java io

Posted 2020-09-27

磁盘io工作机制

1、标准方式

物理磁盘<-->内核空间<-->用户地址空间

2、直接I/O

物理磁盘<-->用户地址空间

数据库是这么干的，从同样是系统软件的角度来看，数据库是跟操作系统平起平坐的。

3、内存映射

物理磁盘<-->内核空间===用户地址空间

2跟3没有真正理解其区别，网上也没找到资料。

性能比较：代码来源ThinKing in java

  1 package pers.jing.io;
  2 
  3 import java.io.*;
  4 import java.nio.IntBuffer;
  5 import java.nio.channels.FileChannel;
  6 
  7 public class MappedIO {
  8     public static final int numberOfInts = 4000000;
  9     public static final int numberOfUbuffInts = 2000000;
 10 
 11     private abstract static class Tester {
 12         private String name;
 13 
 14         public Tester(String name) {
 15             this.name = name;
 16         }
 17 
 18         public void runTest() {
 19             System.out.print(name + ": ");
 20             try {
 21                 long start = System.nanoTime();
 22                 test();
 23                 double duration = System.nanoTime() - start;
 24                 System.out.format("%.2f\n", duration / 1.0e9);
 25             } catch (IOException e) {
 26                 throw new RuntimeException(e);
 27             }
 28         }
 29 
 30         protected abstract void test() throws IOException;
 31     }
 32 
 33     private static Tester[] testers = {
 34             new Tester("Stream Write") {
 35                 @Override
 36                 protected void test() throws IOException {
 37                     DataOutputStream dos = new DataOutputStream(
 38                             new BufferedOutputStream(
 39                                     new FileOutputStream("temp.temp")));
 40                     for (int i = 0; i < numberOfInts; i++) {
 41                         dos.writeInt(i);
 42                     }
 43                     dos.close();
 44                 }
 45             },
 46             new Tester("Mapped Write") {
 47                 @Override
 48                 protected void test() throws IOException {
 49                     FileChannel fc = new RandomAccessFile("temp.temp", "rw")
 50                             .getChannel();
 51                     IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size())
 52                             .asIntBuffer();
 53                     for (int i = 0; i < numberOfInts; i++) {
 54                         ib.put(i);
 55                     }
 56                     ib.clear();
 57                 }
 58             },
 59             new Tester("Stream Read") {
 60                 @Override
 61                 protected void test() throws IOException {
 62                     DataInputStream dis = new DataInputStream(
 63                             new BufferedInputStream(
 64                                     new FileInputStream("temp.temp")));
 65                     for (int i = 0; i < numberOfInts; i++) {
 66                         dis.readInt();
 67                     }
 68                     dis.close();
 69                 }
 70             },
 71             new Tester("Mapped Read") {
 72                 @Override
 73                 protected void test() throws IOException {
 74                     FileChannel fc = new RandomAccessFile("temp.temp", "rw")
 75                             .getChannel();
 76                     IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size())
 77                             .asIntBuffer();
 78                     while (ib.hasRemaining()) {
 79                         ib.get();
 80                     }
 81                     ib.clear();
 82                 }
 83             },
 84             new Tester("Stream rw") {
 85                 @Override
 86                 protected void test() throws IOException {
 87                     RandomAccessFile raf = new RandomAccessFile("temp.temp", "rw");
 88                     raf.writeInt(1);
 89                     for (int i = 0; i < numberOfUbuffInts; i++) {
 90                         raf.seek(raf.length() - 4);
 91                         raf.writeInt(raf.readInt());
 92                     }
 93                     raf.close();
 94                 }
 95             },
 96             new Tester("Mapped rw") {
 97                 @Override
 98                 protected void test() throws IOException {
 99                     FileChannel fc = new RandomAccessFile("temp.temp", "rw")
100                             .getChannel();
101                     IntBuffer ib = fc.map(FileChannel.MapMode.READ_WRITE, 0, fc.size())
102                             .asIntBuffer();
103                     ib.put(0);
104                     for (int i = 1; i < numberOfUbuffInts; i++) {
105                         ib.put(ib.get(i - 1));
106                     }
107                     fc.close();
108                 }
109             }
110     };
111 
112     public static void main(String[] args) {
113         for (Tester tester : testers) {
114             tester.runTest();
115         }
116     }
117 }