Subcommunicators about mpich HOT 4 CLOSED

MPI_Group_incl is a local function, it creates a group structure on your local process. You can later create the communicator via MPI_Comm_create_group to create the communicator. You need make sure the groups are on all participating processes are consistent.

MPI_Comm_split is a collective. It creates the sub-communicator in one step. I recommend the latter unless you want the flexibility of using groups.

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Could you please elaborate on the "You need make sure the groups are on all participating processes are consistent." Does this mean sometimes there can be inconsistence with the participating processes

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If you call the same group construction code on all participating processes, then the resulting groups will be identical even though each process creates its group independently. But you always can create local group via different paths resulting in inconsistent groups at communicator creation time.

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Thanks!

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