--- jsr166/src/jsr166y/LinkedTransferQueue.java 2009/11/02 18:38:37 1.66 +++ jsr166/src/jsr166y/LinkedTransferQueue.java 2011/05/25 16:08:03 1.84 @@ -1,20 +1,19 @@ /* * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at - * http://creativecommons.org/licenses/publicdomain + * http://creativecommons.org/publicdomain/zero/1.0/ */ package jsr166y; -import java.util.concurrent.*; - import java.util.AbstractQueue; import java.util.Collection; -import java.util.ConcurrentModificationException; import java.util.Iterator; import java.util.NoSuchElementException; import java.util.Queue; +import java.util.concurrent.TimeUnit; import java.util.concurrent.locks.LockSupport; + /** * An unbounded {@link TransferQueue} based on linked nodes. * This queue orders elements FIFO (first-in-first-out) with respect @@ -23,10 +22,17 @@ import java.util.concurrent.locks.LockSu * producer. The tail of the queue is that element that has * been on the queue the shortest time for some producer. * - *

Beware that, unlike in most collections, the {@code size} - * method is NOT a constant-time operation. Because of the + *

Beware that, unlike in most collections, the {@code size} method + * is NOT a constant-time operation. Because of the * asynchronous nature of these queues, determining the current number - * of elements requires a traversal of the elements. + * of elements requires a traversal of the elements, and so may report + * inaccurate results if this collection is modified during traversal. + * Additionally, the bulk operations {@code addAll}, + * {@code removeAll}, {@code retainAll}, {@code containsAll}, + * {@code equals}, and {@code toArray} are not guaranteed + * to be performed atomically. For example, an iterator operating + * concurrently with an {@code addAll} operation might view only some + * of the added elements. * *

This class and its iterator implement all of the * optional methods of the {@link Collection} and {@link @@ -206,24 +212,6 @@ public class LinkedTransferQueue exte * additional GC bookkeeping ("write barriers") that are sometimes * more costly than the writes themselves because of contention). * - * Removal of interior nodes (due to timed out or interrupted - * waits, or calls to remove(x) or Iterator.remove) can use a - * scheme roughly similar to that described in Scherer, Lea, and - * Scott's SynchronousQueue. Given a predecessor, we can unsplice - * any node except the (actual) tail of the queue. To avoid - * build-up of cancelled trailing nodes, upon a request to remove - * a trailing node, it is placed in field "cleanMe" to be - * unspliced upon the next call to unsplice any other node. - * Situations needing such mechanics are not common but do occur - * in practice; for example when an unbounded series of short - * timed calls to poll repeatedly time out but never otherwise - * fall off the list because of an untimed call to take at the - * front of the queue. Note that maintaining field cleanMe does - * not otherwise much impact garbage retention even if never - * cleared by some other call because the held node will - * eventually either directly or indirectly lead to a self-link - * once off the list. - * * *** Overview of implementation *** * * We use a threshold-based approach to updates, with a slack @@ -239,15 +227,10 @@ public class LinkedTransferQueue exte * per-thread one available, but even ThreadLocalRandom is too * heavy for these purposes. * - * With such a small slack threshold value, it is rarely - * worthwhile to augment this with path short-circuiting; i.e., - * unsplicing nodes between head and the first unmatched node, or - * similarly for tail, rather than advancing head or tail - * proper. However, it is used (in awaitMatch) immediately before - * a waiting thread starts to block, as a final bit of helping at - * a point when contention with others is extremely unlikely - * (since if other threads that could release it are operating, - * then the current thread wouldn't be blocking). + * With such a small slack threshold value, it is not worthwhile + * to augment this with path short-circuiting (i.e., unsplicing + * interior nodes) except in the case of cancellation/removal (see + * below). * * We allow both the head and tail fields to be null before any * nodes are enqueued; initializing upon first append. This @@ -329,6 +312,70 @@ public class LinkedTransferQueue exte * versa) compared to their predecessors receive additional * chained spins, reflecting longer paths typically required to * unblock threads during phase changes. + * + * + * ** Unlinking removed interior nodes ** + * + * In addition to minimizing garbage retention via self-linking + * described above, we also unlink removed interior nodes. These + * may arise due to timed out or interrupted waits, or calls to + * remove(x) or Iterator.remove. Normally, given a node that was + * at one time known to be the predecessor of some node s that is + * to be removed, we can unsplice s by CASing the next field of + * its predecessor if it still points to s (otherwise s must + * already have been removed or is now offlist). But there are two + * situations in which we cannot guarantee to make node s + * unreachable in this way: (1) If s is the trailing node of list + * (i.e., with null next), then it is pinned as the target node + * for appends, so can only be removed later after other nodes are + * appended. (2) We cannot necessarily unlink s given a + * predecessor node that is matched (including the case of being + * cancelled): the predecessor may already be unspliced, in which + * case some previous reachable node may still point to s. + * (For further explanation see Herlihy & Shavit "The Art of + * Multiprocessor Programming" chapter 9). Although, in both + * cases, we can rule out the need for further action if either s + * or its predecessor are (or can be made to be) at, or fall off + * from, the head of list. + * + * Without taking these into account, it would be possible for an + * unbounded number of supposedly removed nodes to remain + * reachable. Situations leading to such buildup are uncommon but + * can occur in practice; for example when a series of short timed + * calls to poll repeatedly time out but never otherwise fall off + * the list because of an untimed call to take at the front of the + * queue. + * + * When these cases arise, rather than always retraversing the + * entire list to find an actual predecessor to unlink (which + * won't help for case (1) anyway), we record a conservative + * estimate of possible unsplice failures (in "sweepVotes"). + * We trigger a full sweep when the estimate exceeds a threshold + * ("SWEEP_THRESHOLD") indicating the maximum number of estimated + * removal failures to tolerate before sweeping through, unlinking + * cancelled nodes that were not unlinked upon initial removal. + * We perform sweeps by the thread hitting threshold (rather than + * background threads or by spreading work to other threads) + * because in the main contexts in which removal occurs, the + * caller is already timed-out, cancelled, or performing a + * potentially O(n) operation (e.g. remove(x)), none of which are + * time-critical enough to warrant the overhead that alternatives + * would impose on other threads. + * + * Because the sweepVotes estimate is conservative, and because + * nodes become unlinked "naturally" as they fall off the head of + * the queue, and because we allow votes to accumulate even while + * sweeps are in progress, there are typically significantly fewer + * such nodes than estimated. Choice of a threshold value + * balances the likelihood of wasted effort and contention, versus + * providing a worst-case bound on retention of interior nodes in + * quiescent queues. The value defined below was chosen + * empirically to balance these under various timeout scenarios. + * + * Note that we cannot self-link unlinked interior nodes during + * sweeps. However, the associated garbage chains terminate when + * some successor ultimately falls off the head of the list and is + * self-linked. */ /** True if on multiprocessor */ @@ -355,11 +402,19 @@ public class LinkedTransferQueue exte private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; /** + * The maximum number of estimated removal failures (sweepVotes) + * to tolerate before sweeping through the queue unlinking + * cancelled nodes that were not unlinked upon initial + * removal. See above for explanation. The value must be at least + * two to avoid useless sweeps when removing trailing nodes. + */ + static final int SWEEP_THRESHOLD = 32; + + /** * Queue nodes. Uses Object, not E, for items to allow forgetting * them after use. Relies heavily on Unsafe mechanics to minimize - * unnecessary ordering constraints: Writes that intrinsically - * precede or follow CASes use simple relaxed forms. Other - * cleanups use releasing/lazy writes. + * unnecessary ordering constraints: Writes that are intrinsically + * ordered wrt other accesses or CASes use simple relaxed forms. */ static final class Node { final boolean isData; // false if this is a request node @@ -373,13 +428,13 @@ public class LinkedTransferQueue exte } final boolean casItem(Object cmp, Object val) { - assert cmp == null || cmp.getClass() != Node.class; + // assert cmp == null || cmp.getClass() != Node.class; return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); } /** - * Creates a new node. Uses relaxed write because item can only - * be seen if followed by CAS. + * Constructs a new node. Uses relaxed write because item can + * only be seen after publication via casNext. */ Node(Object item, boolean isData) { UNSAFE.putObject(this, itemOffset, item); // relaxed write @@ -395,13 +450,17 @@ public class LinkedTransferQueue exte } /** - * Sets item to self (using a releasing/lazy write) and waiter - * to null, to avoid garbage retention after extracting or - * cancelling. + * Sets item to self and waiter to null, to avoid garbage + * retention after matching or cancelling. Uses relaxed writes + * because order is already constrained in the only calling + * contexts: item is forgotten only after volatile/atomic + * mechanics that extract items. Similarly, clearing waiter + * follows either CAS or return from park (if ever parked; + * else we don't care). */ final void forgetContents() { - UNSAFE.putOrderedObject(this, itemOffset, this); - UNSAFE.putOrderedObject(this, waiterOffset, null); + UNSAFE.putObject(this, itemOffset, this); + UNSAFE.putObject(this, waiterOffset, null); } /** @@ -435,7 +494,7 @@ public class LinkedTransferQueue exte * Tries to artificially match a data node -- used by remove. */ final boolean tryMatchData() { - assert isData; + // assert isData; Object x = item; if (x != null && x != this && casItem(x, null)) { LockSupport.unpark(waiter); @@ -444,27 +503,38 @@ public class LinkedTransferQueue exte return false; } - // Unsafe mechanics - private static final sun.misc.Unsafe UNSAFE = getUnsafe(); - private static final long nextOffset = - objectFieldOffset(UNSAFE, "next", Node.class); - private static final long itemOffset = - objectFieldOffset(UNSAFE, "item", Node.class); - private static final long waiterOffset = - objectFieldOffset(UNSAFE, "waiter", Node.class); - private static final long serialVersionUID = -3375979862319811754L; + + // Unsafe mechanics + private static final sun.misc.Unsafe UNSAFE; + private static final long itemOffset; + private static final long nextOffset; + private static final long waiterOffset; + static { + try { + UNSAFE = getUnsafe(); + Class k = Node.class; + itemOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("item")); + nextOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("next")); + waiterOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("waiter")); + } catch (Exception e) { + throw new Error(e); + } + } } /** head of the queue; null until first enqueue */ transient volatile Node head; - /** predecessor of dangling unspliceable node */ - private transient volatile Node cleanMe; // decl here reduces contention - /** tail of the queue; null until first append */ private transient volatile Node tail; + /** The number of apparent failures to unsplice removed nodes */ + private transient volatile int sweepVotes; + // CAS methods for fields private boolean casTail(Node cmp, Node val) { return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); @@ -474,8 +544,8 @@ public class LinkedTransferQueue exte return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); } - private boolean casCleanMe(Node cmp, Node val) { - return UNSAFE.compareAndSwapObject(this, cleanMeOffset, cmp, val); + private boolean casSweepVotes(int cmp, int val) { + return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); } /* @@ -488,7 +558,7 @@ public class LinkedTransferQueue exte @SuppressWarnings("unchecked") static E cast(Object item) { - assert item == null || item.getClass() != Node.class; + // assert item == null || item.getClass() != Node.class; return (E) item; } @@ -507,7 +577,8 @@ public class LinkedTransferQueue exte throw new NullPointerException(); Node s = null; // the node to append, if needed - retry: for (;;) { // restart on append race + retry: + for (;;) { // restart on append race for (Node h = head, p = h; p != null;) { // find & match first node boolean isData = p.isData; @@ -517,10 +588,8 @@ public class LinkedTransferQueue exte break; if (p.casItem(item, e)) { // match for (Node q = p; q != h;) { - Node n = q.next; // update head by 2 - if (n != null) // unless singleton - q = n; - if (head == h && casHead(h, q)) { + Node n = q.next; // update by 2 unless singleton + if (head == h && casHead(h, n == null ? q : n)) { h.forgetNext(); break; } // advance and retry @@ -605,12 +674,12 @@ public class LinkedTransferQueue exte for (;;) { Object item = s.item; if (item != e) { // matched - assert item != s; + // assert item != s; s.forgetContents(); // avoid garbage return this.cast(item); } if ((w.isInterrupted() || (timed && nanos <= 0)) && - s.casItem(e, s)) { // cancel + s.casItem(e, s)) { // cancel unsplice(pred, s); return e; } @@ -620,9 +689,8 @@ public class LinkedTransferQueue exte randomYields = ThreadLocalRandom.current(); } else if (spins > 0) { // spin - if (--spins == 0) - shortenHeadPath(); // reduce slack before blocking - else if (randomYields.nextInt(CHAINED_SPINS) == 0) + --spins; + if (randomYields.nextInt(CHAINED_SPINS) == 0) Thread.yield(); // occasionally yield } else if (s.waiter == null) { @@ -636,8 +704,6 @@ public class LinkedTransferQueue exte } else { LockSupport.park(this); - s.waiter = null; - spins = -1; // spin if front upon wakeup } } } @@ -658,27 +724,6 @@ public class LinkedTransferQueue exte return 0; } - /** - * Tries (once) to unsplice nodes between head and first unmatched - * or trailing node; failing on contention. - */ - private void shortenHeadPath() { - Node h, hn, p, q; - if ((p = h = head) != null && h.isMatched() && - (q = hn = h.next) != null) { - Node n; - while ((n = q.next) != q) { - if (n == null || !q.isMatched()) { - if (hn != q && h.next == hn) - h.casNext(hn, q); - break; - } - p = q; - q = n; - } - } - } - /* -------------- Traversal methods -------------- */ /** @@ -754,22 +799,61 @@ public class LinkedTransferQueue exte * Moves to next node after prev, or first node if prev null. */ private void advance(Node prev) { - lastPred = lastRet; - lastRet = prev; - for (Node p = (prev == null) ? head : succ(prev); - p != null; p = succ(p)) { - Object item = p.item; - if (p.isData) { - if (item != null && item != p) { - nextItem = LinkedTransferQueue.this.cast(item); - nextNode = p; + /* + * To track and avoid buildup of deleted nodes in the face + * of calls to both Queue.remove and Itr.remove, we must + * include variants of unsplice and sweep upon each + * advance: Upon Itr.remove, we may need to catch up links + * from lastPred, and upon other removes, we might need to + * skip ahead from stale nodes and unsplice deleted ones + * found while advancing. + */ + + Node r, b; // reset lastPred upon possible deletion of lastRet + if ((r = lastRet) != null && !r.isMatched()) + lastPred = r; // next lastPred is old lastRet + else if ((b = lastPred) == null || b.isMatched()) + lastPred = null; // at start of list + else { + Node s, n; // help with removal of lastPred.next + while ((s = b.next) != null && + s != b && s.isMatched() && + (n = s.next) != null && n != s) + b.casNext(s, n); + } + + this.lastRet = prev; + + for (Node p = prev, s, n;;) { + s = (p == null) ? head : p.next; + if (s == null) + break; + else if (s == p) { + p = null; + continue; + } + Object item = s.item; + if (s.isData) { + if (item != null && item != s) { + nextItem = LinkedTransferQueue.cast(item); + nextNode = s; return; } } else if (item == null) break; + // assert s.isMatched(); + if (p == null) + p = s; + else if ((n = s.next) == null) + break; + else if (s == n) + p = null; + else + p.casNext(s, n); } nextNode = null; + nextItem = null; } Itr() { @@ -789,9 +873,12 @@ public class LinkedTransferQueue exte } public final void remove() { - Node p = lastRet; - if (p == null) throw new IllegalStateException(); - findAndRemoveDataNode(lastPred, p); + final Node lastRet = this.lastRet; + if (lastRet == null) + throw new IllegalStateException(); + this.lastRet = null; + if (lastRet.tryMatchData()) + unsplice(lastPred, lastRet); } } @@ -801,99 +888,68 @@ public class LinkedTransferQueue exte * Unsplices (now or later) the given deleted/cancelled node with * the given predecessor. * - * @param pred predecessor of node to be unspliced + * @param pred a node that was at one time known to be the + * predecessor of s, or null or s itself if s is/was at head * @param s the node to be unspliced */ - private void unsplice(Node pred, Node s) { - s.forgetContents(); // clear unneeded fields + final void unsplice(Node pred, Node s) { + s.forgetContents(); // forget unneeded fields /* - * At any given time, exactly one node on list cannot be - * unlinked -- the last inserted node. To accommodate this, if - * we cannot unlink s, we save its predecessor as "cleanMe", - * processing the previously saved version first. Because only - * one node in the list can have a null next, at least one of - * node s or the node previously saved can always be - * processed, so this always terminates. + * See above for rationale. Briefly: if pred still points to + * s, try to unlink s. If s cannot be unlinked, because it is + * trailing node or pred might be unlinked, and neither pred + * nor s are head or offlist, add to sweepVotes, and if enough + * votes have accumulated, sweep. */ - if (pred != null && pred != s) { - while (pred.next == s) { - Node oldpred = (cleanMe == null) ? null : reclean(); - Node n = s.next; - if (n != null) { - if (n != s) - pred.casNext(s, n); - break; + if (pred != null && pred != s && pred.next == s) { + Node n = s.next; + if (n == null || + (n != s && pred.casNext(s, n) && pred.isMatched())) { + for (;;) { // check if at, or could be, head + Node h = head; + if (h == pred || h == s || h == null) + return; // at head or list empty + if (!h.isMatched()) + break; + Node hn = h.next; + if (hn == null) + return; // now empty + if (hn != h && casHead(h, hn)) + h.forgetNext(); // advance head } - if (oldpred == pred || // Already saved - ((oldpred == null || oldpred.next == s) && - casCleanMe(oldpred, pred))) { - break; + if (pred.next != pred && s.next != s) { // recheck if offlist + for (;;) { // sweep now if enough votes + int v = sweepVotes; + if (v < SWEEP_THRESHOLD) { + if (casSweepVotes(v, v + 1)) + break; + } + else if (casSweepVotes(v, 0)) { + sweep(); + break; + } + } } } } } /** - * Tries to unsplice the deleted/cancelled node held in cleanMe - * that was previously uncleanable because it was at tail. - * - * @return current cleanMe node (or null) + * Unlinks matched (typically cancelled) nodes encountered in a + * traversal from head. */ - private Node reclean() { - /* - * cleanMe is, or at one time was, predecessor of a cancelled - * node s that was the tail so could not be unspliced. If it - * is no longer the tail, try to unsplice if necessary and - * make cleanMe slot available. This differs from similar - * code in unsplice() because we must check that pred still - * points to a matched node that can be unspliced -- if not, - * we can (must) clear cleanMe without unsplicing. This can - * loop only due to contention. - */ - Node pred; - while ((pred = cleanMe) != null) { - Node s = pred.next; - Node n; - if (s == null || s == pred || !s.isMatched()) - casCleanMe(pred, null); // already gone - else if ((n = s.next) != null) { - if (n != s) - pred.casNext(s, n); - casCleanMe(pred, null); - } - else + private void sweep() { + for (Node p = head, s, n; p != null && (s = p.next) != null; ) { + if (!s.isMatched()) + // Unmatched nodes are never self-linked + p = s; + else if ((n = s.next) == null) // trailing node is pinned break; - } - return pred; - } - - /** - * Main implementation of Iterator.remove(). Finds - * and unsplices the given data node. - * - * @param possiblePred possible predecessor of s - * @param s the node to remove - */ - final void findAndRemoveDataNode(Node possiblePred, Node s) { - assert s.isData; - if (s.tryMatchData()) { - if (possiblePred != null && possiblePred.next == s) - unsplice(possiblePred, s); // was actual predecessor - else { - for (Node pred = null, p = head; p != null; ) { - if (p == s) { - unsplice(pred, p); - break; - } - if (p.isUnmatchedRequest()) - break; - pred = p; - if ((p = p.next) == pred) { // stale - pred = null; - p = head; - } - } - } + else if (s == n) // stale + // No need to also check for p == s, since that implies s == n + p = head; + else + p.casNext(s, n); } } @@ -972,8 +1028,7 @@ public class LinkedTransferQueue exte * Inserts the specified element at the tail of this queue. * As the queue is unbounded, this method will never return {@code false}. * - * @return {@code true} (as specified by - * {@link BlockingQueue#offer(Object) BlockingQueue.offer}) + * @return {@code true} (as specified by {@link Queue#offer}) * @throws NullPointerException if the specified element is null */ public boolean offer(E e) { @@ -1105,15 +1160,15 @@ public class LinkedTransferQueue exte } /** - * Returns an iterator over the elements in this queue in proper - * sequence, from head to tail. + * Returns an iterator over the elements in this queue in proper sequence. + * The elements will be returned in order from first (head) to last (tail). * *

The returned iterator is a "weakly consistent" iterator that - * will never throw - * {@link ConcurrentModificationException ConcurrentModificationException}, - * and guarantees to traverse elements as they existed upon - * construction of the iterator, and may (but is not guaranteed - * to) reflect any modifications subsequent to construction. + * will never throw {@link java.util.ConcurrentModificationException + * ConcurrentModificationException}, and guarantees to traverse + * elements as they existed upon construction of the iterator, and + * may (but is not guaranteed to) reflect any modifications + * subsequent to construction. * * @return an iterator over the elements in this queue in proper sequence */ @@ -1131,7 +1186,11 @@ public class LinkedTransferQueue exte * @return {@code true} if this queue contains no elements */ public boolean isEmpty() { - return firstOfMode(true) == null; + for (Node p = head; p != null; p = succ(p)) { + if (!p.isMatched()) + return !p.isData; + } + return true; } public boolean hasWaitingConsumer() { @@ -1174,6 +1233,28 @@ public class LinkedTransferQueue exte } /** + * Returns {@code true} if this queue contains the specified element. + * More formally, returns {@code true} if and only if this queue contains + * at least one element {@code e} such that {@code o.equals(e)}. + * + * @param o object to be checked for containment in this queue + * @return {@code true} if this queue contains the specified element + */ + public boolean contains(Object o) { + if (o == null) return false; + for (Node p = head; p != null; p = succ(p)) { + Object item = p.item; + if (p.isData) { + if (item != null && item != p && o.equals(item)) + return true; + } + else if (item == null) + break; + } + return false; + } + + /** * Always returns {@code Integer.MAX_VALUE} because a * {@code LinkedTransferQueue} is not capacity constrained. * @@ -1220,23 +1301,22 @@ public class LinkedTransferQueue exte // Unsafe mechanics - private static final sun.misc.Unsafe UNSAFE = getUnsafe(); - private static final long headOffset = - objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); - private static final long tailOffset = - objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); - private static final long cleanMeOffset = - objectFieldOffset(UNSAFE, "cleanMe", LinkedTransferQueue.class); - - static long objectFieldOffset(sun.misc.Unsafe UNSAFE, - String field, Class klazz) { + private static final sun.misc.Unsafe UNSAFE; + private static final long headOffset; + private static final long tailOffset; + private static final long sweepVotesOffset; + static { try { - return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); - } catch (NoSuchFieldException e) { - // Convert Exception to corresponding Error - NoSuchFieldError error = new NoSuchFieldError(field); - error.initCause(e); - throw error; + UNSAFE = getUnsafe(); + Class k = LinkedTransferQueue.class; + headOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("head")); + tailOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("tail")); + sweepVotesOffset = UNSAFE.objectFieldOffset + (k.getDeclaredField("sweepVotes")); + } catch (Exception e) { + throw new Error(e); } }