Running jobs

This page is intended for the user who is already familiar with the concepts of job scheduling and job scripts, and who wants guidance on submitting jobs to Compute Canada clusters. If you have not worked on a large shared computer cluster before, you should probably read What is a scheduler? first.

On Compute Canada clusters, the job scheduler is the Slurm Workload Manager. Comprehensive documentation for Slurm is maintained by SchedMD. If you are coming to Slurm from PBS/Torque, SGE, LSF, or LoadLeveler, you might find this table of corresponding commands useful.

Use sbatch to submit jobs

The command to submit a job is sbatch:

[someuser@host ~]$ sbatch simple_job.sh
Submitted batch job 123456

A minimal Slurm job script looks like this:

File : simple_job.sh

#!/bin/bash
#SBATCH --time=00:01:00
#SBATCH --account=def-someuser
echo 'Hello, world!'
sleep 30

Directives (or "options") in the job script are prefixed with #SBATCH and must precede all executable commands. All available directives are described on the sbatch manual page. Compute Canada policies require that you supply at least a time limit (--time) and an account name (--account) for each job. (See #Accounts and projects below.)

You can also specify directives as command-line arguments to sbatch. So for example,

[someuser@host ~]$ sbatch --time=00:30:00 simple_job.sh 

will submit the above job script with a time limit of 30 minutes.

Use squeue to list jobs

The squeue command lists pending and running jobs. Supply your username as an argument with -u to list only your own jobs:

[someuser@host ~]$ squeue -u $USER
      JOBID PARTITION      NAME     USER ST   TIME  NODES NODELIST(REASON)
     123456 cpubase_b  simple_j someuser  R   0:03      1 cdr234
     123457 cpubase_b  simple_j someuser PD             1 (Priority)

The ST column of the output shows the status of each job. The two most common states are "PD" for "pending" or "R" for "running". See the man page for more on selecting, formatting, and interpreting the squeue output.

Where does the output go?

By default the output is placed in a file named "slurm-", suffixed with the job ID number and ".out", e.g. slurm-123456.out, in the directory from which the job was submitted. You can use --output to specify a different name or location. Certain replacement symbols can be used in the filename, e.g. %j will be replaced by the job ID number. See sbatch for a complete list.

The following sample script sets a job name (which appears in squeue output) and sends the output to a file with a name constructed from the job name (%x) and the job ID number (%j).

File : name_output.sh

#!/bin/bash
#SBATCH --account=def-someuser
#SBATCH --time=00:01:00
#SBATCH --job-name=test
#SBATCH --output=%x-%j.out
echo 'Hello, world!'

Error output will normally appear in the same file as standard output, just as it would if you were typing commands interactively. If you want to send the standard error channel (stderr) to a separate file, use --error.

Accounts and projects

Every job must have an associated account name corresponding to a Compute Canada Resource Allocation Project, specified using the --account directive:

#SBATCH --account=def-user-ab

If you try to submit a job with sbatch without supplying an account name, you will be shown a list of valid account names to chose from. If you have access to several Resource Allocation Projects and want to know which account name corresponds to a given Resource Allocation Project, log in to CCDB and visit the page for that project. The second field in the display, the group name, is the string you should use with the --account directive. Note that a Resource Allocation Project may only apply to a specific cluster (or set of clusters) and therefore may not be transferable from one cluster to another.

In the illustration below, jobs which are to be accounted against RAP wnp-003-ac should be submitted with --account=def-rdickson-ac.

Examples of job scripts

MPI job

This example script launches four MPI processes, each with 1024 MB of memory. The run time is limited to 5 minutes.

File : mpi_job.sh

#!/bin/bash
#SBATCH --ntasks=4               # number of MPI processes
#SBATCH --mem-per-cpu=1024M      # memory; default unit is megabytes
#SBATCH --time=0-00:05           # time (DD-HH:MM)
srun ./mpi_program               # mpirun or mpiexec also work

One can have detailed control over the location of MPI processes by, for example, requesting a specific number of processes per node. Hybrid MPI/threaded jobs are also possible. For more on these and other options relating to distributed parallel jobs, see Advanced MPI scheduling.

Threaded or OpenMP job

This example script launches a single process with six CPU cores. Bear in mind that for an application to use OpenMP it must be compiled with the appropriate flag, e.g. gcc -fopenmp ... or icc -openmp ...

File : openmp_job.sh

#!/bin/bash
#SBATCH --time=0-0:5
#SBATCH --cpus-per-task=6
export OMP_NUM_THREADS=$SLURM_CPUS_PER_TASK
./ompHello

For more on writing and running parallel programs with OpenMP, see OpenMP.

GPU job

This example is a serial job with one GPU allocated, a memory limit of 4000 MB per node, and a run-time limit of 5 hours. The output filename will include the name of the first node used and the job ID number.

File : simple_gpu_job.sh

#!/bin/bash
#SBATCH --gres=gpu:1              # request GPU "generic resource"
#SBATCH --mem=4000M               # memory per node
#SBATCH --time=0-05:00            # time (DD-HH:MM)
#SBATCH --output=%N-%j.out        # %N for node name, %j for jobID
nvidia-smi

Because no node count is specified in the above example, one node will be allocated. If you were to add --nodes=3, the total memory allocated would be 12000M. The same goes for --gres: If you request three nodes, you will get one GPU per node, for a total of three.

For more on running GPU jobs, see Using GPUs with SLURM.

Array job

Also known as a task array, an array job is a way to submit a whole set of jobs with one command. The individual jobs in the array are distinguished by an environment variable, $SLURM_ARRAY_TASK_ID, which is set to a different value for each instance of the job.

sbatch --array=0-7 ...      # $SLURM_ARRAY_TASK_ID will take values from 0 to 7 inclusive
sbatch --array=1,3,5,7 ...  # $SLURM_ARRAY_TASK_ID will take the listed values
sbatch --array=1-7:2 ...    # Another way to do the same thing
sbatch --array=1-100%10 ... # Allow no more than 10 of the jobs to run simultaneously

Interactive jobs

Though batch submission is the most common and most efficient way to take advantage of our clusters, interactive jobs are also supported. These can be useful for things like:

• Data exploration at the command line
• Interactive "console tools" like R and iPython
• Significant software development, debugging, or compiling

You can start an interactive session on a compute node with salloc. In the following example we request two tasks, which corresponds to two CPU cores, for an hour:

[name@login ~]$ salloc --time=1:0:0 --ntasks=2 
salloc: Granted job allocation 1234567

Then we start a shell (bash) with timeout disabled (--wait 0):

[name@login ~]$ srun --wait 0 --pty bash
[name@node01 ~]$ ...             # do some work
[name@node01 ~]$ exit            # log out of the compute node (terminate srun)
[name@login ~]$ exit             # terminate the allocation
salloc: Relinquishing job allocation 1234567

For more details see Interactive jobs.

Monitoring jobs

By default squeue will show all the jobs the scheduler is managing at the moment. It may run much faster if you ask only about your own jobs with

squeue -u <username>

You can show only running jobs, or only pending jobs:

squeue -u <username> -t RUNNING
squeue -u <username> -t PENDING

You can show detailed information for a specific job with scontrol:

scontrol show job -dd <jobid>

Find information about a completed job with sacct, and optionally, control what it prints using --format:

sacct -j <jobid>
sacct -j <jobid> --format=JobID,JobName,MaxRSS,Elapsed

Use the MaxRSS accounting field to determine how much memory a job needed. The value returned will be the largest resident set size for any of the tasks. If you want to know which task and node this occurred on, print the MaxRSSTask and MaxRSSNode fields also.

The sstat command works on a running job much the same way that sacct works on a completed job.

You can ask to be notified by email of certain job conditions by supplying options to sbatch:

#SBATCH --mail-user=<email_address>
#SBATCH --mail-type=BEGIN
#SBATCH --mail-type=END
#SBATCH --mail-type=FAIL
#SBATCH --mail-type=REQUEUE
#SBATCH --mail-type=ALL

Cancelling jobs

Use scancel with the job ID to cancel a job:

 scancel <jobid>

You can also use it to cancel all your jobs, or all your pending jobs:

scancel -u <username>
scancel -t PENDING -u <username>

Troubleshooting

Avoid hidden characters in job scripts

Preparing a job script with a word processor instead of a text editor is a common cause of trouble. Best practice is to prepare your job script on the cluster using an editor such as nano, vim, or emacs. If you prefer to prepare or alter the script off-line, then:

• Windows users:
  • Use a text editor such as Notepad or Notepad++.
  • After uploading the script, use dos2unix to change Windows end-of-line characters to Linux end-of-line characters.
• Mac users:
  • Open a terminal window and use an editor such as nano, vim, or emacs.

Further reading

• Details on Job scheduling policies at Cedar and Graham.
• Comprehensive documentation is maintained by SchedMD, as well as some tutorials.
  • sbatch command options
• There is also a "Rosetta stone" mapping commands and directives from PBS/Torque, SGE, LSF, and LoadLeveler, to SLURM. NERSC also offers some tables comparing Torque and SLURM.
• Here is a text tutorial from CÉCI, Belgium
• Here is a rather minimal text tutorial from Bright Computing

Message Passing Interface