |
sweep(); |
871 |
> |
break; |
872 |
> |
} |
873 |
> |
} |
874 |
|
} |
875 |
|
} |
876 |
|
} |
877 |
|
} |
878 |
|
|
879 |
|
/** |
880 |
< |
* Tries to unsplice the deleted/cancelled node held in cleanMe |
881 |
< |
* that was previously uncleanable because it was at tail. |
839 |
< |
* |
840 |
< |
* @return current cleanMe node (or null) |
880 |
> |
* Unlinks matched (typically cancelled) nodes encountered in a |
881 |
> |
* traversal from head. |
882 |
|
*/ |
883 |
< |
private Node reclean() { |
884 |
< |
/* |
885 |
< |
* cleanMe is, or at one time was, predecessor of a cancelled |
886 |
< |
* node s that was the tail so could not be unspliced. If it |
887 |
< |
* is no longer the tail, try to unsplice if necessary and |
888 |
< |
* make cleanMe slot available. This differs from similar |
889 |
< |
* code in unsplice() because we must check that pred still |
849 |
< |
* points to a matched node that can be unspliced -- if not, |
850 |
< |
* we can (must) clear cleanMe without unsplicing. This can |
851 |
< |
* loop only due to contention. |
852 |
< |
*/ |
853 |
< |
Node pred; |
854 |
< |
while ((pred = cleanMe) != null) { |
855 |
< |
Node s = pred.next; |
856 |
< |
Node n; |
857 |
< |
if (s == null || s == pred || !s.isMatched()) |
858 |
< |
casCleanMe(pred, null); // already gone |
859 |
< |
else if ((n = s.next) != null) { |
860 |
< |
if (n != s) |
861 |
< |
pred.casNext(s, n); |
862 |
< |
casCleanMe(pred, null); |
863 |
< |
} |
864 |
< |
else |
883 |
> |
private void sweep() { |
884 |
> |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
885 |
> |
if (p == s) // stale |
886 |
> |
p = head; |
887 |
> |
else if (!s.isMatched()) |
888 |
> |
p = s; |
889 |
> |
else if ((n = s.next) == null || s == n) // trailing node is pinned |
890 |
|
break; |
891 |
< |
} |
892 |
< |
return pred; |
868 |
< |
} |
869 |
< |
|
870 |
< |
/** |
871 |
< |
* Main implementation of Iterator.remove(). Find |
872 |
< |
* and unsplice the given data node. |
873 |
< |
* @param possiblePred possible predecessor of s |
874 |
< |
* @param s the node to remove |
875 |
< |
*/ |
876 |
< |
final void findAndRemoveDataNode(Node possiblePred, Node s) { |
877 |
< |
assert s.isData; |
878 |
< |
if (s.tryMatchData()) { |
879 |
< |
if (possiblePred != null && possiblePred.next == s) |
880 |
< |
unsplice(possiblePred, s); // was actual predecessor |
881 |
< |
else { |
882 |
< |
for (Node pred = null, p = head; p != null; ) { |
883 |
< |
if (p == s) { |
884 |
< |
unsplice(pred, p); |
885 |
< |
break; |
886 |
< |
} |
887 |
< |
if (p.isUnmatchedRequest()) |
888 |
< |
break; |
889 |
< |
pred = p; |
890 |
< |
if ((p = p.next) == pred) { // stale |
891 |
< |
pred = null; |
892 |
< |
p = head; |
893 |
< |
} |
894 |
< |
} |
895 |
< |
} |
891 |
> |
else |
892 |
> |
p.casNext(s, n); |
893 |
|
} |
894 |
|
} |
895 |
|
|
1127 |
|
* @return {@code true} if this queue contains no elements |
1128 |
|
*/ |
1129 |
|
public boolean isEmpty() { |
1130 |
< |
return firstOfMode(true) == null; |
1130 |
> |
for (Node p = head; p != null; p = succ(p)) { |
1131 |
> |
if (!p.isMatched()) |
1132 |
> |
return !p.isData; |
1133 |
> |
} |
1134 |
> |
return true; |
1135 |
|
} |
1136 |
|
|
1137 |
|
public boolean hasWaitingConsumer() { |
1225 |
|
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1226 |
|
private static final long tailOffset = |
1227 |
|
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1228 |
< |
private static final long cleanMeOffset = |
1229 |
< |
objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); |
1228 |
> |
private static final long sweepVotesOffset = |
1229 |
> |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1230 |
|
|
1231 |
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1232 |
|
String field, Class<?> klazz) { |