SLURM - forcing MPI to schedule different ranks on different physical CPUs

I am running an experiment on an 8 node cluster under SLURM. Each CPU has 8 physical cores, and is capable of hyperthreading. When running a program with

#SBATCH --nodes=8
#SBATCH --ntasks-per-node=8

mpirun -n 64 bin/hello_world_mpi

it schedules two ranks on the same physical core. Adding the option

#SBATCH --ntasks-per-core=1

gives an error, SLURM saying "Batch job submission failed: Requested node configuration is not available". Is it somehow only allocating 4 physical cores per node? How can I fix this?

You can check the available CPU information in your cluster using sinfo -o%C.

I wasn't able to find any --ntasks-per-cpu for SBATCH in the documentation. You could try the following options for SBATCH --ntasks-per-core. As per documentation:

> --ntasks-per-core=<ntasks>
> Request the maximum ntasks be invoked on each core. Meant to be used with the --ntasks option. Related to --ntasks-per-node except at
> the core level instead of the node level. This option will be
> inherited by srun.

You could also try --cpus-per-task.
> c, --cpus-per-task=<ncpus>
> Advise the Slurm controller that ensuing job steps will require ncpus number of processors per task. Without this option, the
> controller will just try to allocate one processor per task.

Also please note:

> Beginning with 22.05, srun will not inherit the --cpus-per-task
> value requested by salloc or sbatch. It must be requested again with
> the call to srun or set with the SRUN_CPUS_PER_TASK environment
> variable if desired for the task(s).