Question

This is my c++ code.

double start_time = time(NULL);
double start_clock = clock();

#pragma omp parallel for private(i) 
for(i=0;i<max_i;i++)
      PROCESS(i);

double end_time = time(NULL);
double end_clock = clock();

printf("%lf second(s)\n", end_time-start_time);
printf("%lf second(s)\n", (end_clock-start_clock)/CLOCKS_PER_SEC);

and this is the output.

took 2.000000 second(s)
took 11.410000 second(s)

Does anyone know why these are not consistent? Is there any other way of measuring this? BTW, 2 seconds seems more reasonable based on the time I’m seeing here.

Answer 1

The clock() function returns the amount of CPU time used by your process since it started, not the absolute time according to a real-time clock.

In another comment you said that CODE_BLOCK is a parallel loop – which means in your case, it used the equivalent of 11.41 seconds of CPU time in 2 seconds of real (“wall clock”) time. Evidently you’re using the power of about 6 CPUs in parallel.