Question

I have a simple C# and C++ code that computes a sum of dot products.

The C# code is:

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();
        }
    }
}

and the C++ is

#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;
}

The C# version (Release x86 with optimization on, x64 is even slower) output is

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

while C++ (default VS2010 Release build settings) yields

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

Now I would expect the C# code would be a little slower. But 130 times slower seems way too much to me. Can someone please explain to me what is going on here?

EDIT

I am not a C++ programmer and I just took the diffclock code somewhere from the internet without really checking if it’s correct.

Using std::difftime the C++ results are

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

which seems about right.

Answer 1

Your diffclock code is wrong.

If you change your C++ code to use the std::clock and std::difftime it appears to show the actual runtime:

#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;
}

Results:

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

That is running the application in default release mode optimizations, outside of vs2010.

EDIT

As others have pointed out, in C++ using clock() is not the most accurate way to time a function (as in C#, Stopwatch is better than DateTime).

If you’re using windows, you can always use the QueryPerformanceCounter for high-resolution timing.