measuring time of multi-threading program

Question

I measured time with function clock() but it gave bad results. I mean it gives the same results for program with one thread and for the same program running with OpenMP with many threads. But in fact, I notice with my watch that with many threads program counts faster. So I need some wall-clock timer...

My question is: What is better function for this issue? clock_gettime() or mb gettimeofday() ? or mb something else?

if clock_gettime(),then with which clock? CLOCK_REALTIME or CLOCK_MONOTONIC?

using mac os x (snow leopard)

Answer 1

If you want wall-clock time, and clock_gettime() is available, it's a good choice. Use it with CLOCK_MONOTONIC if you're measuring intervals of time, and CLOCK_REALTIME to get the actual time of day.

CLOCK_REALTIME gives you the actual time of day, but is affected by adjustments to the system time -- so if the system time is adjusted while your program runs that will mess up measurements of intervals using it.

CLOCK_MONOTONIC doesn't give you the correct time of day, but it does count at the same rate and is immune to changes to the system time -- so it's ideal for measuring intervals, but useless when correct time of day is needed for display or for timestamps.

Answer 2

I think clock() counts the total CPU usage among all threads, I had this problem too...

The choice of wall-clock timing method is personal preference. I use an inline wrapper function to take time-stamps (take the difference of 2 time-stamps to time your processing). I've used floating point for convenience (units are in seconds, don't have to worry about integer overflow). With multi-threading, there are so many asynchronous events that in my opinion it doesn't make sense to time below 1 microsecond. This has worked very well for me so far :)

Whatever you choose, a wrapper is the easiest way to experiment

inline double my_clock(void) {
  struct timeval t;
  gettimeofday(&t, NULL);
  return (1.0e-6*t.tv_usec + t.tv_sec);
}

usage:

double start_time, end_time;
start_time = my_clock();
//some multi-threaded processing
end_time = my_clock();
printf("time is %lf\n", end_time-start_time);