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1   /**
2    * Licensed to the Apache Software Foundation (ASF) under one
3    * or more contributor license agreements.  See the NOTICE file
4    * distributed with this work for additional information
5    * regarding copyright ownership.  The ASF licenses this file
6    * to you under the Apache License, Version 2.0 (the
7    * "License"); you may not use this file except in compliance
8    * with the License.  You may obtain a copy of the License at
9    *
10   *     http://www.apache.org/licenses/LICENSE-2.0
11   *
12   * Unless required by applicable law or agreed to in writing, software
13   * distributed under the License is distributed on an "AS IS" BASIS,
14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
15   * See the License for the specific language governing permissions and
16   * limitations under the License.
17   */
18  package org.apache.hadoop.hbase.regionserver;
19  
20  import org.apache.hadoop.classification.InterfaceAudience;
21  import org.apache.hadoop.hbase.util.ByteRange;
22  
23  /**
24   * A memstore-local allocation buffer.
25   * <p>
26   * The MemStoreLAB is basically a bump-the-pointer allocator that allocates big (2MB) chunks from
27   * and then doles it out to threads that request slices into the array.
28   * <p>
29   * The purpose of this is to combat heap fragmentation in the regionserver. By ensuring that all
30   * KeyValues in a given memstore refer only to large chunks of contiguous memory, we ensure that
31   * large blocks get freed up when the memstore is flushed.
32   * <p>
33   * Without the MSLAB, the byte array allocated during insertion end up interleaved throughout the
34   * heap, and the old generation gets progressively more fragmented until a stop-the-world compacting
35   * collection occurs.
36   * <p>
37   */
38  @InterfaceAudience.Private
39  public interface MemStoreLAB {
40  
41    /**
42     * Allocate a slice of the given length. If the size is larger than the maximum size specified for
43     * this allocator, returns null.
44     * @param size
45     * @return {@link ByteRange}
46     */
47    ByteRange allocateBytes(int size);
48  
49    /**
50     * Close instance since it won't be used any more, try to put the chunks back to pool
51     */
52    void close();
53  
54    /**
55     * Called when opening a scanner on the data of this MemStoreLAB
56     */
57    void incScannerCount();
58  
59    /**
60     * Called when closing a scanner on the data of this MemStoreLAB
61     */
62    void decScannerCount();
63  }