使用Multiple Thread读取单个文件:应该加快速度吗？

Question

我正在读一个包含500000行的文件.我正在测试看多个线程如何加速进程....

private void multiThreadRead(int num){

    for(int i=1; i<= num; i++) { 
        new Thread(readIndivColumn(i),""+i).start(); 
     } 
}

private Runnable readIndivColumn(final int colNum){
    return new Runnable(){
        @Override
        public void run() {
            // TODO Auto-generated method stub
            try {

                long startTime = System.currentTimeMillis();
                System.out.println("From Thread no:"+colNum+" Start time:"+startTime);

                RandomAccessFile raf = new RandomAccessFile("./src/test/test1.csv","r");
                String line = "";
                //System.out.println("From Thread no:"+colNum);

                while((line = raf.readLine()) != null){
                    //System.out.println(line);
                    //System.out.println(StatUtils.getCellValue(line, colNum));
                }


                long elapsedTime = System.currentTimeMillis() - startTime;

                String formattedTime = String.format("%d min, %d sec",  
                        TimeUnit.MILLISECONDS.toMinutes(elapsedTime), 
                        TimeUnit.MILLISECONDS.toSeconds(elapsedTime) -  
                        TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(elapsedTime)) 
                    );

                System.out.println("From Thread no:"+colNum+" Finished Time:"+formattedTime);
            } 
            catch (Exception e) {
                // TODO Auto-generated catch block
                System.out.println("From Thread no:"+colNum +"===>"+e.getMessage());

                e.printStackTrace();
            }
        }
    };
}

private void sequentialRead(int num){
    try{
        long startTime = System.currentTimeMillis();
        System.out.println("Start time:"+startTime);

        for(int i =0; i < num; i++){
            RandomAccessFile raf = new RandomAccessFile("./src/test/test1.csv","r");
            String line = "";

            while((line = raf.readLine()) != null){
                //System.out.println(line);
            }               
        }

        long elapsedTime = System.currentTimeMillis() - startTime;

        String formattedTime = String.format("%d min, %d sec",  
                TimeUnit.MILLISECONDS.toMinutes(elapsedTime), 
                TimeUnit.MILLISECONDS.toSeconds(elapsedTime) -  
                TimeUnit.MINUTES.toSeconds(TimeUnit.MILLISECONDS.toMinutes(elapsedTime)) 
            );

        System.out.println("Finished Time:"+formattedTime);
    }
    catch (Exception e) {
        e.printStackTrace();
        // TODO: handle exception
    }

}
    public TesterClass() {

    sequentialRead(1);      
    this.multiThreadRead(1);

}

对于num = 1,我得到以下结果:

开始时间:1326224619049

完成时间:2分钟,14秒

顺序读取ENDS ...........

多线程读取开始:

来自线程号:1开始时间:1326224753606

从线号:1完成时间:2分13秒

多线程读取ENDS .....

对于num = 5,我得到以下结果:

    formatted Time:10 min, 20 sec

Sequential read ENDS...........

Multi-Thread read starts:

From Thread no:1 Start time:1326223509574
From Thread no:3 Start time:1326223509574
From Thread no:4 Start time:1326223509574
From Thread no:5 Start time:1326223509574
From Thread no:2 Start time:1326223509574
From Thread no:4 formatted Time:5 min, 54 sec
From Thread no:2 formatted Time:6 min, 0 sec
From Thread no:3 formatted Time:6 min, 7 sec
From Thread no:5 formatted Time:6 min, 23 sec
From Thread no:1 formatted Time:6 min, 23 sec
Multi-Thread read ENDS.....

我的问题是:不应该多线程读取大约.2.13秒？您能解释为什么多线程解决方案需要太长时间吗？

提前致谢.

Answer 1

在并行读取时看到速度减慢的原因是因为磁性硬盘磁头需要为每个线程寻找下一个读取位置(大约需要5ms).因此,使用多个线程进行读取可以有效地在搜索之间反弹磁盘,从而减慢速度.从单个磁盘读取文件的唯一推荐方法是使用一个线程顺序读取.

Answer 2

由于文件读取主要是等待磁盘I/O,因此您遇到的问题是磁盘不会因为许多线程使用它而旋转得更快:)