Java 似乎比 C++ 更快地执行基本算法。为什么？

Question

简介:

使用两种相同的合并排序算法，我测试了 C++(使用 Visual Studios C++ 2010 express)与 Java(使用 NetBeans 7.0)的执行速度。我推测 C++ 执行至少会稍微快一些，但测试显示 C++ 执行比 Java 执行慢 4 - 10 倍。我相信我已经为 C++ 设置了所有速度优化，并且我将作为发布而不是作为调试发布。为什么会出现这种速度差异？

代码:

Java:

public class PerformanceTest1
{
 /**
  * Sorts the array using a merge sort algorithm
  * @param array The array to be sorted
  * @return The sorted array
  */
 public static void sort(double[] array)
 {
      if(array.length > 1)
      {
           int centre;
           double[] left;
           double[] right;
           int arrayPointer = 0;
           int leftPointer = 0;
           int rightPointer = 0;

           centre = (int)Math.floor((array.length) / 2.0);

           left = new double[centre];
           right = new double[array.length - centre];

           System.arraycopy(array,0,left,0,left.length);
           System.arraycopy(array,centre,right,0,right.length);

           sort(left);
           sort(right);

           while((leftPointer < left.length) && (rightPointer < right.length))
           {
                if(left[leftPointer] <= right[rightPointer])
                {
                     array[arrayPointer] = left[leftPointer];
                     leftPointer += 1;
                }
                else
                {
                     array[arrayPointer] = right[rightPointer];
                     rightPointer += 1;
                }
                arrayPointer += 1;
           }
           if(leftPointer < left.length)
           {
                System.arraycopy(left,leftPointer,array,arrayPointer,array.length - arrayPointer);
           }
           else if(rightPointer < right.length)
           {
                System.arraycopy(right,rightPointer,array,arrayPointer,array.length - arrayPointer);
           }
      }
 }

 public static void main(String args[])
 {
      //Number of elements to sort
      int arraySize = 1000000;

      //Create the variables for timing
      double start;
      double end;
      double duration;

      //Build array
      double[] data = new double[arraySize];
      for(int i = 0;i < data.length;i += 1)
      {
           data[i] = Math.round(Math.random() * 10000);
      }

      //Run performance test
      start = System.nanoTime();
      sort(data);
      end = System.nanoTime();

      //Output performance results
      duration = (end - start) / 1E9;
      System.out.println("Duration: " + duration);
 }
}

C++:

#include <iostream>
#include <windows.h>
using namespace std;

//Mergesort
void sort1(double *data,int size)
{
if(size > 1)
{
    int centre;
    double *left;
    int leftSize;
    double *right;
    int rightSize;
    int dataPointer = 0;
    int leftPointer = 0;
    int rightPointer = 0;

    centre = (int)floor((size) / 2.0);
    leftSize = centre;
    left = new double[leftSize];
    for(int i = 0;i < leftSize;i += 1)
    {
        left[i] = data[i];
    }
    rightSize = size - leftSize;
    right = new double[rightSize];
    for(int i = leftSize;i < size;i += 1)
    {
        right[i - leftSize] = data[i];
    }

    sort1(left,leftSize);
    sort1(right,rightSize);

    while((leftPointer < leftSize) && (rightPointer < rightSize))
    {
        if(left[leftPointer] <= right[rightPointer])
        {
            data[dataPointer] = left[leftPointer];
            leftPointer += 1;
        }
        else
        {
            data[dataPointer] = right[rightPointer];
            rightPointer += 1;
        }
        dataPointer += 1;
    }
    if(leftPointer < leftSize)
    {
        for(int i = dataPointer;i < size;i += 1)
        {
            data[i] = left[leftPointer++];
        }
    }
    else if(rightPointer < rightSize)
    {
        for(int i = dataPointer;i < size;i += 1)
        {
            data[i] = right[rightPointer++];
        }
    }
            delete left;
            delete right;
}
}

void main()
{
//Number of elements to sort
int arraySize = 1000000;

//Create the variables for timing
LARGE_INTEGER start; //Starting time
LARGE_INTEGER end; //Ending time
LARGE_INTEGER freq; //Rate of time update
double duration; //end - start
QueryPerformanceFrequency(&freq); //Determinine the frequency of the performance counter (high precision system timer)

//Build array
double *temp2 = new double[arraySize];
QueryPerformanceCounter(&start);
srand((int)start.QuadPart);
for(int i = 0;i < arraySize;i += 1)
{
    double randVal = rand() % 10000;
    temp2[i] = randVal;
}

//Run performance test
QueryPerformanceCounter(&start);
sort1(temp2,arraySize);
QueryPerformanceCounter(&end);
    delete temp2;

//Output performance test results
duration = (double)(end.QuadPart - start.QuadPart) / (double)(freq.QuadPart);
cout << "Duration: " << duration << endl;

//Dramatic pause
system("pause");
}

观察:

对于 10000 个元素，C++ 执行时间大约是 Java 执行时间的 4 倍。 对于 100000 个元素，该比例约为 7:1。 对于 10000000 个元素，该比例约为 10:1。 对于超过 10000000，Java 执行完成，但 C++ 执行停止，我必须手动终止进程。

Answer 1

我认为您运行程序的方式可能有误。当您在 Visual C++ Express 中按 F5 键时，程序在调试器下运行，速度会慢很多。在其他版本的 Visual C++ 2010(例如我使用的 Ultimate)中，尝试按 CTRL+F5(即不调试启动)或尝试运行可执行文件本身(在 Express 中)，您会看到不同之处。

我在我的机器上只做了一个修改就运行了你的程序(添加了 delete[] left; delete[] right; 来消除内存泄漏；否则它会在 32 位中耗尽内存模式!)。我有一个 i7 950。公平地说，我还将相同的数组传递给 Java 中的 Arrays.sort() 和 C++ 中的 std::sort。我使用的数组大小为 10,000,000。

结果如下(以秒为单位):

Java code:            7.13
Java Arrays.sort:     0.93

32 bits
C++ code:             3.57
C++ std::sort         0.81

64 bits
C++ code:             2.77
C++ std::sort         0.76

因此 C++ 代码要快得多，即使是在 Java 和 C++ 中都经过高度调整的标准库，也往往会显示出 C++ 的轻微优势。

编辑:我刚刚在您的原始测试中意识到，您在 Debug模式下运行 C++ 代码。您应该切换到 Release 模式并在调试器外部运行它(正如我在帖子中解释的那样)以获得公平的结果。