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  1. JDK
  2. JDK-7168505

(bf) MappedByteBuffer.load does not load buffer's content into memory

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    • b40
    • x86
    • os_x
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      Description

        FULL PRODUCT VERSION :
        openjdk version "1.7.0-jdk7u4-b21"
        OpenJDK Runtime Environment (build 1.7.0-jdk7u4-b21-20120427)
        OpenJDK 64-Bit Server VM (build 23.0-b21, mixed mode)

        For the reference, used:

        java version "1.6.0_31"
        Java(TM) SE Runtime Environment (build 1.6.0_31-b04-415-11M3635)
        Java HotSpot(TM) 64-Bit Server VM (build 20.6-b01-415, mixed mode)

        ADDITIONAL OS VERSION INFORMATION :
        Darwin Mistral-MacOSX.local 11.4.0 Darwin Kernel Version 11.4.0: Mon Apr 9 19:32:15 PDT 2012; root:xnu-1699.26.8~1/RELEASE_X86_64 x86_64


        A DESCRIPTION OF THE PROBLEM :
        I have some code that performs I/O by means of file channels to compute MD5
        fingerprints of files. The code performs very well on Apple JDK 6 (90% of the
        theoretical disk speed) and very bad on OpenJDK 7 (roughly ten times slower).

        So far I've drilled down the problem to the behaviour of MapByteBuffer.load().
        While on Apple JDK 6 it actually loads file contents in memory (and it's fast),
        it seems to do nothing on OpenJDK 7. I suppose that data are only loaded
        on-demand when the MD5 is computed and it's done in an inefficient way.

        The self-contained attached code creates 20 files whose sizes range from 10MB
        to 100MB and then loads them by means of MapByteBuffer. On Apple JDK 6 it
        reports

            Read 1020 MB, speed 61 MB/sec

        while on OpenJDK 7u4 it reports

            Read 1020 MB, speed 1861 MB/sec

        But the latter is a fake measure, as a system monitor reports no read disk
        activity (and the measure is unrealistic).

        STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
        Execute the attached test case.

        EXPECTED VERSUS ACTUAL BEHAVIOR :
        EXPECTED -
        Similar performance in JDK 6 and OpenJDK 7 is expected.
        ACTUAL -
        OpenJDK 7 performs differently, as it doesn't read data in memory.

        REPRODUCIBILITY :
        This bug can be reproduced always.

        ---------- BEGIN SOURCE ----------
        package it.tidalwave.integritychecker.impl;

        import java.util.Random;
        import java.io.File;
        import java.io.FileInputStream;
        import java.io.FileOutputStream;
        import java.io.IOException;
        import java.nio.ByteBuffer;
        import static java.nio.channels.FileChannel.MapMode.READ_ONLY;

        public class IoPerformanceTest
          {
            private final static double MEGA = 1024 * 1024;
            private final static int MIN_FILE_SIZE = 10 * 1000 * 1000;
            private final static int MAX_FILE_SIZE = 100 * 1000 * 1000;
            
            private File testFileFolder;
            
            public static void main (final String ... args)
              throws Exception
              {
                final IoPerformanceTest test = new IoPerformanceTest();
                test.createTestFiles();
                test.test();
              }
                    
            private void createTestFiles()
              throws IOException
              {
                System.err.println("Creating test files...");
                testFileFolder = new File(System.getProperty("java.io.tmpdir"));
                testFileFolder.mkdirs();
                final Random random = new Random(342345426536L);
                
                for (int f = 0; f < 20; f++)
                  {
                    final File file = new File(testFileFolder, "testfile" + f);
                    System.err.println(">>>> creating " + file.getAbsolutePath());
                    int size = MIN_FILE_SIZE + random.nextInt(MAX_FILE_SIZE - MIN_FILE_SIZE);
                    final byte[] buffer = new byte[size];
                    random.nextBytes(buffer);
                    final FileOutputStream fos = new FileOutputStream(file);
                    fos.write(buffer);
                    fos.close();
                  }
              }
            
            public void test()
              throws Exception
              {
                final long startTime = System.currentTimeMillis();
                long size = 0;
                
                for (int f = 0; f < 20; f++)
                  {
                    final File file = new File(testFileFolder, "testfile" + f).getAbsoluteFile();
                    final FileInputStream fis = new FileInputStream(file);
                    final ByteBuffer byteBuffer = nioRead(fis, (int)file.length());
                    fis.close();
                    size += file.length();
                  }
                
                final long time = System.currentTimeMillis() - startTime;
                System.err.printf("Read %d MB, speed %d MB/sec\n", (int)(size / MEGA), (int)(((size / MEGA) / (time / 1000.0))));
              }
            
            private ByteBuffer nioRead (final FileInputStream fis, final int length)
              throws IOException
              {
                return fis.getChannel().map(READ_ONLY, 0, length).load();
              }
            
            private ByteBuffer ioRead (final FileInputStream fis, final int length)
              throws IOException
              {
                final byte[] bytes = new byte[length];
                fis.read(bytes);
                return ByteBuffer.wrap(bytes);
              }
          }

        ---------- END SOURCE ----------

        CUSTOMER SUBMITTED WORKAROUND :
        Regular IO can be used in place of NIO, but at the expense of allocating memory in the heap, which could be a problem for processing multiple large files at the same time.

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                alanb Alan Bateman
                webbuggrp Webbug Group
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