Hmmm...well, the info is there. There is an envar OMPI_COMM_WORLD_LOCAL_SIZE which tells you how many procs are on this node. If you tell your proc how many cores (or hwthreads) to use, it would be a simple division to get what you want.
You could also detect the number of cores or hwthreads via a call to hwloc, but I don't know if you want to link that deep and MPI doesn't have a function for that purpose. Could be that OpenMP provides a call for that purpose? On Sep 12, 2014, at 7:22 AM, JR Cary <c...@txcorp.com> wrote: > > > On 9/12/14, 7:27 AM, Tim Prince wrote: >> >> On 9/12/2014 6:14 AM, JR Cary wrote: >>> This must be a very old topic. >>> >>> I would like to run mpi with one process per node, e.g., using >>> -cpus-per-rank=1. Then I want to use openmp inside of that. >>> But other times I will run with a rank on each physical core. >>> >>> Inside my code I would like to detect which situation I am in. >>> Is there an openmpi api call to determine that? >>> >> omp_get_num_threads() should work. Unless you want to choose a different >> non-parallel algorithm for this case, a single thread omp parallel region >> works fine. >> You should soon encounter cases where you want intermediate choices, such as >> 1 rank per CPU package and 1 thread per core, even if you stay away from >> platforms with more than 12 cores per CPU. > > I may not understand, so I will try to ask in more detail. > > Suppose I am running on a four-core processor (and my code likes one thread > per core). > > In case 1 I do > > mpiexec -np 2 myexec > > and I want to know that each mpi process should use 2 threads. > > If instead I did > > mpiexec -np 4 myexec > > I want to know that each mpi process should use one thread. > > Will omp_get_num_threads() should return a different value for those two > cases? > > Perhaps I am not invoking mpiexec correctly. > I use MPI_Init_thread(&argc, &argv, MPI_THREAD_FUNNELED, &threadSupport), and > regardless > of what how I invoke mpiexec (-n 1, -n 2, -n 4), I see 2 openmp processes > and 1 openmp threads (have not called omp_set_num_threads). > When I run serial, I see 8 openmp processes and 1 openmp threads. > So I must be missing an arg to mpiexec? > > This is a 4-core haswell with hyperthreading to get 8. > > > > Thx..... > _______________________________________________ > users mailing list > us...@open-mpi.org > Subscription: http://www.open-mpi.org/mailman/listinfo.cgi/users > Link to this post: > http://www.open-mpi.org/community/lists/users/2014/09/25322.php
