[Mpi-forum] Datatypes for all-to-all collectives on intercomms

Michael Knobloch m.knobloch at fz-juelich.de
Tue Apr 30 04:07:52 CDT 2019

Hi all,

when looking at the semantics of all-to-all collectives on intercomms I
came to a point that isn't entirely clear from the standard: Am I'm
allowed to use different datatypes for each communication direction,
i.e. send MPI_INTEGERS from group A to group B and MPI_DOUBLES from B to A?

The standard says (page 168, line 38 for MPI_Alltoall, similar
statements for the other all-to-all collectives): The type signature
associated with sendcount, sendtype, at a process must be equal to
the type signature associated with recvcount, recvtype at any other
process. This implies that the amount of data sent must be equal to the
amount of data received, pairwise between every pair of processes. As
usual, however, the type maps may be different.

This however is only true for an intracommunicator as the statement only
holds for every pair of *communicating* processes, which in the case of
an intercommunicator is not every pair of processes (though the name
implies a different semantics). And the advice to users (page 169, line
23) contradicts that by saying that the counts may be different, but
says nothing about the datatypes.

Michael Knobloch
Juelich Supercomputing Centre
Institute for Advanced Simulation
Forschungszentrum Juelich GmbH
52425 Juelich, Germany

Phone:  +49-2461-61-3546
Fax:    +49-2461-61-6656
E-mail: m.knobloch at fz-juelich.de
WWW:    http://www.fz-juelich.de/jsc/

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