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

Question

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?

Answer 1

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
}