A common abstraction layer for MPI

[LecrisUT]

I am wondering if you know of an equivalent project to flexi-blas but for MPI. Something that would generalize the building process such that you only need to build once instead of all mpi variants (+serial), and then can be dynamically loaded via a module load for example.

If not, what are the roadblocks to developing this? Looking at the mpicc compiler there doesn't seem to do anything fancy at the end of it other than adding -lmpi.

[hzhou]

Traditionally MPI only defines API rather than ABI. So the two major implementation - MPICH and Open MPI - are not binary compatible. There are projects to address this. For example, the MPItrampoline project. However, that essentially creates a third MPI ABI.

MPI 5 introduced an ABI standard. MPICH already supports MPI ABI via an option (--enable-mpi-abi). Open MPI is currently working on it (open-mpi/ompi#13280). So hopefully in a year or two, all MPI new releases will be ABI compatible.

[LecrisUT]

There are projects to address this. For example, the MPItrampoline project. However, that essentially creates a third MPI ABI.

A third mpi ABI is not an issue, it's also how flexiblas works. As long as it can redirect all of its calls to the selected implementation then that's all good.

So hopefully in a year or two, all MPI new releases will be ABI compatible.

Very cool. Would be a nightmare to figure out which MPI to use to build. But what about the serial case? We would like it if that can be covered as well. As in if no mpi library is found or a dummy one is loaded then tun it as if it was built for serial

[hzhou]

But what about the serial case? We would like it if that can be covered as well. As in if no mpi library is found or a dummy one is loaded then tun it as if it was built for serial

Most MPI application won't run properly if the MPI turned into dummy one to my knowledge. But where is a prayer, there is an answer! This project - https://github.com/mpi-forum/mpi-abi-stubs - is part of MPI ABI standardization effort, provides a dummy library one can build against. It won't be functional but it can be later ABI swapped with another ABI-compatible implementation.

[Growl1234]

There is a same concern from Debian side: open-mpi/ompi#14223 (comment).

[ocaisa]

I've actually made a pass at implementing this recently based off of ideas from https://github.com/eschnett/MPItrampoline . Still a WIP, but basic idea seems to work: https://github.com/ocaisa/trampi