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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/publicdomain/zero/1.0/ |
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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.lang.invoke.MethodHandles; |
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import java.lang.invoke.VarHandle; |
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import java.util.AbstractQueue; |
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import java.util.Arrays; |
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import java.util.Collection; |
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import java.util.Iterator; |
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import java.util.NoSuchElementException; |
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import java.util.Objects; |
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import java.util.Queue; |
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import java.util.Spliterator; |
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import java.util.Spliterators; |
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import java.util.concurrent.locks.LockSupport; |
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import java.util.function.Consumer; |
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import java.util.function.Predicate; |
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|
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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} method |
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* 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, and so may report |
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* inaccurate results if this collection is modified during traversal. |
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* |
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* <p>Bulk operations that add, remove, or examine multiple elements, |
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* such as {@link #addAll}, {@link #removeIf} or {@link #forEach}, |
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* are <em>not</em> guaranteed to be performed atomically. |
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* For example, a {@code forEach} traversal concurrent with an {@code |
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* addAll} operation might observe only some of the added elements. |
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* |
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* <p>This class and its iterator implement all of the <em>optional</em> |
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* methods of the {@link Collection} and {@link 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 queue |
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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.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf) |
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* are (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/~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. While this would be a terrible idea |
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* in 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 slack. |
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* However, they may be retried at any time to maintain targets. |
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* Even when using very small slack values, this approach works |
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* well for dual queues because it allows all operations up to the |
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* point of matching or appending an item (hence potentially |
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* allowing progress by another thread) to be read-only, thus not |
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* introducing any further contention. As described below, we |
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* implement this by performing slack maintenance retries only |
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* 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 chains of dead nodes. |
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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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* 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 two phases. The first is implemented |
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* in method xfer, the second in method awaitMatch. |
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* |
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* 1. Traverse until matching or appending (method xfer) |
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* |
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* Conceptually, we simply traverse all nodes starting from head. |
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* If we encounter an unmatched node of opposite mode, we match |
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* it and return, also updating head (by at least 2 hops) to |
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* one past the matched node (or the node itself if it's the |
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* pinned trailing node). Traversals also check for the |
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* possibility of falling off-list, in which case they restart. |
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* |
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* If the trailing node of the list is reached, a match is not |
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* possible. If this call was untimed poll or tryTransfer |
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* (argument "how" is NOW), return empty-handed immediately. |
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* Else a new node is CAS-appended. On successful append, if |
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* this call was ASYNC (e.g. offer), an element was |
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* successfully added to the end of the queue and we return. |
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* |
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* Of course, this naive traversal is O(n) when no match is |
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* possible. We optimize the traversal by maintaining a tail |
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* pointer, which is expected to be "near" the end of the list. |
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* It is only safe to fast-forward to tail (in the presence of |
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* arbitrary concurrent changes) if it is pointing to a node of |
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* the same mode, even if it is dead (in this case no preceding |
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* node could still be matchable by this traversal). If we |
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* need to restart due to falling off-list, we can again |
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* fast-forward to tail, but only if it has changed since the |
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* last traversal (else we might loop forever). If tail cannot |
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* be used, traversal starts at head (but in this case we |
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* expect to be able to match near head). As with head, we |
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* CAS-advance the tail pointer by at least two hops. |
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* |
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* 2. 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 it might 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 after 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 reachable. |
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* Situations leading to such buildup are uncommon but can occur |
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* in practice; for example when a series of short timed calls to |
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* poll repeatedly time out at the trailing node but otherwise |
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* never fall off the list because of an untimed call to take() at |
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* the front of the 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"). |
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* We trigger a full sweep when the estimate exceeds a threshold |
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* ("SWEEP_THRESHOLD") indicating the maximum number of estimated |
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* removal failures to tolerate before sweeping through, unlinking |
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* cancelled nodes that were not unlinked upon initial removal. |
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* We perform sweeps by the thread hitting threshold (rather than |
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* background threads or by spreading work to other threads) |
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* because in the main contexts in which removal occurs, the |
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* caller is timed-out or cancelled, which are not 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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* Because traversal operations on the linked list of nodes are a |
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* natural opportunity to sweep dead nodes, we generally do so, |
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* including all the operations that might remove elements as they |
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* traverse, such as removeIf and Iterator.remove. This largely |
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* eliminates long chains of dead interior nodes, except from |
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* cancelled or timed out blocking operations. |
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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. Writes that are intrinsically ordered wrt |
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* 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 when not waiting for a match |
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|
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/** |
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* Constructs a data node holding item if item is non-null, |
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* else a request node. Uses relaxed write because item can |
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* only be seen after piggy-backing publication via CAS. |
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*/ |
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Node(Object item) { |
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ITEM.set(this, item); |
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isData = (item != null); |
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} |
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|
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/** Constructs a (matched data) dummy node. */ |
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Node() { |
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isData = true; |
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} |
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|
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final boolean casNext(Node cmp, Node val) { |
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// assert val != null; |
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return NEXT.compareAndSet(this, 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 isData == (cmp != null); |
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// assert isData == (val == null); |
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// assert !(cmp instanceof Node); |
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return ITEM.compareAndSet(this, cmp, val); |
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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 selfLink() { |
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// assert isMatched(); |
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NEXT.setRelease(this, this); |
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} |
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|
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final void appendRelaxed(Node next) { |
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// assert next != null; |
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// assert this.next == null; |
461 |
NEXT.set(this, next); |
462 |
} |
463 |
|
464 |
/** |
465 |
* Sets item (of a request node) to self and waiter to null, |
466 |
* to avoid garbage retention after matching or cancelling. |
467 |
* Uses relaxed writes because order is already constrained in |
468 |
* the only calling contexts: item is forgotten only after |
469 |
* volatile/atomic mechanics that extract items, and visitors |
470 |
* of request nodes only ever check whether item is null. |
471 |
* Similarly, clearing waiter follows either CAS or return |
472 |
* from park (if ever parked; else we don't care). |
473 |
*/ |
474 |
final void forgetContents() { |
475 |
// assert isMatched(); |
476 |
if (!isData) |
477 |
ITEM.set(this, this); |
478 |
WAITER.set(this, null); |
479 |
} |
480 |
|
481 |
/** |
482 |
* Returns true if this node has been matched, including the |
483 |
* case of artificial matches due to cancellation. |
484 |
*/ |
485 |
final boolean isMatched() { |
486 |
return isData == (item == null); |
487 |
} |
488 |
|
489 |
/** Tries to CAS-match this node; if successful, wakes waiter. */ |
490 |
final boolean tryMatch(Object cmp, Object val) { |
491 |
if (casItem(cmp, val)) { |
492 |
LockSupport.unpark(waiter); |
493 |
return true; |
494 |
} |
495 |
return false; |
496 |
} |
497 |
|
498 |
/** |
499 |
* Returns true if a node with the given mode cannot be |
500 |
* appended to this node because this node is unmatched and |
501 |
* has opposite data mode. |
502 |
*/ |
503 |
final boolean cannotPrecede(boolean haveData) { |
504 |
boolean d = isData; |
505 |
return d != haveData && d != (item == null); |
506 |
} |
507 |
|
508 |
private static final long serialVersionUID = -3375979862319811754L; |
509 |
} |
510 |
|
511 |
/** |
512 |
* A node from which the first live (non-matched) node (if any) |
513 |
* can be reached in O(1) time. |
514 |
* Invariants: |
515 |
* - all live nodes are reachable from head via .next |
516 |
* - head != null |
517 |
* - (tmp = head).next != tmp || tmp != head |
518 |
* Non-invariants: |
519 |
* - head may or may not be live |
520 |
* - it is permitted for tail to lag behind head, that is, for tail |
521 |
* to not be reachable from head! |
522 |
*/ |
523 |
transient volatile Node head; |
524 |
|
525 |
/** |
526 |
* A node from which the last node on list (that is, the unique |
527 |
* node with node.next == null) can be reached in O(1) time. |
528 |
* Invariants: |
529 |
* - the last node is always reachable from tail via .next |
530 |
* - tail != null |
531 |
* Non-invariants: |
532 |
* - tail may or may not be live |
533 |
* - it is permitted for tail to lag behind head, that is, for tail |
534 |
* to not be reachable from head! |
535 |
* - tail.next may or may not be self-linked. |
536 |
*/ |
537 |
private transient volatile Node tail; |
538 |
|
539 |
/** The number of apparent failures to unsplice cancelled nodes */ |
540 |
private transient volatile int sweepVotes; |
541 |
|
542 |
private boolean casTail(Node cmp, Node val) { |
543 |
// assert cmp != null; |
544 |
// assert val != null; |
545 |
return TAIL.compareAndSet(this, cmp, val); |
546 |
} |
547 |
|
548 |
private boolean casHead(Node cmp, Node val) { |
549 |
return HEAD.compareAndSet(this, cmp, val); |
550 |
} |
551 |
|
552 |
/** Atomic version of ++sweepVotes. */ |
553 |
private int incSweepVotes() { |
554 |
return (int) SWEEPVOTES.getAndAdd(this, 1) + 1; |
555 |
} |
556 |
|
557 |
/** |
558 |
* Tries to CAS pred.next (or head, if pred is null) from c to p. |
559 |
* Caller must ensure that we're not unlinking the trailing node. |
560 |
*/ |
561 |
private boolean tryCasSuccessor(Node pred, Node c, Node p) { |
562 |
// assert p != null; |
563 |
// assert c.isData != (c.item != null); |
564 |
// assert c != p; |
565 |
if (pred != null) |
566 |
return pred.casNext(c, p); |
567 |
if (casHead(c, p)) { |
568 |
c.selfLink(); |
569 |
return true; |
570 |
} |
571 |
return false; |
572 |
} |
573 |
|
574 |
/** |
575 |
* Collapses dead (matched) nodes between pred and q. |
576 |
* @param pred the last known live node, or null if none |
577 |
* @param c the first dead node |
578 |
* @param p the last dead node |
579 |
* @param q p.next: the next live node, or null if at end |
580 |
* @return pred if pred still alive and CAS succeeded; else p |
581 |
*/ |
582 |
private Node skipDeadNodes(Node pred, Node c, Node p, Node q) { |
583 |
// assert pred != c; |
584 |
// assert p != q; |
585 |
// assert c.isMatched(); |
586 |
// assert p.isMatched(); |
587 |
if (q == null) { |
588 |
// Never unlink trailing node. |
589 |
if (c == p) return pred; |
590 |
q = p; |
591 |
} |
592 |
return (tryCasSuccessor(pred, c, q) |
593 |
&& (pred == null || !pred.isMatched())) |
594 |
? pred : p; |
595 |
} |
596 |
|
597 |
/** |
598 |
* Collapses dead (matched) nodes from h (which was once head) to p. |
599 |
* Caller ensures all nodes from h up to and including p are dead. |
600 |
*/ |
601 |
private void skipDeadNodesNearHead(Node h, Node p) { |
602 |
// assert h != null; |
603 |
// assert h != p; |
604 |
// assert p.isMatched(); |
605 |
for (;;) { |
606 |
final Node q; |
607 |
if ((q = p.next) == null) break; |
608 |
else if (!q.isMatched()) { p = q; break; } |
609 |
else if (p == (p = q)) return; |
610 |
} |
611 |
if (casHead(h, p)) |
612 |
h.selfLink(); |
613 |
} |
614 |
|
615 |
/* Possible values for "how" argument in xfer method. */ |
616 |
|
617 |
private static final int NOW = 0; // for untimed poll, tryTransfer |
618 |
private static final int ASYNC = 1; // for offer, put, add |
619 |
private static final int SYNC = 2; // for transfer, take |
620 |
private static final int TIMED = 3; // for timed poll, tryTransfer |
621 |
|
622 |
/** |
623 |
* Implements all queuing methods. See above for explanation. |
624 |
* |
625 |
* @param e the item or null for take |
626 |
* @param haveData true if this is a put, else a take |
627 |
* @param how NOW, ASYNC, SYNC, or TIMED |
628 |
* @param nanos timeout in nanosecs, used only if mode is TIMED |
629 |
* @return an item if matched, else e |
630 |
* @throws NullPointerException if haveData mode but e is null |
631 |
*/ |
632 |
@SuppressWarnings("unchecked") |
633 |
private E xfer(E e, boolean haveData, int how, long nanos) { |
634 |
if (haveData && (e == null)) |
635 |
throw new NullPointerException(); |
636 |
|
637 |
restart: for (Node s = null, t = null, h = null;;) { |
638 |
for (Node p = (t != (t = tail) && t.isData == haveData) ? t |
639 |
: (h = head);; ) { |
640 |
final Node q; final Object item; |
641 |
if (p.isData != haveData |
642 |
&& haveData == ((item = p.item) == null)) { |
643 |
if (h == null) h = head; |
644 |
if (p.tryMatch(item, e)) { |
645 |
if (h != p) skipDeadNodesNearHead(h, p); |
646 |
return (E) item; |
647 |
} |
648 |
} |
649 |
if ((q = p.next) == null) { |
650 |
if (how == NOW) return e; |
651 |
if (s == null) s = new Node(e); |
652 |
if (!p.casNext(null, s)) continue; |
653 |
if (p != t) casTail(t, s); |
654 |
if (how == ASYNC) return e; |
655 |
return awaitMatch(s, p, e, (how == TIMED), nanos); |
656 |
} |
657 |
if (p == (p = q)) continue restart; |
658 |
} |
659 |
} |
660 |
} |
661 |
|
662 |
/** |
663 |
* Spins/yields/blocks until node s is matched or caller gives up. |
664 |
* |
665 |
* @param s the waiting node |
666 |
* @param pred the predecessor of s, or null if unknown (the null |
667 |
* case does not occur in any current calls but may in possible |
668 |
* future extensions) |
669 |
* @param e the comparison value for checking match |
670 |
* @param timed if true, wait only until timeout elapses |
671 |
* @param nanos timeout in nanosecs, used only if timed is true |
672 |
* @return matched item, or e if unmatched on interrupt or timeout |
673 |
*/ |
674 |
private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) { |
675 |
final long deadline = timed ? System.nanoTime() + nanos : 0L; |
676 |
Thread w = Thread.currentThread(); |
677 |
int spins = -1; // initialized after first item and cancel checks |
678 |
ThreadLocalRandom randomYields = null; // bound if needed |
679 |
|
680 |
for (;;) { |
681 |
final Object item; |
682 |
if ((item = s.item) != e) { // matched |
683 |
// assert item != s; |
684 |
s.forgetContents(); // avoid garbage |
685 |
@SuppressWarnings("unchecked") E itemE = (E) item; |
686 |
return itemE; |
687 |
} |
688 |
else if (w.isInterrupted() || (timed && nanos <= 0L)) { |
689 |
// try to cancel and unlink |
690 |
if (s.casItem(e, s.isData ? null : s)) { |
691 |
unsplice(pred, s); |
692 |
return e; |
693 |
} |
694 |
// return normally if lost CAS |
695 |
} |
696 |
else if (spins < 0) { // establish spins at/near front |
697 |
if ((spins = spinsFor(pred, s.isData)) > 0) |
698 |
randomYields = ThreadLocalRandom.current(); |
699 |
} |
700 |
else if (spins > 0) { // spin |
701 |
--spins; |
702 |
if (randomYields.nextInt(CHAINED_SPINS) == 0) |
703 |
Thread.yield(); // occasionally yield |
704 |
} |
705 |
else if (s.waiter == null) { |
706 |
s.waiter = w; // request unpark then recheck |
707 |
} |
708 |
else if (timed) { |
709 |
nanos = deadline - System.nanoTime(); |
710 |
if (nanos > 0L) |
711 |
LockSupport.parkNanos(this, nanos); |
712 |
} |
713 |
else { |
714 |
LockSupport.park(this); |
715 |
} |
716 |
} |
717 |
} |
718 |
|
719 |
/** |
720 |
* Returns spin/yield value for a node with given predecessor and |
721 |
* data mode. See above for explanation. |
722 |
*/ |
723 |
private static int spinsFor(Node pred, boolean haveData) { |
724 |
if (MP && pred != null) { |
725 |
if (pred.isData != haveData) // phase change |
726 |
return FRONT_SPINS + CHAINED_SPINS; |
727 |
if (pred.isMatched()) // probably at front |
728 |
return FRONT_SPINS; |
729 |
if (pred.waiter == null) // pred apparently spinning |
730 |
return CHAINED_SPINS; |
731 |
} |
732 |
return 0; |
733 |
} |
734 |
|
735 |
/* -------------- Traversal methods -------------- */ |
736 |
|
737 |
/** |
738 |
* Returns the first unmatched data node, or null if none. |
739 |
* Callers must recheck if the returned node is unmatched |
740 |
* before using. |
741 |
*/ |
742 |
final Node firstDataNode() { |
743 |
Node first = null; |
744 |
restartFromHead: for (;;) { |
745 |
Node h = head, p = h; |
746 |
for (; p != null;) { |
747 |
final Object item; |
748 |
if ((item = p.item) != null) { |
749 |
if (p.isData) { |
750 |
first = p; |
751 |
break; |
752 |
} |
753 |
} |
754 |
else if (!p.isData) |
755 |
break; |
756 |
final Node q; |
757 |
if ((q = p.next) == null) |
758 |
break; |
759 |
if (p == (p = q)) |
760 |
continue restartFromHead; |
761 |
} |
762 |
if (p != h && casHead(h, p)) |
763 |
h.selfLink(); |
764 |
return first; |
765 |
} |
766 |
} |
767 |
|
768 |
/** |
769 |
* Traverses and counts unmatched nodes of the given mode. |
770 |
* Used by methods size and getWaitingConsumerCount. |
771 |
*/ |
772 |
private int countOfMode(boolean data) { |
773 |
restartFromHead: for (;;) { |
774 |
int count = 0; |
775 |
for (Node p = head; p != null;) { |
776 |
if (!p.isMatched()) { |
777 |
if (p.isData != data) |
778 |
return 0; |
779 |
if (++count == Integer.MAX_VALUE) |
780 |
break; // @see Collection.size() |
781 |
} |
782 |
if (p == (p = p.next)) |
783 |
continue restartFromHead; |
784 |
} |
785 |
return count; |
786 |
} |
787 |
} |
788 |
|
789 |
public String toString() { |
790 |
String[] a = null; |
791 |
restartFromHead: for (;;) { |
792 |
int charLength = 0; |
793 |
int size = 0; |
794 |
for (Node p = head; p != null;) { |
795 |
Object item = p.item; |
796 |
if (p.isData) { |
797 |
if (item != null) { |
798 |
if (a == null) |
799 |
a = new String[4]; |
800 |
else if (size == a.length) |
801 |
a = Arrays.copyOf(a, 2 * size); |
802 |
String s = item.toString(); |
803 |
a[size++] = s; |
804 |
charLength += s.length(); |
805 |
} |
806 |
} else if (item == null) |
807 |
break; |
808 |
if (p == (p = p.next)) |
809 |
continue restartFromHead; |
810 |
} |
811 |
|
812 |
if (size == 0) |
813 |
return "[]"; |
814 |
|
815 |
return Helpers.toString(a, size, charLength); |
816 |
} |
817 |
} |
818 |
|
819 |
private Object[] toArrayInternal(Object[] a) { |
820 |
Object[] x = a; |
821 |
restartFromHead: for (;;) { |
822 |
int size = 0; |
823 |
for (Node p = head; p != null;) { |
824 |
Object item = p.item; |
825 |
if (p.isData) { |
826 |
if (item != null) { |
827 |
if (x == null) |
828 |
x = new Object[4]; |
829 |
else if (size == x.length) |
830 |
x = Arrays.copyOf(x, 2 * (size + 4)); |
831 |
x[size++] = item; |
832 |
} |
833 |
} else if (item == null) |
834 |
break; |
835 |
if (p == (p = p.next)) |
836 |
continue restartFromHead; |
837 |
} |
838 |
if (x == null) |
839 |
return new Object[0]; |
840 |
else if (a != null && size <= a.length) { |
841 |
if (a != x) |
842 |
System.arraycopy(x, 0, a, 0, size); |
843 |
if (size < a.length) |
844 |
a[size] = null; |
845 |
return a; |
846 |
} |
847 |
return (size == x.length) ? x : Arrays.copyOf(x, size); |
848 |
} |
849 |
} |
850 |
|
851 |
/** |
852 |
* Returns an array containing all of the elements in this queue, in |
853 |
* proper sequence. |
854 |
* |
855 |
* <p>The returned array will be "safe" in that no references to it are |
856 |
* maintained by this queue. (In other words, this method must allocate |
857 |
* a new array). The caller is thus free to modify the returned array. |
858 |
* |
859 |
* <p>This method acts as bridge between array-based and collection-based |
860 |
* APIs. |
861 |
* |
862 |
* @return an array containing all of the elements in this queue |
863 |
*/ |
864 |
public Object[] toArray() { |
865 |
return toArrayInternal(null); |
866 |
} |
867 |
|
868 |
/** |
869 |
* Returns an array containing all of the elements in this queue, in |
870 |
* proper sequence; the runtime type of the returned array is that of |
871 |
* the specified array. If the queue fits in the specified array, it |
872 |
* is returned therein. Otherwise, a new array is allocated with the |
873 |
* runtime type of the specified array and the size of this queue. |
874 |
* |
875 |
* <p>If this queue fits in the specified array with room to spare |
876 |
* (i.e., the array has more elements than this queue), the element in |
877 |
* the array immediately following the end of the queue is set to |
878 |
* {@code null}. |
879 |
* |
880 |
* <p>Like the {@link #toArray()} method, this method acts as bridge between |
881 |
* array-based and collection-based APIs. Further, this method allows |
882 |
* precise control over the runtime type of the output array, and may, |
883 |
* under certain circumstances, be used to save allocation costs. |
884 |
* |
885 |
* <p>Suppose {@code x} is a queue known to contain only strings. |
886 |
* The following code can be used to dump the queue into a newly |
887 |
* allocated array of {@code String}: |
888 |
* |
889 |
* <pre> {@code String[] y = x.toArray(new String[0]);}</pre> |
890 |
* |
891 |
* Note that {@code toArray(new Object[0])} is identical in function to |
892 |
* {@code toArray()}. |
893 |
* |
894 |
* @param a the array into which the elements of the queue are to |
895 |
* be stored, if it is big enough; otherwise, a new array of the |
896 |
* same runtime type is allocated for this purpose |
897 |
* @return an array containing all of the elements in this queue |
898 |
* @throws ArrayStoreException if the runtime type of the specified array |
899 |
* is not a supertype of the runtime type of every element in |
900 |
* this queue |
901 |
* @throws NullPointerException if the specified array is null |
902 |
*/ |
903 |
@SuppressWarnings("unchecked") |
904 |
public <T> T[] toArray(T[] a) { |
905 |
Objects.requireNonNull(a); |
906 |
return (T[]) toArrayInternal(a); |
907 |
} |
908 |
|
909 |
/** |
910 |
* Weakly-consistent iterator. |
911 |
* |
912 |
* Lazily updated ancestor is expected to be amortized O(1) remove(), |
913 |
* but O(n) in the worst case, when lastRet is concurrently deleted. |
914 |
*/ |
915 |
final class Itr implements Iterator<E> { |
916 |
private Node nextNode; // next node to return item for |
917 |
private E nextItem; // the corresponding item |
918 |
private Node lastRet; // last returned node, to support remove |
919 |
private Node ancestor; // Helps unlink lastRet on remove() |
920 |
|
921 |
/** |
922 |
* Moves to next node after pred, or first node if pred null. |
923 |
*/ |
924 |
@SuppressWarnings("unchecked") |
925 |
private void advance(Node pred) { |
926 |
for (Node p = (pred == null) ? head : pred.next, c = p; |
927 |
p != null; ) { |
928 |
final Object item; |
929 |
if ((item = p.item) != null && p.isData) { |
930 |
nextNode = p; |
931 |
nextItem = (E) item; |
932 |
if (c != p) |
933 |
tryCasSuccessor(pred, c, p); |
934 |
return; |
935 |
} |
936 |
else if (!p.isData && item == null) |
937 |
break; |
938 |
if (c != p && !tryCasSuccessor(pred, c, c = p)) { |
939 |
pred = p; |
940 |
c = p = p.next; |
941 |
} |
942 |
else if (p == (p = p.next)) { |
943 |
pred = null; |
944 |
c = p = head; |
945 |
} |
946 |
} |
947 |
nextItem = null; |
948 |
nextNode = null; |
949 |
} |
950 |
|
951 |
Itr() { |
952 |
advance(null); |
953 |
} |
954 |
|
955 |
public final boolean hasNext() { |
956 |
return nextNode != null; |
957 |
} |
958 |
|
959 |
public final E next() { |
960 |
final Node p; |
961 |
if ((p = nextNode) == null) throw new NoSuchElementException(); |
962 |
E e = nextItem; |
963 |
advance(lastRet = p); |
964 |
return e; |
965 |
} |
966 |
|
967 |
public void forEachRemaining(Consumer<? super E> action) { |
968 |
Objects.requireNonNull(action); |
969 |
Node q = null; |
970 |
for (Node p; (p = nextNode) != null; advance(q = p)) |
971 |
action.accept(nextItem); |
972 |
if (q != null) |
973 |
lastRet = q; |
974 |
} |
975 |
|
976 |
public final void remove() { |
977 |
final Node lastRet = this.lastRet; |
978 |
if (lastRet == null) |
979 |
throw new IllegalStateException(); |
980 |
this.lastRet = null; |
981 |
if (lastRet.item == null) // already deleted? |
982 |
return; |
983 |
// Advance ancestor, collapsing intervening dead nodes |
984 |
Node pred = ancestor; |
985 |
for (Node p = (pred == null) ? head : pred.next, c = p, q; |
986 |
p != null; ) { |
987 |
if (p == lastRet) { |
988 |
final Object item; |
989 |
if ((item = p.item) != null) |
990 |
p.tryMatch(item, null); |
991 |
if ((q = p.next) == null) q = p; |
992 |
if (c != q) tryCasSuccessor(pred, c, q); |
993 |
ancestor = pred; |
994 |
return; |
995 |
} |
996 |
final Object item; final boolean pAlive; |
997 |
if (pAlive = ((item = p.item) != null && p.isData)) { |
998 |
// exceptionally, nothing to do |
999 |
} |
1000 |
else if (!p.isData && item == null) |
1001 |
break; |
1002 |
if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) { |
1003 |
pred = p; |
1004 |
c = p = p.next; |
1005 |
} |
1006 |
else if (p == (p = p.next)) { |
1007 |
pred = null; |
1008 |
c = p = head; |
1009 |
} |
1010 |
} |
1011 |
// traversal failed to find lastRet; must have been deleted; |
1012 |
// leave ancestor at original location to avoid overshoot; |
1013 |
// better luck next time! |
1014 |
|
1015 |
// assert lastRet.isMatched(); |
1016 |
} |
1017 |
} |
1018 |
|
1019 |
/** A customized variant of Spliterators.IteratorSpliterator */ |
1020 |
final class LTQSpliterator implements Spliterator<E> { |
1021 |
static final int MAX_BATCH = 1 << 25; // max batch array size; |
1022 |
Node current; // current node; null until initialized |
1023 |
int batch; // batch size for splits |
1024 |
boolean exhausted; // true when no more nodes |
1025 |
LTQSpliterator() {} |
1026 |
|
1027 |
public Spliterator<E> trySplit() { |
1028 |
Node p, q; |
1029 |
if ((p = current()) == null || (q = p.next) == null) |
1030 |
return null; |
1031 |
int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH); |
1032 |
Object[] a = null; |
1033 |
do { |
1034 |
final Object item = p.item; |
1035 |
if (p.isData) { |
1036 |
if (item != null) { |
1037 |
if (a == null) |
1038 |
a = new Object[n]; |
1039 |
a[i++] = item; |
1040 |
} |
1041 |
} else if (item == null) { |
1042 |
p = null; |
1043 |
break; |
1044 |
} |
1045 |
if (p == (p = q)) |
1046 |
p = firstDataNode(); |
1047 |
} while (p != null && (q = p.next) != null && i < n); |
1048 |
setCurrent(p); |
1049 |
return (i == 0) ? null : |
1050 |
Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED | |
1051 |
Spliterator.NONNULL | |
1052 |
Spliterator.CONCURRENT)); |
1053 |
} |
1054 |
|
1055 |
public void forEachRemaining(Consumer<? super E> action) { |
1056 |
Objects.requireNonNull(action); |
1057 |
final Node p; |
1058 |
if ((p = current()) != null) { |
1059 |
current = null; |
1060 |
exhausted = true; |
1061 |
forEachFrom(action, p); |
1062 |
} |
1063 |
} |
1064 |
|
1065 |
@SuppressWarnings("unchecked") |
1066 |
public boolean tryAdvance(Consumer<? super E> action) { |
1067 |
Objects.requireNonNull(action); |
1068 |
Node p; |
1069 |
if ((p = current()) != null) { |
1070 |
E e = null; |
1071 |
do { |
1072 |
final Object item = p.item; |
1073 |
final boolean isData = p.isData; |
1074 |
if (p == (p = p.next)) |
1075 |
p = head; |
1076 |
if (isData) { |
1077 |
if (item != null) { |
1078 |
e = (E) item; |
1079 |
break; |
1080 |
} |
1081 |
} |
1082 |
else if (item == null) |
1083 |
p = null; |
1084 |
} while (p != null); |
1085 |
setCurrent(p); |
1086 |
if (e != null) { |
1087 |
action.accept(e); |
1088 |
return true; |
1089 |
} |
1090 |
} |
1091 |
return false; |
1092 |
} |
1093 |
|
1094 |
private void setCurrent(Node p) { |
1095 |
if ((current = p) == null) |
1096 |
exhausted = true; |
1097 |
} |
1098 |
|
1099 |
private Node current() { |
1100 |
Node p; |
1101 |
if ((p = current) == null && !exhausted) |
1102 |
setCurrent(p = firstDataNode()); |
1103 |
return p; |
1104 |
} |
1105 |
|
1106 |
public long estimateSize() { return Long.MAX_VALUE; } |
1107 |
|
1108 |
public int characteristics() { |
1109 |
return (Spliterator.ORDERED | |
1110 |
Spliterator.NONNULL | |
1111 |
Spliterator.CONCURRENT); |
1112 |
} |
1113 |
} |
1114 |
|
1115 |
/** |
1116 |
* Returns a {@link Spliterator} over the elements in this queue. |
1117 |
* |
1118 |
* <p>The returned spliterator is |
1119 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1120 |
* |
1121 |
* <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT}, |
1122 |
* {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}. |
1123 |
* |
1124 |
* @implNote |
1125 |
* The {@code Spliterator} implements {@code trySplit} to permit limited |
1126 |
* parallelism. |
1127 |
* |
1128 |
* @return a {@code Spliterator} over the elements in this queue |
1129 |
* @since 1.8 |
1130 |
*/ |
1131 |
public Spliterator<E> spliterator() { |
1132 |
return new LTQSpliterator(); |
1133 |
} |
1134 |
|
1135 |
/* -------------- Removal methods -------------- */ |
1136 |
|
1137 |
/** |
1138 |
* Unsplices (now or later) the given deleted/cancelled node with |
1139 |
* the given predecessor. |
1140 |
* |
1141 |
* @param pred a node that was at one time known to be the |
1142 |
* predecessor of s |
1143 |
* @param s the node to be unspliced |
1144 |
*/ |
1145 |
final void unsplice(Node pred, Node s) { |
1146 |
// assert pred != null; |
1147 |
// assert pred != s; |
1148 |
// assert s != null; |
1149 |
// assert s.isMatched(); |
1150 |
// assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0; |
1151 |
s.waiter = null; // disable signals |
1152 |
/* |
1153 |
* See above for rationale. Briefly: if pred still points to |
1154 |
* s, try to unlink s. If s cannot be unlinked, because it is |
1155 |
* trailing node or pred might be unlinked, and neither pred |
1156 |
* nor s are head or offlist, add to sweepVotes, and if enough |
1157 |
* votes have accumulated, sweep. |
1158 |
*/ |
1159 |
if (pred != null && pred.next == s) { |
1160 |
Node n = s.next; |
1161 |
if (n == null || |
1162 |
(n != s && pred.casNext(s, n) && pred.isMatched())) { |
1163 |
for (;;) { // check if at, or could be, head |
1164 |
Node h = head; |
1165 |
if (h == pred || h == s) |
1166 |
return; // at head or list empty |
1167 |
if (!h.isMatched()) |
1168 |
break; |
1169 |
Node hn = h.next; |
1170 |
if (hn == null) |
1171 |
return; // now empty |
1172 |
if (hn != h && casHead(h, hn)) |
1173 |
h.selfLink(); // advance head |
1174 |
} |
1175 |
// sweep every SWEEP_THRESHOLD votes |
1176 |
if (pred.next != pred && s.next != s // recheck if offlist |
1177 |
&& (incSweepVotes() & (SWEEP_THRESHOLD - 1)) == 0) |
1178 |
sweep(); |
1179 |
} |
1180 |
} |
1181 |
} |
1182 |
|
1183 |
/** |
1184 |
* Unlinks matched (typically cancelled) nodes encountered in a |
1185 |
* traversal from head. |
1186 |
*/ |
1187 |
private void sweep() { |
1188 |
for (Node p = head, s, n; p != null && (s = p.next) != null; ) { |
1189 |
if (!s.isMatched()) |
1190 |
// Unmatched nodes are never self-linked |
1191 |
p = s; |
1192 |
else if ((n = s.next) == null) // trailing node is pinned |
1193 |
break; |
1194 |
else if (s == n) // stale |
1195 |
// No need to also check for p == s, since that implies s == n |
1196 |
p = head; |
1197 |
else |
1198 |
p.casNext(s, n); |
1199 |
} |
1200 |
} |
1201 |
|
1202 |
/** |
1203 |
* Creates an initially empty {@code LinkedTransferQueue}. |
1204 |
*/ |
1205 |
public LinkedTransferQueue() { |
1206 |
head = tail = new Node(); |
1207 |
} |
1208 |
|
1209 |
/** |
1210 |
* Creates a {@code LinkedTransferQueue} |
1211 |
* initially containing the elements of the given collection, |
1212 |
* added in traversal order of the collection's iterator. |
1213 |
* |
1214 |
* @param c the collection of elements to initially contain |
1215 |
* @throws NullPointerException if the specified collection or any |
1216 |
* of its elements are null |
1217 |
*/ |
1218 |
public LinkedTransferQueue(Collection<? extends E> c) { |
1219 |
Node h = null, t = null; |
1220 |
for (E e : c) { |
1221 |
Node newNode = new Node(Objects.requireNonNull(e)); |
1222 |
if (h == null) |
1223 |
h = t = newNode; |
1224 |
else |
1225 |
t.appendRelaxed(t = newNode); |
1226 |
} |
1227 |
if (h == null) |
1228 |
h = t = new Node(); |
1229 |
head = h; |
1230 |
tail = t; |
1231 |
} |
1232 |
|
1233 |
/** |
1234 |
* Inserts the specified element at the tail of this queue. |
1235 |
* As the queue is unbounded, this method will never block. |
1236 |
* |
1237 |
* @throws NullPointerException if the specified element is null |
1238 |
*/ |
1239 |
public void put(E e) { |
1240 |
xfer(e, true, ASYNC, 0); |
1241 |
} |
1242 |
|
1243 |
/** |
1244 |
* Inserts the specified element at the tail of this queue. |
1245 |
* As the queue is unbounded, this method will never block or |
1246 |
* return {@code false}. |
1247 |
* |
1248 |
* @return {@code true} (as specified by |
1249 |
* {@link java.util.concurrent.BlockingQueue#offer(Object,long,TimeUnit) |
1250 |
* BlockingQueue.offer}) |
1251 |
* @throws NullPointerException if the specified element is null |
1252 |
*/ |
1253 |
public boolean offer(E e, long timeout, TimeUnit unit) { |
1254 |
xfer(e, true, ASYNC, 0); |
1255 |
return true; |
1256 |
} |
1257 |
|
1258 |
/** |
1259 |
* Inserts the specified element at the tail of this queue. |
1260 |
* As the queue is unbounded, this method will never return {@code false}. |
1261 |
* |
1262 |
* @return {@code true} (as specified by {@link Queue#offer}) |
1263 |
* @throws NullPointerException if the specified element is null |
1264 |
*/ |
1265 |
public boolean offer(E e) { |
1266 |
xfer(e, true, ASYNC, 0); |
1267 |
return true; |
1268 |
} |
1269 |
|
1270 |
/** |
1271 |
* Inserts the specified element at the tail of this queue. |
1272 |
* As the queue is unbounded, this method will never throw |
1273 |
* {@link IllegalStateException} or return {@code false}. |
1274 |
* |
1275 |
* @return {@code true} (as specified by {@link Collection#add}) |
1276 |
* @throws NullPointerException if the specified element is null |
1277 |
*/ |
1278 |
public boolean add(E e) { |
1279 |
xfer(e, true, ASYNC, 0); |
1280 |
return true; |
1281 |
} |
1282 |
|
1283 |
/** |
1284 |
* Transfers the element to a waiting consumer immediately, if possible. |
1285 |
* |
1286 |
* <p>More precisely, transfers the specified element immediately |
1287 |
* if there exists a consumer already waiting to receive it (in |
1288 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1289 |
* otherwise returning {@code false} without enqueuing the element. |
1290 |
* |
1291 |
* @throws NullPointerException if the specified element is null |
1292 |
*/ |
1293 |
public boolean tryTransfer(E e) { |
1294 |
return xfer(e, true, NOW, 0) == null; |
1295 |
} |
1296 |
|
1297 |
/** |
1298 |
* Transfers the element to a consumer, waiting if necessary to do so. |
1299 |
* |
1300 |
* <p>More precisely, transfers the specified element immediately |
1301 |
* if there exists a consumer already waiting to receive it (in |
1302 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1303 |
* else inserts the specified element at the tail of this queue |
1304 |
* and waits until the element is received by a consumer. |
1305 |
* |
1306 |
* @throws NullPointerException if the specified element is null |
1307 |
*/ |
1308 |
public void transfer(E e) throws InterruptedException { |
1309 |
if (xfer(e, true, SYNC, 0) != null) { |
1310 |
Thread.interrupted(); // failure possible only due to interrupt |
1311 |
throw new InterruptedException(); |
1312 |
} |
1313 |
} |
1314 |
|
1315 |
/** |
1316 |
* Transfers the element to a consumer if it is possible to do so |
1317 |
* before the timeout elapses. |
1318 |
* |
1319 |
* <p>More precisely, transfers the specified element immediately |
1320 |
* if there exists a consumer already waiting to receive it (in |
1321 |
* {@link #take} or timed {@link #poll(long,TimeUnit) poll}), |
1322 |
* else inserts the specified element at the tail of this queue |
1323 |
* and waits until the element is received by a consumer, |
1324 |
* returning {@code false} if the specified wait time elapses |
1325 |
* before the element can be transferred. |
1326 |
* |
1327 |
* @throws NullPointerException if the specified element is null |
1328 |
*/ |
1329 |
public boolean tryTransfer(E e, long timeout, TimeUnit unit) |
1330 |
throws InterruptedException { |
1331 |
if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null) |
1332 |
return true; |
1333 |
if (!Thread.interrupted()) |
1334 |
return false; |
1335 |
throw new InterruptedException(); |
1336 |
} |
1337 |
|
1338 |
public E take() throws InterruptedException { |
1339 |
E e = xfer(null, false, SYNC, 0); |
1340 |
if (e != null) |
1341 |
return e; |
1342 |
Thread.interrupted(); |
1343 |
throw new InterruptedException(); |
1344 |
} |
1345 |
|
1346 |
public E poll(long timeout, TimeUnit unit) throws InterruptedException { |
1347 |
E e = xfer(null, false, TIMED, unit.toNanos(timeout)); |
1348 |
if (e != null || !Thread.interrupted()) |
1349 |
return e; |
1350 |
throw new InterruptedException(); |
1351 |
} |
1352 |
|
1353 |
public E poll() { |
1354 |
return xfer(null, false, NOW, 0); |
1355 |
} |
1356 |
|
1357 |
/** |
1358 |
* @throws NullPointerException {@inheritDoc} |
1359 |
* @throws IllegalArgumentException {@inheritDoc} |
1360 |
*/ |
1361 |
public int drainTo(Collection<? super E> c) { |
1362 |
Objects.requireNonNull(c); |
1363 |
if (c == this) |
1364 |
throw new IllegalArgumentException(); |
1365 |
int n = 0; |
1366 |
for (E e; (e = poll()) != null; n++) |
1367 |
c.add(e); |
1368 |
return n; |
1369 |
} |
1370 |
|
1371 |
/** |
1372 |
* @throws NullPointerException {@inheritDoc} |
1373 |
* @throws IllegalArgumentException {@inheritDoc} |
1374 |
*/ |
1375 |
public int drainTo(Collection<? super E> c, int maxElements) { |
1376 |
Objects.requireNonNull(c); |
1377 |
if (c == this) |
1378 |
throw new IllegalArgumentException(); |
1379 |
int n = 0; |
1380 |
for (E e; n < maxElements && (e = poll()) != null; n++) |
1381 |
c.add(e); |
1382 |
return n; |
1383 |
} |
1384 |
|
1385 |
/** |
1386 |
* Returns an iterator over the elements in this queue in proper sequence. |
1387 |
* The elements will be returned in order from first (head) to last (tail). |
1388 |
* |
1389 |
* <p>The returned iterator is |
1390 |
* <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>. |
1391 |
* |
1392 |
* @return an iterator over the elements in this queue in proper sequence |
1393 |
*/ |
1394 |
public Iterator<E> iterator() { |
1395 |
return new Itr(); |
1396 |
} |
1397 |
|
1398 |
public E peek() { |
1399 |
restartFromHead: for (;;) { |
1400 |
for (Node p = head; p != null;) { |
1401 |
Object item = p.item; |
1402 |
if (p.isData) { |
1403 |
if (item != null) { |
1404 |
@SuppressWarnings("unchecked") E e = (E) item; |
1405 |
return e; |
1406 |
} |
1407 |
} |
1408 |
else if (item == null) |
1409 |
break; |
1410 |
if (p == (p = p.next)) |
1411 |
continue restartFromHead; |
1412 |
} |
1413 |
return null; |
1414 |
} |
1415 |
} |
1416 |
|
1417 |
/** |
1418 |
* Returns {@code true} if this queue contains no elements. |
1419 |
* |
1420 |
* @return {@code true} if this queue contains no elements |
1421 |
*/ |
1422 |
public boolean isEmpty() { |
1423 |
return firstDataNode() == null; |
1424 |
} |
1425 |
|
1426 |
public boolean hasWaitingConsumer() { |
1427 |
restartFromHead: for (;;) { |
1428 |
for (Node p = head; p != null;) { |
1429 |
Object item = p.item; |
1430 |
if (p.isData) { |
1431 |
if (item != null) |
1432 |
break; |
1433 |
} |
1434 |
else if (item == null) |
1435 |
return true; |
1436 |
if (p == (p = p.next)) |
1437 |
continue restartFromHead; |
1438 |
} |
1439 |
return false; |
1440 |
} |
1441 |
} |
1442 |
|
1443 |
/** |
1444 |
* Returns the number of elements in this queue. If this queue |
1445 |
* contains more than {@code Integer.MAX_VALUE} elements, returns |
1446 |
* {@code Integer.MAX_VALUE}. |
1447 |
* |
1448 |
* <p>Beware that, unlike in most collections, this method is |
1449 |
* <em>NOT</em> a constant-time operation. Because of the |
1450 |
* asynchronous nature of these queues, determining the current |
1451 |
* number of elements requires an O(n) traversal. |
1452 |
* |
1453 |
* @return the number of elements in this queue |
1454 |
*/ |
1455 |
public int size() { |
1456 |
return countOfMode(true); |
1457 |
} |
1458 |
|
1459 |
public int getWaitingConsumerCount() { |
1460 |
return countOfMode(false); |
1461 |
} |
1462 |
|
1463 |
/** |
1464 |
* Removes a single instance of the specified element from this queue, |
1465 |
* if it is present. More formally, removes an element {@code e} such |
1466 |
* that {@code o.equals(e)}, if this queue contains one or more such |
1467 |
* elements. |
1468 |
* Returns {@code true} if this queue contained the specified element |
1469 |
* (or equivalently, if this queue changed as a result of the call). |
1470 |
* |
1471 |
* @param o element to be removed from this queue, if present |
1472 |
* @return {@code true} if this queue changed as a result of the call |
1473 |
*/ |
1474 |
public boolean remove(Object o) { |
1475 |
if (o == null) return false; |
1476 |
restartFromHead: for (;;) { |
1477 |
for (Node p = head, pred = null; p != null; ) { |
1478 |
Node q = p.next; |
1479 |
final Object item; |
1480 |
if ((item = p.item) != null) { |
1481 |
if (p.isData) { |
1482 |
if (o.equals(item) && p.tryMatch(item, null)) { |
1483 |
skipDeadNodes(pred, p, p, q); |
1484 |
return true; |
1485 |
} |
1486 |
pred = p; p = q; continue; |
1487 |
} |
1488 |
} |
1489 |
else if (!p.isData) |
1490 |
break; |
1491 |
for (Node c = p;; q = p.next) { |
1492 |
if (q == null || !q.isMatched()) { |
1493 |
pred = skipDeadNodes(pred, c, p, q); p = q; break; |
1494 |
} |
1495 |
if (p == (p = q)) continue restartFromHead; |
1496 |
} |
1497 |
} |
1498 |
return false; |
1499 |
} |
1500 |
} |
1501 |
|
1502 |
/** |
1503 |
* Returns {@code true} if this queue contains the specified element. |
1504 |
* More formally, returns {@code true} if and only if this queue contains |
1505 |
* at least one element {@code e} such that {@code o.equals(e)}. |
1506 |
* |
1507 |
* @param o object to be checked for containment in this queue |
1508 |
* @return {@code true} if this queue contains the specified element |
1509 |
*/ |
1510 |
public boolean contains(Object o) { |
1511 |
if (o == null) return false; |
1512 |
restartFromHead: for (;;) { |
1513 |
for (Node p = head, pred = null; p != null; ) { |
1514 |
Node q = p.next; |
1515 |
final Object item; |
1516 |
if ((item = p.item) != null) { |
1517 |
if (p.isData) { |
1518 |
if (o.equals(item)) |
1519 |
return true; |
1520 |
pred = p; p = q; continue; |
1521 |
} |
1522 |
} |
1523 |
else if (!p.isData) |
1524 |
break; |
1525 |
for (Node c = p;; q = p.next) { |
1526 |
if (q == null || !q.isMatched()) { |
1527 |
pred = skipDeadNodes(pred, c, p, q); p = q; break; |
1528 |
} |
1529 |
if (p == (p = q)) continue restartFromHead; |
1530 |
} |
1531 |
} |
1532 |
return false; |
1533 |
} |
1534 |
} |
1535 |
|
1536 |
/** |
1537 |
* Always returns {@code Integer.MAX_VALUE} because a |
1538 |
* {@code LinkedTransferQueue} is not capacity constrained. |
1539 |
* |
1540 |
* @return {@code Integer.MAX_VALUE} (as specified by |
1541 |
* {@link java.util.concurrent.BlockingQueue#remainingCapacity() |
1542 |
* BlockingQueue.remainingCapacity}) |
1543 |
*/ |
1544 |
public int remainingCapacity() { |
1545 |
return Integer.MAX_VALUE; |
1546 |
} |
1547 |
|
1548 |
/** |
1549 |
* Saves this queue to a stream (that is, serializes it). |
1550 |
* |
1551 |
* @param s the stream |
1552 |
* @throws java.io.IOException if an I/O error occurs |
1553 |
* @serialData All of the elements (each an {@code E}) in |
1554 |
* the proper order, followed by a null |
1555 |
*/ |
1556 |
private void writeObject(java.io.ObjectOutputStream s) |
1557 |
throws java.io.IOException { |
1558 |
s.defaultWriteObject(); |
1559 |
for (E e : this) |
1560 |
s.writeObject(e); |
1561 |
// Use trailing null as sentinel |
1562 |
s.writeObject(null); |
1563 |
} |
1564 |
|
1565 |
/** |
1566 |
* Reconstitutes this queue from a stream (that is, deserializes it). |
1567 |
* @param s the stream |
1568 |
* @throws ClassNotFoundException if the class of a serialized object |
1569 |
* could not be found |
1570 |
* @throws java.io.IOException if an I/O error occurs |
1571 |
*/ |
1572 |
private void readObject(java.io.ObjectInputStream s) |
1573 |
throws java.io.IOException, ClassNotFoundException { |
1574 |
|
1575 |
// Read in elements until trailing null sentinel found |
1576 |
Node h = null, t = null; |
1577 |
for (Object item; (item = s.readObject()) != null; ) { |
1578 |
@SuppressWarnings("unchecked") |
1579 |
Node newNode = new Node((E) item); |
1580 |
if (h == null) |
1581 |
h = t = newNode; |
1582 |
else |
1583 |
t.appendRelaxed(t = newNode); |
1584 |
} |
1585 |
if (h == null) |
1586 |
h = t = new Node(); |
1587 |
head = h; |
1588 |
tail = t; |
1589 |
} |
1590 |
|
1591 |
/** |
1592 |
* @throws NullPointerException {@inheritDoc} |
1593 |
*/ |
1594 |
public boolean removeIf(Predicate<? super E> filter) { |
1595 |
Objects.requireNonNull(filter); |
1596 |
return bulkRemove(filter); |
1597 |
} |
1598 |
|
1599 |
/** |
1600 |
* @throws NullPointerException {@inheritDoc} |
1601 |
*/ |
1602 |
public boolean removeAll(Collection<?> c) { |
1603 |
Objects.requireNonNull(c); |
1604 |
return bulkRemove(e -> c.contains(e)); |
1605 |
} |
1606 |
|
1607 |
/** |
1608 |
* @throws NullPointerException {@inheritDoc} |
1609 |
*/ |
1610 |
public boolean retainAll(Collection<?> c) { |
1611 |
Objects.requireNonNull(c); |
1612 |
return bulkRemove(e -> !c.contains(e)); |
1613 |
} |
1614 |
|
1615 |
public void clear() { |
1616 |
bulkRemove(e -> true); |
1617 |
} |
1618 |
|
1619 |
/** |
1620 |
* Tolerate this many consecutive dead nodes before CAS-collapsing. |
1621 |
* Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element. |
1622 |
*/ |
1623 |
private static final int MAX_HOPS = 8; |
1624 |
|
1625 |
/** Implementation of bulk remove methods. */ |
1626 |
@SuppressWarnings("unchecked") |
1627 |
private boolean bulkRemove(Predicate<? super E> filter) { |
1628 |
boolean removed = false; |
1629 |
restartFromHead: for (;;) { |
1630 |
int hops = MAX_HOPS; |
1631 |
// c will be CASed to collapse intervening dead nodes between |
1632 |
// pred (or head if null) and p. |
1633 |
for (Node p = head, c = p, pred = null, q; p != null; p = q) { |
1634 |
q = p.next; |
1635 |
final Object item; boolean pAlive; |
1636 |
if (pAlive = ((item = p.item) != null && p.isData)) { |
1637 |
if (filter.test((E) item)) { |
1638 |
if (p.tryMatch(item, null)) |
1639 |
removed = true; |
1640 |
pAlive = false; |
1641 |
} |
1642 |
} |
1643 |
else if (!p.isData && item == null) |
1644 |
break; |
1645 |
if (pAlive || q == null || --hops == 0) { |
1646 |
// p might already be self-linked here, but if so: |
1647 |
// - CASing head will surely fail |
1648 |
// - CASing pred's next will be useless but harmless. |
1649 |
if ((c != p && !tryCasSuccessor(pred, c, c = p)) |
1650 |
|| pAlive) { |
1651 |
// if CAS failed or alive, abandon old pred |
1652 |
hops = MAX_HOPS; |
1653 |
pred = p; |
1654 |
c = q; |
1655 |
} |
1656 |
} else if (p == q) |
1657 |
continue restartFromHead; |
1658 |
} |
1659 |
return removed; |
1660 |
} |
1661 |
} |
1662 |
|
1663 |
/** |
1664 |
* Runs action on each element found during a traversal starting at p. |
1665 |
* If p is null, the action is not run. |
1666 |
*/ |
1667 |
@SuppressWarnings("unchecked") |
1668 |
void forEachFrom(Consumer<? super E> action, Node p) { |
1669 |
for (Node pred = null; p != null; ) { |
1670 |
Node q = p.next; |
1671 |
final Object item; |
1672 |
if ((item = p.item) != null) { |
1673 |
if (p.isData) { |
1674 |
action.accept((E) item); |
1675 |
pred = p; p = q; continue; |
1676 |
} |
1677 |
} |
1678 |
else if (!p.isData) |
1679 |
break; |
1680 |
for (Node c = p;; q = p.next) { |
1681 |
if (q == null || !q.isMatched()) { |
1682 |
pred = skipDeadNodes(pred, c, p, q); p = q; break; |
1683 |
} |
1684 |
if (p == (p = q)) { pred = null; p = head; break; } |
1685 |
} |
1686 |
} |
1687 |
} |
1688 |
|
1689 |
/** |
1690 |
* @throws NullPointerException {@inheritDoc} |
1691 |
*/ |
1692 |
public void forEach(Consumer<? super E> action) { |
1693 |
Objects.requireNonNull(action); |
1694 |
forEachFrom(action, head); |
1695 |
} |
1696 |
|
1697 |
// VarHandle mechanics |
1698 |
private static final VarHandle HEAD; |
1699 |
private static final VarHandle TAIL; |
1700 |
private static final VarHandle SWEEPVOTES; |
1701 |
static final VarHandle ITEM; |
1702 |
static final VarHandle NEXT; |
1703 |
static final VarHandle WAITER; |
1704 |
static { |
1705 |
try { |
1706 |
MethodHandles.Lookup l = MethodHandles.lookup(); |
1707 |
HEAD = l.findVarHandle(LinkedTransferQueue.class, "head", |
1708 |
Node.class); |
1709 |
TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail", |
1710 |
Node.class); |
1711 |
SWEEPVOTES = l.findVarHandle(LinkedTransferQueue.class, "sweepVotes", |
1712 |
int.class); |
1713 |
ITEM = l.findVarHandle(Node.class, "item", Object.class); |
1714 |
NEXT = l.findVarHandle(Node.class, "next", Node.class); |
1715 |
WAITER = l.findVarHandle(Node.class, "waiter", Thread.class); |
1716 |
} catch (ReflectiveOperationException e) { |
1717 |
throw new Error(e); |
1718 |
} |
1719 |
|
1720 |
// Reduce the risk of rare disastrous classloading in first call to |
1721 |
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773 |
1722 |
Class<?> ensureLoaded = LockSupport.class; |
1723 |
} |
1724 |
} |