Back to index...

	/*
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	/*
	* This file is available under and governed by the GNU General Public
	* License version 2 only, as published by the Free Software Foundation.
	* However, the following notice accompanied the original version of this
	* file:
	*
	* Written by Doug Lea, Bill Scherer, and Michael Scott with
	* assistance from members of JCP JSR-166 Expert Group and released to
	* the public domain, as explained at
	* http://creativecommons.org/publicdomain/zero/1.0/
	*/

	package java.util.concurrent;

	import java.lang.invoke.MethodHandles;
	import java.lang.invoke.VarHandle;
	import java.util.concurrent.locks.LockSupport;

	/**
	* A synchronization point at which threads can pair and swap elements
	* within pairs. Each thread presents some object on entry to the
	* {@link #exchange exchange} method, matches with a partner thread,
	* and receives its partner's object on return. An Exchanger may be
	* viewed as a bidirectional form of a {@link SynchronousQueue}.
	* Exchangers may be useful in applications such as genetic algorithms
	* and pipeline designs.
	*
	* <p><b>Sample Usage:</b>
	* Here are the highlights of a class that uses an {@code Exchanger}
	* to swap buffers between threads so that the thread filling the
	* buffer gets a freshly emptied one when it needs it, handing off the
	* filled one to the thread emptying the buffer.
	* <pre> {@code
	* class FillAndEmpty {
	* Exchanger<DataBuffer> exchanger = new Exchanger<>();
	* DataBuffer initialEmptyBuffer = ... a made-up type
	* DataBuffer initialFullBuffer = ...
	*
	* class FillingLoop implements Runnable {
	* public void run() {
	* DataBuffer currentBuffer = initialEmptyBuffer;
	* try {
	* while (currentBuffer != null) {
	* addToBuffer(currentBuffer);
	* if (currentBuffer.isFull())
	* currentBuffer = exchanger.exchange(currentBuffer);
	* }
	* } catch (InterruptedException ex) { ... handle ... }
	* }
	* }
	*
	* class EmptyingLoop implements Runnable {
	* public void run() {
	* DataBuffer currentBuffer = initialFullBuffer;
	* try {
	* while (currentBuffer != null) {
	* takeFromBuffer(currentBuffer);
	* if (currentBuffer.isEmpty())
	* currentBuffer = exchanger.exchange(currentBuffer);
	* }
	* } catch (InterruptedException ex) { ... handle ...}
	* }
	* }
	*
	* void start() {
	* new Thread(new FillingLoop()).start();
	* new Thread(new EmptyingLoop()).start();
	* }
	* }}</pre>
	*
	* <p>Memory consistency effects: For each pair of threads that
	* successfully exchange objects via an {@code Exchanger}, actions
	* prior to the {@code exchange()} in each thread
	* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
	* those subsequent to a return from the corresponding {@code exchange()}
	* in the other thread.
	*
	* @since 1.5
	* @author Doug Lea and Bill Scherer and Michael Scott
	* @param <V> The type of objects that may be exchanged
	*/
	public class Exchanger<V> {

	/*
	* Overview: The core algorithm is, for an exchange "slot",
	* and a participant (caller) with an item:
	*
	* for (;;) {
	* if (slot is empty) { // offer
	* place item in a Node;
	* if (can CAS slot from empty to node) {
	* wait for release;
	* return matching item in node;
	* }
	* }
	* else if (can CAS slot from node to empty) { // release
	* get the item in node;
	* set matching item in node;
	* release waiting thread;
	* }
	* // else retry on CAS failure
	* }
	*
	* This is among the simplest forms of a "dual data structure" --
	* see Scott and Scherer's DISC 04 paper and
	* http://www.cs.rochester.edu/research/synchronization/pseudocode/duals.html
	*
	* This works great in principle. But in practice, like many
	* algorithms centered on atomic updates to a single location, it
	* scales horribly when there are more than a few participants
	* using the same Exchanger. So the implementation instead uses a
	* form of elimination arena, that spreads out this contention by
	* arranging that some threads typically use different slots,
	* while still ensuring that eventually, any two parties will be
	* able to exchange items. That is, we cannot completely partition
	* across threads, but instead give threads arena indices that
	* will on average grow under contention and shrink under lack of
	* contention. We approach this by defining the Nodes that we need
	* anyway as ThreadLocals, and include in them per-thread index
	* and related bookkeeping state. (We can safely reuse per-thread
	* nodes rather than creating them fresh each time because slots
	* alternate between pointing to a node vs null, so cannot
	* encounter ABA problems. However, we do need some care in
	* resetting them between uses.)
	*
	* Implementing an effective arena requires allocating a bunch of
	* space, so we only do so upon detecting contention (except on
	* uniprocessors, where they wouldn't help, so aren't used).
	* Otherwise, exchanges use the single-slot slotExchange method.
	* On contention, not only must the slots be in different
	* locations, but the locations must not encounter memory
	* contention due to being on the same cache line (or more
	* generally, the same coherence unit). Because, as of this
	* writing, there is no way to determine cacheline size, we define
	* a value that is enough for common platforms. Additionally,
	* extra care elsewhere is taken to avoid other false/unintended
	* sharing and to enhance locality, including adding padding (via
	* @Contended) to Nodes, embedding "bound" as an Exchanger field.
	*
	* The arena starts out with only one used slot. We expand the
	* effective arena size by tracking collisions; i.e., failed CASes
	* while trying to exchange. By nature of the above algorithm, the
	* only kinds of collision that reliably indicate contention are
	* when two attempted releases collide -- one of two attempted
	* offers can legitimately fail to CAS without indicating
	* contention by more than one other thread. (Note: it is possible
	* but not worthwhile to more precisely detect contention by
	* reading slot values after CAS failures.) When a thread has
	* collided at each slot within the current arena bound, it tries
	* to expand the arena size by one. We track collisions within
	* bounds by using a version (sequence) number on the "bound"
	* field, and conservatively reset collision counts when a
	* participant notices that bound has been updated (in either
	* direction).
	*
	* The effective arena size is reduced (when there is more than
	* one slot) by giving up on waiting after a while and trying to
	* decrement the arena size on expiration. The value of "a while"
	* is an empirical matter. We implement by piggybacking on the
	* use of spin->yield->block that is essential for reasonable
	* waiting performance anyway -- in a busy exchanger, offers are
	* usually almost immediately released, in which case context
	* switching on multiprocessors is extremely slow/wasteful. Arena
	* waits just omit the blocking part, and instead cancel. The spin
	* count is empirically chosen to be a value that avoids blocking
	* 99% of the time under maximum sustained exchange rates on a
	* range of test machines. Spins and yields entail some limited
	* randomness (using a cheap xorshift) to avoid regular patterns
	* that can induce unproductive grow/shrink cycles. (Using a
	* pseudorandom also helps regularize spin cycle duration by
	* making branches unpredictable.) Also, during an offer, a
	* waiter can "know" that it will be released when its slot has
	* changed, but cannot yet proceed until match is set. In the
	* mean time it cannot cancel the offer, so instead spins/yields.
	* Note: It is possible to avoid this secondary check by changing
	* the linearization point to be a CAS of the match field (as done
	* in one case in the Scott & Scherer DISC paper), which also
	* increases asynchrony a bit, at the expense of poorer collision
	* detection and inability to always reuse per-thread nodes. So
	* the current scheme is typically a better tradeoff.
	*
	* On collisions, indices traverse the arena cyclically in reverse
	* order, restarting at the maximum index (which will tend to be
	* sparsest) when bounds change. (On expirations, indices instead
	* are halved until reaching 0.) It is possible (and has been
	* tried) to use randomized, prime-value-stepped, or double-hash
	* style traversal instead of simple cyclic traversal to reduce
	* bunching. But empirically, whatever benefits these may have
	* don't overcome their added overhead: We are managing operations
	* that occur very quickly unless there is sustained contention,
	* so simpler/faster control policies work better than more
	* accurate but slower ones.
	*
	* Because we use expiration for arena size control, we cannot
	* throw TimeoutExceptions in the timed version of the public
	* exchange method until the arena size has shrunken to zero (or
	* the arena isn't enabled). This may delay response to timeout
	* but is still within spec.
	*
	* Essentially all of the implementation is in methods
	* slotExchange and arenaExchange. These have similar overall
	* structure, but differ in too many details to combine. The
	* slotExchange method uses the single Exchanger field "slot"
	* rather than arena array elements. However, it still needs
	* minimal collision detection to trigger arena construction.
	* (The messiest part is making sure interrupt status and
	* InterruptedExceptions come out right during transitions when
	* both methods may be called. This is done by using null return
	* as a sentinel to recheck interrupt status.)
	*
	* As is too common in this sort of code, methods are monolithic
	* because most of the logic relies on reads of fields that are
	* maintained as local variables so can't be nicely factored --
	* mainly, here, bulky spin->yield->block/cancel code. Note that
	* field Node.item is not declared as volatile even though it is
	* read by releasing threads, because they only do so after CAS
	* operations that must precede access, and all uses by the owning
	* thread are otherwise acceptably ordered by other operations.
	* (Because the actual points of atomicity are slot CASes, it
	* would also be legal for the write to Node.match in a release to
	* be weaker than a full volatile write. However, this is not done
	* because it could allow further postponement of the write,
	* delaying progress.)
	*/

	/**
	* The index distance (as a shift value) between any two used slots
	* in the arena, spacing them out to avoid false sharing.
	*/
	private static final int ASHIFT = 5;

	/**
	* The maximum supported arena index. The maximum allocatable
	* arena size is MMASK + 1. Must be a power of two minus one, less
	* than (1<<(31-ASHIFT)). The cap of 255 (0xff) more than suffices
	* for the expected scaling limits of the main algorithms.
	*/
	private static final int MMASK = 0xff;

	/**
	* Unit for sequence/version bits of bound field. Each successful
	* change to the bound also adds SEQ.
	*/
	private static final int SEQ = MMASK + 1;

	/** The number of CPUs, for sizing and spin control */
	private static final int NCPU = Runtime.getRuntime().availableProcessors();

	/**
	* The maximum slot index of the arena: The number of slots that
	* can in principle hold all threads without contention, or at
	* most the maximum indexable value.
	*/
	static final int FULL = (NCPU >= (MMASK << 1)) ? MMASK : NCPU >>> 1;

	/**
	* The bound for spins while waiting for a match. The actual
	* number of iterations will on average be about twice this value
	* due to randomization. Note: Spinning is disabled when NCPU==1.
	*/
	private static final int SPINS = 1 << 10;

	/**
	* Value representing null arguments/returns from public
	* methods. Needed because the API originally didn't disallow null
	* arguments, which it should have.
	*/
	private static final Object NULL_ITEM = new Object();

	/**
	* Sentinel value returned by internal exchange methods upon
	* timeout, to avoid need for separate timed versions of these
	* methods.
	*/
	private static final Object TIMED_OUT = new Object();

	/**
	* Nodes hold partially exchanged data, plus other per-thread
	* bookkeeping. Padded via @Contended to reduce memory contention.
	*/
	@jdk.internal.vm.annotation.Contended static final class Node {
	int index; // Arena index
	int bound; // Last recorded value of Exchanger.bound
	int collides; // Number of CAS failures at current bound
	int hash; // Pseudo-random for spins
	Object item; // This thread's current item
	volatile Object match; // Item provided by releasing thread
	volatile Thread parked; // Set to this thread when parked, else null
	}

	/** The corresponding thread local class */
	static final class Participant extends ThreadLocal<Node> {
	public Node initialValue() { return new Node(); }
	}

	/**
	* Per-thread state.
	*/
	private final Participant participant;

	/**
	* Elimination array; null until enabled (within slotExchange).
	* Element accesses use emulation of volatile gets and CAS.
	*/
	private volatile Node[] arena;

	/**
	* Slot used until contention detected.
	*/
	private volatile Node slot;

	/**
	* The index of the largest valid arena position, OR'ed with SEQ
	* number in high bits, incremented on each update. The initial
	* update from 0 to SEQ is used to ensure that the arena array is
	* constructed only once.
	*/
	private volatile int bound;

	/**
	* Exchange function when arenas enabled. See above for explanation.
	*
	* @param item the (non-null) item to exchange
	* @param timed true if the wait is timed
	* @param ns if timed, the maximum wait time, else 0L
	* @return the other thread's item; or null if interrupted; or
	* TIMED_OUT if timed and timed out
	*/
	private final Object arenaExchange(Object item, boolean timed, long ns) {
	Node[] a = arena;
	int alen = a.length;
	Node p = participant.get();
	for (int i = p.index;;) { // access slot at i
	int b, m, c;
	int j = (i << ASHIFT) + ((1 << ASHIFT) - 1);
	if (j < 0 \|\| j >= alen)
	j = alen - 1;
	Node q = (Node)AA.getAcquire(a, j);
	if (q != null && AA.compareAndSet(a, j, q, null)) {
	Object v = q.item; // release
	q.match = item;
	Thread w = q.parked;
	if (w != null)
	LockSupport.unpark(w);
	return v;
	}
	else if (i <= (m = (b = bound) & MMASK) && q == null) {
	p.item = item; // offer
	if (AA.compareAndSet(a, j, null, p)) {
	long end = (timed && m == 0) ? System.nanoTime() + ns : 0L;
	Thread t = Thread.currentThread(); // wait
	for (int h = p.hash, spins = SPINS;;) {
	Object v = p.match;
	if (v != null) {
	MATCH.setRelease(p, null);
	p.item = null; // clear for next use
	p.hash = h;
	return v;
	}
	else if (spins > 0) {
	h ^= h << 1; h ^= h >>> 3; h ^= h << 10; // xorshift
	if (h == 0) // initialize hash
	h = SPINS \| (int)t.getId();
	else if (h < 0 && // approx 50% true
	(--spins & ((SPINS >>> 1) - 1)) == 0)
	Thread.yield(); // two yields per wait
	}
	else if (AA.getAcquire(a, j) != p)
	spins = SPINS; // releaser hasn't set match yet
	else if (!t.isInterrupted() && m == 0 &&
	(!timed \|\|
	(ns = end - System.nanoTime()) > 0L)) {
	p.parked = t; // minimize window
	if (AA.getAcquire(a, j) == p) {
	if (ns == 0L)
	LockSupport.park(this);
	else
	LockSupport.parkNanos(this, ns);
	}
	p.parked = null;
	}
	else if (AA.getAcquire(a, j) == p &&
	AA.compareAndSet(a, j, p, null)) {
	if (m != 0) // try to shrink
	BOUND.compareAndSet(this, b, b + SEQ - 1);
	p.item = null;
	p.hash = h;
	i = p.index >>>= 1; // descend
	if (Thread.interrupted())
	return null;
	if (timed && m == 0 && ns <= 0L)
	return TIMED_OUT;
	break; // expired; restart
	}
	}
	}
	else
	p.item = null; // clear offer
	}
	else {
	if (p.bound != b) { // stale; reset
	p.bound = b;
	p.collides = 0;
	i = (i != m \|\| m == 0) ? m : m - 1;
	}
	else if ((c = p.collides) < m \|\| m == FULL \|\|
	!BOUND.compareAndSet(this, b, b + SEQ + 1)) {
	p.collides = c + 1;
	i = (i == 0) ? m : i - 1; // cyclically traverse
	}
	else
	i = m + 1; // grow
	p.index = i;
	}
	}
	}

	/**
	* Exchange function used until arenas enabled. See above for explanation.
	*
	* @param item the item to exchange
	* @param timed true if the wait is timed
	* @param ns if timed, the maximum wait time, else 0L
	* @return the other thread's item; or null if either the arena
	* was enabled or the thread was interrupted before completion; or
	* TIMED_OUT if timed and timed out
	*/
	private final Object slotExchange(Object item, boolean timed, long ns) {
	Node p = participant.get();
	Thread t = Thread.currentThread();
	if (t.isInterrupted()) // preserve interrupt status so caller can recheck
	return null;

	for (Node q;;) {
	if ((q = slot) != null) {
	if (SLOT.compareAndSet(this, q, null)) {
	Object v = q.item;
	q.match = item;
	Thread w = q.parked;
	if (w != null)
	LockSupport.unpark(w);
	return v;
	}
	// create arena on contention, but continue until slot null
	if (NCPU > 1 && bound == 0 &&
	BOUND.compareAndSet(this, 0, SEQ))
	arena = new Node[(FULL + 2) << ASHIFT];
	}
	else if (arena != null)
	return null; // caller must reroute to arenaExchange
	else {
	p.item = item;
	if (SLOT.compareAndSet(this, null, p))
	break;
	p.item = null;
	}
	}

	// await release
	int h = p.hash;
	long end = timed ? System.nanoTime() + ns : 0L;
	int spins = (NCPU > 1) ? SPINS : 1;
	Object v;
	while ((v = p.match) == null) {
	if (spins > 0) {
	h ^= h << 1; h ^= h >>> 3; h ^= h << 10;
	if (h == 0)
	h = SPINS \| (int)t.getId();
	else if (h < 0 && (--spins & ((SPINS >>> 1) - 1)) == 0)
	Thread.yield();
	}
	else if (slot != p)
	spins = SPINS;
	else if (!t.isInterrupted() && arena == null &&
	(!timed \|\| (ns = end - System.nanoTime()) > 0L)) {
	p.parked = t;
	if (slot == p) {
	if (ns == 0L)
	LockSupport.park(this);
	else
	LockSupport.parkNanos(this, ns);
	}
	p.parked = null;
	}
	else if (SLOT.compareAndSet(this, p, null)) {
	v = timed && ns <= 0L && !t.isInterrupted() ? TIMED_OUT : null;
	break;
	}
	}
	MATCH.setRelease(p, null);
	p.item = null;
	p.hash = h;
	return v;
	}

	/**
	* Creates a new Exchanger.
	*/
	public Exchanger() {
	participant = new Participant();
	}

	/**
	* 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
	*/
	@SuppressWarnings("unchecked")
	public V exchange(V x) throws InterruptedException {
	Object v;
	Node[] a;
	Object item = (x == null) ? NULL_ITEM : x; // translate null args
	if (((a = arena) != null \|\|
	(v = slotExchange(item, false, 0L)) == null) &&
	((Thread.interrupted() \|\| // disambiguates null return
	(v = arenaExchange(item, false, 0L)) == null)))
	throw new InterruptedException();
	return (v == NULL_ITEM) ? null : (V)v;
	}

	/**
	* 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 {@code timeout} 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
	*/
	@SuppressWarnings("unchecked")
	public V exchange(V x, long timeout, TimeUnit unit)
	throws InterruptedException, TimeoutException {
	Object v;
	Object item = (x == null) ? NULL_ITEM : x;
	long ns = unit.toNanos(timeout);
	if ((arena != null \|\|
	(v = slotExchange(item, true, ns)) == null) &&
	((Thread.interrupted() \|\|
	(v = arenaExchange(item, true, ns)) == null)))
	throw new InterruptedException();
	if (v == TIMED_OUT)
	throw new TimeoutException();
	return (v == NULL_ITEM) ? null : (V)v;
	}

	// VarHandle mechanics
	private static final VarHandle BOUND;
	private static final VarHandle SLOT;
	private static final VarHandle MATCH;
	private static final VarHandle AA;
	static {
	try {
	MethodHandles.Lookup l = MethodHandles.lookup();
	BOUND = l.findVarHandle(Exchanger.class, "bound", int.class);
	SLOT = l.findVarHandle(Exchanger.class, "slot", Node.class);
	MATCH = l.findVarHandle(Node.class, "match", Object.class);
	AA = MethodHandles.arrayElementVarHandle(Node[].class);
	} catch (ReflectiveOperationException e) {
	throw new ExceptionInInitializerError(e);
	}
	}

	}

Back to index...