[slurm-users] OpenMPI and Slurm clarification?

[Craig]

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.

Thanks.

[Pritchard Jr., Howard]

HI Craig,

 

If you run

 

srun –mpi=list

 

what does slurm report?

 

That will help in determining what argument you want to supply for the –mpi srun option.

 

Howard

[Craig]

srun: MPI types are...

srun: none

srun: openmpi

srun: pmix_v3

srun: pmi2

srun: pmix

but I'm not sure that tells me much about how I am supposed to be building OpenMPI?

[Pritchard Jr., Howard]

HI Craig,

 

Your use of the –with-pmix on the open mpi configure line is important.  Without  any args to this configure option open mpi configure will first check if there’s an external pmix which is newer than the one that is included in the openmpi release tarball.  If it is not, the internal pmix is built.

 

You can check in the config.log whether the internal PMix or an external one was used.

 

If you want to be extra careful, find the location of the PMIx v3 used to build the SLURM PMIx plugin, and then rebuild your open mpi 4.1.5 with

 

./configure … --with-pmix=path_to_pmix_used_for_slurm_pmix_plugin_build ….

 

But you may be okay without doing this.  You can check this by running your open mpi job with

 

srun –mpi=pmix_v3 -N2 foo

 

and see if it behaves as expected.

 

I’m not sure what the “openmpi” result from srun –mpi=list is about.

[Craig]

conf.log...

checking if user requested PMI support

result: no

checking if user requested internal PMIx support(yes)

result: no

checking for pmix.h in /usr

result: not found

checking for pmix.h in /usr/include

result: not found

WARNING: discovered external PMIx version is less than internal version 3.x

WARNING: using internal PMIx

So is looks like it used the internal version (which is what I was aiming for) and that's ok by me since it seems to be working, but if I'm really supposed to be using the same one that SLURM used then I'm gonna have to figure out a way to determine what that was/is.

[Pritchard Jr., Howard]

HI Craig,

 

Its not essential to use the pmix lib used to build the SLURM pmix plugin but it does reduce likelihood of problems.

I don’t know how, but there is some way that the admin installing SLURM can “name” the available pmix –mpi options.

For instance on one of our systems, the admin has built multiple variants of the pmix plugin:

 

MPI plugin types are...

       cray_shasta

       none

       pmi2

       pmix

specific pmix plugin versions available: pmix_v2,pmix_v3,pmix_v314,pmix_v4,pmix_v422

 

This naming convention has helped us with “decoupling” building of Open MPI from SLURM build, but does mean some coordination with the sys admins.

We’re using SLURM 22.05.6

 

Hope this helps,

[Craig]

Ok, thanks.  "Coordinating" with sys admins is problematic so I guess I'll just continue with the internal pmix and keep an eye out for problems. 

At least I know I'm not doing anything blatantly stupid.

[Paul Brunk]

Hi:

 

I’ve just started watching “the ABCs of OpenMPI” videos (they and slides are available at https://www.open-mpi.org/video/?category=general ).  I’ve not finished part 1 (of 3) yet, and already have seen very-well-explained material specifically about the building of OpenMPI, Slurm, and related libraries.

 

These are a fantastic resource!

 

--

Paul Brunk, system administrator

Advanced Computing Resource Center

Enterprise IT Svcs, the University of Georgia

 

 

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

 

https://slurm.schedmd.com/mpi_guide.html#open_mpi <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.