Question

I have a program written in Fortran and I have more than 100 subroutines. However, I have around 30 subroutines where there are open-mp codes present. I was wondering what is the best procedure to compile these subroutines. When I used the all the files to compile at once then I found that open mp compiled code runs even slower than the one without open-mp. Should I compile the subroutines with open-mp tags separately ? What is the best practice under these conditions ?

Thank you so much.

Best Regards,
Jdbaba

Answer 1

The OpenMP-aware compilers look for the OpenMP pragma (the open signs after a comment symbol at the begin of the line). Therefore, sources without OpenMP code compiled with an OpenMP-aware compiler should result on the exact or very close object files (and executable).

Edit: One should note that as stated by Hristo Iliev below, enabling OpenMP could affect the serial code, for example by using OpenMP versions of libraries that may differ in algorithm (to be more effective in parallel) and optimizations.

Most likely, the problem here is more related to your code algorithms.
Or perhaps you did not compile with the same optimization flags when comparing OpenMP and non-OpenMP versions.