MPI communicator for sub-range of MPI_COMM_WORLD

Question

What is a simple way to create a (sub)communicator containing consecutive ranks [rStart, ..., last rank of MPI_COMM_WORLD] of MPI_COMM_WORLD?

rStart is >= 0, i.e., first rStart ranks need to be excluded.

Answer 1

The simplest code is to have

MPI_Comm_split(MPI_COMM_WORLD, rank < rStart, rank, &new_comm);

run on all ranks of MPI_COMM_WORLD. It will create two communicators - all ranks starting with rStart will get the one you desire, the others can just MPI_Comm_free their communicator.

If you cannot easily have the excluded ranks run the same code, you can use MPI_Comm_create_group, but then you have to also create the group first.