On 12/3/21 5:53 PM, Jed Brown wrote:

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Ah, interesting. I guess mpi_comm_split() might also have the advantage that each rank only needs to know if it is in the group or not, rather than needing an array of all participating ranks, as mpi_comm_create() and mpi_comm_create_group() do. That's exactly why it's often more convenient. But it is collective on the parent communicator.

Yes, I was a bit concerned about that after reading this (from https://cvw.cac.cornell.edu/MPIAdvTopics/subdividing): " ...the MPI specification states that the call to MPI_Comm_create must be executed by all processes in the input communicator (in our case, MPI_COMM_WORLD), and that all processes must pass the same value for the group argument (grp), even if they do not belong to the new group. This can be a dire problem with a very large number of processes, such as are found in petascale systems, so MPI_COMM_CREATE_GROUP was introduced in MPI-3 to alleviate this problem." - Adrian -- Dr Adrian Croucher Senior Research Fellow Department of Engineering Science University of Auckland, New Zealand email: [email protected] tel: +64 (0)9 923 4611