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Diffstat (limited to 'libjava/classpath/external/jsr166/java/util/concurrent/Exchanger.java')
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diff --git a/libjava/classpath/external/jsr166/java/util/concurrent/Exchanger.java b/libjava/classpath/external/jsr166/java/util/concurrent/Exchanger.java new file mode 100644 index 00000000000..fb917f4321e --- /dev/null +++ b/libjava/classpath/external/jsr166/java/util/concurrent/Exchanger.java @@ -0,0 +1,656 @@ +/* + * 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/licenses/publicdomain + */ + +package java.util.concurrent; +import java.util.concurrent.atomic.*; +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>(); + * 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> { + /* + * Algorithm Description: + * + * The basic idea is to maintain a "slot", which is a reference to + * a Node containing both an Item to offer and a "hole" waiting to + * get filled in. If an incoming "occupying" thread sees that the + * slot is null, it CAS'es (compareAndSets) a Node there and waits + * for another to invoke exchange. That second "fulfilling" thread + * sees that the slot is non-null, and so CASes it back to null, + * also exchanging items by CASing the hole, plus waking up the + * occupying thread if it is blocked. In each case CAS'es may + * fail because a slot at first appears non-null but is null upon + * CAS, or vice-versa. So threads may need to retry these + * actions. + * + * This simple approach works great when there are only a few + * threads using an Exchanger, but performance rapidly + * deteriorates due to CAS contention on the single slot when + * there are lots of threads using an exchanger. So instead we use + * an "arena"; basically a kind of hash table with a dynamically + * varying number of slots, any one of which can be used by + * threads performing an exchange. Incoming threads pick slots + * based on a hash of their Thread ids. If an incoming thread + * fails to CAS in its chosen slot, it picks an alternative slot + * instead. And similarly from there. If a thread successfully + * CASes into a slot but no other thread arrives, it tries + * another, heading toward the zero slot, which always exists even + * if the table shrinks. The particular mechanics controlling this + * are as follows: + * + * Waiting: Slot zero is special in that it is the only slot that + * exists when there is no contention. A thread occupying slot + * zero will block if no thread fulfills it after a short spin. + * In other cases, occupying threads eventually give up and try + * another slot. Waiting threads spin for a while (a period that + * should be a little less than a typical context-switch time) + * before either blocking (if slot zero) or giving up (if other + * slots) and restarting. There is no reason for threads to block + * unless there are unlikely to be any other threads present. + * Occupants are mainly avoiding memory contention so sit there + * quietly polling for a shorter period than it would take to + * block and then unblock them. Non-slot-zero waits that elapse + * because of lack of other threads waste around one extra + * context-switch time per try, which is still on average much + * faster than alternative approaches. + * + * Sizing: Usually, using only a few slots suffices to reduce + * contention. Especially with small numbers of threads, using + * too many slots can lead to just as poor performance as using + * too few of them, and there's not much room for error. The + * variable "max" maintains the number of slots actually in + * use. It is increased when a thread sees too many CAS + * failures. (This is analogous to resizing a regular hash table + * based on a target load factor, except here, growth steps are + * just one-by-one rather than proportional.) Growth requires + * contention failures in each of three tried slots. Requiring + * multiple failures for expansion copes with the fact that some + * failed CASes are not due to contention but instead to simple + * races between two threads or thread pre-emptions occurring + * between reading and CASing. Also, very transient peak + * contention can be much higher than the average sustainable + * levels. The max limit is decreased on average 50% of the times + * that a non-slot-zero wait elapses without being fulfilled. + * Threads experiencing elapsed waits move closer to zero, so + * eventually find existing (or future) threads even if the table + * has been shrunk due to inactivity. The chosen mechanics and + * thresholds for growing and shrinking are intrinsically + * entangled with indexing and hashing inside the exchange code, + * and can't be nicely abstracted out. + * + * Hashing: Each thread picks its initial slot to use in accord + * with a simple hashcode. The sequence is the same on each + * encounter by any given thread, but effectively random across + * threads. Using arenas encounters the classic cost vs quality + * tradeoffs of all hash tables. Here, we use a one-step FNV-1a + * hash code based on the current thread's Thread.getId(), along + * with a cheap approximation to a mod operation to select an + * index. The downside of optimizing index selection in this way + * is that the code is hardwired to use a maximum table size of + * 32. But this value more than suffices for known platforms and + * applications. + * + * Probing: On sensed contention of a selected slot, we probe + * sequentially through the table, analogously to linear probing + * after collision in a hash table. (We move circularly, in + * reverse order, to mesh best with table growth and shrinkage + * rules.) Except that to minimize the effects of false-alarms + * and cache thrashing, we try the first selected slot twice + * before moving. + * + * Padding: Even with contention management, slots are heavily + * contended, so use cache-padding to avoid poor memory + * performance. Because of this, slots are lazily constructed + * only when used, to avoid wasting this space unnecessarily. + * While isolation of locations is not much of an issue at first + * in an application, as time goes on and garbage-collectors + * perform compaction, slots are very likely to be moved adjacent + * to each other, which can cause much thrashing of cache lines on + * MPs unless padding is employed. + * + * This is an improvement of the algorithm described in the paper + * "A Scalable Elimination-based Exchange Channel" by William + * Scherer, Doug Lea, and Michael Scott in Proceedings of SCOOL05 + * workshop. Available at: http://hdl.handle.net/1802/2104 + */ + + /** The number of CPUs, for sizing and spin control */ + private static final int NCPU = Runtime.getRuntime().availableProcessors(); + + /** + * The capacity of the arena. Set to a value that provides more + * than enough space to handle contention. On small machines + * most slots won't be used, but it is still not wasted because + * the extra space provides some machine-level address padding + * to minimize interference with heavily CAS'ed Slot locations. + * And on very large machines, performance eventually becomes + * bounded by memory bandwidth, not numbers of threads/CPUs. + * This constant cannot be changed without also modifying + * indexing and hashing algorithms. + */ + private static final int CAPACITY = 32; + + /** + * The value of "max" that will hold all threads without + * contention. When this value is less than CAPACITY, some + * otherwise wasted expansion can be avoided. + */ + private static final int FULL = + Math.max(0, Math.min(CAPACITY, NCPU / 2) - 1); + + /** + * The number of times to spin (doing nothing except polling a + * memory location) before blocking or giving up while waiting to + * be fulfilled. Should be zero on uniprocessors. On + * multiprocessors, this value should be large enough so that two + * threads exchanging items as fast as possible block only when + * one of them is stalled (due to GC or preemption), but not much + * longer, to avoid wasting CPU resources. Seen differently, this + * value is a little over half the number of cycles of an average + * context switch time on most systems. The value here is + * approximately the average of those across a range of tested + * systems. + */ + private static final int SPINS = (NCPU == 1) ? 0 : 2000; + + /** + * The number of times to spin before blocking in timed waits. + * Timed waits spin more slowly because checking the time takes + * time. The best value relies mainly on the relative rate of + * System.nanoTime vs memory accesses. The value is empirically + * derived to work well across a variety of systems. + */ + private static final int TIMED_SPINS = SPINS / 20; + + /** + * Sentinel item representing cancellation of a wait due to + * interruption, timeout, or elapsed spin-waits. This value is + * placed in holes on cancellation, and used as a return value + * from waiting methods to indicate failure to set or get hole. + */ + private static final Object CANCEL = new Object(); + + /** + * Value representing null arguments/returns from public + * methods. This disambiguates from internal requirement that + * holes start out as null to mean they are not yet set. + */ + private static final Object NULL_ITEM = new Object(); + + /** + * Nodes hold partially exchanged data. This class + * opportunistically subclasses AtomicReference to represent the + * hole. So get() returns hole, and compareAndSet CAS'es value + * into hole. This class cannot be parameterized as "V" because + * of the use of non-V CANCEL sentinels. + */ + private static final class Node extends AtomicReference<Object> { + /** The element offered by the Thread creating this node. */ + public final Object item; + + /** The Thread waiting to be signalled; null until waiting. */ + public volatile Thread waiter; + + /** + * Creates node with given item and empty hole. + * @param item the item + */ + public Node(Object item) { + this.item = item; + } + } + + /** + * A Slot is an AtomicReference with heuristic padding to lessen + * cache effects of this heavily CAS'ed location. While the + * padding adds noticeable space, all slots are created only on + * demand, and there will be more than one of them only when it + * would improve throughput more than enough to outweigh using + * extra space. + */ + private static final class Slot extends AtomicReference<Object> { + // Improve likelihood of isolation on <= 64 byte cache lines + long q0, q1, q2, q3, q4, q5, q6, q7, q8, q9, qa, qb, qc, qd, qe; + } + + /** + * Slot array. Elements are lazily initialized when needed. + * Declared volatile to enable double-checked lazy construction. + */ + private volatile Slot[] arena = new Slot[CAPACITY]; + + /** + * The maximum slot index being used. The value sometimes + * increases when a thread experiences too many CAS contentions, + * and sometimes decreases when a spin-wait elapses. Changes + * are performed only via compareAndSet, to avoid stale values + * when a thread happens to stall right before setting. + */ + private final AtomicInteger max = new AtomicInteger(); + + /** + * Main exchange function, handling the different policy variants. + * Uses Object, not "V" as argument and return value to simplify + * handling of sentinel values. Callers from public methods decode + * and cast accordingly. + * + * @param item the (non-null) item to exchange + * @param timed true if the wait is timed + * @param nanos if timed, the maximum wait time + * @return the other thread's item, or CANCEL if interrupted or timed out + */ + private Object doExchange(Object item, boolean timed, long nanos) { + Node me = new Node(item); // Create in case occupying + int index = hashIndex(); // Index of current slot + int fails = 0; // Number of CAS failures + + for (;;) { + Object y; // Contents of current slot + Slot slot = arena[index]; + if (slot == null) // Lazily initialize slots + createSlot(index); // Continue loop to reread + else if ((y = slot.get()) != null && // Try to fulfill + slot.compareAndSet(y, null)) { + Node you = (Node)y; // Transfer item + if (you.compareAndSet(null, item)) { + LockSupport.unpark(you.waiter); + return you.item; + } // Else cancelled; continue + } + else if (y == null && // Try to occupy + slot.compareAndSet(null, me)) { + if (index == 0) // Blocking wait for slot 0 + return timed? awaitNanos(me, slot, nanos): await(me, slot); + Object v = spinWait(me, slot); // Spin wait for non-0 + if (v != CANCEL) + return v; + me = new Node(item); // Throw away cancelled node + int m = max.get(); + if (m > (index >>>= 1)) // Decrease index + max.compareAndSet(m, m - 1); // Maybe shrink table + } + else if (++fails > 1) { // Allow 2 fails on 1st slot + int m = max.get(); + if (fails > 3 && m < FULL && max.compareAndSet(m, m + 1)) + index = m + 1; // Grow on 3rd failed slot + else if (--index < 0) + index = m; // Circularly traverse + } + } + } + + /** + * Returns a hash index for the current thread. Uses a one-step + * FNV-1a hash code (http://www.isthe.com/chongo/tech/comp/fnv/) + * based on the current thread's Thread.getId(). These hash codes + * have more uniform distribution properties with respect to small + * moduli (here 1-31) than do other simple hashing functions. + * + * <p>To return an index between 0 and max, we use a cheap + * approximation to a mod operation, that also corrects for bias + * due to non-power-of-2 remaindering (see {@link + * java.util.Random#nextInt}). Bits of the hashcode are masked + * with "nbits", the ceiling power of two of table size (looked up + * in a table packed into three ints). If too large, this is + * retried after rotating the hash by nbits bits, while forcing new + * top bit to 0, which guarantees eventual termination (although + * with a non-random-bias). This requires an average of less than + * 2 tries for all table sizes, and has a maximum 2% difference + * from perfectly uniform slot probabilities when applied to all + * possible hash codes for sizes less than 32. + * + * @return a per-thread-random index, 0 <= index < max + */ + private final int hashIndex() { + long id = Thread.currentThread().getId(); + int hash = (((int)(id ^ (id >>> 32))) ^ 0x811c9dc5) * 0x01000193; + + int m = max.get(); + int nbits = (((0xfffffc00 >> m) & 4) | // Compute ceil(log2(m+1)) + ((0x000001f8 >>> m) & 2) | // The constants hold + ((0xffff00f2 >>> m) & 1)); // a lookup table + int index; + while ((index = hash & ((1 << nbits) - 1)) > m) // May retry on + hash = (hash >>> nbits) | (hash << (33 - nbits)); // non-power-2 m + return index; + } + + /** + * Creates a new slot at given index. Called only when the slot + * appears to be null. Relies on double-check using builtin + * locks, since they rarely contend. This in turn relies on the + * arena array being declared volatile. + * + * @param index the index to add slot at + */ + private void createSlot(int index) { + // Create slot outside of lock to narrow sync region + Slot newSlot = new Slot(); + Slot[] a = arena; + synchronized (a) { + if (a[index] == null) + a[index] = newSlot; + } + } + + /** + * Tries to cancel a wait for the given node waiting in the given + * slot, if so, helping clear the node from its slot to avoid + * garbage retention. + * + * @param node the waiting node + * @param the slot it is waiting in + * @return true if successfully cancelled + */ + private static boolean tryCancel(Node node, Slot slot) { + if (!node.compareAndSet(null, CANCEL)) + return false; + if (slot.get() == node) // pre-check to minimize contention + slot.compareAndSet(node, null); + return true; + } + + // Three forms of waiting. Each just different enough not to merge + // code with others. + + /** + * Spin-waits for hole for a non-0 slot. Fails if spin elapses + * before hole filled. Does not check interrupt, relying on check + * in public exchange method to abort if interrupted on entry. + * + * @param node the waiting node + * @return on success, the hole; on failure, CANCEL + */ + private static Object spinWait(Node node, Slot slot) { + int spins = SPINS; + for (;;) { + Object v = node.get(); + if (v != null) + return v; + else if (spins > 0) + --spins; + else + tryCancel(node, slot); + } + } + + /** + * Waits for (by spinning and/or blocking) and gets the hole + * filled in by another thread. Fails if interrupted before + * hole filled. + * + * When a node/thread is about to block, it sets its waiter field + * and then rechecks state at least one more time before actually + * parking, thus covering race vs fulfiller noticing that waiter + * is non-null so should be woken. + * + * Thread interruption status is checked only surrounding calls to + * park. The caller is assumed to have checked interrupt status + * on entry. + * + * @param node the waiting node + * @return on success, the hole; on failure, CANCEL + */ + private static Object await(Node node, Slot slot) { + Thread w = Thread.currentThread(); + int spins = SPINS; + for (;;) { + Object v = node.get(); + if (v != null) + return v; + else if (spins > 0) // Spin-wait phase + --spins; + else if (node.waiter == null) // Set up to block next + node.waiter = w; + else if (w.isInterrupted()) // Abort on interrupt + tryCancel(node, slot); + 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(); + } +} |
