Loris, It’s simple, tell folks only to use -n for mpi jobs, and -c otherwise (default).
It’s a big deal if folks use -n when it’s not an mpi program. This is because the non mpi program is launched n times (instead of once with internal threads) and will stomp over logs and output files (uncoordinated) leading to poor performance and incorrect results. Best, Chris > On Feb 22, 2018, at 1:52 AM, Loris Bennett <loris.benn...@fu-berlin.de> wrote: > > Hi Chris, > > Christopher Samuel <ch...@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.benn...@fu-berlin.de >
