[slurm-users] OpenMPI and Slurm clarification?

Craig cfreese at super.org
Mon Mar 27 18:28:08 UTC 2023

Can someone please clarify the "best practices" for building OpenMPI 
compatible with Slurm?

https://slurm.schedmd.com/mpi_guide.html#open_mpi tells me what I _can_ 
do but I'm unclear as to what I _should_ do.

I've built OpenMPI 4.1.5 with:   --with-pmix --with-libevent=internal  
--with-hwloc=internal  --with-slurm.  If I run an MPI program on my 
cluster (slurm 18.08.8) with "srun -N2 foo" it seems to work fine. 
(slurm.conf has MpiDefault=pmix).

If I "srun --mpi=openmpi -N2 foo" it chokes with:

    OPAL_ERROR: Unreachable in file
    ../../../../../opal/mca/pmix/pmix3/pmix3x_client.c at line 112
    This application appears to have been direct launched using "srun",
    but OMPI was not build with SLURM's PMI support and therefore cannot
    execute.  There are several options for building PMI support under
    SLURM, depending upon the SLURM version you are using:

    version 16.05 or later: you can use SLURM's PMIx support. THis
    require that you configure and uild SLURM --with-pmix.

So I guess the question is, what is the "right" way to build OpenMPI 
with Slurm.  Is the fact that my non-Slurm pmix works "correct" or am I 
just getting lucky that the various software I have just happens to be 
compatible.  If I build OpenMPI am I supposed to use Slurm's 
pmix/libevent/hwloc or is that optional.  If it's optional when/why 
might I choose to do so.  If I need Slurm's versions is there some way 
to find which pmix/libevent/hwloc my current Slurm install is using? 
Note: my sysadmins are not going to be helpful as they think Slurm 18 
and OpenMPI 4.0.2a is adequate for users' needs :^(.

I like the idea of _not_ tying my OpenMPI to the installed Slurm just in 
case our support people ever decide to upgrade system software.


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