Question

I need to “time” or benchmark a number crunching application written in C/C++. The problem is that the machine where I run the program is usually full of people doing similar things, so the CPUs are always at full load.

I thought about using functions from time.h liket “get time of the day” (don’t remember the exact syntax, sorry) and similars, but I am afraid they would not be good for this case, am I right?

And the program “time” from bash, gave me some errors long time ago.

Also the problem is, that sometimes I need to get timings in the range of 0.5 secs and so on.

Anybody has a hint?

P.S.: compiler is gcc and in some cases nvcc (NVIDIA)
P.S.2: in my benchmarks I just want to measure the execution time between two parts of the main function

Answer 1

From your other recent questions, you seem to be using MPI for parallelisation. Assuming this question is within the same context, then the simplest way to time your application would be to use MPI_Wtime().

From the man page:

This subroutine returns the current
value of time as a double precision
floating point number of seconds. This
value represents elapsed time since
some point in the past. This time in
the past will not change during the
life of the task. You are responsible
for converting the number of seconds
into other units if you prefer.

Example usage:

#include "mpi.h"

int main(int argc, char **argv)
{
    int rc, taskid;
    double t_start, t_end;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&taskid); 

    t_start = MPI_Wtime();

    /* .... your computation kernel .... */

    t_end = MPI_Wtime();

    /* make sure all processes have completed */
    MPI_Barrier(MPI_COMM_WORLD);

    if (taskid == 0) {
        printf("Elapsed time: %1.2f seconds\n", t_start - t_end);
    }

    MPI_Finalize();
    return 0;
} 

The advantage of this is that we let the underlying MPI library handle platform specific ways of handling time, although you might want to use MPI_Wtick() to determine the resolution of the timer used on each platform.