[petsc-users] how to use petsc4py with mpi subcommunicators?

[Rodrigo Felicio]

Hello all,

Sorry for the newbie question, but is there a way of making petsc4py work with an MPI group or subcommunicator?  I  saw a solution posted back in 2010 (http://lists.mcs.anl.gov/pipermail/petsc-users/2010-May/006382.html), but it does not work for me. Indeed, if I try to use petsc4py.init(comm=newcom), then my sample code prints a msg "Attempting to use an MPI routine before initializing MPI".  Below I attach both the output and the source of the python code.

kind regards
Rodrigo


time mpirun -n 5 python split_comm_ex2.py
Global: rank 0 of 5. New comm : rank  0 of  3
Global: rank 1 of 5. New comm : rank  0 of  2
Global: rank 2 of 5. New comm : rank  1 of  3
Global: rank 3 of 5. New comm : rank  1 of  2
Global: rank 4 of 5. New comm : rank  2 of  3
Attempting to use an MPI routine before initializing MPI
Attempting to use an MPI routine before initializing MPI

real    0m0.655s
user    0m1.122s
sys     0m1.047s

And the python code:

from mpi4py import MPI

comm = MPI.COMM_WORLD
world_rank = comm.rank
world_size = comm.size

color = world_rank % 2

newcomm = comm.Split(color)
newcomm_rank = newcomm.rank
newcomm_size = newcomm.size

for i in range(world_size):
    comm.Barrier()
    if (world_rank == i):
        print ("Global: rank %d of %d. New comm : rank % d of  %d" %
               (world_rank, world_size, newcomm_rank, newcomm_size))

if newcomm.rank == 0:
    import petsc4py
    petsc4py.init(comm=newcomm)
    from petsc4py import PETSc

    pcomm = PETSc.COMM_WORLD
    print('pcomm size is {}/{}'.format(pcomm.rank, pcomm.size))

newcomm.Free()

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