java - 2核Mac上有多个Java线程-慢

Question

我一直在为操作系统类(class)编写以下代码，但结果有些奇怪。该代码创建x线程并同时运行它们，以便将两个平方矩阵相乘。每个线程将输入矩阵的Number_of_rows/Number_of_threads行相乘。

当在具有1 ... 8个线程的1024X1024矩阵上运行它时，我得到最快的乘法运算是在仅使用一个线程的情况下发生的。我希望带有i5处理器(2核)的MacBook Pro将利用两个核，并且在使用两个线程时会产生更快的结果。
使用一个线程，运行时间从大约9.2秒开始，使用8个线程从大约9.6秒变为27秒。

知道为什么会这样吗？

顺便说一句，关于代码的一些事情:

一种。假设两个矩阵具有相同的尺寸并且为正方形。

b。假设线程数<=行数/列数。

public class MatrixMultThread implements Runnable {
    final static int MATRIX_SIZE = 1024;
    final static int MAX_THREADS = MATRIX_SIZE;
    private float[][] a;
    private float[][] b;
    private float[][] res;
    private int startIndex;
    private int endIndex;


public MatrixMultThread(float[][] a, float[][]b, float[][] res, int startIndex, int endIndex) {
    this.a = a;
    this.b = b;
    this.res = res;
    this.startIndex = startIndex;
    this.endIndex = endIndex;
}


public void run() {
    float value = 0;
    for (int k = startIndex; k < endIndex; k++) {
        for (int i = 0; i < a.length; i++) {
            for (int j = 0; j < a.length; j++) {
                value += a[k][j]*b[j][i];
            }
            res[k][i] = value;
            value = 0;
        }
    }
}


public static float[][] mult(float[][] a, float[][] b, int threadCount){

    // Get number of rows per each thread.
    int rowsPerThread = (int) Math.ceil(MATRIX_SIZE / threadCount);
    float[][] res = new float[MATRIX_SIZE][MATRIX_SIZE];

    // Create thread array
    Thread[] threadsArray = new Thread[threadCount];
    int rowCounter = 0;
    for (int i = 0; i < threadCount; i++) {
        threadsArray[i] = new Thread(new MatrixMultThread(a,b,res,rowCounter, Math.max(rowCounter + rowsPerThread, MATRIX_SIZE)));
        threadsArray[i].start();
        rowCounter += rowsPerThread;
    }


    // Wait for all threads to end before finishing execution.
    for (int i = 0; i < threadCount; i++) {
        try {
            threadsArray[i].join();
        } catch (InterruptedException e) {
            System.out.println("join failed");
        }
    }
    return res;
}


public static void main(String args[]) {
    // Create matrices and random generator
    Random randomGenerator = new Random();
    float[][] a = new float[MATRIX_SIZE][MATRIX_SIZE];
    float[][] b = new float[MATRIX_SIZE][MATRIX_SIZE];

    // Initialize two matrices with initial values from 1 to 10.
    for (int i = 0; i < a.length; i++) {
        for (int j = 0; j < a.length; j++) {
            a[i][j] = randomGenerator.nextFloat() * randomGenerator.nextInt(100);
            b[i][j] = randomGenerator.nextFloat() * randomGenerator.nextInt(100);
        }
    }
    long startTime;
    for (int i = 1; i <= 8; i++) {
        startTime = System.currentTimeMillis();
        mult(a,b,i);
        System.out.println("Total running time is: " + (System.currentTimeMillis() - startTime) + " ms");
    }
}

}

Answer 1

首先，一些日志记录会有所帮助。我为此做了记录，并发现了您逻辑中的错误。

这是日志

Starting execution for thread count: 1
Start index: 0
End index: 1024
Starting execution: MatrixMultiplier: 0
Ending executionMatrixMultiplier: 0
Total running time is: 6593 ms


Starting execution for thread count: 2
Start index: 0
End index: 1024  <------ This is the problem area
Start index: 512
End index: 1024
Starting execution: MatrixMultiplier: 1
Starting execution: MatrixMultiplier: 0

您所有迭代中的第一个线程每次都在执行整个乘法。这就是为什么您看不到结果。找出错误。