Rick, There are few requirements to use any communicator in a collective operation (including MPI_Reduce), and most of them are common sense. All ranks in the communicator must call the collective, and the different ranks should all reach the collective together (no unmatched blocking communications should exists before the collective on some ranks). To focus on the MPI_Reduce, all participants should also provide the same root rank, which should be bounded by 0 and by the communicator size, the same MPI_Op, and the same amount of data.
If I might I would suggest a good MPI-related book, "Using MPI ..." by Gropp, Lusk and Skjellum (ISBN 978-0262527392). The examples are freely available online at http://www.mcs.anl.gov/research/projects/mpi/usingmpi/. George. On Wed, Oct 19, 2016 at 5:50 PM, Marlborough, Rick <rmarlboro...@aaccorp.com > wrote: > Designation: Non-Export Controlled Content > > Folks; > > My MPI_COMM_WORLD size is 2000. I have created a > communicator based on a small subset. I have used this communicator in > MPI_Barrier calls and it seems to work fine. Now I want to use this in > MPI_Reduce. When I do this I get errors like “invalid root” or more > mysteriously “on communicator MPI communicator 7 group from 0… invalid > argument of some kind.”. can someone specify how to use a communicator > (that is not COMM_WORLD) with MPI_Reduce and specify rank properly. > > > > Thanx > > Rick > > 3.1.1001 > > _______________________________________________ > users mailing list > users@lists.open-mpi.org > https://rfd.newmexicoconsortium.org/mailman/listinfo/users >
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