On Mon, Aug 12, 2024 at 10:01 AM Paul Edmon <pedmon@cfa.harvard.edu> wrote:

Certainly a strange setup. I would probably talk with who ever is providing MPI for you and ask them to build it against Slurm properly. As in order to get correct process binding you definitely want to have it integrated properly with slurm either via PMI2 or PMIx. If you just use the bare hostlist, your ranks may not end up properly bound to the specific cores they are supposed to be allocated. So definitely proceed with caution and validate your ranks are being laid out properly, as you will be relying on mpirun/mpiexec to bootstrap rather than the scheduler.

-Paul Edmon-

On 8/12/2024 9:55 AM, Jeffrey Layton wrote:

Paul,

I tend not to rely on the MPI being built with Slurm :) I find that the systems I use haven't done that. :( I'm not exactly sure why, but that is the way it is :)

Up to now, using scontrol has always worked for me. However, a new system is not cooperating (it is running on the submittal host and not the compute nodes) and I'm trying to debug it. My first step was to check that the job was getting the compute nodes names (the list of nodes from Slurm is empty). This led to my question about the "canonical" way to get the hostlist (I'm checking using the hostlist and just relying on Slurm being integrated into the mpi - both don't work since the hostlist is empty).

It looks like there is a canonical way to do it as you mentioned. FAQ worthy? Definitely for my own Slurm FAQ. Others will decide if it is worthy for Slurm docs :)

Thanks everyone for your help!

Jeff

On Mon, Aug 12, 2024 at 9:36 AM Paul Edmon via slurm-users <slurm-users@lists.schedmd.com> wrote:

Normally MPI will just pick up the host list from Slurm itself. You just need to build MPI against Slurm and it will just grab it. Typically this is transparent to the user. Normally you shouldn't need to pass a host list at all. See: https://slurm.schedmd.com/mpi_guide.html

The canonical way to do it if you need to would be the scontrol show hostnames command against the $SLURM_JOB_NODELIST (https://slurm.schedmd.com/scontrol.html#OPT_hostnames). That will give you the list of hosts your job is set to run on.

-Paul Edmon-

On 8/12/2024 8:34 AM, Jeffrey Layton via slurm-users wrote:

Thanks! I admit I'm not that experienced in Bash. I will give this a whirl as a test.

In the meantime, let ask, what is the "canonical" way to create the host list? It would be nice to have this in the Slurm FAQ somewhere.

Thanks!

Jeff

On Fri, Aug 9, 2024 at 1:32 PM Hermann Schwärzler via slurm-users <slurm-users@lists.schedmd.com> wrote:

Hi Paul,

On 8/9/24 18:45, Paul Edmon via slurm-users wrote:
> As I recall I think OpenMPI needs a list that has an entry on each line,
> rather than one seperated by a space. See:
>
> [root@holy7c26401 ~]# echo $SLURM_JOB_NODELIST
> holy7c[26401-26405]
> [root@holy7c26401 ~]# scontrol show hostnames $SLURM_JOB_NODELIST
> holy7c26401
> holy7c26402
> holy7c26403
> holy7c26404
> holy7c26405
>
> [root@holy7c26401 ~]# list=$(scontrol show hostname $SLURM_NODELIST)
> [root@holy7c26401 ~]# echo $list
> holy7c26401 holy7c26402 holy7c26403 holy7c26404 holy7c26405

proper quoting does wonders here (please consult the man-page of bash).
If you try

echo "$list"

you will see that you will get

holy7c26401
holy7c26402
holy7c26403
holy7c26404
holy7c26405

So you *can* pass this around in a variable if you use "$variable"
whenever you provide it to a utility.

Regards,
Hermann

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