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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.AbstractQueue; |
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import java.util.Collection; |
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import java.util.ConcurrentModificationException; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Queue; |
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import java.util.concurrent.locks.LockSupport; |
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/** |
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* An unbounded {@link TransferQueue} based on linked nodes. |
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* This queue orders elements FIFO (first-in-first-out) with respect |
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* to any given producer. The <em>head</em> of the queue is that |
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* element that has been on the queue the longest time for some |
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* producer. The <em>tail</em> of the queue is that element that has |
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* been on the queue the shortest time for some producer. |
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* |
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* <p>Beware that, unlike in most collections, the {@code size} |
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* method is <em>NOT</em> a constant-time operation. Because of the |
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* asynchronous nature of these queues, determining the current number |
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* of elements requires a traversal of the elements. |
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* |
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* <p>This class and its iterator implement all of the |
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* <em>optional</em> methods of the {@link Collection} and {@link |
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* Iterator} interfaces. |
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* |
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* <p>Memory consistency effects: As with other concurrent |
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* collections, actions in a thread prior to placing an object into a |
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* {@code LinkedTransferQueue} |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* actions subsequent to the access or removal of that element from |
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* the {@code LinkedTransferQueue} in another thread. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../technotes/guides/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.7 |
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* @author Doug Lea |
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* @param <E> the type of elements held in this collection |
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*/ |
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public class LinkedTransferQueue<E> extends AbstractQueue<E> |
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implements TransferQueue<E>, java.io.Serializable { |
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private static final long serialVersionUID = -3223113410248163686L; |
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|
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/* |
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* *** Overview of Dual Queues with Slack *** |
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* |
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* Dual Queues, introduced by Scherer and Scott |
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* (http://www.cs.rice.edu/~wns1/papers/2004-DISC-DDS.pdf) are |
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* (linked) queues in which nodes may represent either data or |
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* requests. When a thread tries to enqueue a data node, but |
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* encounters a request node, it instead "matches" and removes it; |
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* and vice versa for enqueuing requests. Blocking Dual Queues |
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* arrange that threads enqueuing unmatched requests block until |
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* other threads provide the match. Dual Synchronous Queues (see |
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* Scherer, Lea, & Scott |
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* http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf) |
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* additionally arrange that threads enqueuing unmatched data also |
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* block. Dual Transfer Queues support all of these modes, as |
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* dictated by callers. |
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* |
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* A FIFO dual queue may be implemented using a variation of the |
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* Michael & Scott (M&S) lock-free queue algorithm |
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* (http://www.cs.rochester.edu/u/scott/papers/1996_PODC_queues.pdf). |
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* It maintains two pointer fields, "head", pointing to a |
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* (matched) node that in turn points to the first actual |
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* (unmatched) queue node (or null if empty); and "tail" that |
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* points to the last node on the queue (or again null if |
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* empty). For example, here is a possible queue with four data |
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* elements: |
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* |
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* head tail |
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* | | |
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* v v |
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* M -> U -> U -> U -> U |
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* |
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* The M&S queue algorithm is known to be prone to scalability and |
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* overhead limitations when maintaining (via CAS) these head and |
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* tail pointers. This has led to the development of |
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* contention-reducing variants such as elimination arrays (see |
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* Moir et al http://portal.acm.org/citation.cfm?id=1074013) and |
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* optimistic back pointers (see Ladan-Mozes & Shavit |
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* http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf). |
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* However, the nature of dual queues enables a simpler tactic for |
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* improving M&S-style implementations when dual-ness is needed. |
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* |
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* In a dual queue, each node must atomically maintain its match |
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* status. While there are other possible variants, we implement |
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* this here as: for a data-mode node, matching entails CASing an |
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* "item" field from a non-null data value to null upon match, and |
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* vice-versa for request nodes, CASing from null to a data |
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* value. (Note that the linearization properties of this style of |
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* queue are easy to verify -- elements are made available by |
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* linking, and unavailable by matching.) Compared to plain M&S |
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* queues, this property of dual queues requires one additional |
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* successful atomic operation per enq/deq pair. But it also |
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* enables lower cost variants of queue maintenance mechanics. (A |
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* variation of this idea applies even for non-dual queues that |
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* support deletion of interior elements, such as |
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* j.u.c.ConcurrentLinkedQueue.) |
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* |
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* Once a node is matched, its match status can never again |
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* change. We may thus arrange that the linked list of them |
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* contain a prefix of zero or more matched nodes, followed by a |
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* suffix of zero or more unmatched nodes. (Note that we allow |
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* both the prefix and suffix to be zero length, which in turn |
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* means that we do not use a dummy header.) If we were not |
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* concerned with either time or space efficiency, we could |
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* correctly perform enqueue and dequeue operations by traversing |
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* from a pointer to the initial node; CASing the item of the |
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* first unmatched node on match and CASing the next field of the |
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* trailing node on appends. (Plus some special-casing when |
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* initially empty). While this would be a terrible idea in |
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* itself, it does have the benefit of not requiring ANY atomic |
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* updates on head/tail fields. |
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* |
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* We introduce here an approach that lies between the extremes of |
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* never versus always updating queue (head and tail) pointers. |
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* This offers a tradeoff between sometimes requiring extra |
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* traversal steps to locate the first and/or last unmatched |
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* nodes, versus the reduced overhead and contention of fewer |
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* updates to queue pointers. For example, a possible snapshot of |
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* a queue is: |
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* |
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* head tail |
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* | | |
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* v v |
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* M -> M -> U -> U -> U -> U |
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* |
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* The best value for this "slack" (the targeted maximum distance |
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* between the value of "head" and the first unmatched node, and |
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* similarly for "tail") is an empirical matter. We have found |
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* that using very small constants in the range of 1-3 work best |
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* over a range of platforms. Larger values introduce increasing |
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* costs of cache misses and risks of long traversal chains, while |
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* smaller values increase CAS contention and overhead. |
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* |
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* Dual queues with slack differ from plain M&S dual queues by |
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* virtue of only sometimes updating head or tail pointers when |
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* matching, appending, or even traversing nodes; in order to |
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* maintain a targeted slack. The idea of "sometimes" may be |
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* operationalized in several ways. The simplest is to use a |
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* per-operation counter incremented on each traversal step, and |
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* to try (via CAS) to update the associated queue pointer |
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* whenever the count exceeds a threshold. Another, that requires |
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* more overhead, is to use random number generators to update |
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* with a given probability per traversal step. |
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* |
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* In any strategy along these lines, because CASes updating |
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* fields may fail, the actual slack may exceed targeted |
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* slack. However, they may be retried at any time to maintain |
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* targets. Even when using very small slack values, this |
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* approach works well for dual queues because it allows all |
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* operations up to the point of matching or appending an item |
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* (hence potentially allowing progress by another thread) to be |
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* read-only, thus not introducing any further contention. As |
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* described below, we implement this by performing slack |
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* maintenance retries only after these points. |
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* |
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* As an accompaniment to such techniques, traversal overhead can |
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* be further reduced without increasing contention of head |
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* pointer updates: Threads may sometimes shortcut the "next" link |
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* path from the current "head" node to be closer to the currently |
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* known first unmatched node, and similarly for tail. Again, this |
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* may be triggered with using thresholds or randomization. |
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* |
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* These ideas must be further extended to avoid unbounded amounts |
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* of costly-to-reclaim garbage caused by the sequential "next" |
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* links of nodes starting at old forgotten head nodes: As first |
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* described in detail by Boehm |
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* (http://portal.acm.org/citation.cfm?doid=503272.503282) if a GC |
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* delays noticing that any arbitrarily old node has become |
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* garbage, all newer dead nodes will also be unreclaimed. |
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* (Similar issues arise in non-GC environments.) To cope with |
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* this in our implementation, upon CASing to advance the head |
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* pointer, we set the "next" link of the previous head to point |
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* only to itself; thus limiting the length of connected dead lists. |
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* (We also take similar care to wipe out possibly garbage |
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* retaining values held in other Node fields.) However, doing so |
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* adds some further complexity to traversal: If any "next" |
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* pointer links to itself, it indicates that the current thread |
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* has lagged behind a head-update, and so the traversal must |
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* continue from the "head". Traversals trying to find the |
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* current tail starting from "tail" may also encounter |
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* self-links, in which case they also continue at "head". |
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* |
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* It is tempting in slack-based scheme to not even use CAS for |
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* updates (similarly to Ladan-Mozes & Shavit). However, this |
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* cannot be done for head updates under the above link-forgetting |
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* mechanics because an update may leave head at a detached node. |
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* And while direct writes are possible for tail updates, they |
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* increase the risk of long retraversals, and hence long garbage |
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* chains, which can be much more costly than is worthwhile |
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* considering that the cost difference of performing a CAS vs |
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* write is smaller when they are not triggered on each operation |
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* (especially considering that writes and CASes equally require |
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* additional GC bookkeeping ("write barriers") that are sometimes |
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* more costly than the writes themselves because of contention). |
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* |
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* *** Overview of implementation *** |
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* |
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* We use a threshold-based approach to updates, with a slack |
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* threshold of two -- that is, we update head/tail when the |
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* current pointer appears to be two or more steps away from the |
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* first/last node. The slack value is hard-wired: a path greater |
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* than one is naturally implemented by checking equality of |
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* traversal pointers except when the list has only one element, |
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* in which case we keep slack threshold at one. Avoiding tracking |
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* explicit counts across method calls slightly simplifies an |
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* already-messy implementation. Using randomization would |
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* probably work better if there were a low-quality dirt-cheap |
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* per-thread one available, but even ThreadLocalRandom is too |
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* heavy for these purposes. |
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* |
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* With such a small slack threshold value, it is not worthwhile |
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* to augment this with path short-circuiting (i.e., unsplicing |
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* interior nodes) except in the case of cancellation/removal (see |
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* below). |
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* |
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* We allow both the head and tail fields to be null before any |
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* nodes are enqueued; initializing upon first append. This |
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* simplifies some other logic, as well as providing more |
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* efficient explicit control paths instead of letting JVMs insert |
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* implicit NullPointerExceptions when they are null. While not |
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* currently fully implemented, we also leave open the possibility |
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* of re-nulling these fields when empty (which is complicated to |
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* arrange, for little benefit.) |
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* |
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* All enqueue/dequeue operations are handled by the single method |
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* "xfer" with parameters indicating whether to act as some form |
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* of offer, put, poll, take, or transfer (each possibly with |
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* timeout). The relative complexity of using one monolithic |
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* method outweighs the code bulk and maintenance problems of |
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* using separate methods for each case. |
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* |
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* Operation consists of up to three phases. The first is |
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* implemented within method xfer, the second in tryAppend, and |
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* the third in method awaitMatch. |
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* |
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* 1. Try to match an existing node |
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* |
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* Starting at head, skip already-matched nodes until finding |
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* an unmatched node of opposite mode, if one exists, in which |
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* case matching it and returning, also if necessary updating |
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* head to one past the matched node (or the node itself if the |
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* list has no other unmatched nodes). If the CAS misses, then |
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* a loop retries advancing head by two steps until either |
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* success or the slack is at most two. By requiring that each |
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* attempt advances head by two (if applicable), we ensure that |
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* the slack does not grow without bound. Traversals also check |
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* if the initial head is now off-list, in which case they |
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* start at the new head. |
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* |
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* If no candidates are found and the call was untimed |
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* poll/offer, (argument "how" is NOW) return. |
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* |
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* 2. Try to append a new node (method tryAppend) |
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* |
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* Starting at current tail pointer, find the actual last node |
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* and try to append a new node (or if head was null, establish |
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* the first node). Nodes can be appended only if their |
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* predecessors are either already matched or are of the same |
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* mode. If we detect otherwise, then a new node with opposite |
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* mode must have been appended during traversal, so we must |
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* restart at phase 1. The traversal and update steps are |
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* otherwise similar to phase 1: Retrying upon CAS misses and |
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* checking for staleness. In particular, if a self-link is |
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* encountered, then we can safely jump to a node on the list |
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* by continuing the traversal at current head. |
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* |
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* On successful append, if the call was ASYNC, return. |
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* |
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* 3. Await match or cancellation (method awaitMatch) |
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* |
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* Wait for another thread to match node; instead cancelling if |
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* the current thread was interrupted or the wait timed out. On |
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* multiprocessors, we use front-of-queue spinning: If a node |
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* appears to be the first unmatched node in the queue, it |
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* spins a bit before blocking. In either case, before blocking |
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* it tries to unsplice any nodes between the current "head" |
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* and the first unmatched node. |
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* |
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* Front-of-queue spinning vastly improves performance of |
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* heavily contended queues. And so long as it is relatively |
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* brief and "quiet", spinning does not much impact performance |
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* of less-contended queues. During spins threads check their |
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* interrupt status and generate a thread-local random number |
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* to decide to occasionally perform a Thread.yield. While |
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* yield has underdefined specs, we assume that might it help, |
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* and will not hurt in limiting impact of spinning on busy |
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* systems. We also use smaller (1/2) spins for nodes that are |
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* not known to be front but whose predecessors have not |
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* blocked -- these "chained" spins avoid artifacts of |
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* front-of-queue rules which otherwise lead to alternating |
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* nodes spinning vs blocking. Further, front threads that |
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* represent phase changes (from data to request node or vice |
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* versa) compared to their predecessors receive additional |
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* chained spins, reflecting longer paths typically required to |
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* unblock threads during phase changes. |
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* |
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* |
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* ** Unlinking removed interior nodes ** |
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* |
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* In addition to minimizing garbage retention via self-linking |
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* described above, we also unlink removed interior nodes. These |
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* may arise due to timed out or interrupted waits, or calls to |
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* remove(x) or Iterator.remove. Normally, given a node that was |
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* at one time known to be the predecessor of some node s that is |
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* to be removed, we can unsplice s by CASing the next field of |
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* its predecessor if it still points to s (otherwise s must |
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* already have been removed or is now offlist). But there are two |
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* situations in which we cannot guarantee to make node s |
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* unreachable in this way: (1) If s is the trailing node of list |
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* (i.e., with null next), then it is pinned as the target node |
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* for appends, so can only be removed later when other nodes are |
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* appended. (2) We cannot necessarily unlink s given a |
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* predecessor node that is matched (including the case of being |
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* cancelled): the predecessor may already be unspliced, in which |
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* case some previous reachable node may still point to s. |
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* (For further explanation see Herlihy & Shavit "The Art of |
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* Multiprocessor Programming" chapter 9). Although, in both |
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* cases, we can rule out the need for further action if either s |
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* or its predecessor are (or can be made to be) at, or fall off |
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* from, the head of list. |
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* |
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* Without taking these into account, it would be possible for an |
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* unbounded number of supposedly removed nodes to remain |
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* reachable. Situations leading to such buildup are uncommon but |
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* can occur in practice; for example when a series of short timed |
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* calls to poll repeatedly time out but never otherwise fall off |
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* the list because of an untimed call to take at the front of the |
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* queue. |
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* |
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* When these cases arise, rather than always retraversing the |
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* entire list to find an actual predecessor to unlink (which |
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* won't help for case (1) anyway), we record a conservative |
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* estimate of possible unsplice failures (in "sweepVotes). We |
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* trigger a full sweep when the estimate exceeds a threshold |
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* indicating the maximum number of estimated removal failures to |
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* tolerate before sweeping through, unlinking cancelled nodes |
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* that were not unlinked upon initial removal. We perform sweeps |
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* by the thread hitting threshold (rather than background threads |
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* or by spreading work to other threads) because in the main |
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* contexts in which removal occurs, the caller is already |
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* timed-out, cancelled, or performing a potentially O(n) |
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* operation (i.e., remove(x)), none of which are time-critical |
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* enough to warrant the overhead that alternatives would impose |
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* on other threads. |
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* |
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* Because the sweepVotes estimate is conservative, and because |
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* nodes become unlinked "naturally" as they fall off the head of |
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* the queue, and because we allow votes to accumulate even while |
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* sweeps are in progress, there are typically significantly fewer |
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* such nodes than estimated. Choice of a threshold value |
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* balances the likelihood of wasted effort and contention, versus |
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* providing a worst-case bound on retention of interior nodes in |
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* quiescent queues. The value defined below was chosen |
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* empirically to balance these under various timeout scenarios. |
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* |
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* Note that we cannot self-link unlinked interior nodes during |
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* sweeps. However, the associated garbage chains terminate when |
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* some successor ultimately falls off the head of the list and is |
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* self-linked. |
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*/ |
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|
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/** True if on multiprocessor */ |
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private static final boolean MP = |
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Runtime.getRuntime().availableProcessors() > 1; |
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|
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/** |
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* The number of times to spin (with randomly interspersed calls |
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* to Thread.yield) on multiprocessor before blocking when a node |
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* is apparently the first waiter in the queue. See above for |
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* explanation. Must be a power of two. The value is empirically |
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* derived -- it works pretty well across a variety of processors, |
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* numbers of CPUs, and OSes. |
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*/ |
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private static final int FRONT_SPINS = 1 << 7; |
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|
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/** |
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* The number of times to spin before blocking when a node is |
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* preceded by another node that is apparently spinning. Also |
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* serves as an increment to FRONT_SPINS on phase changes, and as |
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* base average frequency for yielding during spins. Must be a |
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* power of two. |
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*/ |
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private static final int CHAINED_SPINS = FRONT_SPINS >>> 1; |
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|
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/** |
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* The maximum number of estimated removal failures (sweepVotes) |
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* to tolerate before sweeping through the queue unlinking |
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* cancelled nodes that were not unlinked upon initial |
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* removal. See above for explanation. The value must be at least |
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* two to avoid useless sweeps when removing trailing nodes. |
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*/ |
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static final int SWEEP_THRESHOLD = 32; |
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|
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/** |
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* Queue nodes. Uses Object, not E, for items to allow forgetting |
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* them after use. Relies heavily on Unsafe mechanics to minimize |
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* unnecessary ordering constraints: Writes that are intrinsically |
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* ordered wrt other accesses or CASes use simple relaxed forms. |
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*/ |
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static final class Node { |
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final boolean isData; // false if this is a request node |
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volatile Object item; // initially non-null if isData; CASed to match |
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volatile Node next; |
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volatile Thread waiter; // null until waiting |
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|
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// CAS methods for fields |
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final boolean casNext(Node cmp, Node val) { |
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return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val); |
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} |
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|
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final boolean casItem(Object cmp, Object val) { |
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assert cmp == null || cmp.getClass() != Node.class; |
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return UNSAFE.compareAndSwapObject(this, itemOffset, cmp, val); |
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} |
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|
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/** |
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* Creates a new node. Uses relaxed write because item can only |
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* be seen if followed by CAS. |
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*/ |
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Node(Object item, boolean isData) { |
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UNSAFE.putObject(this, itemOffset, item); // relaxed write |
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this.isData = isData; |
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} |
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|
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/** |
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* Links node to itself to avoid garbage retention. Called |
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* only after CASing head field, so uses relaxed write. |
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*/ |
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final void forgetNext() { |
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UNSAFE.putObject(this, nextOffset, this); |
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} |
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|
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/** |
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* Sets item to self and waiter to null, to avoid garbage |
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* retention after matching or cancelling. Uses relaxed writes |
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* bacause order is already constrained in the only calling |
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* contexts: item is forgotten only after volatile/atomic |
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* mechanics that extract items. Similarly, clearing waiter |
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* follows either CAS or return from park (if ever parked; |
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* else we don't care). |
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*/ |
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final void forgetContents() { |
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UNSAFE.putObject(this, itemOffset, this); |
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UNSAFE.putObject(this, waiterOffset, null); |
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} |
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|
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/** |
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* Returns true if this node has been matched, including the |
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* case of artificial matches due to cancellation. |
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*/ |
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final boolean isMatched() { |
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Object x = item; |
464 |
return (x == this) || ((x == null) == isData); |
465 |
} |
466 |
|
467 |
/** |
468 |
* Returns true if this is an unmatched request node. |
469 |
*/ |
470 |
final boolean isUnmatchedRequest() { |
471 |
return !isData && item == null; |
472 |
} |
473 |
|
474 |
/** |
475 |
* Returns true if a node with the given mode cannot be |
476 |
* appended to this node because this node is unmatched and |
477 |
* has opposite data mode. |
478 |
*/ |
479 |
final boolean cannotPrecede(boolean haveData) { |
480 |
boolean d = isData; |
481 |
Object x; |
482 |
return d != haveData && (x = item) != this && (x != null) == d; |
483 |
} |
484 |
|
485 |
/** |
486 |
* Tries to artificially match a data node -- used by remove. |
487 |
*/ |
488 |
final boolean tryMatchData() { |
489 |
assert isData; |
490 |
Object x = item; |
491 |
if (x != null && x != this && casItem(x, null)) { |
492 |
LockSupport.unpark(waiter); |
493 |
return true; |
494 |
} |
495 |
return false; |
496 |
} |
497 |
|
498 |
// Unsafe mechanics |
499 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
500 |
private static final long nextOffset = |
501 |
objectFieldOffset(UNSAFE, "next", Node.class); |
502 |
private static final long itemOffset = |
503 |
objectFieldOffset(UNSAFE, "item", Node.class); |
504 |
private static final long waiterOffset = |
505 |
objectFieldOffset(UNSAFE, "waiter", Node.class); |
506 |
|
507 |
private static final long serialVersionUID = -3375979862319811754L; |
508 |
} |
509 |
|
510 |
/** head of the queue; null until first enqueue */ |
511 |
transient volatile Node head; |
512 |
|
513 |
/** tail of the queue; null until first append */ |
514 |
private transient volatile Node tail; |
515 |
|
516 |
/** The number of apparent failures to unsplice removed nodes */ |
517 |
private transient volatile int sweepVotes; |
518 |
|
519 |
// CAS methods for fields |
520 |
private boolean casTail(Node cmp, Node val) { |
521 |
return UNSAFE.compareAndSwapObject(this, tailOffset, cmp, val); |
522 |
} |
523 |
|
524 |
private boolean casHead(Node cmp, Node val) { |
525 |
return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val); |
526 |
} |
527 |
|
528 |
private boolean casSweepVotes(int cmp, int val) { |
529 |
return UNSAFE.compareAndSwapInt(this, sweepVotesOffset, cmp, val); |
530 |
} |
531 |
|
532 |
/* |
533 |
* Possible values for "how" argument in xfer method. |
534 |
*/ |
535 |
private static final int NOW = 0; // for untimed poll, tryTransfer |
536 |
private static final int ASYNC = 1; // for offer, put, add |
537 |
private static final int SYNC = 2; // for transfer, take |
538 |
private static final int TIMED = 3; // for timed poll, tryTransfer |
539 |
|
540 |
@SuppressWarnings("unchecked") |
541 |
static <E> E cast(Object item) { |
542 |
assert item == null || item.getClass() != Node.class; |
543 |
return (E) item; |
544 |
} |
545 |
|
546 |
/** |
547 |
* Implements all queuing methods. See above for explanation. |
548 |
* |
549 |
* @param e the item or null for take |
550 |
* @param haveData true if this is a put, else a take |
551 |
* @param how NOW, ASYNC, SYNC, or TIMED |
552 |
* @param nanos timeout in nanosecs, used only if mode is TIMED |
553 |
* @return an item if matched, else e |
554 |
* @throws NullPointerException if haveData mode but e is null |
555 |
*/ |
556 |
private E xfer(E e, boolean haveData, int how, long nanos) { |
557 |
if (haveData && (e == null)) |
558 |
throw new NullPointerException(); |
559 |
Node s = null; // the node to append, if needed |
560 |
|
561 |
retry: for (;;) { // restart on append race |
562 |
|
563 |
for (Node h = head, p = h; p != null;) { // find & match first node |
564 |
boolean isData = p.isData; |
565 |
Object item = p.item; |
566 |
if (item != p && (item != null) == isData) { // unmatched |
567 |
if (isData == haveData) // can't match |
568 |
break; |
569 |
if (p.casItem(item, e)) { // match |
570 |
for (Node q = p; q != h;) { |
571 |
Node n = q.next; // update by 2 unless singleton |
572 |
if (head == h && casHead(h, n == null? q : n)) { |
573 |
h.forgetNext(); |
574 |
break; |
575 |
} // advance and retry |
576 |
if ((h = head) == null || |
577 |
(q = h.next) == null || !q.isMatched()) |
578 |
break; // unless slack < 2 |
579 |
} |
580 |
LockSupport.unpark(p.waiter); |
581 |
return this.<E>cast(item); |
582 |
} |
583 |
} |
584 |
Node n = p.next; |
585 |
p = (p != n) ? n : (h = head); // Use head if p offlist |
586 |
} |
587 |
|
588 |
if (how != NOW) { // No matches available |
589 |
if (s == null) |
590 |
s = new Node(e, haveData); |
591 |
Node pred = tryAppend(s, haveData); |
592 |
if (pred == null) |
593 |
continue retry; // lost race vs opposite mode |
594 |
if (how != ASYNC) |
595 |
return awaitMatch(s, pred, e, (how == TIMED), nanos); |
596 |
} |
597 |
return e; // not waiting |
598 |
} |
599 |
} |
600 |
|
601 |
/** |
602 |
* Tries to append node s as tail. |
603 |
* |
604 |
* @param s the node to append |
605 |
* @param haveData true if appending in data mode |
606 |
* @return null on failure due to losing race with append in |
607 |
* different mode, else s's predecessor, or s itself if no |
608 |
* predecessor |
609 |
*/ |
610 |
private Node tryAppend(Node s, boolean haveData) { |
611 |
for (Node t = tail, p = t;;) { // move p to last node and append |
612 |
Node n, u; // temps for reads of next & tail |
613 |
if (p == null && (p = head) == null) { |
614 |
if (casHead(null, s)) |
615 |
return s; // initialize |
616 |
} |
617 |
else if (p.cannotPrecede(haveData)) |
618 |
return null; // lost race vs opposite mode |
619 |
else if ((n = p.next) != null) // not last; keep traversing |
620 |
p = p != t && t != (u = tail) ? (t = u) : // stale tail |
621 |
(p != n) ? n : null; // restart if off list |
622 |
else if (!p.casNext(null, s)) |
623 |
p = p.next; // re-read on CAS failure |
624 |
else { |
625 |
if (p != t) { // update if slack now >= 2 |
626 |
while ((tail != t || !casTail(t, s)) && |
627 |
(t = tail) != null && |
628 |
(s = t.next) != null && // advance and retry |
629 |
(s = s.next) != null && s != t); |
630 |
} |
631 |
return p; |
632 |
} |
633 |
} |
634 |
} |
635 |
|
636 |
/** |
637 |
* Spins/yields/blocks until node s is matched or caller gives up. |
638 |
* |
639 |
* @param s the waiting node |
640 |
* @param pred the predecessor of s, or s itself if it has no |
641 |
* predecessor, or null if unknown (the null case does not occur |
642 |
* in any current calls but may in possible future extensions) |
643 |
* @param e the comparison value for checking match |
644 |
* @param timed if true, wait only until timeout elapses |
645 |
* @param nanos timeout in nanosecs, used only if timed is true |
646 |
* @return matched item, or e if unmatched on interrupt or timeout |
647 |
*/ |
648 |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
649 |
long lastTime = timed ? System.nanoTime() : 0L; |
650 |
Thread w = Thread.currentThread(); |
651 |
int spins = -1; // initialized after first item and cancel checks |
652 |
ThreadLocalRandom randomYields = null; // bound if needed |
653 |
|
654 |
for (;;) { |
655 |
Object item = s.item; |
656 |
if (item != e) { // matched |
657 |
assert item != s; |
658 |
s.forgetContents(); // avoid garbage |
659 |
return this.<E>cast(item); |
660 |
} |
661 |
if ((w.isInterrupted() || (timed && nanos <= 0)) && |
662 |
s.casItem(e, s)) { // cancel |
663 |
unsplice(pred, s); |
664 |
return e; |
665 |
} |
666 |
|
667 |
if (spins < 0) { // establish spins at/near front |
668 |
if ((spins = spinsFor(pred, s.isData)) > 0) |
669 |
randomYields = ThreadLocalRandom.current(); |
670 |
} |
671 |
else if (spins > 0) { // spin |
672 |
--spins; |
673 |
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
674 |
Thread.yield(); // occasionally yield |
675 |
} |
676 |
else if (s.waiter == null) { |
677 |
s.waiter = w; // request unpark then recheck |
678 |
} |
679 |
else if (timed) { |
680 |
long now = System.nanoTime(); |
681 |
if ((nanos -= now - lastTime) > 0) |
682 |
LockSupport.parkNanos(this, nanos); |
683 |
lastTime = now; |
684 |
} |
685 |
else { |
686 |
LockSupport.park(this); |
687 |
} |
688 |
} |
689 |
} |
690 |
|
691 |
/** |
692 |
* Returns spin/yield value for a node with given predecessor and |
693 |
* data mode. See above for explanation. |
694 |
*/ |
695 |
private static int spinsFor(Node pred, boolean haveData) { |
696 |
if (MP && pred != null) { |
697 |
if (pred.isData != haveData) // phase change |
698 |
return FRONT_SPINS + CHAINED_SPINS; |
699 |
if (pred.isMatched()) // probably at front |
700 |
return FRONT_SPINS; |
701 |
if (pred.waiter == null) // pred apparently spinning |
702 |
return CHAINED_SPINS; |
703 |
} |
704 |
return 0; |
705 |
} |
706 |
|
707 |
/* -------------- Traversal methods -------------- */ |
708 |
|
709 |
/** |
710 |
* Returns the successor of p, or the head node if p.next has been |
711 |
* linked to self, which will only be true if traversing with a |
712 |
* stale pointer that is now off the list. |
713 |
*/ |
714 |
final Node succ(Node p) { |
715 |
Node next = p.next; |
716 |
return (p == next) ? head : next; |
717 |
} |
718 |
|
719 |
/** |
720 |
* Returns the first unmatched node of the given mode, or null if |
721 |
* none. Used by methods isEmpty, hasWaitingConsumer. |
722 |
*/ |
723 |
private Node firstOfMode(boolean isData) { |
724 |
for (Node p = head; p != null; p = succ(p)) { |
725 |
if (!p.isMatched()) |
726 |
return (p.isData == isData) ? p : null; |
727 |
} |
728 |
return null; |
729 |
} |
730 |
|
731 |
/** |
732 |
* Returns the item in the first unmatched node with isData; or |
733 |
* null if none. Used by peek. |
734 |
*/ |
735 |
private E firstDataItem() { |
736 |
for (Node p = head; p != null; p = succ(p)) { |
737 |
Object item = p.item; |
738 |
if (p.isData) { |
739 |
if (item != null && item != p) |
740 |
return this.<E>cast(item); |
741 |
} |
742 |
else if (item == null) |
743 |
return null; |
744 |
} |
745 |
return null; |
746 |
} |
747 |
|
748 |
/** |
749 |
* Traverses and counts unmatched nodes of the given mode. |
750 |
* Used by methods size and getWaitingConsumerCount. |
751 |
*/ |
752 |
private int countOfMode(boolean data) { |
753 |
int count = 0; |
754 |
for (Node p = head; p != null; ) { |
755 |
if (!p.isMatched()) { |
756 |
if (p.isData != data) |
757 |
return 0; |
758 |
if (++count == Integer.MAX_VALUE) // saturated |
759 |
break; |
760 |
} |
761 |
Node n = p.next; |
762 |
if (n != p) |
763 |
p = n; |
764 |
else { |
765 |
count = 0; |
766 |
p = head; |
767 |
} |
768 |
} |
769 |
return count; |
770 |
} |
771 |
|
772 |
final class Itr implements Iterator<E> { |
773 |
private Node nextNode; // next node to return item for |
774 |
private E nextItem; // the corresponding item |
775 |
private Node lastRet; // last returned node, to support remove |
776 |
private Node lastPred; // predecessor to unlink lastRet |
777 |
|
778 |
/** |
779 |
* Moves to next node after prev, or first node if prev null. |
780 |
*/ |
781 |
private void advance(Node prev) { |
782 |
lastPred = lastRet; |
783 |
lastRet = prev; |
784 |
for (Node p = (prev == null) ? head : succ(prev); |
785 |
p != null; p = succ(p)) { |
786 |
Object item = p.item; |
787 |
if (p.isData) { |
788 |
if (item != null && item != p) { |
789 |
nextItem = LinkedTransferQueue.this.<E>cast(item); |
790 |
nextNode = p; |
791 |
return; |
792 |
} |
793 |
} |
794 |
else if (item == null) |
795 |
break; |
796 |
} |
797 |
nextNode = null; |
798 |
} |
799 |
|
800 |
Itr() { |
801 |
advance(null); |
802 |
} |
803 |
|
804 |
public final boolean hasNext() { |
805 |
return nextNode != null; |
806 |
} |
807 |
|
808 |
public final E next() { |
809 |
Node p = nextNode; |
810 |
if (p == null) throw new NoSuchElementException(); |
811 |
E e = nextItem; |
812 |
advance(p); |
813 |
return e; |
814 |
} |
815 |
|
816 |
public final void remove() { |
817 |
Node p = lastRet; |
818 |
if (p == null) throw new IllegalStateException(); |
819 |
if (p.tryMatchData()) |
820 |
unsplice(lastPred, p); |
821 |
} |
822 |
} |
823 |
|
824 |
/* -------------- Removal methods -------------- */ |
825 |
|
826 |
/** |
827 |
* Unsplices (now or later) the given deleted/cancelled node with |
828 |
* the given predecessor. |
829 |
* |
830 |
* @param pred a node that was at one time known to be the |
831 |
* predecessor of s, or null or s itself if s is/was at head |
832 |
* @param s the node to be unspliced |
833 |
*/ |
834 |
final void unsplice(Node pred, Node s) { |
835 |
s.forgetContents(); // forget unneeded fields |
836 |
/* |
837 |
* See above for rationale. Briefly: if pred still points to |
838 |
* s, try to unlink s. If s cannot be unlinked, because it is |
839 |
* trailing node or pred might be unlinked, and neither pred |
840 |
* nor s are head or offlist, add to sweepVotes, and if enough |
841 |
* votes have accumulated, sweep. |
842 |
*/ |
843 |
if (pred != null && pred != s && pred.next == s) { |
844 |
Node n = s.next; |
845 |
if (n == null || |
846 |
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
847 |
for (;;) { // check if at, or could be, head |
848 |
Node h = head; |
849 |
if (h == pred || h == s || h == null) |
850 |
return; // at head or list empty |
851 |
if (!h.isMatched()) |
852 |
break; |
853 |
Node hn = h.next; |
854 |
if (hn == null) |
855 |
return; // now empty |
856 |
if (hn != h && casHead(h, hn)) |
857 |
h.forgetNext(); // advance head |
858 |
} |
859 |
if (pred.next != pred && s.next != s) { // recheck if offlist |
860 |
for (;;) { // sweep now if enough votes |
861 |
int v = sweepVotes; |
862 |
if (v < SWEEP_THRESHOLD) { |
863 |
if (casSweepVotes(v, v + 1)) |
864 |
break; |
865 |
} |
866 |
else if (casSweepVotes(v, 0)) { |
867 |
sweep(); |
868 |
break; |
869 |
} |
870 |
} |
871 |
} |
872 |
} |
873 |
} |
874 |
} |
875 |
|
876 |
/** |
877 |
* Unlink matched nodes encountered in a traversal from head |
878 |
*/ |
879 |
private void sweep() { |
880 |
Node p = head, s, n; |
881 |
while (p != null && (s = p.next) != null && (n = s.next) != null) { |
882 |
if (p == s || s == n) |
883 |
p = head; // stale |
884 |
else if (s.isMatched()) |
885 |
p.casNext(s, n); |
886 |
else |
887 |
p = s; |
888 |
} |
889 |
} |
890 |
|
891 |
/** |
892 |
* Main implementation of remove(Object) |
893 |
*/ |
894 |
private boolean findAndRemove(Object e) { |
895 |
if (e != null) { |
896 |
for (Node pred = null, p = head; p != null; ) { |
897 |
Object item = p.item; |
898 |
if (p.isData) { |
899 |
if (item != null && item != p && e.equals(item) && |
900 |
p.tryMatchData()) { |
901 |
unsplice(pred, p); |
902 |
return true; |
903 |
} |
904 |
} |
905 |
else if (item == null) |
906 |
break; |
907 |
pred = p; |
908 |
if ((p = p.next) == pred) { // stale |
909 |
pred = null; |
910 |
p = head; |
911 |
} |
912 |
} |
913 |
} |
914 |
return false; |
915 |
} |
916 |
|
917 |
|
918 |
/** |
919 |
* Creates an initially empty {@code LinkedTransferQueue}. |
920 |
*/ |
921 |
public LinkedTransferQueue() { |
922 |
} |
923 |
|
924 |
/** |
925 |
* Creates a {@code LinkedTransferQueue} |
926 |
* initially containing the elements of the given collection, |
927 |
* added in traversal order of the collection's iterator. |
928 |
* |
929 |
* @param c the collection of elements to initially contain |
930 |
* @throws NullPointerException if the specified collection or any |
931 |
* of its elements are null |
932 |
*/ |
933 |
public LinkedTransferQueue(Collection<? extends E> c) { |
934 |
this(); |
935 |
addAll(c); |
936 |
} |
937 |
|
938 |
/** |
939 |
* Inserts the specified element at the tail of this queue. |
940 |
* As the queue is unbounded, this method will never block. |
941 |
* |
942 |
* @throws NullPointerException if the specified element is null |
943 |
*/ |
944 |
public void put(E e) { |
945 |
xfer(e, true, ASYNC, 0); |
946 |
} |
947 |
|
948 |
/** |
949 |
* Inserts the specified element at the tail of this queue. |
950 |
* As the queue is unbounded, this method will never block or |
951 |
* return {@code false}. |
952 |
* |
953 |
* @return {@code true} (as specified by |
954 |
* {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer}) |
955 |
* @throws NullPointerException if the specified element is null |
956 |
*/ |
957 |
public boolean offer(E e, long timeout, TimeUnit unit) { |
958 |
xfer(e, true, ASYNC, 0); |
959 |
return true; |
960 |
} |
961 |
|
962 |
/** |
963 |
* Inserts the specified element at the tail of this queue. |
964 |
* As the queue is unbounded, this method will never return {@code false}. |
965 |
* |
966 |
* @return {@code true} (as specified by |
967 |
* {@link BlockingQueue#offer(Object) BlockingQueue.offer}) |
968 |
* @throws NullPointerException if the specified element is null |
969 |
*/ |
970 |
public boolean offer(E e) { |
971 |
xfer(e, true, ASYNC, 0); |
972 |
return true; |
973 |
} |
974 |
|
975 |
/** |
976 |
* Inserts the specified element at the tail of this queue. |
977 |
* As the queue is unbounded, this method will never throw |
978 |
* {@link IllegalStateException} or return {@code false}. |
979 |
* |
980 |
* @return {@code true} (as specified by {@link Collection#add}) |
981 |
* @throws NullPointerException if the specified element is null |
982 |
*/ |
983 |
public boolean add(E e) { |
984 |
xfer(e, true, ASYNC, 0); |
985 |
return true; |
986 |
} |
987 |
|
988 |
/** |
989 |
* Transfers the element to a waiting consumer immediately, if possible. |
990 |
* |
991 |
* <p>More precisely, transfers the specified element immediately |
992 |
* if there exists a consumer already waiting to receive it (in |
993 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
994 |
* otherwise returning {@code false} without enqueuing the element. |
995 |
* |
996 |
* @throws NullPointerException if the specified element is null |
997 |
*/ |
998 |
public boolean tryTransfer(E e) { |
999 |
return xfer(e, true, NOW, 0) == null; |
1000 |
} |
1001 |
|
1002 |
/** |
1003 |
* Transfers the element to a consumer, waiting if necessary to do so. |
1004 |
* |
1005 |
* <p>More precisely, transfers the specified element immediately |
1006 |
* if there exists a consumer already waiting to receive it (in |
1007 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1008 |
* else inserts the specified element at the tail of this queue |
1009 |
* and waits until the element is received by a consumer. |
1010 |
* |
1011 |
* @throws NullPointerException if the specified element is null |
1012 |
*/ |
1013 |
public void transfer(E e) throws InterruptedException { |
1014 |
if (xfer(e, true, SYNC, 0) != null) { |
1015 |
Thread.interrupted(); // failure possible only due to interrupt |
1016 |
throw new InterruptedException(); |
1017 |
} |
1018 |
} |
1019 |
|
1020 |
/** |
1021 |
* Transfers the element to a consumer if it is possible to do so |
1022 |
* before the timeout elapses. |
1023 |
* |
1024 |
* <p>More precisely, transfers the specified element immediately |
1025 |
* if there exists a consumer already waiting to receive it (in |
1026 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1027 |
* else inserts the specified element at the tail of this queue |
1028 |
* and waits until the element is received by a consumer, |
1029 |
* returning {@code false} if the specified wait time elapses |
1030 |
* before the element can be transferred. |
1031 |
* |
1032 |
* @throws NullPointerException if the specified element is null |
1033 |
*/ |
1034 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1035 |
throws InterruptedException { |
1036 |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1037 |
return true; |
1038 |
if (!Thread.interrupted()) |
1039 |
return false; |
1040 |
throw new InterruptedException(); |
1041 |
} |
1042 |
|
1043 |
public E take() throws InterruptedException { |
1044 |
E e = xfer(null, false, SYNC, 0); |
1045 |
if (e != null) |
1046 |
return e; |
1047 |
Thread.interrupted(); |
1048 |
throw new InterruptedException(); |
1049 |
} |
1050 |
|
1051 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1052 |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1053 |
if (e != null || !Thread.interrupted()) |
1054 |
return e; |
1055 |
throw new InterruptedException(); |
1056 |
} |
1057 |
|
1058 |
public E poll() { |
1059 |
return xfer(null, false, NOW, 0); |
1060 |
} |
1061 |
|
1062 |
/** |
1063 |
* @throws NullPointerException {@inheritDoc} |
1064 |
* @throws IllegalArgumentException {@inheritDoc} |
1065 |
*/ |
1066 |
public int drainTo(Collection<? super E> c) { |
1067 |
if (c == null) |
1068 |
throw new NullPointerException(); |
1069 |
if (c == this) |
1070 |
throw new IllegalArgumentException(); |
1071 |
int n = 0; |
1072 |
E e; |
1073 |
while ( (e = poll()) != null) { |
1074 |
c.add(e); |
1075 |
++n; |
1076 |
} |
1077 |
return n; |
1078 |
} |
1079 |
|
1080 |
/** |
1081 |
* @throws NullPointerException {@inheritDoc} |
1082 |
* @throws IllegalArgumentException {@inheritDoc} |
1083 |
*/ |
1084 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1085 |
if (c == null) |
1086 |
throw new NullPointerException(); |
1087 |
if (c == this) |
1088 |
throw new IllegalArgumentException(); |
1089 |
int n = 0; |
1090 |
E e; |
1091 |
while (n < maxElements && (e = poll()) != null) { |
1092 |
c.add(e); |
1093 |
++n; |
1094 |
} |
1095 |
return n; |
1096 |
} |
1097 |
|
1098 |
/** |
1099 |
* Returns an iterator over the elements in this queue in proper |
1100 |
* sequence, from head to tail. |
1101 |
* |
1102 |
* <p>The returned iterator is a "weakly consistent" iterator that |
1103 |
* will never throw |
1104 |
* {@link ConcurrentModificationException ConcurrentModificationException}, |
1105 |
* and guarantees to traverse elements as they existed upon |
1106 |
* construction of the iterator, and may (but is not guaranteed |
1107 |
* to) reflect any modifications subsequent to construction. |
1108 |
* |
1109 |
* @return an iterator over the elements in this queue in proper sequence |
1110 |
*/ |
1111 |
public Iterator<E> iterator() { |
1112 |
return new Itr(); |
1113 |
} |
1114 |
|
1115 |
public E peek() { |
1116 |
return firstDataItem(); |
1117 |
} |
1118 |
|
1119 |
/** |
1120 |
* Returns {@code true} if this queue contains no elements. |
1121 |
* |
1122 |
* @return {@code true} if this queue contains no elements |
1123 |
*/ |
1124 |
public boolean isEmpty() { |
1125 |
return firstOfMode(true) == null; |
1126 |
} |
1127 |
|
1128 |
public boolean hasWaitingConsumer() { |
1129 |
return firstOfMode(false) != null; |
1130 |
} |
1131 |
|
1132 |
/** |
1133 |
* Returns the number of elements in this queue. If this queue |
1134 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
1135 |
* {@code Integer.MAX_VALUE}. |
1136 |
* |
1137 |
* <p>Beware that, unlike in most collections, this method is |
1138 |
* <em>NOT</em> a constant-time operation. Because of the |
1139 |
* asynchronous nature of these queues, determining the current |
1140 |
* number of elements requires an O(n) traversal. |
1141 |
* |
1142 |
* @return the number of elements in this queue |
1143 |
*/ |
1144 |
public int size() { |
1145 |
return countOfMode(true); |
1146 |
} |
1147 |
|
1148 |
public int getWaitingConsumerCount() { |
1149 |
return countOfMode(false); |
1150 |
} |
1151 |
|
1152 |
/** |
1153 |
* Removes a single instance of the specified element from this queue, |
1154 |
* if it is present. More formally, removes an element {@code e} such |
1155 |
* that {@code o.equals(e)}, if this queue contains one or more such |
1156 |
* elements. |
1157 |
* Returns {@code true} if this queue contained the specified element |
1158 |
* (or equivalently, if this queue changed as a result of the call). |
1159 |
* |
1160 |
* @param o element to be removed from this queue, if present |
1161 |
* @return {@code true} if this queue changed as a result of the call |
1162 |
*/ |
1163 |
public boolean remove(Object o) { |
1164 |
return findAndRemove(o); |
1165 |
} |
1166 |
|
1167 |
/** |
1168 |
* Always returns {@code Integer.MAX_VALUE} because a |
1169 |
* {@code LinkedTransferQueue} is not capacity constrained. |
1170 |
* |
1171 |
* @return {@code Integer.MAX_VALUE} (as specified by |
1172 |
* {@link BlockingQueue#remainingCapacity()}) |
1173 |
*/ |
1174 |
public int remainingCapacity() { |
1175 |
return Integer.MAX_VALUE; |
1176 |
} |
1177 |
|
1178 |
/** |
1179 |
* Saves the state to a stream (that is, serializes it). |
1180 |
* |
1181 |
* @serialData All of the elements (each an {@code E}) in |
1182 |
* the proper order, followed by a null |
1183 |
* @param s the stream |
1184 |
*/ |
1185 |
private void writeObject(java.io.ObjectOutputStream s) |
1186 |
throws java.io.IOException { |
1187 |
s.defaultWriteObject(); |
1188 |
for (E e : this) |
1189 |
s.writeObject(e); |
1190 |
// Use trailing null as sentinel |
1191 |
s.writeObject(null); |
1192 |
} |
1193 |
|
1194 |
/** |
1195 |
* Reconstitutes the Queue instance from a stream (that is, |
1196 |
* deserializes it). |
1197 |
* |
1198 |
* @param s the stream |
1199 |
*/ |
1200 |
private void readObject(java.io.ObjectInputStream s) |
1201 |
throws java.io.IOException, ClassNotFoundException { |
1202 |
s.defaultReadObject(); |
1203 |
for (;;) { |
1204 |
@SuppressWarnings("unchecked") E item = (E) s.readObject(); |
1205 |
if (item == null) |
1206 |
break; |
1207 |
else |
1208 |
offer(item); |
1209 |
} |
1210 |
} |
1211 |
|
1212 |
// Unsafe mechanics |
1213 |
|
1214 |
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe(); |
1215 |
private static final long headOffset = |
1216 |
objectFieldOffset(UNSAFE, "head", LinkedTransferQueue.class); |
1217 |
private static final long tailOffset = |
1218 |
objectFieldOffset(UNSAFE, "tail", LinkedTransferQueue.class); |
1219 |
private static final long sweepVotesOffset = |
1220 |
objectFieldOffset(UNSAFE, "sweepVotes", LinkedTransferQueue.class); |
1221 |
|
1222 |
static long objectFieldOffset(sun.misc.Unsafe UNSAFE, |
1223 |
String field, Class<?> klazz) { |
1224 |
try { |
1225 |
return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field)); |
1226 |
} catch (NoSuchFieldException e) { |
1227 |
// Convert Exception to corresponding Error |
1228 |
NoSuchFieldError error = new NoSuchFieldError(field); |
1229 |
error.initCause(e); |
1230 |
throw error; |
1231 |
} |
1232 |
} |
1233 |
} |