Java大文件磁盘IO性能

Question

我的硬盘上有两个(每个2GB)文件,想要将它们相互比较:

• 使用Windows资源管理器复制原始文件大约需要.2-4分钟(即读写 - 在同一物理和逻辑磁盘上).
• java.io.FileInputStream两次读取并在每个字节的字节上比较字节数组需要20多分钟.
• java.io.BufferedInputStream 缓冲区为64kb,文件以块的形式读取然后进行比较.

• 比较完成是一个紧凑的循环

  int numRead = Math.min(numRead[0], numRead[1]);
  for (int k = 0; k < numRead; k++)
  {
     if (buffer[1][k] != buffer[0][k])
     {
        return buffer[0][k] - buffer[1][k];
     }
  }
  

我该怎么做才能加快速度呢？NIO应该比普通的流更快吗？Java无法使用DMA/SATA技术,而是做一些缓慢的OS-API调用吗？

编辑:
谢谢你的答案.我做了一些基于它们的实验.安德烈亚斯表示

流或nio方法没有太大差别.
更重要的是正确的缓冲区大小.

我的实验证实了这一点.由于文件是以大块读取的,因此即使是额外的缓冲区(BufferedInputStream)也不会提供任何内容.优化比较是可能的,并且我通过32次展开获得了最佳结果,但与磁盘读取相比,花费的时间比较小,因此加速很小.看起来我无能为力;-(

Answer 1

我尝试了三种不同的方法来比较两个相同的3,8 gb文件,缓冲区大小介于8 kb和1 MB之间.第一种方法只使用了两个缓冲输入流

第二种方法使用一个线程池,它读入两个不同的线程并在第三个线程中进行比较.这会以高CPU利用率为代价获得略高的吞吐量.对于那些短期运行的任务,线程池的管理需要大量的开销.

第三种方法使用nio,由laginimaineb发布

正如您所看到的,一般方法没有太大差异.更重要的是正确的缓冲区大小.

奇怪的是,我使用线程读取的字节少了1个字节.我无法发现错误.

comparing just with two streams
I was equal, even after 3684070360 bytes and reading for 704813 ms (4,98MB/sec * 2) with a buffer size of 8 kB
I was equal, even after 3684070360 bytes and reading for 578563 ms (6,07MB/sec * 2) with a buffer size of 16 kB
I was equal, even after 3684070360 bytes and reading for 515422 ms (6,82MB/sec * 2) with a buffer size of 32 kB
I was equal, even after 3684070360 bytes and reading for 534532 ms (6,57MB/sec * 2) with a buffer size of 64 kB
I was equal, even after 3684070360 bytes and reading for 422953 ms (8,31MB/sec * 2) with a buffer size of 128 kB
I was equal, even after 3684070360 bytes and reading for 793359 ms (4,43MB/sec * 2) with a buffer size of 256 kB
I was equal, even after 3684070360 bytes and reading for 746344 ms (4,71MB/sec * 2) with a buffer size of 512 kB
I was equal, even after 3684070360 bytes and reading for 669969 ms (5,24MB/sec * 2) with a buffer size of 1024 kB
comparing with threads
I was equal, even after 3684070359 bytes and reading for 602391 ms (5,83MB/sec * 2) with a buffer size of 8 kB
I was equal, even after 3684070359 bytes and reading for 523156 ms (6,72MB/sec * 2) with a buffer size of 16 kB
I was equal, even after 3684070359 bytes and reading for 527547 ms (6,66MB/sec * 2) with a buffer size of 32 kB
I was equal, even after 3684070359 bytes and reading for 276750 ms (12,69MB/sec * 2) with a buffer size of 64 kB
I was equal, even after 3684070359 bytes and reading for 493172 ms (7,12MB/sec * 2) with a buffer size of 128 kB
I was equal, even after 3684070359 bytes and reading for 696781 ms (5,04MB/sec * 2) with a buffer size of 256 kB
I was equal, even after 3684070359 bytes and reading for 727953 ms (4,83MB/sec * 2) with a buffer size of 512 kB
I was equal, even after 3684070359 bytes and reading for 741000 ms (4,74MB/sec * 2) with a buffer size of 1024 kB
comparing with nio
I was equal, even after 3684070360 bytes and reading for 661313 ms (5,31MB/sec * 2) with a buffer size of 8 kB
I was equal, even after 3684070360 bytes and reading for 656156 ms (5,35MB/sec * 2) with a buffer size of 16 kB
I was equal, even after 3684070360 bytes and reading for 491781 ms (7,14MB/sec * 2) with a buffer size of 32 kB
I was equal, even after 3684070360 bytes and reading for 317360 ms (11,07MB/sec * 2) with a buffer size of 64 kB
I was equal, even after 3684070360 bytes and reading for 643078 ms (5,46MB/sec * 2) with a buffer size of 128 kB
I was equal, even after 3684070360 bytes and reading for 865016 ms (4,06MB/sec * 2) with a buffer size of 256 kB
I was equal, even after 3684070360 bytes and reading for 716796 ms (4,90MB/sec * 2) with a buffer size of 512 kB
I was equal, even after 3684070360 bytes and reading for 652016 ms (5,39MB/sec * 2) with a buffer size of 1024 kB

使用的代码:

import junit.framework.Assert;
import org.junit.Before;
import org.junit.Test;

import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.text.DecimalFormat;
import java.text.NumberFormat;
import java.util.Arrays;
import java.util.concurrent.*;

public class FileCompare {

    private static final int MIN_BUFFER_SIZE = 1024 * 8;
    private static final int MAX_BUFFER_SIZE = 1024 * 1024;
    private String fileName1;
    private String fileName2;
    private long start;
    private long totalbytes;

    @Before
    public void createInputStream() {
        fileName1 = "bigFile.1";
        fileName2 = "bigFile.2";
    }

    @Test
    public void compareTwoFiles() throws IOException {
        System.out.println("comparing just with two streams");
        int currentBufferSize = MIN_BUFFER_SIZE;
        while (currentBufferSize <= MAX_BUFFER_SIZE) {
            compareWithBufferSize(currentBufferSize);
            currentBufferSize *= 2;
        }
    }

    @Test
    public void compareTwoFilesFutures() 
            throws IOException, ExecutionException, InterruptedException {
        System.out.println("comparing with threads");
        int myBufferSize = MIN_BUFFER_SIZE;
        while (myBufferSize <= MAX_BUFFER_SIZE) {
            start = System.currentTimeMillis();
            totalbytes = 0;
            compareWithBufferSizeFutures(myBufferSize);
            myBufferSize *= 2;
        }
    }

    @Test
    public void compareTwoFilesNio() throws IOException {
        System.out.println("comparing with nio");
        int myBufferSize = MIN_BUFFER_SIZE;
        while (myBufferSize <= MAX_BUFFER_SIZE) {
            start = System.currentTimeMillis();
            totalbytes = 0;
            boolean wasEqual = isEqualsNio(myBufferSize);

            if (wasEqual) {
                printAfterEquals(myBufferSize);
            } else {
                Assert.fail("files were not equal");
            }

            myBufferSize *= 2;
        }

    }

    private void compareWithBufferSize(int myBufferSize) throws IOException {
        final BufferedInputStream inputStream1 =
                new BufferedInputStream(
                        new FileInputStream(new File(fileName1)),
                        myBufferSize);
        byte[] buff1 = new byte[myBufferSize];
        final BufferedInputStream inputStream2 =
                new BufferedInputStream(
                        new FileInputStream(new File(fileName2)),
                        myBufferSize);
        byte[] buff2 = new byte[myBufferSize];
        int read1;

        start = System.currentTimeMillis();
        totalbytes = 0;
        while ((read1 = inputStream1.read(buff1)) != -1) {
            totalbytes += read1;
            int read2 = inputStream2.read(buff2);
            if (read1 != read2) {
                break;
            }
            if (!Arrays.equals(buff1, buff2)) {
                break;
            }
        }
        if (read1 == -1) {
            printAfterEquals(myBufferSize);
        } else {
            Assert.fail("files were not equal");
        }
        inputStream1.close();
        inputStream2.close();
    }

    private void compareWithBufferSizeFutures(int myBufferSize)
            throws ExecutionException, InterruptedException, IOException {
        final BufferedInputStream inputStream1 =
                new BufferedInputStream(
                        new FileInputStream(
                                new File(fileName1)),
                        myBufferSize);
        final BufferedInputStream inputStream2 =
                new BufferedInputStream(
                        new FileInputStream(
                                new File(fileName2)),
                        myBufferSize);

        final boolean wasEqual = isEqualsParallel(myBufferSize, inputStream1, inputStream2);

        if (wasEqual) {
            printAfterEquals(myBufferSize);
        } else {
            Assert.fail("files were not equal");
        }
        inputStream1.close();
        inputStream2.close();
    }

    private boolean isEqualsParallel(int myBufferSize
            , final BufferedInputStream inputStream1
            , final BufferedInputStream inputStream2)
            throws InterruptedException, ExecutionException {
        final byte[] buff1Even = new byte[myBufferSize];
        final byte[] buff1Odd = new byte[myBufferSize];
        final byte[] buff2Even = new byte[myBufferSize];
        final byte[] buff2Odd = new byte[myBufferSize];
        final Callable<Integer> read1Even = new Callable<Integer>() {
            public Integer call() throws Exception {
                return inputStream1.read(buff1Even);
            }
        };
        final Callable<Integer> read2Even = new Callable<Integer>() {
            public Integer call() throws Exception {
                return inputStream2.read(buff2Even);
            }
        };
        final Callable<Integer> read1Odd = new Callable<Integer>() {
            public Integer call() throws Exception {
                return inputStream1.read(buff1Odd);
            }
        };
        final Callable<Integer> read2Odd = new Callable<Integer>() {
            public Integer call() throws Exception {
                return inputStream2.read(buff2Odd);
            }
        };
        final Callable<Boolean> oddEqualsArray = new Callable<Boolean>() {
            public Boolean call() throws Exception {
                return Arrays.equals(buff1Odd, buff2Odd);
            }
        };
        final Callable<Boolean> evenEqualsArray = new Callable<Boolean>() {
            public Boolean call() throws Exception {
                return Arrays.equals(buff1Even, buff2Even);
            }
        };

        ExecutorService executor = Executors.newCachedThreadPool();
        boolean isEven = true;
        Future<Integer> read1 = null;
        Future<Integer> read2 = null;
        Future<Boolean> isEqual = null;
        int lastSize = 0;
        while (true) {
            if (isEqual != null) {
                if (!isEqual.get()) {
                    return false;
                } else if (lastSize == -1) {
                    return true;
                }
            }
            if (read1 != null) {
                lastSize = read1.get();
                totalbytes += lastSize;
                final int size2 = read2.get();
                if (lastSize != size2) {
                    return false;
                }
            }
            isEven = !isEven;
            if (isEven) {
                if (read1 != null) {
                    isEqual = executor.submit(oddEqualsArray);
                }
                read1 = executor.submit(read1Even);
                read2 = executor.submit(read2Even);
            } else {
                if (read1 != null) {
                    isEqual = executor.submit(evenEqualsArray);
                }
                read1 = executor.submit(read1Odd);
                read2 = executor.submit(read2Odd);
            }
        }
    }

    private boolean isEqualsNio(int myBufferSize) throws IOException {
        FileChannel first = null, seconde = null;
        try {
            first = new FileInputStream(fileName1).getChannel();
            seconde = new FileInputStream(fileName2).getChannel();
            if (first.size() != seconde.size()) {
                return false;
            }
            ByteBuffer firstBuffer = ByteBuffer.allocateDirect(myBufferSize);
            ByteBuffer secondBuffer = ByteBuffer.allocateDirect(myBufferSize);
            int firstRead, secondRead;
            while (first.position() < first.size()) {
                firstRead = first.read(firstBuffer);
                totalbytes += firstRead;
                secondRead = seconde.read(secondBuffer);
                if (firstRead != secondRead) {
                    return false;
                }
                if (!nioBuffersEqual(firstBuffer, secondBuffer, firstRead)) {
                    return false;
                }
            }
            return true;
        } finally {
            if (first != null) {
                first.close();
            }
            if (seconde != null) {
                seconde.close();
            }
        }
    }

    private static boolean nioBuffersEqual(ByteBuffer first, ByteBuffer second, final int length) {
        if (first.limit() != second.limit() || length > first.limit()) {
            return false;
        }
        first.rewind();
        second.rewind();
        for (int i = 0; i < length; i++) {
            if (first.get() != second.get()) {
                return false;
            }
        }
        return true;
    }

    private void printAfterEquals(int myBufferSize) {
        NumberFormat nf = new DecimalFormat("#.00");
        final long dur = System.currentTimeMillis() - start;
        double seconds = dur / 1000d;
        double megabytes = totalbytes / 1024 / 1024;
        double rate = (megabytes) / seconds;
        System.out.println("I was equal, even after " + totalbytes
                + " bytes and reading for " + dur
                + " ms (" + nf.format(rate) + "MB/sec * 2)" +
                " with a buffer size of " + myBufferSize / 1024 + " kB");
    }
}

Answer 2

有了这么大的文件,你可以用java.nio获得更好的性能.

此外,使用java流读取单个字节可能非常慢.使用字节数组(我自己的经验中的2-6K元素,ymmv,因为它看起来像平台/应用程序特定)将显着提高您使用流的读取性能.

Answer 3

使用Java读取和写入文件也同样快.您可以使用FileChannels.至于比较文件,显然这需要花费大量时间来比较字节和字节这里是一个使用FileChannels和ByteBuffers的例子(可以进一步优化):

public static boolean compare(String firstPath, String secondPath, final int BUFFER_SIZE) throws IOException {
    FileChannel firstIn = null, secondIn = null;
    try {
        firstIn = new FileInputStream(firstPath).getChannel();
        secondIn = new FileInputStream(secondPath).getChannel();
        if (firstIn.size() != secondIn.size())
            return false;
        ByteBuffer firstBuffer = ByteBuffer.allocateDirect(BUFFER_SIZE);
        ByteBuffer secondBuffer = ByteBuffer.allocateDirect(BUFFER_SIZE);
        int firstRead, secondRead;
        while (firstIn.position() < firstIn.size()) {
            firstRead = firstIn.read(firstBuffer);
            secondRead = secondIn.read(secondBuffer);
            if (firstRead != secondRead)
                return false;
            if (!buffersEqual(firstBuffer, secondBuffer, firstRead))
                return false;
        }
        return true;
    } finally {
        if (firstIn != null) firstIn.close();
        if (secondIn != null) firstIn.close();
    }
}

private static boolean buffersEqual(ByteBuffer first, ByteBuffer second, final int length) {
    if (first.limit() != second.limit())
        return false;
    if (length > first.limit())
        return false;
    first.rewind(); second.rewind();
    for (int i=0; i<length; i++)
        if (first.get() != second.get())
            return false;
    return true;
}

Answer 4

以下是关于在java中读取文件的不同方法的相对优点的好文章.可能有一些用处:

如何快速阅读文件

Answer 5

修改NIO比较功能后,我得到以下结果.

I was equal, even after 4294967296 bytes and reading for 304594 ms (13.45MB/sec * 2) with a buffer size of 1024 kB
I was equal, even after 4294967296 bytes and reading for 225078 ms (18.20MB/sec * 2) with a buffer size of 4096 kB
I was equal, even after 4294967296 bytes and reading for 221351 ms (18.50MB/sec * 2) with a buffer size of 16384 kB

注意:这意味着正在以37 MB/s的速率读取文件

在更快的驱动器上运行相同的东西

I was equal, even after 4294967296 bytes and reading for 178087 ms (23.00MB/sec * 2) with a buffer size of 1024 kB
I was equal, even after 4294967296 bytes and reading for 119084 ms (34.40MB/sec * 2) with a buffer size of 4096 kB
I was equal, even after 4294967296 bytes and reading for 109549 ms (37.39MB/sec * 2) with a buffer size of 16384 kB

注意:这意味着正在以74.8 MB/s的速率读取文件

private static boolean nioBuffersEqual(ByteBuffer first, ByteBuffer second, final int length) {
    if (first.limit() != second.limit() || length > first.limit()) {
        return false;
    }
    first.rewind();
    second.rewind();
    int i;
    for (i = 0; i < length-7; i+=8) {
        if (first.getLong() != second.getLong()) {
            return false;
        }
    }
    for (; i < length; i++) {
        if (first.get() != second.get()) {
            return false;
        }
    }
    return true;
}