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/* |
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* Written by Doug Lea, Bill Scherer, and Michael Scott with |
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* assistance from members of JCP JSR-166 Expert Group and released to |
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* 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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/** |
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* A synchronization point at which threads can pair and swap elements |
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* within pairs. Each thread presents some object on entry to the |
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* {@link #exchange exchange} method, matches with a partner thread, |
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* and receives its partner's object on return. An Exchanger may be |
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* viewed as a bidirectional form of a {@link SynchronousQueue}. |
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* Exchangers may be useful in applications such as genetic algorithms |
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* and pipeline designs. |
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* |
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* <p><b>Sample Usage:</b> |
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* Here are the highlights of a class that uses an {@code Exchanger} |
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* to swap buffers between threads so that the thread filling the |
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* buffer gets a freshly emptied one when it needs it, handing off the |
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* filled one to the thread emptying the buffer. |
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* <pre> {@code |
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* class FillAndEmpty { |
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* Exchanger<DataBuffer> exchanger = new Exchanger<>(); |
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* DataBuffer initialEmptyBuffer = ... a made-up type |
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* DataBuffer initialFullBuffer = ... |
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* |
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* class FillingLoop implements Runnable { |
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* public void run() { |
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* DataBuffer currentBuffer = initialEmptyBuffer; |
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* try { |
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* while (currentBuffer != null) { |
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* addToBuffer(currentBuffer); |
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* if (currentBuffer.isFull()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
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* } |
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* } catch (InterruptedException ex) { ... handle ... } |
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* } |
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* } |
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* |
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* class EmptyingLoop implements Runnable { |
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* public void run() { |
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* DataBuffer currentBuffer = initialFullBuffer; |
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* try { |
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* while (currentBuffer != null) { |
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* takeFromBuffer(currentBuffer); |
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* if (currentBuffer.isEmpty()) |
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* currentBuffer = exchanger.exchange(currentBuffer); |
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* } |
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* } catch (InterruptedException ex) { ... handle ...} |
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* } |
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* } |
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* |
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* void start() { |
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* new Thread(new FillingLoop()).start(); |
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* new Thread(new EmptyingLoop()).start(); |
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* } |
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* }}</pre> |
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* |
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* <p>Memory consistency effects: For each pair of threads that |
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* successfully exchange objects via an {@code Exchanger}, actions |
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* prior to the {@code exchange()} in each thread |
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
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* those subsequent to a return from the corresponding {@code exchange()} |
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* in the other thread. |
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* |
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* @since 1.5 |
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* @author Doug Lea and Bill Scherer and Michael Scott |
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* @param <V> The type of objects that may be exchanged |
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*/ |
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public class Exchanger<V> { |
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|
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/* |
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* Overview: The core algorithm is, for an exchange "slot", |
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* and a participant (caller) with an item: |
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* |
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* for (;;) { |
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* if (slot is empty) { // offer |
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* place item in a Node; |
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* if (can CAS slot from empty to node) { |
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* wait for release; |
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* return matching item in node; |
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* } |
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* } |
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* else if (can CAS slot from node to empty) { // release |
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* get the item in node; |
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* set matching item in node; |
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* release waiting thread; |
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* } |
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* // else retry on CAS failure |
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* } |
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* |
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* This is among the simplest forms of a "dual data structure" -- |
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* see Scott and Scherer's DISC 04 paper and |
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* http://www.cs.rochester.edu/research/synchronization/pseudocode/duals.html |
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* |
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* This works great in principle. But in practice, like many |
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* algorithms centered on atomic updates to a single location, it |
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* scales horribly when there are more than a few participants |
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* using the same Exchanger. So the implementation instead uses a |
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* form of elimination arena, that spreads out this contention by |
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* arranging that some threads typically use different slots, |
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* while still ensuring that eventually, any two parties will be |
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* able to exchange items. That is, we cannot completely partition |
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* across threads, but instead give threads arena indices that |
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* will on average grow under contention and shrink under lack of |
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* contention. We approach this by defining the Nodes that we need |
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* anyway as ThreadLocals, and include in them per-thread index |
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* and related bookkeeping state. (We can safely reuse per-thread |
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* nodes rather than creating them fresh each time because slots |
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* alternate between pointing to a node vs null, so cannot |
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* encounter ABA problems. However, we do need some care in |
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* resetting them between uses.) |
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* |
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* Implementing an effective arena requires allocating a bunch of |
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* space, so we only do so upon detecting contention (except on |
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* uniprocessors, where they wouldn't help, so aren't used). |
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* Otherwise, exchanges use the single-slot slotExchange method. |
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* On contention, not only must the slots be in different |
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* locations, but the locations must not encounter memory |
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* contention due to being on the same cache line (or more |
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* generally, the same coherence unit). Because, as of this |
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* writing, there is no way to determine cacheline size, we define |
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* a value that is enough for common platforms. Additionally, |
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* extra care elsewhere is taken to avoid other false/unintended |
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* sharing and to enhance locality, including adding padding (via |
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* @Contended) to Nodes, embedding "bound" as an Exchanger field, |
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* and reworking some park/unpark mechanics compared to |
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* LockSupport versions. |
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* |
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* The arena starts out with only one used slot. We expand the |
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* effective arena size by tracking collisions; i.e., failed CASes |
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* while trying to exchange. By nature of the above algorithm, the |
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* only kinds of collision that reliably indicate contention are |
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* when two attempted releases collide -- one of two attempted |
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* offers can legitimately fail to CAS without indicating |
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* contention by more than one other thread. (Note: it is possible |
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* but not worthwhile to more precisely detect contention by |
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* reading slot values after CAS failures.) When a thread has |
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* collided at each slot within the current arena bound, it tries |
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* to expand the arena size by one. We track collisions within |
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* bounds by using a version (sequence) number on the "bound" |
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* field, and conservatively reset collision counts when a |
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* participant notices that bound has been updated (in either |
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* direction). |
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* |
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* The effective arena size is reduced (when there is more than |
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* one slot) by giving up on waiting after a while and trying to |
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* decrement the arena size on expiration. The value of "a while" |
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* is an empirical matter. We implement by piggybacking on the |
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* use of spin->yield->block that is essential for reasonable |
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* waiting performance anyway -- in a busy exchanger, offers are |
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* usually almost immediately released, in which case context |
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* switching on multiprocessors is extremely slow/wasteful. Arena |
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* waits just omit the blocking part, and instead cancel. The spin |
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* count is empirically chosen to be a value that avoids blocking |
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* 99% of the time under maximum sustained exchange rates on a |
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* range of test machines. Spins and yields entail some limited |
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* randomness (using a cheap xorshift) to avoid regular patterns |
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* that can induce unproductive grow/shrink cycles. (Using a |
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* pseudorandom also helps regularize spin cycle duration by |
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* making branches unpredictable.) Also, during an offer, a |
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* waiter can "know" that it will be released when its slot has |
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* changed, but cannot yet proceed until match is set. In the |
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* mean time it cannot cancel the offer, so instead spins/yields. |
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* Note: It is possible to avoid this secondary check by changing |
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* the linearization point to be a CAS of the match field (as done |
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* in one case in the Scott & Scherer DISC paper), which also |
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* increases asynchrony a bit, at the expense of poorer collision |
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* detection and inability to always reuse per-thread nodes. So |
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* the current scheme is typically a better tradeoff. |
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* |
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* On collisions, indices traverse the arena cyclically in reverse |
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* order, restarting at the maximum index (which will tend to be |
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* sparsest) when bounds change. (On expirations, indices instead |
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* are halved until reaching 0.) It is possible (and has been |
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* tried) to use randomized, prime-value-stepped, or double-hash |
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* style traversal instead of simple cyclic traversal to reduce |
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* bunching. But empirically, whatever benefits these may have |
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* don't overcome their added overhead: We are managing operations |
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* that occur very quickly unless there is sustained contention, |
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* so simpler/faster control policies work better than more |
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* accurate but slower ones. |
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* |
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* Because we use expiration for arena size control, we cannot |
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* throw TimeoutExceptions in the timed version of the public |
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* exchange method until the arena size has shrunken to zero (or |
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* the arena isn't enabled). This may delay response to timeout |
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* but is still within spec. |
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* |
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* Essentially all of the implementation is in methods |
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* slotExchange and arenaExchange. These have similar overall |
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* structure, but differ in too many details to combine. The |
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* slotExchange method uses the single Exchanger field "slot" |
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* rather than arena array elements. However, it still needs |
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* minimal collision detection to trigger arena construction. |
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* (The messiest part is making sure interrupt status and |
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* InterruptedExceptions come out right during transitions when |
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* both methods may be called. This is done by using null return |
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* as a sentinel to recheck interrupt status.) |
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* |
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* As is too common in this sort of code, methods are monolithic |
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* because most of the logic relies on reads of fields that are |
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* maintained as local variables so can't be nicely factored -- |
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* mainly, here, bulky spin->yield->block/cancel code), and |
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* heavily dependent on intrinsics (Unsafe) to use inlined |
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* embedded CAS and related memory access operations (that tend |
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* not to be as readily inlined by dynamic compilers when they are |
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* hidden behind other methods that would more nicely name and |
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* encapsulate the intended effects). This includes the use of |
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* putOrderedX to clear fields of the per-thread Nodes between |
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* uses. Note that field Node.item is not declared as volatile |
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* even though it is read by releasing threads, because they only |
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* do so after CAS operations that must precede access, and all |
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* uses by the owning thread are otherwise acceptably ordered by |
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* other operations. (Because the actual points of atomicity are |
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* slot CASes, it would also be legal for the write to Node.match |
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* in a release to be weaker than a full volatile write. However, |
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* this is not done because it could allow further postponement of |
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* the write, delaying progress.) |
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*/ |
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|
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/** |
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* The byte distance (as a shift value) between any two used slots |
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* in the arena. 1 << ASHIFT should be at least cacheline size. |
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*/ |
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private static final int ASHIFT = 7; |
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|
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/** |
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* The maximum supported arena index. The maximum allocatable |
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* arena size is MMASK + 1. Must be a power of two minus one, less |
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* than (1<<(31-ASHIFT)). The cap of 255 (0xff) more than suffices |
235 |
* for the expected scaling limits of the main algorithms. |
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*/ |
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private static final int MMASK = 0xff; |
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|
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/** |
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* Unit for sequence/version bits of bound field. Each successful |
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* change to the bound also adds SEQ. |
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*/ |
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private static final int SEQ = MMASK + 1; |
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|
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/** The number of CPUs, for sizing and spin control */ |
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private static final int NCPU = Runtime.getRuntime().availableProcessors(); |
247 |
|
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/** |
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* The maximum slot index of the arena: The number of slots that |
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* can in principle hold all threads without contention, or at |
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* most the maximum indexable value. |
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*/ |
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static final int FULL = (NCPU >= (MMASK << 1)) ? MMASK : NCPU >>> 1; |
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|
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/** |
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* The bound for spins while waiting for a match. The actual |
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* number of iterations will on average be about twice this value |
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* due to randomization. Note: Spinning is disabled when NCPU==1. |
259 |
*/ |
260 |
private static final int SPINS = 1 << 10; |
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|
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/** |
263 |
* Value representing null arguments/returns from public |
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* methods. Needed because the API originally didn't disallow null |
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* arguments, which it should have. |
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*/ |
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private static final Object NULL_ITEM = new Object(); |
268 |
|
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/** |
270 |
* Sentinel value returned by internal exchange methods upon |
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* timeout, to avoid need for separate timed versions of these |
272 |
* methods. |
273 |
*/ |
274 |
private static final Object TIMED_OUT = new Object(); |
275 |
|
276 |
/** |
277 |
* Nodes hold partially exchanged data, plus other per-thread |
278 |
* bookkeeping. Padded via @Contended to reduce memory contention. |
279 |
*/ |
280 |
@jdk.internal.vm.annotation.Contended static final class Node { |
281 |
int index; // Arena index |
282 |
int bound; // Last recorded value of Exchanger.bound |
283 |
int collides; // Number of CAS failures at current bound |
284 |
int hash; // Pseudo-random for spins |
285 |
Object item; // This thread's current item |
286 |
volatile Object match; // Item provided by releasing thread |
287 |
volatile Thread parked; // Set to this thread when parked, else null |
288 |
} |
289 |
|
290 |
/** The corresponding thread local class */ |
291 |
static final class Participant extends ThreadLocal<Node> { |
292 |
public Node initialValue() { return new Node(); } |
293 |
} |
294 |
|
295 |
/** |
296 |
* Per-thread state. |
297 |
*/ |
298 |
private final Participant participant; |
299 |
|
300 |
/** |
301 |
* Elimination array; null until enabled (within slotExchange). |
302 |
* Element accesses use emulation of volatile gets and CAS. |
303 |
*/ |
304 |
private volatile Node[] arena; |
305 |
|
306 |
/** |
307 |
* Slot used until contention detected. |
308 |
*/ |
309 |
private volatile Node slot; |
310 |
|
311 |
/** |
312 |
* The index of the largest valid arena position, OR'ed with SEQ |
313 |
* number in high bits, incremented on each update. The initial |
314 |
* update from 0 to SEQ is used to ensure that the arena array is |
315 |
* constructed only once. |
316 |
*/ |
317 |
private volatile int bound; |
318 |
|
319 |
/** |
320 |
* Exchange function when arenas enabled. See above for explanation. |
321 |
* |
322 |
* @param item the (non-null) item to exchange |
323 |
* @param timed true if the wait is timed |
324 |
* @param ns if timed, the maximum wait time, else 0L |
325 |
* @return the other thread's item; or null if interrupted; or |
326 |
* TIMED_OUT if timed and timed out |
327 |
*/ |
328 |
private final Object arenaExchange(Object item, boolean timed, long ns) { |
329 |
Node[] a = arena; |
330 |
Node p = participant.get(); |
331 |
for (int i = p.index;;) { // access slot at i |
332 |
int b, m, c; long j; // j is raw array offset |
333 |
Node q = (Node)U.getObjectVolatile(a, j = (i << ASHIFT) + ABASE); |
334 |
if (q != null && U.compareAndSwapObject(a, j, q, null)) { |
335 |
Object v = q.item; // release |
336 |
q.match = item; |
337 |
Thread w = q.parked; |
338 |
if (w != null) |
339 |
U.unpark(w); |
340 |
return v; |
341 |
} |
342 |
else if (i <= (m = (b = bound) & MMASK) && q == null) { |
343 |
p.item = item; // offer |
344 |
if (U.compareAndSwapObject(a, j, null, p)) { |
345 |
long end = (timed && m == 0) ? System.nanoTime() + ns : 0L; |
346 |
Thread t = Thread.currentThread(); // wait |
347 |
for (int h = p.hash, spins = SPINS;;) { |
348 |
Object v = p.match; |
349 |
if (v != null) { |
350 |
U.putOrderedObject(p, MATCH, null); |
351 |
p.item = null; // clear for next use |
352 |
p.hash = h; |
353 |
return v; |
354 |
} |
355 |
else if (spins > 0) { |
356 |
h ^= h << 1; h ^= h >>> 3; h ^= h << 10; // xorshift |
357 |
if (h == 0) // initialize hash |
358 |
h = SPINS | (int)t.getId(); |
359 |
else if (h < 0 && // approx 50% true |
360 |
(--spins & ((SPINS >>> 1) - 1)) == 0) |
361 |
Thread.yield(); // two yields per wait |
362 |
} |
363 |
else if (U.getObjectVolatile(a, j) != p) |
364 |
spins = SPINS; // releaser hasn't set match yet |
365 |
else if (!t.isInterrupted() && m == 0 && |
366 |
(!timed || |
367 |
(ns = end - System.nanoTime()) > 0L)) { |
368 |
U.putObject(t, BLOCKER, this); // emulate LockSupport |
369 |
p.parked = t; // minimize window |
370 |
if (U.getObjectVolatile(a, j) == p) |
371 |
U.park(false, ns); |
372 |
p.parked = null; |
373 |
U.putObject(t, BLOCKER, null); |
374 |
} |
375 |
else if (U.getObjectVolatile(a, j) == p && |
376 |
U.compareAndSwapObject(a, j, p, null)) { |
377 |
if (m != 0) // try to shrink |
378 |
U.compareAndSwapInt(this, BOUND, b, b + SEQ - 1); |
379 |
p.item = null; |
380 |
p.hash = h; |
381 |
i = p.index >>>= 1; // descend |
382 |
if (Thread.interrupted()) |
383 |
return null; |
384 |
if (timed && m == 0 && ns <= 0L) |
385 |
return TIMED_OUT; |
386 |
break; // expired; restart |
387 |
} |
388 |
} |
389 |
} |
390 |
else |
391 |
p.item = null; // clear offer |
392 |
} |
393 |
else { |
394 |
if (p.bound != b) { // stale; reset |
395 |
p.bound = b; |
396 |
p.collides = 0; |
397 |
i = (i != m || m == 0) ? m : m - 1; |
398 |
} |
399 |
else if ((c = p.collides) < m || m == FULL || |
400 |
!U.compareAndSwapInt(this, BOUND, b, b + SEQ + 1)) { |
401 |
p.collides = c + 1; |
402 |
i = (i == 0) ? m : i - 1; // cyclically traverse |
403 |
} |
404 |
else |
405 |
i = m + 1; // grow |
406 |
p.index = i; |
407 |
} |
408 |
} |
409 |
} |
410 |
|
411 |
/** |
412 |
* Exchange function used until arenas enabled. See above for explanation. |
413 |
* |
414 |
* @param item the item to exchange |
415 |
* @param timed true if the wait is timed |
416 |
* @param ns if timed, the maximum wait time, else 0L |
417 |
* @return the other thread's item; or null if either the arena |
418 |
* was enabled or the thread was interrupted before completion; or |
419 |
* TIMED_OUT if timed and timed out |
420 |
*/ |
421 |
private final Object slotExchange(Object item, boolean timed, long ns) { |
422 |
Node p = participant.get(); |
423 |
Thread t = Thread.currentThread(); |
424 |
if (t.isInterrupted()) // preserve interrupt status so caller can recheck |
425 |
return null; |
426 |
|
427 |
for (Node q;;) { |
428 |
if ((q = slot) != null) { |
429 |
if (U.compareAndSwapObject(this, SLOT, q, null)) { |
430 |
Object v = q.item; |
431 |
q.match = item; |
432 |
Thread w = q.parked; |
433 |
if (w != null) |
434 |
U.unpark(w); |
435 |
return v; |
436 |
} |
437 |
// create arena on contention, but continue until slot null |
438 |
if (NCPU > 1 && bound == 0 && |
439 |
U.compareAndSwapInt(this, BOUND, 0, SEQ)) |
440 |
arena = new Node[(FULL + 2) << ASHIFT]; |
441 |
} |
442 |
else if (arena != null) |
443 |
return null; // caller must reroute to arenaExchange |
444 |
else { |
445 |
p.item = item; |
446 |
if (U.compareAndSwapObject(this, SLOT, null, p)) |
447 |
break; |
448 |
p.item = null; |
449 |
} |
450 |
} |
451 |
|
452 |
// await release |
453 |
int h = p.hash; |
454 |
long end = timed ? System.nanoTime() + ns : 0L; |
455 |
int spins = (NCPU > 1) ? SPINS : 1; |
456 |
Object v; |
457 |
while ((v = p.match) == null) { |
458 |
if (spins > 0) { |
459 |
h ^= h << 1; h ^= h >>> 3; h ^= h << 10; |
460 |
if (h == 0) |
461 |
h = SPINS | (int)t.getId(); |
462 |
else if (h < 0 && (--spins & ((SPINS >>> 1) - 1)) == 0) |
463 |
Thread.yield(); |
464 |
} |
465 |
else if (slot != p) |
466 |
spins = SPINS; |
467 |
else if (!t.isInterrupted() && arena == null && |
468 |
(!timed || (ns = end - System.nanoTime()) > 0L)) { |
469 |
U.putObject(t, BLOCKER, this); |
470 |
p.parked = t; |
471 |
if (slot == p) |
472 |
U.park(false, ns); |
473 |
p.parked = null; |
474 |
U.putObject(t, BLOCKER, null); |
475 |
} |
476 |
else if (U.compareAndSwapObject(this, SLOT, p, null)) { |
477 |
v = timed && ns <= 0L && !t.isInterrupted() ? TIMED_OUT : null; |
478 |
break; |
479 |
} |
480 |
} |
481 |
U.putOrderedObject(p, MATCH, null); |
482 |
p.item = null; |
483 |
p.hash = h; |
484 |
return v; |
485 |
} |
486 |
|
487 |
/** |
488 |
* Creates a new Exchanger. |
489 |
*/ |
490 |
public Exchanger() { |
491 |
participant = new Participant(); |
492 |
} |
493 |
|
494 |
/** |
495 |
* Waits for another thread to arrive at this exchange point (unless |
496 |
* the current thread is {@linkplain Thread#interrupt interrupted}), |
497 |
* and then transfers the given object to it, receiving its object |
498 |
* in return. |
499 |
* |
500 |
* <p>If another thread is already waiting at the exchange point then |
501 |
* it is resumed for thread scheduling purposes and receives the object |
502 |
* passed in by the current thread. The current thread returns immediately, |
503 |
* receiving the object passed to the exchange by that other thread. |
504 |
* |
505 |
* <p>If no other thread is already waiting at the exchange then the |
506 |
* current thread is disabled for thread scheduling purposes and lies |
507 |
* dormant until one of two things happens: |
508 |
* <ul> |
509 |
* <li>Some other thread enters the exchange; or |
510 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
511 |
* the current thread. |
512 |
* </ul> |
513 |
* <p>If the current thread: |
514 |
* <ul> |
515 |
* <li>has its interrupted status set on entry to this method; or |
516 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
517 |
* for the exchange, |
518 |
* </ul> |
519 |
* then {@link InterruptedException} is thrown and the current thread's |
520 |
* interrupted status is cleared. |
521 |
* |
522 |
* @param x the object to exchange |
523 |
* @return the object provided by the other thread |
524 |
* @throws InterruptedException if the current thread was |
525 |
* interrupted while waiting |
526 |
*/ |
527 |
@SuppressWarnings("unchecked") |
528 |
public V exchange(V x) throws InterruptedException { |
529 |
Object v; |
530 |
Object item = (x == null) ? NULL_ITEM : x; // translate null args |
531 |
if ((arena != null || |
532 |
(v = slotExchange(item, false, 0L)) == null) && |
533 |
((Thread.interrupted() || // disambiguates null return |
534 |
(v = arenaExchange(item, false, 0L)) == null))) |
535 |
throw new InterruptedException(); |
536 |
return (v == NULL_ITEM) ? null : (V)v; |
537 |
} |
538 |
|
539 |
/** |
540 |
* Waits for another thread to arrive at this exchange point (unless |
541 |
* the current thread is {@linkplain Thread#interrupt interrupted} or |
542 |
* the specified waiting time elapses), and then transfers the given |
543 |
* object to it, receiving its object in return. |
544 |
* |
545 |
* <p>If another thread is already waiting at the exchange point then |
546 |
* it is resumed for thread scheduling purposes and receives the object |
547 |
* passed in by the current thread. The current thread returns immediately, |
548 |
* receiving the object passed to the exchange by that other thread. |
549 |
* |
550 |
* <p>If no other thread is already waiting at the exchange then the |
551 |
* current thread is disabled for thread scheduling purposes and lies |
552 |
* dormant until one of three things happens: |
553 |
* <ul> |
554 |
* <li>Some other thread enters the exchange; or |
555 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts} |
556 |
* the current thread; or |
557 |
* <li>The specified waiting time elapses. |
558 |
* </ul> |
559 |
* <p>If the current thread: |
560 |
* <ul> |
561 |
* <li>has its interrupted status set on entry to this method; or |
562 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting |
563 |
* for the exchange, |
564 |
* </ul> |
565 |
* then {@link InterruptedException} is thrown and the current thread's |
566 |
* interrupted status is cleared. |
567 |
* |
568 |
* <p>If the specified waiting time elapses then {@link |
569 |
* TimeoutException} is thrown. If the time is less than or equal |
570 |
* to zero, the method will not wait at all. |
571 |
* |
572 |
* @param x the object to exchange |
573 |
* @param timeout the maximum time to wait |
574 |
* @param unit the time unit of the {@code timeout} argument |
575 |
* @return the object provided by the other thread |
576 |
* @throws InterruptedException if the current thread was |
577 |
* interrupted while waiting |
578 |
* @throws TimeoutException if the specified waiting time elapses |
579 |
* before another thread enters the exchange |
580 |
*/ |
581 |
@SuppressWarnings("unchecked") |
582 |
public V exchange(V x, long timeout, TimeUnit unit) |
583 |
throws InterruptedException, TimeoutException { |
584 |
Object v; |
585 |
Object item = (x == null) ? NULL_ITEM : x; |
586 |
long ns = unit.toNanos(timeout); |
587 |
if ((arena != null || |
588 |
(v = slotExchange(item, true, ns)) == null) && |
589 |
((Thread.interrupted() || |
590 |
(v = arenaExchange(item, true, ns)) == null))) |
591 |
throw new InterruptedException(); |
592 |
if (v == TIMED_OUT) |
593 |
throw new TimeoutException(); |
594 |
return (v == NULL_ITEM) ? null : (V)v; |
595 |
} |
596 |
|
597 |
// Unsafe mechanics |
598 |
private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
599 |
private static final long BOUND; |
600 |
private static final long SLOT; |
601 |
private static final long MATCH; |
602 |
private static final long BLOCKER; |
603 |
private static final int ABASE; |
604 |
static { |
605 |
try { |
606 |
BOUND = U.objectFieldOffset |
607 |
(Exchanger.class.getDeclaredField("bound")); |
608 |
SLOT = U.objectFieldOffset |
609 |
(Exchanger.class.getDeclaredField("slot")); |
610 |
|
611 |
MATCH = U.objectFieldOffset |
612 |
(Node.class.getDeclaredField("match")); |
613 |
|
614 |
BLOCKER = U.objectFieldOffset |
615 |
(Thread.class.getDeclaredField("parkBlocker")); |
616 |
|
617 |
int scale = U.arrayIndexScale(Node[].class); |
618 |
if ((scale & (scale - 1)) != 0 || scale > (1 << ASHIFT)) |
619 |
throw new Error("Unsupported array scale"); |
620 |
// ABASE absorbs padding in front of element 0 |
621 |
ABASE = U.arrayBaseOffset(Node[].class) + (1 << ASHIFT); |
622 |
} catch (ReflectiveOperationException e) { |
623 |
throw new Error(e); |
624 |
} |
625 |
} |
626 |
|
627 |
} |