Hi Bob, Thanks for the detailed bug report and test cases. I confirmed that the failure you are seeing on the MPICH trunk. This is likely related to changes we made to support MPI-3 MPI_Comm_create_group(). I created a ticket to track this: https://trac.mpich.org/projects/mpich/ticket/1768 ~Jim. On 12/12/12 5:38 PM, Bob Cernohous wrote:

I've had a hang reported on BG/Q after about 2K MPI_Comm_create's.

It hangs on the latest 2 releases (mpich2 v1.5.x and v1.4.x) on BG/Q.

It also hangs on linux: 64bit (MPI over PAMI) MPICH2 library.

On older mpich 1.? (BG/P) it failed with 'too many communicators' and didn't hang, which is what they expected.

It seems like it's stuck in the while (*context_id == 0) loop repeatedly calling allreduce and never settling on a context id in commutil.c. I didn't do a lot of debug but seems like it's in vanilla mpich code, not something we modified.

ftmain.f90 fails if you run it on >2k ranks (creates one comm per rank). This was the original customer testcase.

ftmain2.f90 fails by looping so you can run on fewer ranks.

I just noticed that with --np 1, I get the 'too many communicators' from ftmain2. But --np 2 and up hangs.

stdout[0]: check_newcomm do-start 0 , repeat 2045 , total 2046 stderr[0]: Abort(1) on node 0 (rank 0 in comm 1140850688): Fatal error in PMPI_Comm_create: Other MPI error, error stack: stderr[0]: PMPI_Comm_create(609).........: MPI_Comm_create(MPI_COMM_WORLD, group=0xc80700f6, new_comm=0x1dbfffb520) failed stderr[0]: PMPI_Comm_create(590).........: stderr[0]: MPIR_Comm_create_intra(250)...: stderr[0]: MPIR_Get_contextid(521).......: stderr[0]: MPIR_Get_contextid_sparse(752): Too many communicators