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 }