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