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Re: OpenMPI 5.0 to be 32-bit only ?

Currently I'm favouring accepting a move to 64-bit OpenMPI as a fait
accompli as part of code cleanups for 5.X (post Bookworm), and Debian moving
to MPICH on at least 32-bit archs - I'd favour OpenMPI on 64-bit archs for
better incoming-code-and-compatability support.

I'd like to hear the case otherwise.
The case we should make is that "no one cares about 32-bit builds" from 
the starting post in the GitHub issue is not true for Debian.
We do care that it *builds*, even if it might not be actually used.
I've been making this point, mostly in the context of avoiding a future where no MPI is available on 32-bit
(and by implication, essentially forking Debian into a toy 32-bit world and a properly-supported 64-bit one).
[1] was about the benefits of switching the two architectures that were 
using MPICH to OpenMPI two years ago. The mentioned "makes packages like 
octave build" is due to sundials build depending on mpi-default-dev but 
requiring ompi-c.pc [2].

m68k and sh4 are building with nocheck, whether or not there might be
additional/different test failures in packages with MPICH is unknown.

Having different MPI implementations on different architectures again 
would be painful for us, especially if it would be on release architectures.

If it would be architecturally hard for upstream to continue supporting 
32-bit then that's how it is, otherwise the current status quo of 32-bit 
OpenMPI is good enough for us and a possible compromise might be if 
upstream says "32-bit patches are welcome" and requires an
configure flag when building for 32-bit archs.

Such a flag is possible and being debated.

The challenge is key dependencies will move on to being 64-bit only, too: PMIx, etc.

The point of going  64-bit only is to clean up data structures and remove technical debt: Hence 5.x will start a cleanup and removal of 32-bit code.

The next point release may work on 32-bit by just bypassing the compilation flag; ongoing support starts meaning more invasive patches need to be carried by us.

Best regards

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=853029#18
[2] https://buildd.debian.org/status/fetch.php?pkg=sundials&arch=m68k&ver=5.7.0%2Bdfsg-1~exp1&stamp=1626976038&raw=0

Alastair McKinstry, 
GPG: 82383CE9165B347C787081A2CBE6BB4E5D9AD3A5
ph: +353 87 6847928 e: alastair@sceal.ie, im: @sceal.ie:mckinstry

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