Array jobs

More often than not, problems involve running similar programs in parallel where there is no dependency or communication among the processes. Their results are thus combined as the final output, e.g. Monte Carlo simulations.

In Slurm context, job arrays are the way to run several instances of a single-threaded program, known as the embarrassingly parallel paradigm.


Array jobs allow you to parallelize your computations. They are used when you need to run the same job many times with only slight changes among the jobs. For example, you need to run 1000 jobs each with a different seed value for the random number generator. Or perhaps you need to apply the same computation to a collection of data sets. These can be done by submitting a single array job.

Slurm job array is a collection of jobs that are to be executed with identical parameters. This means that there is one single batch script that is to be run as many times as indicated by the --array directive, e.g.:

#SBATCH --array=0-4

creates an array of 5 jobs (tasks) with index values 0, 1, 2, 3, 4.

The array tasks are copies of the “master” batch script that are automatically submitted to Slurm. Slurm provides a unique environment variable SLURM_ARRAY_TASK_ID to each task which could be used for handling input/output files to each task.


You can alternatively pass the --array option as a command-line argument to sbatch.


When running array job you’re basically running identical copies of a single job. Thus it is increasingly important to know how your code behaves with respect to the file system:

  • Does it use libraries/environment stored in the work directory?
  • How much input data does it need?
  • How much output data does the job create?

For example, running an array job with hundreds of workers that uses a Python environment stored in the work disk can inadvertently cause a lot of filesystem load as there will be hundreds of thousands of file calls.

If you’re unsure how your job will behave, ask admins for help.

Your first array job

Let’s see a job array in action. Create a file with your favorite editor and name it as you wish. Let’s name it and write it as follows.

#SBATCH --job-name=slurm_env_var
#SBATCH --output=/scratch/work/%u/task_number_%A_%a.out
#SBATCH --array=0-15
#SBATCH --time=00:15:00
#SBATCH --mem=200
# You may put the commands below:

# Job step
srun echo "I am array task number" $SLURM_ARRAY_TASK_ID

Submitting the job script to Slurm with sbatch, you will get the message:

$ Submitted batch job 52608925

The jobid in the message is that of the “master” job - which is also used in the batch script using %A for the output files (with the slurm --output option). To create unique output files for each task in the job array, %a is used that is filled in with the array task ID.

Once the jobs are completed, the output files will be created in your work directory, with the help %u to determine your user name:

$ ls $WRKDIR
task_number_52608925_12.out  task_number_52608925_3.out   task_number_52608925_8.out  task_number_52608925_9.out
task_number_52608925_0.out   task_number_52608925_13.out  task_number_52608925_4.out
task_number_52608925_1.out   task_number_52608925_14.out  task_number_52608925_5.out
task_number_52608925_10.out  task_number_52608925_15.out  task_number_52608925_6.out
task_number_52608925_11.out  task_number_52608925_2.out   task_number_52608925_7.out

You can cat one of the files to see the output of each task:

$ cat task_number_52608925_5.out
I am array task number 5


If your current directory is your home directory, please remember to direct your results to your work directory. You can keep your scripts/source codes in you home directory since it is backed up daily and should keep your calculations and analyses on your work directory.

More examples

The following examples give you an idea on how to use job arrays for different use cases and how to utilize the $SLURM_ARRAY_TASK_ID environment variable.

Reading input files

In many cases, you would like to process several data files, that is, pass different input files to your code to be processed. This can be achieved by using $SLURM_ARRAY_TASK_ID envinronment variable.

You could utilize to process several data files. In this case, In the example below, the is used to change to the right directory, make the application read the correct input file, and to generate output in a unique directory. This script is submitted with sbatch

#SBATCH -n 1
#SBATCH -t 04:00:00
#SBATCH --mem-per-cpu=1G
#SBATCH --array=0-29

# Each array task runs the same program, but with a different input file.
# e.g. srun ./my_application -input input_data_$SLURM_ARRAY_TASK_ID

Hardcoding arguments in the batch script

One way to pass arguments to your code is by hardcoding them in the batch script you want to submit to Slurm.

Assume you would like to run the Pi estimation code for 5 different seed values, each for 2.5 million iterations. You could assign a seed value to each task in you job array and save each output to a file. Having calculated all estimations, you could take the average of all the Pi values to arrive at a more accurate estimate. An example of such a batch script is as follows.

#SBATCH --job-name=pi_estimation
#SBATCH --output=pi.out.log --open-mode=append
#SBATCH --array=0-4
#SBATCH --time=01:00:00
#SBATCH --mem=500
# Note that all jobs will write to the same file.  This makes less
# files, but will be hard to tell the outputs apart.


    0)  SEED=123 ;;
    1)  SEED=38  ;;
    2)  SEED=22  ;;
    3)  SEED=60  ;;
    4)  SEED=432 ;;

srun python ~/hpc-examples/slurm/ 2500000 --seed=$SEED > pi_$SEED.json

Save the script as e.g. and submit to Slurm:

$ sbatch
Submitted batch job 52655434

Once finished, 5 files will be created in your current directory each containing the Pi estimation; total number of iterations (sum of iteration per task); and total number of successes):

$ cat pi_22.json
{"successes": 1963163, "pi_estimate": 3.1410608, "iterations": 2500000}

Reading parameters from one file

Another way to pass arguments to your code via script is to save the arguments to a file and have your script read the arguments from it.

Drawing on the previous example, let’s assume you now want to run with different iterations. You can create a file, say iterations.txt and have all the values written to it, e.g.:

$ cat iterations.txt

You can modify the previous script to have it read the iterations.txt one line at a time and pass it on to Here, sed is used to get each line. Alternatively you can use any other command-line utility in its stead, e.g. awk. Do not worry if you don’t know how sed works - Google search and man sed always help. Also note that the line numbers start at 1, not 0.

#SBATCH -n 1
#SBATCH --output=pi.2.out.log --open-mode=append
#SBATCH --array=1-4
#SBATCH --time=01:00:00
#SBATCH --mem=500

iteration=`sed -n "${n} p" iterations.txt`      # Get n-th line (1-indexed) of the file
srun python ~/hpc-examples/slurm/ ${iteration} > pi_iter_${n}.json

You can additionally do this procedure in a more complex way, e.g. read in multiple arguments from a csv file, etc.

(Advanced) Grouping runs together in bigger chunks

If your jobs are many and too short (a few minutes), using array jobs may induce too much overhead in scheduling. Or you may simply have too many runs and creating too many array jobs again is not recommended.


A good target time for the array jobs would be approximately 30 minutes, so please try to combine your tasks so that each job would at least take this long.

The workaround is exploiting shell’s capabilities. For example, assume you want to run the Pi script with 50 different seed values. You could define a chunk size of 10 and 5 array jobs. Even with as little as 5 array jobs, you can run 50 simulations.

This method demands for more knowledge of shell scripting which will definitely be worth your while.

#SBATCH -n 1
#SBATCH --output=pi.3.out.log --open-mode=append
#SBATCH --array=1-5
#SBATCH --time=01:00:00
#SBATCH --mem=500

# Define and create a new directory (and an intermediate one) in your work directory
mkdir -p ${DIRECTORY}

indexes=`seq $((n*CHUNKSIZE)) $(((n + 1)*CHUNKSIZE - 1))`

for i in $indexes
    srun python ~/hpc-examples/slurm/ 1500000 --seed=$i > ${DIRECTORY}/pi_$i.json


The array indices need not be sequential, e.g. if you discover that after the array job is finished, the job task id’s 2 and 5 failed, you can relaunch just those jobs with --array=2,5. To do this, you can simply pass the --array option as a command-line argument to sbatch.


  1. Using the example from the interactive tutorial exercises, create a job array that calculates a combination of different iterations and seed values, and save them all to different files. Look at the outputs and see how you would combine them together. Advanced: average them all to arrive at a more accurate Pi.
  2. Make an array job that runs from interactive tutorial exercises with five different values of memory (5M, 50M, 100M, 200M, 500M) using one of the techniques above - this is the memory that the memory-hog script requests, not the requested from Slurm (each job in an array requests the same memory).
  3. Make a job array which runs every other index, e.g. the array can be indexed as 1, 3, 5…(sbatch manual page can be of help)
  4. Think about your typical work. How could you split your stuff into trivial pieces that can be run with array jobs?

What’s next?

See also

For more information, you can see the CSC guide on array jobs

Please check the quick reference when needed.

If you need more detailed information about running on Triton, see the main page Running programs on Triton.

The next tutorial is about GPU computing.