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Fixed race causing an incorrect removal cause (fixes #412)
This bug manifests due to an incorrectly optimized `Map.remove(key)` implementation when there is no CacheWriter specified. If there is, an alternative implementation is used which does not suffer from this mistake. This change now uses that version in all cases. The explicit removal and an eviction of the same entry can occur concurrently. If the removal wins the race, it is responsible for retiring the entry. However even after that entry is discarded, an further evictions may be necessary to meet the bounding criteria (e.g. weighted entries). Therefore the eviction thread eagerly discards the entry from the policy's data structures so that it can continue its evaluation. While the entry is synchronized in both cases, the eviction thread could lose the removal but retire the entry first. This lead to the removal seeing a dead entry and miscommunicating the cause to the listener. There is a modest performance difference in a microbenchmark because of a slightly coarser locking. However most of the differences between libraries is whether they decide to be linearizable or perform an optimistic check to no-op if the entry is absent. The optimistic check avoids locking but also can lead to a removal to not block waiting for an in-flight computation to complete. While a valid choice, some users have expressed a desire for removal to be pessimistic and is rare enough that we follow their wishes.
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caffeine/src/jmh/java/com/github/benmanes/caffeine/cache/PutRemoveBenchmark.java
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/* | ||
* Copyright 2014 Ben Manes. All Rights Reserved. | ||
* | ||
* 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.github.benmanes.caffeine.cache; | ||
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import java.util.Random; | ||
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import org.openjdk.jmh.annotations.Benchmark; | ||
import org.openjdk.jmh.annotations.Group; | ||
import org.openjdk.jmh.annotations.GroupThreads; | ||
import org.openjdk.jmh.annotations.Level; | ||
import org.openjdk.jmh.annotations.Param; | ||
import org.openjdk.jmh.annotations.Scope; | ||
import org.openjdk.jmh.annotations.Setup; | ||
import org.openjdk.jmh.annotations.State; | ||
import org.openjdk.jmh.annotations.TearDown; | ||
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import site.ycsb.generator.NumberGenerator; | ||
import site.ycsb.generator.ScrambledZipfianGenerator; | ||
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/** | ||
* A benchmark that loosely evaluates the put and removal performance of the cache. The cache is | ||
* written using a Zipf distribution of keys is used to incur lock contention on popular entries. | ||
* <p> | ||
* The performance of this benchmark is expected to be unrealistic due to removing entries. A cache | ||
* that optimistically skips locking when the entry is absent receives a benefit, but sacrifices | ||
* linearizability with a computation. In real-world usages the rate of explicit writes is | ||
* relatively rare compared to reads. Thus, this benchmark is only for diagnosing performance | ||
* concerns and should not be used to compare implementations. | ||
* <p> | ||
* <pre>{@code | ||
* ./gradlew jmh -PincludePattern=PutRemoveBenchmark | ||
* }</pre> | ||
* | ||
* @author [email protected] (Ben Manes) | ||
*/ | ||
@State(Scope.Group) | ||
public class PutRemoveBenchmark { | ||
private static final int SIZE = (2 << 14); | ||
private static final int MASK = SIZE - 1; | ||
private static final int ITEMS = SIZE / 3; | ||
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@Param({ | ||
"Caffeine", | ||
"LinkedHashMap_Lru", | ||
"ConcurrentHashMap", | ||
"ConcurrentLinkedHashMap", | ||
"Guava", | ||
"Cache2k", | ||
"Ehcache3", | ||
}) | ||
CacheType cacheType; | ||
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BasicCache<Integer, Boolean> cache; | ||
Integer[] ints; | ||
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@State(Scope.Thread) | ||
public static class ThreadState { | ||
static final Random random = new Random(); | ||
int index = random.nextInt(); | ||
} | ||
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@Setup | ||
public void setup() { | ||
ints = new Integer[SIZE]; | ||
cache = cacheType.create(2 * SIZE); | ||
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// Enforce full initialization of internal structures | ||
for (int i = 0; i < 2 * SIZE; i++) { | ||
cache.put(i, Boolean.TRUE); | ||
} | ||
cache.clear(); | ||
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// Populate with a realistic access distribution | ||
NumberGenerator generator = new ScrambledZipfianGenerator(ITEMS); | ||
for (int i = 0; i < SIZE; i++) { | ||
ints[i] = generator.nextValue().intValue(); | ||
cache.put(ints[i], Boolean.TRUE); | ||
} | ||
} | ||
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@TearDown(Level.Iteration) | ||
public void tearDown() { | ||
cache.cleanUp(); | ||
} | ||
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@Benchmark @Group @GroupThreads(4) | ||
public void put(ThreadState threadState) { | ||
cache.put(ints[threadState.index++ & MASK], Boolean.TRUE); | ||
} | ||
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@Benchmark @Group @GroupThreads(4) | ||
public void remove(ThreadState threadState) { | ||
cache.remove(ints[threadState.index++ & MASK]); | ||
} | ||
} |
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