Time spends in CPU faster than in reality

I am wondering why my entire application runs in less than 8 seconds while the time obtained from clock_gettime is 19.3468 seconds which is more than two times as much as what happens in reality. Where is the problem from?

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time_start);

... // many calculations

clock_gettime(CLOCK_PROCESS_CPUTIME_ID, &time_stop);
double time_diff=(time_stop.tv_sec-time_start.tv_sec+
    (1e-9)*(time_stop.tv_nsec-time_start.tv_nsec);

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Update:

I am not using any OpenMP explicitly.

Answer

CLOCK_MONOTONIC should be used if you want to measure total elapsed time, including time spent blocked waiting for IO, but it will also include slowdowns caused by other processes getting scheduled while your program is trying to run.