我正在制作一个多线程应用程序来处理图像，但顺序版本更快，为什么？

Question

我在线程中有静态 BufferedImage 并让每个线程修改图像的某些部分。处理非常独立，因为每个像素都是单独完成的，但顺序版本更快我该如何解决？

编辑 1：代码如下所示

public static void main(String[] args) throws IOException, InterruptedException {
    long startime, endtime;
    long Time;
    startime = System.currentTimeMillis();
    //also tried to use nano

    WorkingThread T1 = new WorkingThread("input.jpg");
    WorkingThread T2 = new WorkingThread();
    WorkingThread T3 = new WorkingThread();
    WorkingThread T4 = new WorkingThread();

    T1.start();
    T2.start();
    T3.start();
    T4.start();

    T1.join();
    T2.join();
    T3.join();
    T4.join();

    endtime = System.currentTimeMillis();
    Time = endtime - startime;
    System.out.println("The time consumed in miliseconds is  " + Time);
}

public class WorkingThread extends Thread {

  public static BufferedImage img;
  public static int p;
  public int nb;
  public static int width, height;

  public WorkingThread(String file) throws IOException {
    File f = new File(file);
    img = ImageIO.read(f);
    width = img.getWidth();
    height = img.getHeight();
    p = 1;
    nb = 0;
}

  public WorkingThread() {
    nb = p;
    p++;
}

  public void run() {

    int start=nb*height/p;
    int end = (nb + 1) * height/p;
    //the image will be split according to y axis in this case
    for(y=start; y < end ; y++) {
       for(x =0; x<width; x++) {
            pixel = img.getRGB(x, y);
            //processing
            img.setRGB(x, y, pixel);
        }
  }
}

Answer 1

正如评论已经指出的那样，您的基准测试中的缺陷是线程设置和拆卸的开销大于顺序执行代码实际花费的时间.

通过插入一些花费大量时间的愚蠢代码，如下所示：

//processing
for (int i = 0; i < 1000; i++) {
    pixel = ((~pixel ^ pixel) | pixel) & pixel;
}

...在 for x/y 循环中并增加 i 的最大值，你会看到最终多线程版本将是比顺序的快。

PS：我使用了大约 2500 x 3300 像素的图像。

我正在制作一个多线程应用程序来处理图像，但顺序版本更快，为什么？

I am making a multithreaded application to process an image yet the sequential version is faster, why?

java

multithreading

bufferedimage