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