Github user hanm commented on a diff in the pull request: https://github.com/apache/zookeeper/pull/590#discussion_r219896267 --- Diff: src/java/main/org/apache/zookeeper/server/watch/BitHashSet.java --- @@ -0,0 +1,156 @@ +/** + * Licensed to the Apache Software Foundation (ASF) under one + * or more contributor license agreements. See the NOTICE file + * distributed with this work for additional information + * regarding copyright ownership. The ASF licenses this file + * to you 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 org.apache.zookeeper.server.watch; + +import java.util.BitSet; +import java.util.Set; +import java.util.HashSet; +import java.util.Iterator; +import java.lang.Iterable; + +import org.apache.zookeeper.server.util.BitMap; + +/** + * Using BitSet to store all the elements, and use HashSet to cache limited + * number of elements to find a balance between memory and time complexity. + * + * Without HashSet, we need to to use O(N) time to get the elements, N is + * the bit numbers in elementBits. But we need to keep the size small to make + * sure it doesn't cost too much in memory, there is a tradeoff between + * memory and time complexity. + * + * Previously, was deciding to dynamically switch between SparseBitSet and + * HashSet based on the memory consumption, but it will take time to copy + * data over and may have some herd effect of keep copying data from one + * data structure to anther. The current solution can do a very good job + * given most of the paths have limited number of elements. + */ +public class BitHashSet implements Iterable<Integer> { + + static final long serialVersionUID = 6382565447128283568L; + + /** + * Change to SparseBitSet if we we want to optimize more, the number of + * elements on a single server is usually limited, so BitSet should be + * fine. + */ + private final BitSet elementBits = new BitSet(); + private final Set<Integer> cache = new HashSet<Integer>(); + + private final int cacheSize; + + // To record how many elements in this set. + private int elementCount = 0; + + public BitHashSet() { + this(Integer.getInteger("zookeeper.bitHashCacheSize", 10)); + } + + public BitHashSet(int cacheSize) { + this.cacheSize = cacheSize; + } + + public synchronized boolean add(Integer elementBit) { + if (elementBit == null || elementBits.get(elementBit)) { + return false; + } + if (cache.size() < cacheSize) { + cache.add(elementBit); + } + elementBits.set(elementBit); + elementCount++; + return true; + } + + /** + * Remove the watches, and return the number of watches being removed. + */ + public synchronized int remove(Set<Integer> bitSet, BitSet bits) { + cache.removeAll(bitSet); + elementBits.andNot(bits); + int elementCountBefore = elementCount; + elementCount = elementBits.cardinality(); + return elementCountBefore - elementCount; + } + + public synchronized boolean remove(Integer elementBit) { + if (elementBit == null || !elementBits.get(elementBit)) { + return false; + } + + cache.remove(elementBit); + elementBits.clear(elementBit); + elementCount--; + return true; + } + + public synchronized boolean contains(Integer elementBit) { + if (elementBit == null) { + return false; + } + return elementBits.get(elementBit); --- End diff -- would be good to add a comment at the declaration of `cache` variable, stating that its purpose is to optimize iteration.
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