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There are two steps involved in writing a job script to run an OpenMP (OMP) job in SLURM:

1. Specify the number of cores to reserve for your OMP job

2. Communicate that information to OpenMP by defining the environment variable OMP_NUM_THREADS

 There are several ways to reserve a given number of CPU cores in SLURM. For example

number of physical CPU cores = number of nodes X * number of tasks-per-node X * number of cpus-per-task  

In a strictly OMP job (for hybrid jobs see the How-to Submit a Hybrid Job article) the number of nodes will always be 1.  We will leave the number of tasks-per-node set to its default value of 1 as this option is best reserved for controlling the number of MPI processes.  So the number of cpus-per-task will control the number of physical CPU cores reserved for our job.  Most nodes on Talapas have 28 physical CPU cores per node (dual E5-2690v4 Intel chips).  Let's reserve all of them for our job:

#!/bin/bash
#SBATCH --partition=short   ### Partition
#SBATCH --job-name=HelloOMP ### Job Name
#SBATCH --time=00:10:00     ### WallTime
#SBATCH --nodes=1           ### Number of Nodes
#SBATCH --ntasks-per-node=1 ### Number of tasks (MPI processes)
#SBATCH --cpus-per-task=28  ### Number of threads per task (OMP threads)
#SBATCH --account=hpcrcf    ### Account used for job submission

export OMP_NUM_THREADS=$SLURM_CPUS_PER_TASK

./hello_omp

Note that we define the value of OMP_NUM_THREADS to be equal to the value of the SLURM defined environment variable $SLURM_CPUS_PER_TASK. By defining OMP_NUM_THREADS in the this way we ensure that any change to the parameter --cpus-per-task in our script will automatically be communicated to OMP.

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