| /** |
| * 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.hadoop.hdfs.server.namenode; |
| |
| import java.util.HashMap; |
| import java.util.Map; |
| |
| import org.apache.commons.logging.Log; |
| import org.apache.commons.logging.LogFactory; |
| |
| /** |
| * Caches frequently used names to facilitate reuse. |
| * (example: byte[] representation of the file name in {@link INode}). |
| * |
| * This class is used by initially adding all the file names. Cache |
| * tracks the number of times a name is used in a transient map. It promotes |
| * a name used more than {@code useThreshold} to the cache. |
| * |
| * One all the names are added, {@link #initialized()} should be called to |
| * finish initialization. The transient map where use count is tracked is |
| * discarded and cache is ready for use. |
| * |
| * <p> |
| * This class must be synchronized externally. |
| * |
| * @param <K> name to be added to the cache |
| */ |
| class NameCache<K> { |
| /** |
| * Class for tracking use count of a name |
| */ |
| private class UseCount { |
| int count; |
| final K value; // Internal value for the name |
| |
| UseCount(final K value) { |
| count = 1; |
| this.value = value; |
| } |
| |
| void increment() { |
| count++; |
| } |
| |
| int get() { |
| return count; |
| } |
| } |
| |
| static final Log LOG = LogFactory.getLog(NameCache.class.getName()); |
| |
| /** indicates initialization is in progress */ |
| private boolean initialized = false; |
| |
| /** names used more than {@code useThreshold} is added to the cache */ |
| private final int useThreshold; |
| |
| /** of times a cache look up was successful */ |
| private int lookups = 0; |
| |
| /** Cached names */ |
| final HashMap<K, K> cache = new HashMap<K, K>(); |
| |
| /** Names and with number of occurrences tracked during initialization */ |
| Map<K, UseCount> transientMap = new HashMap<K, UseCount>(); |
| |
| /** |
| * Constructor |
| * @param useThreshold names occurring more than this is promoted to the |
| * cache |
| */ |
| NameCache(int useThreshold) { |
| this.useThreshold = useThreshold; |
| } |
| |
| /** |
| * Add a given name to the cache or track use count. |
| * exist. If the name already exists, then the internal value is returned. |
| * |
| * @param name name to be looked up |
| * @return internal value for the name if found; otherwise null |
| */ |
| K put(final K name) { |
| K internal = cache.get(name); |
| if (internal != null) { |
| lookups++; |
| return internal; |
| } |
| |
| // Track the usage count only during initialization |
| if (!initialized) { |
| UseCount useCount = transientMap.get(name); |
| if (useCount != null) { |
| useCount.increment(); |
| if (useCount.get() >= useThreshold) { |
| promote(name); |
| } |
| return useCount.value; |
| } |
| useCount = new UseCount(name); |
| transientMap.put(name, useCount); |
| } |
| return null; |
| } |
| |
| /** |
| * Lookup count when a lookup for a name returned cached object |
| * @return number of successful lookups |
| */ |
| int getLookupCount() { |
| return lookups; |
| } |
| |
| /** |
| * Size of the cache |
| * @return Number of names stored in the cache |
| */ |
| int size() { |
| return cache.size(); |
| } |
| |
| /** |
| * Mark the name cache as initialized. The use count is no longer tracked |
| * and the transient map used for initializing the cache is discarded to |
| * save heap space. |
| */ |
| void initialized() { |
| LOG.info("initialized with " + size() + " entries " + lookups + " lookups"); |
| this.initialized = true; |
| transientMap.clear(); |
| transientMap = null; |
| } |
| |
| /** Promote a frequently used name to the cache */ |
| private void promote(final K name) { |
| transientMap.remove(name); |
| cache.put(name, name); |
| lookups += useThreshold; |
| } |
| |
| public void reset() { |
| initialized = false; |
| cache.clear(); |
| if (transientMap == null) { |
| transientMap = new HashMap<K, UseCount>(); |
| } else { |
| transientMap.clear(); |
| } |
| } |
| } |