View Javadoc

1   /**
2    * Copyright 2009 The Apache Software Foundation
3    *
4    * Licensed to the Apache Software Foundation (ASF) under one
5    * or more contributor license agreements.  See the NOTICE file
6    * distributed with this work for additional information
7    * regarding copyright ownership.  The ASF licenses this file
8    * to you under the Apache License, Version 2.0 (the
9    * "License"); you may not use this file except in compliance
10   * with the License.  You may obtain a copy of the License at
11   *
12   *     http://www.apache.org/licenses/LICENSE-2.0
13   *
14   * Unless required by applicable law or agreed to in writing, software
15   * distributed under the License is distributed on an "AS IS" BASIS,
16   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
17   * See the License for the specific language governing permissions and
18   * limitations under the License.
19   */
20  package org.apache.hadoop.hbase.io.hfile;
21  
22  import java.util.LinkedList;
23  import java.util.PriorityQueue;
24  
25  import org.apache.hadoop.hbase.io.HeapSize;
26  
27  /**
28   * A memory-bound queue that will grow until an element brings
29   * total size >= maxSize.  From then on, only entries that are sorted larger
30   * than the smallest current entry will be inserted/replaced.
31   *
32   * <p>Use this when you want to find the largest elements (according to their
33   * ordering, not their heap size) that consume as close to the specified
34   * maxSize as possible.  Default behavior is to grow just above rather than
35   * just below specified max.
36   *
37   * <p>Object used in this queue must implement {@link HeapSize} as well as
38   * {@link Comparable}.
39   */
40  public class CachedBlockQueue implements HeapSize {
41  
42    private PriorityQueue<CachedBlock> queue;
43  
44    private long heapSize;
45    private long maxSize;
46  
47    /**
48     * @param maxSize the target size of elements in the queue
49     * @param blockSize expected average size of blocks
50     */
51    public CachedBlockQueue(long maxSize, long blockSize) {
52      int initialSize = (int)(maxSize / blockSize);
53      if(initialSize == 0) initialSize++;
54      queue = new PriorityQueue<CachedBlock>(initialSize);
55      heapSize = 0;
56      this.maxSize = maxSize;
57    }
58  
59    /**
60     * Attempt to add the specified cached block to this queue.
61     *
62     * <p>If the queue is smaller than the max size, or if the specified element
63     * is ordered before the smallest element in the queue, the element will be
64     * added to the queue.  Otherwise, there is no side effect of this call.
65     * @param cb block to try to add to the queue
66     */
67    public void add(CachedBlock cb) {
68      if(heapSize < maxSize) {
69        queue.add(cb);
70        heapSize += cb.heapSize();
71      } else {
72        CachedBlock head = queue.peek();
73        if(cb.compareTo(head) > 0) {
74          heapSize += cb.heapSize();
75          heapSize -= head.heapSize();
76          if(heapSize > maxSize) {
77            queue.poll();
78          } else {
79            heapSize += head.heapSize();
80          }
81          queue.add(cb);
82        }
83      }
84    }
85  
86    /**
87     * @return a sorted List of all elements in this queue, in descending order
88     */
89    public LinkedList<CachedBlock> get() {
90      LinkedList<CachedBlock> blocks = new LinkedList<CachedBlock>();
91      while (!queue.isEmpty()) {
92        blocks.addFirst(queue.poll());
93      }
94      return blocks;
95    }
96  
97    /**
98     * Total size of all elements in this queue.
99     * @return size of all elements currently in queue, in bytes
100    */
101   public long heapSize() {
102     return heapSize;
103   }
104 }