mirror of git://gcc.gnu.org/git/gcc.git
657 lines
28 KiB
Java
657 lines
28 KiB
Java
/*
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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/licenses/publicdomain
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*/
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package java.util.concurrent;
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import java.util.concurrent.atomic.*;
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import java.util.concurrent.locks.LockSupport;
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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<DataBuffer>();
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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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* }
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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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* Algorithm Description:
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*
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* The basic idea is to maintain a "slot", which is a reference to
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* a Node containing both an Item to offer and a "hole" waiting to
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* get filled in. If an incoming "occupying" thread sees that the
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* slot is null, it CAS'es (compareAndSets) a Node there and waits
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* for another to invoke exchange. That second "fulfilling" thread
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* sees that the slot is non-null, and so CASes it back to null,
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* also exchanging items by CASing the hole, plus waking up the
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* occupying thread if it is blocked. In each case CAS'es may
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* fail because a slot at first appears non-null but is null upon
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* CAS, or vice-versa. So threads may need to retry these
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* actions.
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*
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* This simple approach works great when there are only a few
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* threads using an Exchanger, but performance rapidly
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* deteriorates due to CAS contention on the single slot when
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* there are lots of threads using an exchanger. So instead we use
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* an "arena"; basically a kind of hash table with a dynamically
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* varying number of slots, any one of which can be used by
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* threads performing an exchange. Incoming threads pick slots
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* based on a hash of their Thread ids. If an incoming thread
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* fails to CAS in its chosen slot, it picks an alternative slot
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* instead. And similarly from there. If a thread successfully
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* CASes into a slot but no other thread arrives, it tries
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* another, heading toward the zero slot, which always exists even
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* if the table shrinks. The particular mechanics controlling this
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* are as follows:
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*
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* Waiting: Slot zero is special in that it is the only slot that
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* exists when there is no contention. A thread occupying slot
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* zero will block if no thread fulfills it after a short spin.
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* In other cases, occupying threads eventually give up and try
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* another slot. Waiting threads spin for a while (a period that
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* should be a little less than a typical context-switch time)
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* before either blocking (if slot zero) or giving up (if other
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* slots) and restarting. There is no reason for threads to block
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* unless there are unlikely to be any other threads present.
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* Occupants are mainly avoiding memory contention so sit there
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* quietly polling for a shorter period than it would take to
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* block and then unblock them. Non-slot-zero waits that elapse
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* because of lack of other threads waste around one extra
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* context-switch time per try, which is still on average much
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* faster than alternative approaches.
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*
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* Sizing: Usually, using only a few slots suffices to reduce
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* contention. Especially with small numbers of threads, using
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* too many slots can lead to just as poor performance as using
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* too few of them, and there's not much room for error. The
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* variable "max" maintains the number of slots actually in
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* use. It is increased when a thread sees too many CAS
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* failures. (This is analogous to resizing a regular hash table
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* based on a target load factor, except here, growth steps are
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* just one-by-one rather than proportional.) Growth requires
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* contention failures in each of three tried slots. Requiring
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* multiple failures for expansion copes with the fact that some
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* failed CASes are not due to contention but instead to simple
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* races between two threads or thread pre-emptions occurring
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* between reading and CASing. Also, very transient peak
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* contention can be much higher than the average sustainable
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* levels. The max limit is decreased on average 50% of the times
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* that a non-slot-zero wait elapses without being fulfilled.
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* Threads experiencing elapsed waits move closer to zero, so
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* eventually find existing (or future) threads even if the table
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* has been shrunk due to inactivity. The chosen mechanics and
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* thresholds for growing and shrinking are intrinsically
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* entangled with indexing and hashing inside the exchange code,
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* and can't be nicely abstracted out.
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*
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* Hashing: Each thread picks its initial slot to use in accord
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* with a simple hashcode. The sequence is the same on each
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* encounter by any given thread, but effectively random across
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* threads. Using arenas encounters the classic cost vs quality
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* tradeoffs of all hash tables. Here, we use a one-step FNV-1a
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* hash code based on the current thread's Thread.getId(), along
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* with a cheap approximation to a mod operation to select an
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* index. The downside of optimizing index selection in this way
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* is that the code is hardwired to use a maximum table size of
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* 32. But this value more than suffices for known platforms and
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* applications.
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*
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* Probing: On sensed contention of a selected slot, we probe
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* sequentially through the table, analogously to linear probing
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* after collision in a hash table. (We move circularly, in
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* reverse order, to mesh best with table growth and shrinkage
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* rules.) Except that to minimize the effects of false-alarms
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* and cache thrashing, we try the first selected slot twice
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* before moving.
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*
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* Padding: Even with contention management, slots are heavily
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* contended, so use cache-padding to avoid poor memory
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* performance. Because of this, slots are lazily constructed
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* only when used, to avoid wasting this space unnecessarily.
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* While isolation of locations is not much of an issue at first
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* in an application, as time goes on and garbage-collectors
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* perform compaction, slots are very likely to be moved adjacent
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* to each other, which can cause much thrashing of cache lines on
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* MPs unless padding is employed.
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*
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* This is an improvement of the algorithm described in the paper
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* "A Scalable Elimination-based Exchange Channel" by William
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* Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05
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* workshop. Available at: http://hdl.handle.net/1802/2104
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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();
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/**
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* The capacity of the arena. Set to a value that provides more
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* than enough space to handle contention. On small machines
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* most slots won't be used, but it is still not wasted because
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* the extra space provides some machine-level address padding
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* to minimize interference with heavily CAS'ed Slot locations.
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* And on very large machines, performance eventually becomes
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* bounded by memory bandwidth, not numbers of threads/CPUs.
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* This constant cannot be changed without also modifying
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* indexing and hashing algorithms.
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*/
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private static final int CAPACITY = 32;
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/**
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* The value of "max" that will hold all threads without
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* contention. When this value is less than CAPACITY, some
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* otherwise wasted expansion can be avoided.
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*/
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private static final int FULL =
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Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1);
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/**
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* The number of times to spin (doing nothing except polling a
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* memory location) before blocking or giving up while waiting to
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* be fulfilled. Should be zero on uniprocessors. On
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* multiprocessors, this value should be large enough so that two
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* threads exchanging items as fast as possible block only when
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* one of them is stalled (due to GC or preemption), but not much
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* longer, to avoid wasting CPU resources. Seen differently, this
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* value is a little over half the number of cycles of an average
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* context switch time on most systems. The value here is
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* approximately the average of those across a range of tested
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* systems.
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*/
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private static final int SPINS = (NCPU == 1) ? 0 : 2000;
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/**
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* The number of times to spin before blocking in timed waits.
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* Timed waits spin more slowly because checking the time takes
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* time. The best value relies mainly on the relative rate of
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* System.nanoTime vs memory accesses. The value is empirically
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* derived to work well across a variety of systems.
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*/
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private static final int TIMED_SPINS = SPINS / 20;
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/**
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* Sentinel item representing cancellation of a wait due to
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* interruption, timeout, or elapsed spin-waits. This value is
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* placed in holes on cancellation, and used as a return value
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* from waiting methods to indicate failure to set or get hole.
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*/
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private static final Object CANCEL = new Object();
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/**
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* Value representing null arguments/returns from public
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* methods. This disambiguates from internal requirement that
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* holes start out as null to mean they are not yet set.
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*/
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private static final Object NULL_ITEM = new Object();
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/**
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* Nodes hold partially exchanged data. This class
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* opportunistically subclasses AtomicReference to represent the
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* hole. So get() returns hole, and compareAndSet CAS'es value
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* into hole. This class cannot be parameterized as "V" because
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* of the use of non-V CANCEL sentinels.
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*/
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private static final class Node extends AtomicReference<Object> {
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/** The element offered by the Thread creating this node. */
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public final Object item;
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/** The Thread waiting to be signalled; null until waiting. */
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public volatile Thread waiter;
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/**
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* Creates node with given item and empty hole.
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* @param item the item
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*/
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public Node(Object item) {
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this.item = item;
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}
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}
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/**
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* A Slot is an AtomicReference with heuristic padding to lessen
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* cache effects of this heavily CAS'ed location. While the
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* padding adds noticeable space, all slots are created only on
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* demand, and there will be more than one of them only when it
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* would improve throughput more than enough to outweigh using
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* extra space.
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*/
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private static final class Slot extends AtomicReference<Object> {
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// Improve likelihood of isolation on <= 64 byte cache lines
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long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe;
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}
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/**
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* Slot array. Elements are lazily initialized when needed.
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* Declared volatile to enable double-checked lazy construction.
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*/
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private volatile Slot[] arena = new Slot[CAPACITY];
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/**
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* The maximum slot index being used. The value sometimes
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* increases when a thread experiences too many CAS contentions,
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* and sometimes decreases when a spin-wait elapses. Changes
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* are performed only via compareAndSet, to avoid stale values
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* when a thread happens to stall right before setting.
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*/
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private final AtomicInteger max = new AtomicInteger();
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/**
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* Main exchange function, handling the different policy variants.
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* Uses Object, not "V" as argument and return value to simplify
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* handling of sentinel values. Callers from public methods decode
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* and cast accordingly.
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*
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* @param item the (non-null) item to exchange
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* @param timed true if the wait is timed
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* @param nanos if timed, the maximum wait time
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* @return the other thread's item, or CANCEL if interrupted or timed out
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*/
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private Object doExchange(Object item, boolean timed, long nanos) {
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Node me = new Node(item); // Create in case occupying
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int index = hashIndex(); // Index of current slot
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int fails = 0; // Number of CAS failures
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for (;;) {
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Object y; // Contents of current slot
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Slot slot = arena[index];
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if (slot == null) // Lazily initialize slots
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createSlot(index); // Continue loop to reread
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else if ((y = slot.get()) != null && // Try to fulfill
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slot.compareAndSet(y, null)) {
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Node you = (Node)y; // Transfer item
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if (you.compareAndSet(null, item)) {
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LockSupport.unpark(you.waiter);
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return you.item;
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} // Else cancelled; continue
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}
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else if (y == null && // Try to occupy
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slot.compareAndSet(null, me)) {
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if (index == 0) // Blocking wait for slot 0
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return timed? awaitNanos(me, slot, nanos): await(me, slot);
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Object v = spinWait(me, slot); // Spin wait for non-0
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if (v != CANCEL)
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return v;
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me = new Node(item); // Throw away cancelled node
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int m = max.get();
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if (m > (index >>>= 1)) // Decrease index
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max.compareAndSet(m, m - 1); // Maybe shrink table
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}
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else if (++fails > 1) { // Allow 2 fails on 1st slot
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int m = max.get();
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if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1))
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index = m + 1; // Grow on 3rd failed slot
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else if (--index < 0)
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index = m; // Circularly traverse
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}
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}
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}
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/**
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* Returns a hash index for the current thread. Uses a one-step
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* FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/)
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* based on the current thread's Thread.getId(). These hash codes
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* have more uniform distribution properties with respect to small
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* moduli (here 1-31) than do other simple hashing functions.
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*
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* <p>To return an index between 0 and max, we use a cheap
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* approximation to a mod operation, that also corrects for bias
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* due to non-power-of-2 remaindering (see {@link
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* java.util.Random#nextInt}). Bits of the hashcode are masked
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* with "nbits", the ceiling power of two of table size (looked up
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* in a table packed into three ints). If too large, this is
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* retried after rotating the hash by nbits bits, while forcing new
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* top bit to 0, which guarantees eventual termination (although
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* with a non-random-bias). This requires an average of less than
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* 2 tries for all table sizes, and has a maximum 2% difference
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* from perfectly uniform slot probabilities when applied to all
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* possible hash codes for sizes less than 32.
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*
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* @return a per-thread-random index, 0 <= index < max
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*/
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private final int hashIndex() {
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long id = Thread.currentThread().getId();
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int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193;
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int m = max.get();
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int nbits = (((0xfffffc00 >> m) & 4) | // Compute ceil(log2(m+1))
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((0x000001f8 >>> m) & 2) | // The constants hold
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((0xffff00f2 >>> m) & 1)); // a lookup table
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int index;
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while ((index = hash & ((1 << nbits) - 1)) > m) // May retry on
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hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m
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return index;
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}
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/**
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* Creates a new slot at given index. Called only when the slot
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* appears to be null. Relies on double-check using builtin
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* locks, since they rarely contend. This in turn relies on the
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* arena array being declared volatile.
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*
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* @param index the index to add slot at
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*/
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private void createSlot(int index) {
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// Create slot outside of lock to narrow sync region
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Slot newSlot = new Slot();
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Slot[] a = arena;
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synchronized (a) {
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if (a[index] == null)
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a[index] = newSlot;
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}
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}
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/**
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* Tries to cancel a wait for the given node waiting in the given
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* slot, if so, helping clear the node from its slot to avoid
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* garbage retention.
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*
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* @param node the waiting node
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* @param the slot it is waiting in
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* @return true if successfully cancelled
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*/
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private static boolean tryCancel(Node node, Slot slot) {
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if (!node.compareAndSet(null, CANCEL))
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return false;
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if (slot.get() == node) // pre-check to minimize contention
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slot.compareAndSet(node, null);
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return true;
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}
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// Three forms of waiting. Each just different enough not to merge
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// code with others.
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/**
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* Spin-waits for hole for a non-0 slot. Fails if spin elapses
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* before hole filled. Does not check interrupt, relying on check
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* in public exchange method to abort if interrupted on entry.
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*
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* @param node the waiting node
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* @return on success, the hole; on failure, CANCEL
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*/
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private static Object spinWait(Node node, Slot slot) {
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int spins = SPINS;
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for (;;) {
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Object v = node.get();
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if (v != null)
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return v;
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else if (spins > 0)
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--spins;
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else
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tryCancel(node, slot);
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}
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}
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/**
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* Waits for (by spinning and/or blocking) and gets the hole
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* filled in by another thread. Fails if interrupted before
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* hole filled.
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*
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* When a node/thread is about to block, it sets its waiter field
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* and then rechecks state at least one more time before actually
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* parking, thus covering race vs fulfiller noticing that waiter
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* is non-null so should be woken.
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*
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* Thread interruption status is checked only surrounding calls to
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* park. The caller is assumed to have checked interrupt status
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* on entry.
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*
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* @param node the waiting node
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* @return on success, the hole; on failure, CANCEL
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*/
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private static Object await(Node node, Slot slot) {
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Thread w = Thread.currentThread();
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int spins = SPINS;
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for (;;) {
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Object v = node.get();
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if (v != null)
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return v;
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else if (spins > 0) // Spin-wait phase
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--spins;
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else if (node.waiter == null) // Set up to block next
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node.waiter = w;
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else if (w.isInterrupted()) // Abort on interrupt
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tryCancel(node, slot);
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else // Block
|
|
LockSupport.park(node);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Waits for (at index 0) and gets the hole filled in by another
|
|
* thread. Fails if timed out or interrupted before hole filled.
|
|
* Same basic logic as untimed version, but a bit messier.
|
|
*
|
|
* @param node the waiting node
|
|
* @param nanos the wait time
|
|
* @return on success, the hole; on failure, CANCEL
|
|
*/
|
|
private Object awaitNanos(Node node, Slot slot, long nanos) {
|
|
int spins = TIMED_SPINS;
|
|
long lastTime = 0;
|
|
Thread w = null;
|
|
for (;;) {
|
|
Object v = node.get();
|
|
if (v != null)
|
|
return v;
|
|
long now = System.nanoTime();
|
|
if (w == null)
|
|
w = Thread.currentThread();
|
|
else
|
|
nanos -= now - lastTime;
|
|
lastTime = now;
|
|
if (nanos > 0) {
|
|
if (spins > 0)
|
|
--spins;
|
|
else if (node.waiter == null)
|
|
node.waiter = w;
|
|
else if (w.isInterrupted())
|
|
tryCancel(node, slot);
|
|
else
|
|
LockSupport.parkNanos(node, nanos);
|
|
}
|
|
else if (tryCancel(node, slot) && !w.isInterrupted())
|
|
return scanOnTimeout(node);
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Sweeps through arena checking for any waiting threads. Called
|
|
* only upon return from timeout while waiting in slot 0. When a
|
|
* thread gives up on a timed wait, it is possible that a
|
|
* previously-entered thread is still waiting in some other
|
|
* slot. So we scan to check for any. This is almost always
|
|
* overkill, but decreases the likelihood of timeouts when there
|
|
* are other threads present to far less than that in lock-based
|
|
* exchangers in which earlier-arriving threads may still be
|
|
* waiting on entry locks.
|
|
*
|
|
* @param node the waiting node
|
|
* @return another thread's item, or CANCEL
|
|
*/
|
|
private Object scanOnTimeout(Node node) {
|
|
Object y;
|
|
for (int j = arena.length - 1; j >= 0; --j) {
|
|
Slot slot = arena[j];
|
|
if (slot != null) {
|
|
while ((y = slot.get()) != null) {
|
|
if (slot.compareAndSet(y, null)) {
|
|
Node you = (Node)y;
|
|
if (you.compareAndSet(null, node.item)) {
|
|
LockSupport.unpark(you.waiter);
|
|
return you.item;
|
|
}
|
|
}
|
|
}
|
|
}
|
|
}
|
|
return CANCEL;
|
|
}
|
|
|
|
/**
|
|
* Creates a new Exchanger.
|
|
*/
|
|
public Exchanger() {
|
|
}
|
|
|
|
/**
|
|
* Waits for another thread to arrive at this exchange point (unless
|
|
* the current thread is {@linkplain Thread#interrupt interrupted}),
|
|
* and then transfers the given object to it, receiving its object
|
|
* in return.
|
|
*
|
|
* <p>If another thread is already waiting at the exchange point then
|
|
* it is resumed for thread scheduling purposes and receives the object
|
|
* passed in by the current thread. The current thread returns immediately,
|
|
* receiving the object passed to the exchange by that other thread.
|
|
*
|
|
* <p>If no other thread is already waiting at the exchange then the
|
|
* current thread is disabled for thread scheduling purposes and lies
|
|
* dormant until one of two things happens:
|
|
* <ul>
|
|
* <li>Some other thread enters the exchange; or
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts} the current
|
|
* thread.
|
|
* </ul>
|
|
* <p>If the current thread:
|
|
* <ul>
|
|
* <li>has its interrupted status set on entry to this method; or
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
|
* for the exchange,
|
|
* </ul>
|
|
* then {@link InterruptedException} is thrown and the current thread's
|
|
* interrupted status is cleared.
|
|
*
|
|
* @param x the object to exchange
|
|
* @return the object provided by the other thread
|
|
* @throws InterruptedException if the current thread was
|
|
* interrupted while waiting
|
|
*/
|
|
public V exchange(V x) throws InterruptedException {
|
|
if (!Thread.interrupted()) {
|
|
Object v = doExchange(x == null? NULL_ITEM : x, false, 0);
|
|
if (v == NULL_ITEM)
|
|
return null;
|
|
if (v != CANCEL)
|
|
return (V)v;
|
|
Thread.interrupted(); // Clear interrupt status on IE throw
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
|
|
/**
|
|
* Waits for another thread to arrive at this exchange point (unless
|
|
* the current thread is {@linkplain Thread#interrupt interrupted} or
|
|
* the specified waiting time elapses), and then transfers the given
|
|
* object to it, receiving its object in return.
|
|
*
|
|
* <p>If another thread is already waiting at the exchange point then
|
|
* it is resumed for thread scheduling purposes and receives the object
|
|
* passed in by the current thread. The current thread returns immediately,
|
|
* receiving the object passed to the exchange by that other thread.
|
|
*
|
|
* <p>If no other thread is already waiting at the exchange then the
|
|
* current thread is disabled for thread scheduling purposes and lies
|
|
* dormant until one of three things happens:
|
|
* <ul>
|
|
* <li>Some other thread enters the exchange; or
|
|
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
|
* the current thread; or
|
|
* <li>The specified waiting time elapses.
|
|
* </ul>
|
|
* <p>If the current thread:
|
|
* <ul>
|
|
* <li>has its interrupted status set on entry to this method; or
|
|
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
|
* for the exchange,
|
|
* </ul>
|
|
* then {@link InterruptedException} is thrown and the current thread's
|
|
* interrupted status is cleared.
|
|
*
|
|
* <p>If the specified waiting time elapses then {@link
|
|
* TimeoutException} is thrown. If the time is less than or equal
|
|
* to zero, the method will not wait at all.
|
|
*
|
|
* @param x the object to exchange
|
|
* @param timeout the maximum time to wait
|
|
* @param unit the time unit of the <tt>timeout</tt> argument
|
|
* @return the object provided by the other thread
|
|
* @throws InterruptedException if the current thread was
|
|
* interrupted while waiting
|
|
* @throws TimeoutException if the specified waiting time elapses
|
|
* before another thread enters the exchange
|
|
*/
|
|
public V exchange(V x, long timeout, TimeUnit unit)
|
|
throws InterruptedException, TimeoutException {
|
|
if (!Thread.interrupted()) {
|
|
Object v = doExchange(x == null? NULL_ITEM : x,
|
|
true, unit.toNanos(timeout));
|
|
if (v == NULL_ITEM)
|
|
return null;
|
|
if (v != CANCEL)
|
|
return (V)v;
|
|
if (!Thread.interrupted())
|
|
throw new TimeoutException();
|
|
}
|
|
throw new InterruptedException();
|
|
}
|
|
}
|