[petsc-dev] mpi_comm_create_group()

[Adrian Croucher]

hi

This question is not strictly PETSc-related but I figure you guys are 
probably most likely to know the answer.

I need to do some simple computations involving only a small subset of 
the parallel processes in my simulation.

 From reading the MPI documentation and a few tutorials it looks like 
mpi_comm_create_group() is probably the thing to use for creating the 
MPI communicator in this case. Since there are only a few processes in 
the group, from what I can gather this function should be more efficient 
than mpi_comm_create(), as it's only collective on the group, not the 
whole parent communicator (MPI_COMM_WORLD).

1) Is it correct that mpi_comm_create_group() should be a better option 
in this case than mpi_comm_create()? I had a grep through the PETSc 
source code and there are some calls to mpi_comm_create() but none to 
mpi_comm_create_group(). But maybe the use case is different.

2) If mpi_comm_create_group() is the better option, is it necessary to 
call it on all processes, or only the ones in the group? The tutorial at 
https://mpitutorial.com/tutorials/introduction-to-groups-and-communicators/ 
calls it on all processes, but other stuff I've read suggests you only 
need to call it on processes in the group. It seems to work either way, 
but you have to use the communicator a little differently.

- Adrian

-- 
Dr Adrian Croucher
Senior Research Fellow
Department of Engineering Science
University of Auckland, New Zealand
email: a.croucher at auckland.ac.nz
tel: +64 (0)9 923 4611