001 /*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.util.concurrent;
018
019 import com.google.common.annotations.Beta;
020 import com.google.common.base.Functions;
021 import com.google.common.base.Preconditions;
022 import com.google.common.base.Supplier;
023 import com.google.common.collect.Iterables;
024 import com.google.common.collect.MapMaker;
025 import com.google.common.math.IntMath;
026 import com.google.common.primitives.Ints;
027
028 import java.math.RoundingMode;
029 import java.util.Arrays;
030 import java.util.Collections;
031 import java.util.List;
032 import java.util.concurrent.ConcurrentMap;
033 import java.util.concurrent.Semaphore;
034 import java.util.concurrent.locks.Lock;
035 import java.util.concurrent.locks.ReadWriteLock;
036 import java.util.concurrent.locks.ReentrantLock;
037 import java.util.concurrent.locks.ReentrantReadWriteLock;
038
039 /**
040 * A striped {@code Lock/Semaphore/ReadWriteLock}. This offers the underlying lock striping
041 * similar to that of {@code ConcurrentHashMap} in a reusable form, and extends it for
042 * semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock
043 * into many <i>stripes</i>, increasing the granularity of a single lock and allowing independent
044 * operations to lock different stripes and proceed concurrently, instead of creating contention
045 * for a single lock.
046 *
047 * <p>The guarantee provided by this class is that equal keys lead to the same lock (or semaphore),
048 * i.e. {@code if (key1.equals(key2))} then {@code striped.get(key1) == striped.get(key2)}
049 * (assuming {@link Object#hashCode()} is correctly implemented for the keys). Note
050 * that if {@code key1} is <strong>not</strong> equal to {@code key2}, it is <strong>not</strong>
051 * guaranteed that {@code striped.get(key1) != striped.get(key2)}; the elements might nevertheless
052 * be mapped to the same lock. The lower the number of stripes, the higher the probability of this
053 * happening.
054 *
055 * <p>There are three flavors of this class: {@code Striped<Lock>}, {@code Striped<Semaphore>},
056 * and {@code Striped<ReadWriteLock>}. For each type, two implementations are offered:
057 * {@linkplain #lock(int) strong} and {@linkplain #lazyWeakLock(int) weak}
058 * {@code Striped<Lock>}, {@linkplain #semaphore(int, int) strong} and {@linkplain
059 * #lazyWeakSemaphore(int, int) weak} {@code Striped<Semaphore>}, and {@linkplain
060 * #readWriteLock(int) strong} and {@linkplain #lazyWeakReadWriteLock(int) weak}
061 * {@code Striped<ReadWriteLock>}. <i>Strong</i> means that all stripes (locks/semaphores) are
062 * initialized eagerly, and are not reclaimed unless {@code Striped} itself is reclaimable.
063 * <i>Weak</i> means that locks/semaphores are created lazily, and they are allowed to be reclaimed
064 * if nobody is holding on to them. This is useful, for example, if one wants to create a {@code
065 * Striped<Lock>} of many locks, but worries that in most cases only a small portion of these
066 * would be in use.
067 *
068 * <p>Prior to this class, one might be tempted to use {@code Map<K, Lock>}, where {@code K}
069 * represents the task. This maximizes concurrency by having each unique key mapped to a unique
070 * lock, but also maximizes memory footprint. On the other extreme, one could use a single lock
071 * for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of
072 * choosing either of these extremes, {@code Striped} allows the user to trade between required
073 * concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily
074 * create a very compact {@code Striped<Lock>} of {@code availableProcessors() * 4} stripes,
075 * instead of possibly thousands of locks which could be created in a {@code Map<K, Lock>}
076 * structure.
077 *
078 * @author Dimitris Andreou
079 */
080 @Beta
081 public abstract class Striped<L> {
082 private Striped() {}
083
084 /**
085 * Returns the stripe that corresponds to the passed key. It is always guaranteed that if
086 * {@code key1.equals(key2)}, then {@code get(key1) == get(key2)}.
087 *
088 * @param key an arbitrary, non-null key
089 * @return the stripe that the passed key corresponds to
090 */
091 public abstract L get(Object key);
092
093 /**
094 * Returns the stripe at the specified index. Valid indexes are 0, inclusively, to
095 * {@code size()}, exclusively.
096 *
097 * @param index the index of the stripe to return; must be in {@code [0...size())}
098 * @return the stripe at the specified index
099 */
100 public abstract L getAt(int index);
101
102 /**
103 * Returns the index to which the given key is mapped, so that getAt(indexFor(key)) == get(key).
104 */
105 abstract int indexFor(Object key);
106
107 /**
108 * Returns the total number of stripes in this instance.
109 */
110 public abstract int size();
111
112 /**
113 * Returns the stripes that correspond to the passed objects, in ascending (as per
114 * {@link #getAt(int)}) order. Thus, threads that use the stripes in the order returned
115 * by this method are guaranteed to not deadlock each other.
116 *
117 * <p>It should be noted that using a {@code Striped<L>} with relatively few stripes, and
118 * {@code bulkGet(keys)} with a relative large number of keys can cause an excessive number
119 * of shared stripes (much like the birthday paradox, where much fewer than anticipated birthdays
120 * are needed for a pair of them to match). Please consider carefully the implications of the
121 * number of stripes, the intended concurrency level, and the typical number of keys used in a
122 * {@code bulkGet(keys)} operation. See <a href="http://www.mathpages.com/home/kmath199.htm">Balls
123 * in Bins model</a> for mathematical formulas that can be used to estimate the probability of
124 * collisions.
125 *
126 * @param keys arbitrary non-null keys
127 * @return the stripes corresponding to the objects (one per each object, derived by delegating
128 * to {@link #get(Object)}; may contain duplicates), in an increasing index order.
129 */
130 public Iterable<L> bulkGet(Iterable<?> keys) {
131 // Initially using the array to store the keys, then reusing it to store the respective L's
132 final Object[] array = Iterables.toArray(keys, Object.class);
133 int[] stripes = new int[array.length];
134 for (int i = 0; i < array.length; i++) {
135 stripes[i] = indexFor(array[i]);
136 }
137 Arrays.sort(stripes);
138 for (int i = 0; i < array.length; i++) {
139 array[i] = getAt(stripes[i]);
140 }
141 /*
142 * Note that the returned Iterable holds references to the returned stripes, to avoid
143 * error-prone code like:
144 *
145 * Striped<Lock> stripedLock = Striped.lazyWeakXXX(...)'
146 * Iterable<Lock> locks = stripedLock.bulkGet(keys);
147 * for (Lock lock : locks) {
148 * lock.lock();
149 * }
150 * operation();
151 * for (Lock lock : locks) {
152 * lock.unlock();
153 * }
154 *
155 * If we only held the int[] stripes, translating it on the fly to L's, the original locks
156 * might be garbage collected after locking them, ending up in a huge mess.
157 */
158 @SuppressWarnings("unchecked") // we carefully replaced all keys with their respective L's
159 List<L> asList = (List<L>) Arrays.asList(array);
160 return Collections.unmodifiableList(asList);
161 }
162
163 // Static factories
164
165 /**
166 * Creates a {@code Striped<Lock>} with eagerly initialized, strongly referenced locks, with the
167 * specified fairness. Every lock is reentrant.
168 *
169 * @param stripes the minimum number of stripes (locks) required
170 * @return a new {@code Striped<Lock>}
171 */
172 public static Striped<Lock> lock(int stripes) {
173 return new CompactStriped<Lock>(stripes, new Supplier<Lock>() {
174 public Lock get() {
175 return new PaddedLock();
176 }
177 });
178 }
179
180 /**
181 * Creates a {@code Striped<Lock>} with lazily initialized, weakly referenced locks, with the
182 * specified fairness. Every lock is reentrant.
183 *
184 * @param stripes the minimum number of stripes (locks) required
185 * @return a new {@code Striped<Lock>}
186 */
187 public static Striped<Lock> lazyWeakLock(int stripes) {
188 return new LazyStriped<Lock>(stripes, new Supplier<Lock>() {
189 public Lock get() {
190 return new ReentrantLock(false);
191 }
192 });
193 }
194
195 /**
196 * Creates a {@code Striped<Semaphore>} with eagerly initialized, strongly referenced semaphores,
197 * with the specified number of permits and fairness.
198 *
199 * @param stripes the minimum number of stripes (semaphores) required
200 * @param permits the number of permits in each semaphore
201 * @return a new {@code Striped<Semaphore>}
202 */
203 public static Striped<Semaphore> semaphore(int stripes, final int permits) {
204 return new CompactStriped<Semaphore>(stripes, new Supplier<Semaphore>() {
205 public Semaphore get() {
206 return new PaddedSemaphore(permits);
207 }
208 });
209 }
210
211 /**
212 * Creates a {@code Striped<Semaphore>} with lazily initialized, weakly referenced semaphores,
213 * with the specified number of permits and fairness.
214 *
215 * @param stripes the minimum number of stripes (semaphores) required
216 * @param permits the number of permits in each semaphore
217 * @return a new {@code Striped<Semaphore>}
218 */
219 public static Striped<Semaphore> lazyWeakSemaphore(int stripes, final int permits) {
220 return new LazyStriped<Semaphore>(stripes, new Supplier<Semaphore>() {
221 public Semaphore get() {
222 return new Semaphore(permits, false);
223 }
224 });
225 }
226
227 /**
228 * Creates a {@code Striped<ReadWriteLock>} with eagerly initialized, strongly referenced
229 * read-write locks, with the specified fairness. Every lock is reentrant.
230 *
231 * @param stripes the minimum number of stripes (locks) required
232 * @return a new {@code Striped<ReadWriteLock>}
233 */
234 public static Striped<ReadWriteLock> readWriteLock(int stripes) {
235 return new CompactStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER);
236 }
237
238 /**
239 * Creates a {@code Striped<ReadWriteLock>} with lazily initialized, weakly referenced
240 * read-write locks, with the specified fairness. Every lock is reentrant.
241 *
242 * @param stripes the minimum number of stripes (locks) required
243 * @return a new {@code Striped<ReadWriteLock>}
244 */
245 public static Striped<ReadWriteLock> lazyWeakReadWriteLock(int stripes) {
246 return new LazyStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER);
247 }
248
249 // ReentrantReadWriteLock is large enough to make padding probably unnecessary
250 private static final Supplier<ReadWriteLock> READ_WRITE_LOCK_SUPPLIER =
251 new Supplier<ReadWriteLock>() {
252 public ReadWriteLock get() {
253 return new ReentrantReadWriteLock();
254 }
255 };
256
257 private abstract static class PowerOfTwoStriped<L> extends Striped<L> {
258 /** Capacity (power of two) minus one, for fast mod evaluation */
259 final int mask;
260
261 PowerOfTwoStriped(int stripes) {
262 Preconditions.checkArgument(stripes > 0, "Stripes must be positive");
263 this.mask = stripes > Ints.MAX_POWER_OF_TWO ? ALL_SET : ceilToPowerOfTwo(stripes) - 1;
264 }
265
266 @Override final int indexFor(Object key) {
267 int hash = smear(key.hashCode());
268 return hash & mask;
269 }
270
271 @Override public final L get(Object key) {
272 return getAt(indexFor(key));
273 }
274 }
275
276 /**
277 * Implementation of Striped where 2^k stripes are represented as an array of the same length,
278 * eagerly initialized.
279 */
280 private static class CompactStriped<L> extends PowerOfTwoStriped<L> {
281 /** Size is a power of two. */
282 private final Object[] array;
283
284 private CompactStriped(int stripes, Supplier<L> supplier) {
285 super(stripes);
286 Preconditions.checkArgument(stripes <= Ints.MAX_POWER_OF_TWO, "Stripes must be <= 2^30)");
287
288 this.array = new Object[mask + 1];
289 for (int i = 0; i < array.length; i++) {
290 array[i] = supplier.get();
291 }
292 }
293
294 @SuppressWarnings("unchecked") // we only put L's in the array
295 @Override public L getAt(int index) {
296 return (L) array[index];
297 }
298
299 @Override public int size() {
300 return array.length;
301 }
302 }
303
304 /**
305 * Implementation of Striped where up to 2^k stripes can be represented, using a Cache
306 * where the key domain is [0..2^k). To map a user key into a stripe, we take a k-bit slice of the
307 * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced.
308 */
309 private static class LazyStriped<L> extends PowerOfTwoStriped<L> {
310 final ConcurrentMap<Integer, L> cache;
311 final int size;
312
313 LazyStriped(int stripes, Supplier<L> supplier) {
314 super(stripes);
315 this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1;
316 this.cache = new MapMaker().weakValues().makeComputingMap(Functions.forSupplier(supplier));
317 }
318
319 @Override public L getAt(int index) {
320 Preconditions.checkElementIndex(index, size());
321 return cache.get(index);
322 }
323
324 @Override public int size() {
325 return size;
326 }
327 }
328
329 /**
330 * A bit mask were all bits are set.
331 */
332 private static final int ALL_SET = ~0;
333
334 private static int ceilToPowerOfTwo(int x) {
335 return 1 << IntMath.log2(x, RoundingMode.CEILING);
336 }
337
338 /*
339 * This method was written by Doug Lea with assistance from members of JCP
340 * JSR-166 Expert Group and released to the public domain, as explained at
341 * http://creativecommons.org/licenses/publicdomain
342 *
343 * As of 2010/06/11, this method is identical to the (package private) hash
344 * method in OpenJDK 7's java.util.HashMap class.
345 */
346 // Copied from java/com/google/common/collect/Hashing.java
347 private static int smear(int hashCode) {
348 hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12);
349 return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4);
350 }
351
352 private static class PaddedLock extends ReentrantLock {
353 /*
354 * Padding from 40 into 64 bytes, same size as cache line. Might be beneficial to add
355 * a fourth long here, to minimize chance of interference between consecutive locks,
356 * but I couldn't observe any benefit from that.
357 */
358 @SuppressWarnings("unused")
359 long q1, q2, q3;
360
361 PaddedLock() {
362 super(false);
363 }
364 }
365
366 private static class PaddedSemaphore extends Semaphore {
367 // See PaddedReentrantLock comment
368 @SuppressWarnings("unused")
369 long q1, q2, q3;
370
371 PaddedSemaphore(int permits) {
372 super(permits, false);
373 }
374 }
375 }