On Dec 12, 2011, at 9:45 AM, Josh Hursey wrote:
> For MPI_Comm_split, all processes in the input communicator (oldcomm
> or MPI_COMM_WORLD in your case) must call the operation since it is
> collective over the input communicator. In your program rank 0 is not
> calling the operation, so MPI_Comm_split is waiting for it to
> If you want rank 0 to be excluded from the any of the communicators,
> you can give it a special color that is distinct from all other ranks.
> Upon return from MPI_Comm_split, rank 0 will be given a new
> communicator containing just one processes, itself. If you do not
> intend to use that communicator you can free it immediately
You can also specify MPI_UNDEFINED as your color, in which case the output communicator in that process will be MPI_COMM_NULL. See MPI-2.2 p205.
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