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Slurm advanced guide

Job array

Job arrays offer a mechanism for submitting and managing collections of similar jobs quickly and easily; job arrays with millions of tasks can be submitted in milliseconds (subject to configured size limits). All jobs must have the same initial options (e.g. size, time limit, etc.), however it is possible to change some of these options after the job has begun execution using the scontrol command specifying the JobID of the array or individual ArrayJobID.

Full documentation:


#SBATCH --array=1,2,4,8
#SBATCH --array=0,100:5 # equivalent to 5,10,15,20...
#SBATCH --array=1-50000%200 # 200 jobs max at the time

Example 1: Deal with a lot of files

#SBATCH --array=0-29  # 30 jobs for 30 fastq files


Example 2: Deal with a lot of parameters

#SBATCH --array=0-4  # 5 jobs for 5 different parameters

PARAMS=(1 3 5 7 9)
srun a_software ${PARAMS[$SLURM_ARRAY_TASK_ID]} a_file.ext

Job dependency

For example, there is a dependency between the fastqc result and multiqc

#SBATCH --array=0-29  # 30 jobs

$ sbatch
Submitted batch job 3161045

multiqc .
sbatch --dependency=afterok:3161045


  • --nodes/-N: number of nodes (default is 1)
  • --ntasks/-n: number of tasks (default is 1)
$ srun hostname
$ srun --ntasks 2 hostname
$ srun --nodes 2 --ntasks 2 hostname


#SBATCH --ntasks=3

for INPUT in "${INPUTS[@]}"; do
     srun --ntasks=1 fastqc $INPUT &

srun --ntasks=1 multiqc .


--ntasks=<number>, -n

Declaration of the number of parallel tasks (default 1)

For srun: Specify the number of tasks to run. Request that srun allocate resources for ntasks tasks. The default is one task per node, but note that the --cpus-per-task option will change this default. This option applies to job and step allocations.

For sbatch: sbatch does not launch tasks, it requests an allocation of resources and submits a batch script. This option advises the Slurm controller that job steps run within the allocation will launch a maximum of number tasks and to provide for sufficient resources. The default is one task per node, but note that the --cpus-per-task option will change this default.

--cpus-per-task=<ncpus>, --cpus, ``-c

Request a number of CPUs par tasks (default 1) (Cf ntask)

--nodes=<minnodes[-maxnodes]>, -N

Request that a minimum of minnodes nodes be allocated to this job (Cf ntask)

--ntasks vs --cpus-per-task

If it's still confusing, please have a look at this thread of discussion: here