[Gretl-devel] Re: MPI and OpenMP (and openblas?)

Hi, here's a question about what is really affected by setting the number of threads used by OpenMP via an environment variable and/or via the option to an MPI block (as explained in the gretl + MPI documentation). My test script has an MPI block as its main part which calls a certain worker function. Basically it's a VAR bootstrap, so making heavy use of mnormal(), varsimul() and mols(). The MPI block is specified with --omp-threads=1. I thought that means that no multithreading is done at the OpenMP level. My blas is openblas and shows up with the option blas_parallel = "OpenMP". However: When I run this script (on a Jan-11 Windows snapshot) with the environment set to OMP_NUM_THREADS=1 it takes about 1/3 _less_ time than with OMP_NUM_THREADS=4, on a 4-physical-cores CPU. This happens for any variation of the numbers of parallel MPI processes. OK, so given our recent discussion about the problems of openblas multithreading, the ranking of the timings is maybe not so surprising anymore. But: why does OMP_NUM_THREADS=1 have an additional effect over --omp-threads=1 when everything happens inside the MPI block ? I hope the problem was clearly explained.