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March 2021

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ECP Community BOF Days: MPICH for Exascale: Supporting MPI-4 and ECP
by Raffenetti, Kenneth J. 24 Mar '21

24 Mar '21

Hi everyone, The MPICH team will host a Birds of a Feather session during next week's ECP Community BoF Days. This virtual event will run similar to our annual MPICH BoF at Supercomputing. Come join us to learn about developments in MPICH and its derivatives. Registration is free! Wednesday, March 31st 1:00pm Eastern Registration link: https://exascaleproject.zoomgov.com/meeting/register/vJItduqrpjMsGGTTf2_YAV… More info: https://github.com/pmodels/mpich/wiki/2021-ECP-Community-BoF Best regards, The MPICH Team

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by Lisandro Dalcin 08 Mar '21

08 Mar '21

I'm entangled in a discussion with Open MPI folks about whether the struct datatype constructor should or should not add padding depending on the oldtypes having or not explicit lb/ub markers. https://github.com/open-mpi/ompi/issues/8560 Please see the attached example. It basically creates a simple struct datatype with type signature {unsigned short, unsigned char} in different ways (which I would expect to be all equivalent). Running it with latest MPICH 3.4.1 $ for i in `seq 0 5`; do mpicc -DCASE=$i align.c; ./a.out; done c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 For Open MPI, up to v4.0.x, that was the case, too. However, with latest Open 4.1.0, now I get: $ for i in `seq 0 5`; do mpicc -DCASE=$i align.c; ./a.out; done c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=4 c-sizeof=4 mpi-extent=3 c-sizeof=4 mpi-extent=2 Open MPI folks claim that their behavior is correct and in line with the MPI standard. Maybe they have a point regarding the ub/lb definition in page 105 of MPI-3.1, despite such behavior being quite unintuitive. On the other hand, MPICH's struct datatype constructor always adjusts the ub to account for padding. Open MPI obviously don't. I don't see any wording in the standard where this could be implementation-defined behavior. If Open MPI folks are right, then MPICH is wrong, or the other way around. But we cannot have the two major competing implementations disagree on this. Regards, -- Lisandro Dalcin ============ Senior Research Scientist Extreme Computing Research Center (ECRC) King Abdullah University of Science and Technology (KAUST) http://ecrc.kaust.edu.sa/

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