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⏎ java/util/concurrent/Exchanger.java
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
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* 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);
}
}
}
⏎ java/util/concurrent/Exchanger.java
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