206 |
|
* additional GC bookkeeping ("write barriers") that are sometimes |
207 |
|
* more costly than the writes themselves because of contention). |
208 |
|
* |
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 |
– |
* |
209 |
|
* *** Overview of implementation *** |
210 |
|
* |
211 |
|
* We use a threshold-based approach to updates, with a slack |
221 |
|
* per-thread one available, but even ThreadLocalRandom is too |
222 |
|
* heavy for these purposes. |
223 |
|
* |
224 |
< |
* With such a small slack threshold value, it is rarely |
225 |
< |
* worthwhile to augment this with path short-circuiting; i.e., |
226 |
< |
* unsplicing nodes between head and the first unmatched node, or |
227 |
< |
* 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). |
224 |
> |
* With such a small slack threshold value, it is not worthwhile |
225 |
> |
* to augment this with path short-circuiting (i.e., unsplicing |
226 |
> |
* interior nodes) except in the case of cancellation/removal (see |
227 |
> |
* below). |
228 |
|
* |
229 |
|
* We allow both the head and tail fields to be null before any |
230 |
|
* nodes are enqueued; initializing upon first append. This |
306 |
|
* versa) compared to their predecessors receive additional |
307 |
|
* chained spins, reflecting longer paths typically required to |
308 |
|
* unblock threads during phase changes. |
309 |
+ |
* |
310 |
+ |
* |
311 |
+ |
* ** Unlinking removed interior nodes ** |
312 |
+ |
* |
313 |
+ |
* In addition to minimizing garbage retention via self-linking |
314 |
+ |
* described above, we also unlink removed interior nodes. These |
315 |
+ |
* may arise due to timed out or interrupted waits, or calls to |
316 |
+ |
* remove(x) or Iterator.remove. Normally, given a node that was |
317 |
+ |
* at one time known to be the predecessor of some node s that is |
318 |
+ |
* to be removed, we can unsplice s by CASing the next field of |
319 |
+ |
* its predecessor if it still points to s (otherwise s must |
320 |
+ |
* already have been removed or is now offlist). But there are two |
321 |
+ |
* situations in which we cannot guarantee to make node s |
322 |
+ |
* unreachable in this way: (1) If s is the trailing node of list |
323 |
+ |
* (i.e., with null next), then it is pinned as the target node |
324 |
+ |
* for appends, so can only be removed later when other nodes are |
325 |
+ |
* appended. (2) We cannot necessarily unlink s given a |
326 |
+ |
* predecessor node that is matched (including the case of being |
327 |
+ |
* cancelled): the predecessor may already be already unspliced, |
328 |
+ |
* in which case some previous reachable node may still point to |
329 |
+ |
* s. (For further explanation see Herlihy & Shavit "The Art of |
330 |
+ |
* Multiprocessor Programming" chapter 9). Although, in both |
331 |
+ |
* cases, we can rule out the need for further action if either s |
332 |
+ |
* or its predecessor are (or can be made to be) at, or fall off |
333 |
+ |
* from, the head of list. |
334 |
+ |
* |
335 |
+ |
* Without taking these into account, it would be possible for an |
336 |
+ |
* unbounded number of supposedly removed nodes to remain |
337 |
+ |
* reachable. Situations leading to such buildup are uncommon but |
338 |
+ |
* can occur in practice; for example when a series of short timed |
339 |
+ |
* calls to poll repeatedly time out but never otherwise fall off |
340 |
+ |
* the list because of an untimed call to take at the front of the |
341 |
+ |
* queue. |
342 |
+ |
* |
343 |
+ |
* When these cases arise, rather than always retraversing the |
344 |
+ |
* entire list to find an actual predecessor to unlink (which |
345 |
+ |
* won't help for case (1) anyway), we record a conservative |
346 |
+ |
* estimate of possible unsplice failures (in "sweepVotes). We |
347 |
+ |
* trigger a full sweep when the estimate exceeds a threshold |
348 |
+ |
* indicating the maximum number of estimated removal failures to |
349 |
+ |
* tolerate before sweeping through, unlinking cancelled nodes |
350 |
+ |
* that were not unlinked upon initial removal. We perform sweeps |
351 |
+ |
* by the thread hitting threshold (rather than background threads |
352 |
+ |
* or by spreading work to other threads) because in the main |
353 |
+ |
* contexts in which removal occurs, the caller is already |
354 |
+ |
* timed-out, cancelled, or performing a potentially O(n) |
355 |
+ |
* operation (i.e., remove(x)), none of which are time-critical |
356 |
+ |
* enough to warrant the overhead that alternatives would impose |
357 |
+ |
* on other threads. |
358 |
+ |
* |
359 |
+ |
* Because the sweepVotes estimate is conservative, and because |
360 |
+ |
* nodes become unlinked "naturally" as they fall off the head of |
361 |
+ |
* the queue, and because we allow votes to accumulate even while |
362 |
+ |
* sweeps are in progress, there are typically signficantly fewer |
363 |
+ |
* such nodes than estimated. Choice of a threshold value |
364 |
+ |
* balances the likelihood of wasted effort and contention, versus |
365 |
+ |
* providing a worst-case bound on retention of interior nodes in |
366 |
+ |
* quiescent queues. The value defined below was chosen |
367 |
+ |
* empirically to balance these under various timeout scenarios. |
368 |
+ |
* |
369 |
+ |
* Note that we cannot self-link unlinked interior nodes during |
370 |
+ |
* sweeps. However, the associated garbage chains terminate when |
371 |
+ |
* some successor ultimately falls off the head of the list and is |
372 |
+ |
* self-linked. |
373 |
|
*/ |
374 |
|
|
375 |
|
/** True if on multiprocessor */ |
396 |
|
private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
397 |
|
|
398 |
|
/** |
399 |
+ |
* The maximum number of estimated removal failures (sweepVotes) |
400 |
+ |
* to tolerate before sweeping through the queue unlinking |
401 |
+ |
* cancelled nodes that were not unlinked upon initial |
402 |
+ |
* removal. See above for explanation. The value must be at least |
403 |
+ |
* two to avoid useless sweeps when removing trailing nodes. |
404 |
+ |
*/ |
405 |
+ |
static final int SWEEP_THRESHOLD = 32; |
406 |
+ |
|
407 |
+ |
/** |
408 |
|
* Queue nodes. Uses Object, not E, for items to allow forgetting |
409 |
|
* them after use. Relies heavily on Unsafe mechanics to minimize |
410 |
< |
* unnecessary ordering constraints: Writes that intrinsically |
411 |
< |
* precede or follow CASes use simple relaxed forms. Other |
362 |
< |
* cleanups use releasing/lazy writes. |
410 |
> |
* unnecessary ordering constraints: Writes that are intrinsically |
411 |
> |
* ordered wrt other accesses or CASes use simple relaxed forms. |
412 |
|
*/ |
413 |
|
static final class Node { |
414 |
|
final boolean isData; // false if this is a request node |
444 |
|
} |
445 |
|
|
446 |
|
/** |
447 |
< |
* Sets item to self (using a releasing/lazy write) and waiter |
448 |
< |
* to null, to avoid garbage retention after extracting or |
449 |
< |
* cancelling. |
447 |
> |
* Sets item to self and waiter to null, to avoid garbage |
448 |
> |
* retention after matching or cancelling. Uses relaxed writes |
449 |
> |
* bacause order is already constrained in the only calling |
450 |
> |
* contexts: item is forgotten only after volatile/atomic |
451 |
> |
* mechanics that extract items. Similarly, clearing waiter |
452 |
> |
* follows either CAS or return from park (if ever parked; |
453 |
> |
* else we don't care). |
454 |
|
*/ |
455 |
|
final void forgetContents() { |
456 |
< |
UNSAFE.putOrderedObject(this, itemOffset, this); |
457 |
< |
UNSAFE.putOrderedObject(this, waiterOffset, null); |
456 |
> |
UNSAFE.putObject(this, itemOffset, this); |
457 |
> |
UNSAFE.putObject(this, waiterOffset, null); |
458 |
|
} |
459 |
|
|
460 |
|
/** |
512 |
|
/** head of the queue; null until first enqueue */ |
513 |
|
transient volatile Node head; |
514 |
|
|
462 |
– |
/** predecessor of dangling unspliceable node */ |
463 |
– |
private transient volatile Node cleanMe; // decl here reduces contention |
464 |
– |
|
515 |
|
/** tail of the queue; null until first append */ |
516 |
|
private transient volatile Node tail; |
517 |
|
|
518 |
+ |
/** The number of apparent failures to unsplice removed nodes */ |
519 |
+ |
private transient volatile int sweepVotes; |
520 |
+ |
|
521 |
|
// CAS methods for fields |
522 |
|
private boolean casTail(Node cmp, Node val) { |
523 |
|
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
527 |
|
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
528 |
|
} |
529 |
|
|
530 |
< |
private boolean casCleanMe(Node cmp, Node val) { |
531 |
< |
return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); |
530 |
> |
private boolean casSweepVotes(int cmp, int val) { |
531 |
> |
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
532 |
|
} |
533 |
|
|
534 |
|
/* |
570 |
|
break; |
571 |
|
if (p.casItem(item, e)) { // match |
572 |
|
for (Node q = p; q != h;) { |
573 |
< |
Node n = q.next; // update head by 2 |
574 |
< |
if (n != null) // unless singleton |
522 |
< |
q = n; |
523 |
< |
if (head == h && casHead(h, q)) { |
573 |
> |
Node n = q.next; // update by 2 unless singleton |
574 |
> |
if (head == h && casHead(h, n == null? q : n)) { |
575 |
|
h.forgetNext(); |
576 |
|
break; |
577 |
|
} // advance and retry |
661 |
|
return this.<E>cast(item); |
662 |
|
} |
663 |
|
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
664 |
< |
s.casItem(e, s)) { // cancel |
664 |
> |
s.casItem(e, s)) { // cancel |
665 |
|
unsplice(pred, s); |
666 |
|
return e; |
667 |
|
} |
671 |
|
randomYields = ThreadLocalRandom.current(); |
672 |
|
} |
673 |
|
else if (spins > 0) { // spin |
674 |
< |
if (--spins == 0) |
675 |
< |
shortenHeadPath(); // reduce slack before blocking |
625 |
< |
else if (randomYields.nextInt(CHAINED_SPINS) == 0) |
674 |
> |
--spins; |
675 |
> |
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
676 |
|
Thread.yield(); // occasionally yield |
677 |
|
} |
678 |
|
else if (s.waiter == null) { |
686 |
|
} |
687 |
|
else { |
688 |
|
LockSupport.park(this); |
639 |
– |
s.waiter = null; |
640 |
– |
spins = -1; // spin if front upon wakeup |
689 |
|
} |
690 |
|
} |
691 |
|
} |
706 |
|
return 0; |
707 |
|
} |
708 |
|
|
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 |
– |
|
709 |
|
/* -------------- Traversal methods -------------- */ |
710 |
|
|
711 |
|
/** |
818 |
|
public final void remove() { |
819 |
|
Node p = lastRet; |
820 |
|
if (p == null) throw new IllegalStateException(); |
821 |
< |
findAndRemoveDataNode(lastPred, p); |
821 |
> |
if (p.tryMatchData()) |
822 |
> |
unsplice(lastPred, p); |
823 |
|
} |
824 |
|
} |
825 |
|
|
829 |
|
* Unsplices (now or later) the given deleted/cancelled node with |
830 |
|
* the given predecessor. |
831 |
|
* |
832 |
< |
* @param pred predecessor of node to be unspliced |
832 |
> |
* @param pred a node that was at one time known to be the |
833 |
> |
* predecessor of s, or null or s itself if s is/was at head |
834 |
|
* @param s the node to be unspliced |
835 |
|
*/ |
836 |
< |
private void unsplice(Node pred, Node s) { |
837 |
< |
s.forgetContents(); // clear unneeded fields |
836 |
> |
final void unsplice(Node pred, Node s) { |
837 |
> |
s.forgetContents(); // forget unneeded fields |
838 |
|
/* |
839 |
< |
* At any given time, exactly one node on list cannot be |
840 |
< |
* unlinked -- the last inserted node. To accommodate this, if |
841 |
< |
* we cannot unlink s, we save its predecessor as "cleanMe", |
842 |
< |
* processing the previously saved version first. Because only |
843 |
< |
* 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. |
839 |
> |
* See above for rationale. Briefly: if pred still points to |
840 |
> |
* s, try to unlink s. If s cannot be unlinked, because it is |
841 |
> |
* trailing node or pred might be unlinked, and neither pred |
842 |
> |
* nor s are head or offlist, add to sweepVotes, and if enough |
843 |
> |
* votes have accumulated, sweep. |
844 |
|
*/ |
845 |
< |
if (pred != null && pred != s) { |
846 |
< |
while (pred.next == s) { |
847 |
< |
Node oldpred = (cleanMe == null) ? null : reclean(); |
848 |
< |
Node n = s.next; |
849 |
< |
if (n != null) { |
850 |
< |
if (n != s) |
851 |
< |
pred.casNext(s, n); |
852 |
< |
break; |
845 |
> |
if (pred != null && pred != s && pred.next == s) { |
846 |
> |
Node n = s.next; |
847 |
> |
if (n == null || |
848 |
> |
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
849 |
> |
for (;;) { // check if at, or could be, head |
850 |
> |
Node h = head; |
851 |
> |
if (h == pred || h == s || h == null) |
852 |
> |
return; // at head or list empty |
853 |
> |
if (!h.isMatched()) |
854 |
> |
break; |
855 |
> |
Node hn = h.next; |
856 |
> |
if (hn == null) |
857 |
> |
return; // now empty |
858 |
> |
if (hn != h && casHead(h, hn)) |
859 |
> |
h.forgetNext(); // advance head |
860 |
|
} |
861 |
< |
if (oldpred == pred || // Already saved |
862 |
< |
((oldpred == null || oldpred.next == s) && |
863 |
< |
casCleanMe(oldpred, pred))) { |
864 |
< |
break; |
861 |
> |
if (pred.next != pred && s.next != s) { // recheck if offlist |
862 |
> |
for (;;) { // sweep now if enough votes |
863 |
> |
int v = sweepVotes; |
864 |
> |
if (v < SWEEP_THRESHOLD) { |
865 |
> |
if (casSweepVotes(v, v + 1)) |
866 |
> |
break; |
867 |
> |
} |
868 |
> |
else if (casSweepVotes(v, 0)) { |
869 |
> |
sweep(); |
870 |
> |
break; |
871 |
> |
} |
872 |
> |
} |
873 |
|
} |
874 |
|
} |
875 |
|
} |
876 |
|
} |
877 |
|
|
878 |
|
/** |
879 |
< |
* Tries to unsplice the deleted/cancelled node held in cleanMe |
838 |
< |
* that was previously uncleanable because it was at tail. |
839 |
< |
* |
840 |
< |
* @return current cleanMe node (or null) |
879 |
> |
* Unlink matched nodes encountered in a traversal from head |
880 |
|
*/ |
881 |
< |
private Node reclean() { |
882 |
< |
/* |
883 |
< |
* cleanMe is, or at one time was, predecessor of a cancelled |
884 |
< |
* node s that was the tail so could not be unspliced. If it |
885 |
< |
* is no longer the tail, try to unsplice if necessary and |
886 |
< |
* make cleanMe slot available. This differs from similar |
887 |
< |
* 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 |
< |
} |
881 |
> |
private void sweep() { |
882 |
> |
Node p = head, s, n; |
883 |
> |
while (p != null && (s = p.next) != null && (n = s.next) != null) { |
884 |
> |
if (p == s || s == n) |
885 |
> |
p = head; // stale |
886 |
> |
else if (s.isMatched()) |
887 |
> |
p.casNext(s, n); |
888 |
|
else |
889 |
< |
break; |
866 |
< |
} |
867 |
< |
return pred; |
868 |
< |
} |
869 |
< |
|
870 |
< |
/** |
871 |
< |
* Main implementation of Iterator.remove(). Finds |
872 |
< |
* and unsplices the given data node. |
873 |
< |
* |
874 |
< |
* @param possiblePred possible predecessor of s |
875 |
< |
* @param s the node to remove |
876 |
< |
*/ |
877 |
< |
final void findAndRemoveDataNode(Node possiblePred, Node s) { |
878 |
< |
assert s.isData; |
879 |
< |
if (s.tryMatchData()) { |
880 |
< |
if (possiblePred != null && possiblePred.next == s) |
881 |
< |
unsplice(possiblePred, s); // was actual predecessor |
882 |
< |
else { |
883 |
< |
for (Node pred = null, p = head; p != null; ) { |
884 |
< |
if (p == s) { |
885 |
< |
unsplice(pred, p); |
886 |
< |
break; |
887 |
< |
} |
888 |
< |
if (p.isUnmatchedRequest()) |
889 |
< |
break; |
890 |
< |
pred = p; |
891 |
< |
if ((p = p.next) == pred) { // stale |
892 |
< |
pred = null; |
893 |
< |
p = head; |
894 |
< |
} |
895 |
< |
} |
896 |
< |
} |
889 |
> |
p = s; |
890 |
|
} |
891 |
|
} |
892 |
|
|
1218 |
|
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1219 |
|
private static final long tailOffset = |
1220 |
|
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1221 |
< |
private static final long cleanMeOffset = |
1222 |
< |
objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); |
1221 |
> |
private static final long sweepVotesOffset = |
1222 |
> |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1223 |
|
|
1224 |
|
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1225 |
|
String field, Class<?> klazz) { |