如何在C#和C++中测量经过的时间

Question

我有一个简单的C#和C++代码来计算点积的总和.

C#代码是:

using System;

namespace DotPerfTestCS
{
    class Program
    {
        struct Point3D
        {
            public double X, Y, Z;

            public Point3D(double x, double y, double z)
            {
                X = x;
                Y = y;
                Z = z;
            }
        }

        static void RunTest()
        {
            unchecked
            {
                const int numPoints = 100000;
                const int numIters = 100000000;

                Point3D[] pts = new Point3D[numPoints];
                for (int i = 0; i < numPoints; i++) pts[i] = new Point3D(i, i + 1, i + 2);

                var begin = DateTime.Now;
                double sum = 0.0;
                var u = new Point3D(1, 2, 3);
                for (int i = 0; i < numIters; i++)
                {
                    var v = pts[i % numPoints];
                    sum += u.X * v.X + u.Y * v.Y + u.Z * v.Z;
                }
                var end = DateTime.Now;
                Console.WriteLine("Sum: {0} Time elapsed: {1} ms", sum, (end - begin).TotalMilliseconds);
            }
        }

        static void Main(string[] args)
        {
            for (int i = 0; i < 5; i++) RunTest();
        }
    }
}

而C++是

#include <iostream>
#include <vector>
#include <time.h>

using namespace std;

typedef struct point3d
{
    double x, y, z;

    point3d(double x, double y, double z)
    {
        this->x = x;
        this->y = y;
        this->z = z;
    }
} point3d_t;

double diffclock(clock_t clock1,clock_t clock2)
{
    double diffticks=clock1-clock2;
    double diffms=(diffticks*10)/CLOCKS_PER_SEC;
    return diffms;
}

void runTest()
{
    const int numPoints = 100000;
    const int numIters = 100000000;

    vector<point3d_t> pts;
    for (int i = 0; i < numPoints; i++) pts.push_back(point3d_t(i, i + 1, i + 2));

    auto begin = clock();
    double sum = 0.0, dum = 0.0;
    point3d_t u(1, 2, 3);
    for (int i = 0; i < numIters; i++) 
    {
        point3d_t v = pts[i % numPoints];
        sum += u.x * v.x + u.y * v.y + u.z * v.z;
    }
    auto end = clock();
    cout << "Sum: " << sum << " Time elapsed: " << double(diffclock(end,begin)) << " ms" << endl;

}

int main()
{
    for (int i = 0; i < 5; i++) runTest();
    return 0;
}

C#版本(发布x86,优化开启,x64甚至更慢)输出

Sum: 30000500000000 Time elapsed: 551.0299 ms 
Sum: 30000500000000 Time elapsed: 551.0315 ms 
Sum: 30000500000000 Time elapsed: 552.0294 ms
Sum: 30000500000000 Time elapsed: 551.0316 ms 
Sum: 30000500000000 Time elapsed: 550.0315 ms

而C++(默认VS2010发布版本设置)产生

Sum: 3.00005e+013 Time elapsed: 4.27 ms
Sum: 3.00005e+013 Time elapsed: 4.27 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms
Sum: 3.00005e+013 Time elapsed: 4.25 ms

现在我希望C#代码会慢一些.但对我来说,慢了130倍似乎太过分了.有人可以向我解释这里发生了什么吗？

编辑

我不是C++程序员,我只是从互联网上的某处获取了diffclock代码而没有真正检查它是否正确.

使用std :: difftime,C++结果是

Sum: 3.00005e+013 Time elapsed: 457 ms
Sum: 3.00005e+013 Time elapsed: 452 ms
Sum: 3.00005e+013 Time elapsed: 451 ms
Sum: 3.00005e+013 Time elapsed: 451 ms
Sum: 3.00005e+013 Time elapsed: 451 ms

这似乎是正确的.

Answer 1

你的diffclock代码是错误的.

如果您更改C++代码以使用它std::clock,std::difftime它似乎显示实际的运行时:

#include <iostream>
#include <vector>
#include <ctime>

using namespace std;

typedef struct point3d
{
    double x, y, z;

    point3d(double x, double y, double z)
    {
        this->x = x;
        this->y = y;
        this->z = z;
    }
} point3d_t;

void runTest()
{
    const int numPoints = 100000;
    const int numIters = 100000000;

    vector<point3d_t> pts;
    for (int i = 0; i < numPoints; i++) pts.push_back(point3d_t(i, i + 1, i + 2));

    auto begin = clock();
    double sum = 0.0, dum = 0.0;
    point3d_t u(1, 2, 3);
    for (int i = 0; i < numIters; i++) 
    {
        point3d_t v = pts[i % numPoints];
        sum += u.x * v.x + u.y * v.y + u.z * v.z;
    }
    auto end = clock();
    cout << "Sum: " << sum << " Time elapsed: " << double(std::difftime(end,begin)) << " ms" << endl;

}

int main()
{
    for (int i = 0; i < 5; i++) runTest();
    return 0;
}

结果:

Sum: 3.00005e+013 Time elapsed: 346 ms
Sum: 3.00005e+013 Time elapsed: 344 ms
Sum: 3.00005e+013 Time elapsed: 346 ms
Sum: 3.00005e+013 Time elapsed: 347 ms
Sum: 3.00005e+013 Time elapsed: 347 ms

这是在vs2010之外的默认发布模式优化中运行应用程序.

编辑

正如其他人所指出的那样,在C++中使用clock()并不是计算时间的最准确方法(如在C#中,Stopwatch优于DateTime).

如果您使用的是Windows,则可以始终使用QueryPerformanceCounter进行高分辨率计时.

Answer 2

我相信你会发现你的diffclock实现产生十分之一秒,而不是毫秒(假设CLOCKS_PER_SECOND准确命名).纠正这个问题,C#实现运行速度大约慢了30%,这似乎是合适的.