| /* |
| * Copyright (C) 2011 The Guava Authors |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); |
| * you may not use this file except in compliance with the License. |
| * You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software |
| * distributed under the License is distributed on an "AS IS" BASIS, |
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| * See the License for the specific language governing permissions and |
| * limitations under the License. |
| */ |
| |
| package com.google.common.util.concurrent; |
| |
| import com.google.common.annotations.Beta; |
| import com.google.common.annotations.VisibleForTesting; |
| import com.google.common.base.MoreObjects; |
| import com.google.common.base.Preconditions; |
| import com.google.common.base.Supplier; |
| import com.google.common.collect.ImmutableList; |
| import com.google.common.collect.Iterables; |
| import com.google.common.collect.MapMaker; |
| import com.google.common.math.IntMath; |
| import com.google.common.primitives.Ints; |
| |
| import java.lang.ref.Reference; |
| import java.lang.ref.ReferenceQueue; |
| import java.lang.ref.WeakReference; |
| import java.math.RoundingMode; |
| import java.util.Arrays; |
| import java.util.Collections; |
| import java.util.List; |
| import java.util.concurrent.ConcurrentMap; |
| import java.util.concurrent.Semaphore; |
| import java.util.concurrent.atomic.AtomicReferenceArray; |
| import java.util.concurrent.locks.Lock; |
| import java.util.concurrent.locks.ReadWriteLock; |
| import java.util.concurrent.locks.ReentrantLock; |
| import java.util.concurrent.locks.ReentrantReadWriteLock; |
| |
| /** |
| * A striped {@code Lock/Semaphore/ReadWriteLock}. This offers the underlying lock striping |
| * similar to that of {@code ConcurrentHashMap} in a reusable form, and extends it for |
| * semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock |
| * into many <i>stripes</i>, increasing the granularity of a single lock and allowing independent |
| * operations to lock different stripes and proceed concurrently, instead of creating contention |
| * for a single lock. |
| * |
| * <p>The guarantee provided by this class is that equal keys lead to the same lock (or semaphore), |
| * i.e. {@code if (key1.equals(key2))} then {@code striped.get(key1) == striped.get(key2)} |
| * (assuming {@link Object#hashCode()} is correctly implemented for the keys). Note |
| * that if {@code key1} is <strong>not</strong> equal to {@code key2}, it is <strong>not</strong> |
| * guaranteed that {@code striped.get(key1) != striped.get(key2)}; the elements might nevertheless |
| * be mapped to the same lock. The lower the number of stripes, the higher the probability of this |
| * happening. |
| * |
| * <p>There are three flavors of this class: {@code Striped<Lock>}, {@code Striped<Semaphore>}, |
| * and {@code Striped<ReadWriteLock>}. For each type, two implementations are offered: |
| * {@linkplain #lock(int) strong} and {@linkplain #lazyWeakLock(int) weak} |
| * {@code Striped<Lock>}, {@linkplain #semaphore(int, int) strong} and {@linkplain |
| * #lazyWeakSemaphore(int, int) weak} {@code Striped<Semaphore>}, and {@linkplain |
| * #readWriteLock(int) strong} and {@linkplain #lazyWeakReadWriteLock(int) weak} |
| * {@code Striped<ReadWriteLock>}. <i>Strong</i> means that all stripes (locks/semaphores) are |
| * initialized eagerly, and are not reclaimed unless {@code Striped} itself is reclaimable. |
| * <i>Weak</i> means that locks/semaphores are created lazily, and they are allowed to be reclaimed |
| * if nobody is holding on to them. This is useful, for example, if one wants to create a {@code |
| * Striped<Lock>} of many locks, but worries that in most cases only a small portion of these |
| * would be in use. |
| * |
| * <p>Prior to this class, one might be tempted to use {@code Map<K, Lock>}, where {@code K} |
| * represents the task. This maximizes concurrency by having each unique key mapped to a unique |
| * lock, but also maximizes memory footprint. On the other extreme, one could use a single lock |
| * for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of |
| * choosing either of these extremes, {@code Striped} allows the user to trade between required |
| * concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily |
| * create a very compact {@code Striped<Lock>} of {@code availableProcessors() * 4} stripes, |
| * instead of possibly thousands of locks which could be created in a {@code Map<K, Lock>} |
| * structure. |
| * |
| * @author Dimitris Andreou |
| * @since 13.0 |
| */ |
| @Beta |
| public abstract class Striped<L> { |
| /** |
| * If there are at least this many stripes, we assume the memory usage of a ConcurrentMap will be |
| * smaller than a large array. (This assumes that in the lazy case, most stripes are unused. As |
| * always, if many stripes are in use, a non-lazy striped makes more sense.) |
| */ |
| private static final int LARGE_LAZY_CUTOFF = 1024; |
| |
| private Striped() {} |
| |
| /** |
| * Returns the stripe that corresponds to the passed key. It is always guaranteed that if |
| * {@code key1.equals(key2)}, then {@code get(key1) == get(key2)}. |
| * |
| * @param key an arbitrary, non-null key |
| * @return the stripe that the passed key corresponds to |
| */ |
| public abstract L get(Object key); |
| |
| /** |
| * Returns the stripe at the specified index. Valid indexes are 0, inclusively, to |
| * {@code size()}, exclusively. |
| * |
| * @param index the index of the stripe to return; must be in {@code [0...size())} |
| * @return the stripe at the specified index |
| */ |
| public abstract L getAt(int index); |
| |
| /** |
| * Returns the index to which the given key is mapped, so that getAt(indexFor(key)) == get(key). |
| */ |
| abstract int indexFor(Object key); |
| |
| /** |
| * Returns the total number of stripes in this instance. |
| */ |
| public abstract int size(); |
| |
| /** |
| * Returns the stripes that correspond to the passed objects, in ascending (as per |
| * {@link #getAt(int)}) order. Thus, threads that use the stripes in the order returned |
| * by this method are guaranteed to not deadlock each other. |
| * |
| * <p>It should be noted that using a {@code Striped<L>} with relatively few stripes, and |
| * {@code bulkGet(keys)} with a relative large number of keys can cause an excessive number |
| * of shared stripes (much like the birthday paradox, where much fewer than anticipated birthdays |
| * are needed for a pair of them to match). Please consider carefully the implications of the |
| * number of stripes, the intended concurrency level, and the typical number of keys used in a |
| * {@code bulkGet(keys)} operation. See <a href="http://www.mathpages.com/home/kmath199.htm">Balls |
| * in Bins model</a> for mathematical formulas that can be used to estimate the probability of |
| * collisions. |
| * |
| * @param keys arbitrary non-null keys |
| * @return the stripes corresponding to the objects (one per each object, derived by delegating |
| * to {@link #get(Object)}; may contain duplicates), in an increasing index order. |
| */ |
| public Iterable<L> bulkGet(Iterable<?> keys) { |
| // Initially using the array to store the keys, then reusing it to store the respective L's |
| final Object[] array = Iterables.toArray(keys, Object.class); |
| if (array.length == 0) { |
| return ImmutableList.of(); |
| } |
| int[] stripes = new int[array.length]; |
| for (int i = 0; i < array.length; i++) { |
| stripes[i] = indexFor(array[i]); |
| } |
| Arrays.sort(stripes); |
| // optimize for runs of identical stripes |
| int previousStripe = stripes[0]; |
| array[0] = getAt(previousStripe); |
| for (int i = 1; i < array.length; i++) { |
| int currentStripe = stripes[i]; |
| if (currentStripe == previousStripe) { |
| array[i] = array[i - 1]; |
| } else { |
| array[i] = getAt(currentStripe); |
| previousStripe = currentStripe; |
| } |
| } |
| /* |
| * Note that the returned Iterable holds references to the returned stripes, to avoid |
| * error-prone code like: |
| * |
| * Striped<Lock> stripedLock = Striped.lazyWeakXXX(...)' |
| * Iterable<Lock> locks = stripedLock.bulkGet(keys); |
| * for (Lock lock : locks) { |
| * lock.lock(); |
| * } |
| * operation(); |
| * for (Lock lock : locks) { |
| * lock.unlock(); |
| * } |
| * |
| * If we only held the int[] stripes, translating it on the fly to L's, the original locks |
| * might be garbage collected after locking them, ending up in a huge mess. |
| */ |
| @SuppressWarnings("unchecked") // we carefully replaced all keys with their respective L's |
| List<L> asList = (List<L>) Arrays.asList(array); |
| return Collections.unmodifiableList(asList); |
| } |
| |
| // Static factories |
| |
| /** |
| * Creates a {@code Striped<Lock>} with eagerly initialized, strongly referenced locks. |
| * Every lock is reentrant. |
| * |
| * @param stripes the minimum number of stripes (locks) required |
| * @return a new {@code Striped<Lock>} |
| */ |
| public static Striped<Lock> lock(int stripes) { |
| return new CompactStriped<Lock>(stripes, new Supplier<Lock>() { |
| @Override public Lock get() { |
| return new PaddedLock(); |
| } |
| }); |
| } |
| |
| /** |
| * Creates a {@code Striped<Lock>} with lazily initialized, weakly referenced locks. |
| * Every lock is reentrant. |
| * |
| * @param stripes the minimum number of stripes (locks) required |
| * @return a new {@code Striped<Lock>} |
| */ |
| public static Striped<Lock> lazyWeakLock(int stripes) { |
| return lazy(stripes, new Supplier<Lock>() { |
| @Override public Lock get() { |
| return new ReentrantLock(false); |
| } |
| }); |
| } |
| |
| private static <L> Striped<L> lazy(int stripes, Supplier<L> supplier) { |
| return stripes < LARGE_LAZY_CUTOFF |
| ? new SmallLazyStriped<L>(stripes, supplier) |
| : new LargeLazyStriped<L>(stripes, supplier); |
| } |
| |
| /** |
| * Creates a {@code Striped<Semaphore>} with eagerly initialized, strongly referenced semaphores, |
| * with the specified number of permits. |
| * |
| * @param stripes the minimum number of stripes (semaphores) required |
| * @param permits the number of permits in each semaphore |
| * @return a new {@code Striped<Semaphore>} |
| */ |
| public static Striped<Semaphore> semaphore(int stripes, final int permits) { |
| return new CompactStriped<Semaphore>(stripes, new Supplier<Semaphore>() { |
| @Override public Semaphore get() { |
| return new PaddedSemaphore(permits); |
| } |
| }); |
| } |
| |
| /** |
| * Creates a {@code Striped<Semaphore>} with lazily initialized, weakly referenced semaphores, |
| * with the specified number of permits. |
| * |
| * @param stripes the minimum number of stripes (semaphores) required |
| * @param permits the number of permits in each semaphore |
| * @return a new {@code Striped<Semaphore>} |
| */ |
| public static Striped<Semaphore> lazyWeakSemaphore(int stripes, final int permits) { |
| return lazy(stripes, new Supplier<Semaphore>() { |
| @Override public Semaphore get() { |
| return new Semaphore(permits, false); |
| } |
| }); |
| } |
| |
| /** |
| * Creates a {@code Striped<ReadWriteLock>} with eagerly initialized, strongly referenced |
| * read-write locks. Every lock is reentrant. |
| * |
| * @param stripes the minimum number of stripes (locks) required |
| * @return a new {@code Striped<ReadWriteLock>} |
| */ |
| public static Striped<ReadWriteLock> readWriteLock(int stripes) { |
| return new CompactStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER); |
| } |
| |
| /** |
| * Creates a {@code Striped<ReadWriteLock>} with lazily initialized, weakly referenced |
| * read-write locks. Every lock is reentrant. |
| * |
| * @param stripes the minimum number of stripes (locks) required |
| * @return a new {@code Striped<ReadWriteLock>} |
| */ |
| public static Striped<ReadWriteLock> lazyWeakReadWriteLock(int stripes) { |
| return lazy(stripes, READ_WRITE_LOCK_SUPPLIER); |
| } |
| |
| // ReentrantReadWriteLock is large enough to make padding probably unnecessary |
| private static final Supplier<ReadWriteLock> READ_WRITE_LOCK_SUPPLIER = |
| new Supplier<ReadWriteLock>() { |
| @Override public ReadWriteLock get() { |
| return new ReentrantReadWriteLock(); |
| } |
| }; |
| |
| private abstract static class PowerOfTwoStriped<L> extends Striped<L> { |
| /** Capacity (power of two) minus one, for fast mod evaluation */ |
| final int mask; |
| |
| PowerOfTwoStriped(int stripes) { |
| Preconditions.checkArgument(stripes > 0, "Stripes must be positive"); |
| this.mask = stripes > Ints.MAX_POWER_OF_TWO ? ALL_SET : ceilToPowerOfTwo(stripes) - 1; |
| } |
| |
| @Override final int indexFor(Object key) { |
| int hash = smear(key.hashCode()); |
| return hash & mask; |
| } |
| |
| @Override public final L get(Object key) { |
| return getAt(indexFor(key)); |
| } |
| } |
| |
| /** |
| * Implementation of Striped where 2^k stripes are represented as an array of the same length, |
| * eagerly initialized. |
| */ |
| private static class CompactStriped<L> extends PowerOfTwoStriped<L> { |
| /** Size is a power of two. */ |
| private final Object[] array; |
| |
| private CompactStriped(int stripes, Supplier<L> supplier) { |
| super(stripes); |
| Preconditions.checkArgument(stripes <= Ints.MAX_POWER_OF_TWO, "Stripes must be <= 2^30)"); |
| |
| this.array = new Object[mask + 1]; |
| for (int i = 0; i < array.length; i++) { |
| array[i] = supplier.get(); |
| } |
| } |
| |
| @SuppressWarnings("unchecked") // we only put L's in the array |
| @Override public L getAt(int index) { |
| return (L) array[index]; |
| } |
| |
| @Override public int size() { |
| return array.length; |
| } |
| } |
| |
| /** |
| * Implementation of Striped where up to 2^k stripes can be represented, using an |
| * AtomicReferenceArray of size 2^k. To map a user key into a stripe, we take a k-bit slice of the |
| * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced. |
| */ |
| @VisibleForTesting static class SmallLazyStriped<L> extends PowerOfTwoStriped<L> { |
| final AtomicReferenceArray<ArrayReference<? extends L>> locks; |
| final Supplier<L> supplier; |
| final int size; |
| final ReferenceQueue<L> queue = new ReferenceQueue<L>(); |
| |
| SmallLazyStriped(int stripes, Supplier<L> supplier) { |
| super(stripes); |
| this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1; |
| this.locks = new AtomicReferenceArray<ArrayReference<? extends L>>(size); |
| this.supplier = supplier; |
| } |
| |
| @Override public L getAt(int index) { |
| if (size != Integer.MAX_VALUE) { |
| Preconditions.checkElementIndex(index, size()); |
| } // else no check necessary, all index values are valid |
| ArrayReference<? extends L> existingRef = locks.get(index); |
| L existing = existingRef == null ? null : existingRef.get(); |
| if (existing != null) { |
| return existing; |
| } |
| L created = supplier.get(); |
| ArrayReference<L> newRef = new ArrayReference<L>(created, index, queue); |
| while (!locks.compareAndSet(index, existingRef, newRef)) { |
| // we raced, we need to re-read and try again |
| existingRef = locks.get(index); |
| existing = existingRef == null ? null : existingRef.get(); |
| if (existing != null) { |
| return existing; |
| } |
| } |
| drainQueue(); |
| return created; |
| } |
| |
| // N.B. Draining the queue is only necessary to ensure that we don't accumulate empty references |
| // in the array. We could skip this if we decide we don't care about holding on to Reference |
| // objects indefinitely. |
| private void drainQueue() { |
| Reference<? extends L> ref; |
| while ((ref = queue.poll()) != null) { |
| // We only ever register ArrayReferences with the queue so this is always safe. |
| ArrayReference<? extends L> arrayRef = (ArrayReference<? extends L>) ref; |
| // Try to clear out the array slot, n.b. if we fail that is fine, in either case the |
| // arrayRef will be out of the array after this step. |
| locks.compareAndSet(arrayRef.index, arrayRef, null); |
| } |
| } |
| |
| @Override public int size() { |
| return size; |
| } |
| |
| private static final class ArrayReference<L> extends WeakReference<L> { |
| final int index; |
| |
| ArrayReference(L referent, int index, ReferenceQueue<L> queue) { |
| super(referent, queue); |
| this.index = index; |
| } |
| } |
| } |
| |
| /** |
| * Implementation of Striped where up to 2^k stripes can be represented, using a ConcurrentMap |
| * where the key domain is [0..2^k). To map a user key into a stripe, we take a k-bit slice of the |
| * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced. |
| */ |
| @VisibleForTesting static class LargeLazyStriped<L> extends PowerOfTwoStriped<L> { |
| final ConcurrentMap<Integer, L> locks; |
| final Supplier<L> supplier; |
| final int size; |
| |
| LargeLazyStriped(int stripes, Supplier<L> supplier) { |
| super(stripes); |
| this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1; |
| this.supplier = supplier; |
| this.locks = new MapMaker().weakValues().makeMap(); |
| } |
| |
| @Override public L getAt(int index) { |
| if (size != Integer.MAX_VALUE) { |
| Preconditions.checkElementIndex(index, size()); |
| } // else no check necessary, all index values are valid |
| L existing = locks.get(index); |
| if (existing != null) { |
| return existing; |
| } |
| L created = supplier.get(); |
| existing = locks.putIfAbsent(index, created); |
| return MoreObjects.firstNonNull(existing, created); |
| } |
| |
| @Override public int size() { |
| return size; |
| } |
| } |
| |
| /** |
| * A bit mask were all bits are set. |
| */ |
| private static final int ALL_SET = ~0; |
| |
| private static int ceilToPowerOfTwo(int x) { |
| return 1 << IntMath.log2(x, RoundingMode.CEILING); |
| } |
| |
| /* |
| * This method was written by Doug Lea with assistance from members of JCP |
| * JSR-166 Expert Group and released to the public domain, as explained at |
| * http://creativecommons.org/licenses/publicdomain |
| * |
| * As of 2010/06/11, this method is identical to the (package private) hash |
| * method in OpenJDK 7's java.util.HashMap class. |
| */ |
| // Copied from java/com/google/common/collect/Hashing.java |
| private static int smear(int hashCode) { |
| hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12); |
| return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4); |
| } |
| |
| private static class PaddedLock extends ReentrantLock { |
| /* |
| * Padding from 40 into 64 bytes, same size as cache line. Might be beneficial to add |
| * a fourth long here, to minimize chance of interference between consecutive locks, |
| * but I couldn't observe any benefit from that. |
| */ |
| @SuppressWarnings("unused") |
| long q1, q2, q3; |
| |
| PaddedLock() { |
| super(false); |
| } |
| } |
| |
| private static class PaddedSemaphore extends Semaphore { |
| // See PaddedReentrantLock comment |
| @SuppressWarnings("unused") |
| long q1, q2, q3; |
| |
| PaddedSemaphore(int permits) { |
| super(permits, false); |
| } |
| } |
| } |