1 /*
   2  * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.
   8  *
   9  * This code is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  12  * version 2 for more details (a copy is included in the LICENSE file that
  13  * accompanied this code).
  14  *
  15  * You should have received a copy of the GNU General Public License version
  16  * 2 along with this work; if not, write to the Free Software Foundation,
  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  18  *
  19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  20  * or visit www.oracle.com if you need additional information or have any
  21  * questions.
  22  *
  23  */
  24 
  25 package gc.g1.humongousObjects;
  26 
  27 import sun.hotspot.WhiteBox;
  28 
  29 public class Helpers {
  30 
  31     // In case of 128 byte padding
  32     private static final int MAX_PADDING_SIZE = 128;
  33 
  34     /**
  35      * Detects amount of extra bytes required to allocate a byte array.
  36      * Allocating a byte[n] array takes more then just n bytes in the heap.
  37      * Extra bytes are required to store object reference and the length.
  38      * This amount depends on bitness and other factors.
  39      *
  40      * @return byte[] memory overhead
  41      */
  42     public static int detectByteArrayAllocationOverhead() {
  43 
  44         WhiteBox whiteBox = WhiteBox.getWhiteBox();
  45 
  46         int zeroLengthByteArraySize = (int) whiteBox.getObjectSize(new byte[0]);
  47 
  48         // Since we do not know is there any padding in zeroLengthByteArraySize we cannot just take byte[0] size as overhead
  49         for (int i = 1; i < MAX_PADDING_SIZE + 1; ++i) {
  50             int realAllocationSize = (int) whiteBox.getObjectSize(new byte[i]);
  51             if (realAllocationSize != zeroLengthByteArraySize) {
  52                 // It means we did not have any padding on previous step
  53                 return zeroLengthByteArraySize - (i - 1);
  54             }
  55         }
  56         throw new Error("We cannot find byte[] memory overhead - should not reach here");
  57     }
  58 }