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From lvfangmin <...@git.apache.org>
Subject [GitHub] zookeeper pull request #590: [ZOOKEEPER-1177] Add the memory optimized watch...
Date Wed, 12 Sep 2018 06:44:27 GMT
Github user lvfangmin commented on a diff in the pull request:

    https://github.com/apache/zookeeper/pull/590#discussion_r216910219
  
    --- Diff: src/java/main/org/apache/zookeeper/server/watch/WatchManagerOptimized.java ---
    @@ -0,0 +1,355 @@
    +/**
    + * 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.io.PrintWriter;
    +import java.util.HashMap;
    +import java.util.BitSet;
    +import java.util.HashSet;
    +import java.util.ArrayList;
    +import java.util.LinkedHashMap;
    +import java.util.Map;
    +import java.util.Map.Entry;
    +import java.util.Random;
    +import java.util.Set;
    +import java.util.Iterator;
    +import java.lang.Iterable;
    +import java.util.concurrent.ConcurrentHashMap;
    +import java.util.concurrent.locks.ReadWriteLock;
    +import java.util.concurrent.locks.ReentrantReadWriteLock;
    +
    +import org.apache.zookeeper.WatchedEvent;
    +import org.apache.zookeeper.Watcher;
    +import org.apache.zookeeper.Watcher.Event.EventType;
    +import org.apache.zookeeper.Watcher.Event.KeeperState;
    +import org.apache.zookeeper.server.ServerCnxn;
    +import org.apache.zookeeper.server.util.BitMap;
    +import org.slf4j.Logger;
    +import org.slf4j.LoggerFactory;
    +
    +/**
    + * Optimized in memory and time complexity, compared to WatchManager, both the
    + * memory consumption and time complexity improved a lot, but it cannot
    + * efficiently remove the watcher when the session or socket is closed, for
    + * majority usecase this is not a problem.
    + *
    + * Changed made compared to WatchManager:
    + *
    + * - Use HashSet and BitSet to store the watchers to find a balance between
    + *   memory usage and time complexity
    + * - Use ReadWriteLock instead of synchronized to reduce lock retention
    + * - Lazily clean up the closed watchers
    + */
    +public class WatchManagerOptimized
    +        implements IWatchManager, DeadWatcherListener {
    +
    +    private static final Logger LOG =
    +            LoggerFactory.getLogger(WatchManagerOptimized.class);
    +
    +    private final ConcurrentHashMap<String, BitHashSet> pathWatches =
    +            new ConcurrentHashMap<String, BitHashSet>();
    +
    +    // watcher to bit id mapping
    +    private final BitMap<Watcher> watcherBitIdMap = new BitMap<Watcher>();
    +
    +    // used to lazily remove the dead watchers
    +    private final WatcherCleaner watcherCleaner;
    +
    +    private final ReentrantReadWriteLock addRemovePathRWLock = new ReentrantReadWriteLock();
    +
    +    public WatchManagerOptimized() {
    +        watcherCleaner = new WatcherCleaner(this);
    +        watcherCleaner.start();
    +    }
    +
    +    @Override
    +    public boolean addWatch(String path, Watcher watcher) {
    +        boolean result = false;
    +        addRemovePathRWLock.readLock().lock();
    +        try {
    +            // avoid race condition of adding a on flying dead watcher
    +            if (isDeadWatcher(watcher)) {
    +                LOG.debug("Ignoring addWatch with closed cnxn");
    +            } else {
    +                Integer bit = watcherBitIdMap.add(watcher);
    +                BitHashSet watchers = pathWatches.get(path);
    +                if (watchers == null) {
    +                    watchers = new BitHashSet();
    +                    BitHashSet existingWatchers = pathWatches.putIfAbsent(path, watchers);
    +                    if (existingWatchers != null) {
    +                        watchers = existingWatchers;
    +                    }
    +                }
    +                result = watchers.add(bit);
    +            }
    +        } finally {
    +            addRemovePathRWLock.readLock().unlock();
    +        }
    +        return result;
    +    }
    +
    +    @Override
    +    public boolean containsWatcher(String path, Watcher watcher) {
    +        BitHashSet watchers = pathWatches.get(path);
    +        if (watchers == null || !watchers.contains(watcherBitIdMap.getBit(watcher)))
{
    +            return false;
    +        }
    +        return true;
    +    }
    +
    +    @Override
    +    public boolean removeWatcher(String path, Watcher watcher) {
    +        addRemovePathRWLock.writeLock().lock();
    +        try {
    +            BitHashSet list = pathWatches.get(path);
    +            if (list == null || !list.remove(watcherBitIdMap.getBit(watcher))) {
    +                return false;
    +            }
    +            if (list.isEmpty()) {
    +                pathWatches.remove(path);
    +            }
    +            return true;
    +        } finally {
    +            addRemovePathRWLock.writeLock().unlock();
    +        }
    +    }
    +
    +    @Override
    +    public void removeWatcher(Watcher watcher) {
    +        Integer watcherBit;
    +        addRemovePathRWLock.writeLock().lock();
    --- End diff --
    
    Missed this comment last time, what we need here is that, as long as we called addDeadWatcher,
there will be no watches added related with this dead watcher. Code executed after line 136
means the watcher will be marked as stale, after we release this lock, any on flying addWatcher
for this dead watcher will be rejected, so it guarantees when we call addDeadWatcher there
will be no race condition between removing and adding watch.
    
    And I need to move addDeadWatcher out of the locking block, since the WatchCleaner might
block on it to avoid OOM issue if the cleaner cannot catch up of cleaning the dead watchers.


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