Yes, if after seven years of MPI_NEIGHBOR_ALLTOALL, neither the user know whether their MPI library is wrong nor all implementors are sure how to implement this routine for 1 or 2 processes in cyclic Cartesian direction, then some wording is missing in the MPI standard. Best regards Rolf ----- Anthony Skjellum <[email protected]> wrote:

Rolf let’s open a Ticket

Anthony Skjellum, PhD 205-807-4968

...

On Sep 27, 2019, at 6:09 PM, Rolf Rabenseifner via mpi-forum <[email protected]> wrote:

Dear MPI collective WG,

you may try to resolve the problem with a maybe wrong MPI specification for MPI_NEIGHBOR_ALLTOALL/ALLGATHER

Dear MPI Forum member,

you may own/use an MPI implementation that implements MPI_NEIGHBOR_ALLTOALL/ALLGATHER with race conditions if #nprocs in one dimension is only 1 or 2 and periodic==true

The problem was reported as a bug of the OpenMPI library by Simone Chiochetti from DICAM at the University of Trento, but seems to be a bug in the MPI specification, or at least an advice to implementors is missing.

I produced a set of animated slides. Please look at them in presentation mode with animation.

Have fun with a problem that clearly prevents the use of MPI_NEIGHBOR_... routines with cyclic boundary condition if one wants to verify that mpirun -np 1 is doing the same as the sequential code.

Best regards Rolf

-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] . High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) . <neighbor_mpi-3_bug.pptx> _______________________________________________ mpi-forum mailing list [email protected] https://lists.mpi-forum.org/mailman/listinfo/mpi-forum

-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] . High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 . University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 . Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner . Nobelstr. 19, D-70550 Stuttgart, Germany . . . . (Office: Room 1.307) .