[slurm-users] ntasks and cpus-per-task

[Miguel Gutiérrez Páez]

Hi,

That was just I thought, ntasks for mpi and cpus-per-task for
multithreading.
So, for example, if every node has 24 cores, is there any difference
between these commands?

sbatch --ntasks 24 [...]
sbatch --ntasks 1 --cpus-per-task 24 [...]

regards.

El jue., 22 feb. 2018 a las 9:52, Loris Bennett (<loris.bennett at fu-berlin.de>)
escribió:

> Hi Chris,
>
> Christopher Samuel <chris at csamuel.org> writes:
>
> > On 22/02/18 18:49, Miguel Gutiérrez Páez wrote:
> >
> >> What's the real meaning of ntasks? Has cpus-per-task and ntasks the
> >> same meaning in sbatch and srun?
> >
> > --ntasks is for parallel distributed jobs, where you can run lots of
> > independent processes that collaborate using some form of communication
> > between the processes (usually MPI for HPC).
> >
> > So inside your batch script you would use "srun" to start up the tasks.
> >
> > However, unless you code is written to make use of that interface then
> > it's not really going to help you, and so for any multithreaded
> > application you need to use --cpus-per-task instead.
>
> [snip (11 lines)]
>
> But does it make any difference for a multithreaded program if I have
>
>   #SBATCH --ntasks=4
>   #SBATCH --nodes=1-1
>
> rather than
>
>   #SBATCH --ntasks=1
>   #SBATCH --cpus-per-task=4
>
> Up to now I have only thought of --cpus-per-task in connection with
> hybrid MPI/OpenMP jobs, which we don't actually have.  Thus I tend to
> tell users to think always in terms of tasks, regardless of whether
> these are MPI processes or just threads.
>
> One downside of my approach is that if the user forgets to specify
> --nodes and --ntasks is greater than 1, non-MPI jobs can be assigned to
> multiple nodes.
>
> Cheers,
>
> Loris
>
> --
> Dr. Loris Bennett (Mr.)
> ZEDAT, Freie Universität Berlin         Email loris.bennett at fu-berlin.de
>
>
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