Question

Given n threads, is there a way that I can calculate the amount of overhead (e.g. # of cycles) that is required to implement a specific directive in OpenMP.

For example, given the code below

 #pragma omp parallel
 {
    #pragma omp for
    for( int i=0 ; i < m ; i++ )
       a[i] = b[i] + c[i];
 }

Can I calculate somehow how much overhead is required to create these threads?

Answer 1

Yes, you can. Please take a look at EPCC benchmark. Although this code is a bit older, it measures the various overhead of OpenMP’s constructs, including omp parallel for and omp critical.

Basic approach is somewhat very simple and straightforward. You measure a baseline serial time without any OpenMP, and just include a OpenMP pragma that you want to measure. Then, subtract the elapsed times. This is exactly how EPCC benchmark measures the overhead. See the source like ‘syncbench.c’.

Please note that the overhead is expressed as time, rather than the # of cycles. I also tried to measure # of cycles, but OpenMP parallel constructs’ overhead may include blocked time due to synchronizations. Hence, # of cycles may not reflect the real overhead of OpenMP.