View Javadoc

1   /**
2    *
3    * Licensed to the Apache Software Foundation (ASF) under one
4    * or more contributor license agreements.  See the NOTICE file
5    * distributed with this work for additional information
6    * regarding copyright ownership.  The ASF licenses this file
7    * to you under the Apache License, Version 2.0 (the
8    * "License"); you may not use this file except in compliance
9    * with the License.  You may obtain a copy of the License at
10   *
11   *     http://www.apache.org/licenses/LICENSE-2.0
12   *
13   * Unless required by applicable law or agreed to in writing, software
14   * distributed under the License is distributed on an "AS IS" BASIS,
15   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
16   * See the License for the specific language governing permissions and
17   * limitations under the License.
18   */
19  package org.apache.hadoop.hbase.io.hfile;
20  
21  import com.google.common.collect.MinMaxPriorityQueue;
22  
23  import org.apache.hadoop.classification.InterfaceAudience;
24  import org.apache.hadoop.hbase.io.HeapSize;
25  
26  /**
27   * A memory-bound queue that will grow until an element brings
28   * total size >= maxSize.  From then on, only entries that are sorted larger
29   * than the smallest current entry will be inserted/replaced.
30   *
31   * <p>Use this when you want to find the largest elements (according to their
32   * ordering, not their heap size) that consume as close to the specified
33   * maxSize as possible.  Default behavior is to grow just above rather than
34   * just below specified max.
35   *
36   * <p>Object used in this queue must implement {@link HeapSize} as well as
37   * {@link Comparable}.
38   */
39  @InterfaceAudience.Private
40  public class LruCachedBlockQueue implements HeapSize {
41  
42    private MinMaxPriorityQueue<LruCachedBlock> 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 LruCachedBlockQueue(long maxSize, long blockSize) {
52      int initialSize = (int)(maxSize / blockSize);
53      if(initialSize == 0) initialSize++;
54      queue = MinMaxPriorityQueue.expectedSize(initialSize).create();
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(LruCachedBlock cb) {
68      if(heapSize < maxSize) {
69        queue.add(cb);
70        heapSize += cb.heapSize();
71      } else {
72        LruCachedBlock 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 The next element in this queue, or {@code null} if the queue is
88     * empty.
89     */
90    public LruCachedBlock poll() {
91      return queue.poll();
92    }
93  
94    /**
95     * @return The last element in this queue, or {@code null} if the queue is
96     * empty.
97     */
98    public LruCachedBlock pollLast() {
99      return queue.pollLast();
100   }
101 
102   /**
103    * Total size of all elements in this queue.
104    * @return size of all elements currently in queue, in bytes
105    */
106   public long heapSize() {
107     return heapSize;
108   }
109 }