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scrontab (Slurm crontab)

Traditional cron functionality has been replaced at NERSC with the Slurm crontab tool called scrontab. This combines the same functionality as cron with the resiliency of the batch system. Jobs are run on a pool of nodes, so unlike with regular cron, a single node going down won't keep your scrontab job from running. You can also find and modify your scrontab job on any login node.

You can edit your scrontab script with

scrontab -e

once you save your script, it will automatically be scheduled by the batch system. By default, vi is the editor for scrontab, if you desire a different editor, you can set the EDITOR environment variable (e.g. export EDITOR=/global/common/software/nersc/bin/emacs).

You can view your existing scripts with

scrontab -l

Example Scrontab Script

Each script should includes traditional Slurm flags like -A and -t. Here's an example scrontab script that will run every three hours (note the #SCRON --open-mode=append line which will tell slurm to append any new output to the output file):

#SCRON -q cron
#SCRON -A <account>
#SCRON -t 00:30:00
#SCRON -o output-%j.out
#SCRON --open-mode=append
0 */3 * * * <full_path_to_your_script>

Scrontab times are in UTC

Currently scrontab times on Perlmutter are in UTC.

Long-Running Scrontab Jobs

Projects often need long running processes to manage their work at NERSC (e.g. a listener process to facilitate external data movement). For now we are supporting these via the workflow queue which allows a much longer run time. However, jobs in this queue may get interrupted by maintenances or login nodes going offline. Since it's generally desirable to have these jobs restart as soon as possible, we recommend that you set the start up time to be fairly frequent (i.e. once an hour) and add the singleton flag to that scrontab's flags:

#SCRON --qos=workflow
#SCRON --account=<account>
#SCRON --time=30-00:00:00
#SCRON --dependency=singleton
#SCRON --name=my_data_movement_helper
0 * * * * <full_path_to_your_script>

This means slurm will check every hour whether an instance of your job is running, and if not, it will start it.

Use singleton for long running jobs

You must use --dependency=singleton for long running jobs to avoid slurm starting multiple instances of the same job every time your scrontab file is edited.

Monitoring Your Scrontab Jobs

You can monitor your scrontab jobs with

squeue --me -q cron -O JobID,EligibleTime

This will show the next time the batch system will run your job. If the scrontab job is set to repeat, the system will automatically reschedule the next job. Additionally, if you modify your scrontab job, slurm will automatically cancel the old job and resubmit an new one.

Canceling a Scrontab job

To remove a scontab job from your running jobs you can edit the scontab file with scrontab -e and comment out all the lines associated with the entry.

Using scancel on a scontab job

The scancel command will give a warning when attempting to remove a job started with scrontab. 

perlmutter$ scancel 555
scancel: error: Kill job error on job id 555: Cannot scancel a scrontab job without the --hurry flag, or modify scrontab jobs through scontrol
By canceling a scontab job with the --hurry flag, the entry in the scrontab file will be prepended with #DISABLED. These comments will needs to be removed before the job will be able to start again.