c++benchmarking

How to Calculate Execution Time of a Code Snippet in C++


I have to compute execution time of a C++ code snippet in seconds. It must be working either on Windows or Unix machines.

I use code the following code to do this. (import before)

clock_t startTime = clock();
// some code here
// to compute its execution duration in runtime
cout << double( clock() - startTime ) / (double)CLOCKS_PER_SEC<< " seconds." << endl;

However for small inputs or short statements such as a = a + 1, I get "0 seconds" result. I think it must be something like 0.0000001 seconds or something like that.

I remember that System.nanoTime() in Java works pretty well in this case. However I can't get same exact functionality from clock() function of C++.

Do you have a solution?


Solution

  • You can use this function I wrote. You call GetTimeMs64(), and it returns the number of milliseconds elapsed since the unix epoch using the system clock - the just like time(NULL), except in milliseconds.

    It works on both windows and linux; it is thread safe.

    Note that the granularity is 15 ms on windows; on linux it is implementation dependent, but it usually 15 ms as well.

    #ifdef _WIN32
    #include <Windows.h>
    #else
    #include <sys/time.h>
    #include <ctime>
    #endif
    
    /* Remove if already defined */
    typedef long long int64; typedef unsigned long long uint64;
    
    /* Returns the amount of milliseconds elapsed since the UNIX epoch. Works on both
     * windows and linux. */
    
    uint64 GetTimeMs64()
    {
    #ifdef _WIN32
     /* Windows */
     FILETIME ft;
     LARGE_INTEGER li;
    
     /* Get the amount of 100 nano seconds intervals elapsed since January 1, 1601 (UTC) and copy it
      * to a LARGE_INTEGER structure. */
     GetSystemTimeAsFileTime(&ft);
     li.LowPart = ft.dwLowDateTime;
     li.HighPart = ft.dwHighDateTime;
    
     uint64 ret = li.QuadPart;
     ret -= 116444736000000000LL; /* Convert from file time to UNIX epoch time. */
     ret /= 10000; /* From 100 nano seconds (10^-7) to 1 millisecond (10^-3) intervals */
    
     return ret;
    #else
     /* Linux */
     struct timeval tv;
    
     gettimeofday(&tv, NULL);
    
     uint64 ret = tv.tv_usec;
     /* Convert from micro seconds (10^-6) to milliseconds (10^-3) */
     ret /= 1000;
    
     /* Adds the seconds (10^0) after converting them to milliseconds (10^-3) */
     ret += (tv.tv_sec * 1000);
    
     return ret;
    #endif
    }