On 2023-11-23 12:13, Emilio Pozuelo Monfort wrote:
Hi, On 23/11/2023 09:36, Alastair McKinstry wrote:Hi,OpenMPI has a new upstream release 5.0.0. It is in experimental now; the SOVERSION for libraries remains 40.X (minor version increment), there is an SOVERSION increment for private libraries only so in theory this is not an ABI transition. However 5.0.0 drops 32-bit system support.The default MPI implementation for each architecture is set in mpi-defaults; this allows a per-arch MPI choice; in practice we currently use OpenMPI for all archs. The other choice is MPICH.So the question becomes: do we switch MPI for just 32-bit archs, or all? What are the release teams opinion on this transition?Having the same implementation across the board makes things easier for testing purposes et al, however I don't see that as a blocker for not having separate implementations.
True, in one sense it's simpler to have the same default MPI. But we've set up our package infrastructure so that in principle it should not matter. One architecture does not (or should not) depend on another, so it shouldn't break packages just because we'd have different MPI implementations on different architectures. On the contrary, "actively" using both implementations could lead to more robust packages overall as MPI bugs get fixed against both implementations.
What are your thoughts on it? Is there a strong reason why we should stick with OpenMPI for 64bit releases? Or from a different POV, what are the risks of changing the implementation? Introducing a different set of bugs?
One point to consider is that upstream developers of several of our numerical libraries have time and again suggested to us that we use mpich instead of openmpi, even before this v5 shift. They perceive (rightly or wrongly) that mpich is more robust, more reliable.
It would be useful to know whether that changes with v5, or whether their complaints are historical and openmpi has already fixed the bugs that concerned them. mpich has had its own share of bugs over the years. My memory told me RMA support was an issue in openmpi, but when I checked my facts, it was mpich that had to be fixed (https://github.com/pmodels/mpich/issues/6110)
Drew