如何在C ++中计算函数的时间执行和CPU加速

Question

I have finished writing a function and I want to compare time and cpu execution of the function with other function. This is code to calculate time execution but I not sure it accuracy. Do you have accuracy code to calculate time and cpu spending for one function in C++?

//Only time execution. CPU spending?
 #include "stdafx.h"
#include <iostream>
#include <time.h>

using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
    clock_t start, end;
    start = clock();

    for(int i=0;i<65536;i++)
    cout<<i<<endl;

    end = clock();
    cout << "Time required for execution: "
    << (double)(end-start)/CLOCKS_PER_SEC
    << " seconds." << "\n\n";
    return 0;
}

Answer 1

In C++11 you should use std::chrono::high_resolution_clock which under the hood uses the clock source with the smallest tick period provided by the system:

#include <chrono>

auto start = std::chrono::high_resolution_clock::now();
//...
auto end = std::chrono::high_resolution_clock::now();
auto elapsed = std::chrono::duration_cast<std::chrono::milliseconds>(end - start);

Answer 2

For this particular code, I'm pretty sure your code is quite OK.

For short periods, you may need to use more precise timing functions, such as Windows' QueryPerformanceCounter and QueryPerformanceFrequency to get higher precision timing.

Answer 3

The easiest way I know of to get precise time estimates is to use an OpenMP function:

#include <stdio.h>
#include <omp.h>
int main() {
    double dtime = omp_get_wtime();
    foo();
    dtime = omp_get_wtime() - dtime;
    printf("time in seconds %f\n", dtime);
}

In gcc compile with -fopenmp. In visual studio turn on OpenMP support under C++/Language Support. BTW, OpenMP now works in Visual Studio 2012 express. For CPU time profiling you can try http://developer.amd.com/tools-and-sdks/heterogeneous-computing/amd-codeanalyst-performance-analyzer/