[mpich-discuss] MPI_Intercomm_create() for merging two spawned groups

Carsten Clauss c.clauss at fz-juelich.de
Fri Sep 26 10:50:10 CDT 2014

Dear all,

I have a code where two processes (forming the original MPI_COMM_WORLD) 
each spawn one additional child process (using MPI_COMM_SELF as spawning 

Now I want to create an intra-comm that covers all of these four processes.

For doing so, I initially merge the two inter-comms resulting from the 
spawn calls into two new intra-comms (by using MPI_Intercomm_merge()).

Then I create via MPI_intercomm_create() a new inter-comm that connects 
these two by using the original world communicator as peer-com.

Finally, I merge the resulting inter-comm into the desired intra-comm.

When using Open MPI, my code (it's derived from the MPICH test 
spaiccreate2.c, see attachment) works fine on my local machine.
However, when running it with MPICH-3.1.2, I get the following error 

PMPI_Intercomm_create(601).....: MPI_Intercomm_create(comm=0x84000006, 
local_leader=1, MPI_COMM_WORLD, remote_leader=1, tag=123, 
newintercomm=0x7fff8323ee3c) failed
MPID_GPID_ToLpidArray(461).....: Internal MPI error: Unknown gpid 
Fatal error in PMPI_Intercomm_create: Internal MPI error!, error stack:
PMPI_Intercomm_create(601).....: MPI_Intercomm_create(comm=0x84000004, 
local_leader=1, MPI_COMM_WORLD, remote_leader=0, tag=123, 
newintercomm=0x7fff6e0b4c7c) failed
MPID_GPID_ToLpidArray(461).....: Internal MPI error: Unknown gpid 

Here are my questions:
1) Is the above mentioned approach the right way to reach my goal?
2) Is the semantics of the attached code MPI compliant?
3) What is the reason for the error message when using MPICH?

Thanks in advance and with kind regards,

Carsten Clauss
ParTec Cluster Competence Center GmbH

Possartstrasse 20
D-81679 Muenchen

Geschäftsführer RA. Dipl.-Ing. Bernhard Frohwitter Eingetragen beim
Amtsgericht München HRB 151545 Steuer-Nr. 08/32305, Ust-ID DE235527064
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