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Interactive Slurm jobs

Interactive jobs allow a user to interact with applications on the compute nodes. With an interactive job, you request time and resources to work on a compute node directly, which is different to a batch job where you submit your job to a queue for later execution.

You can use two commands to create an interactive session: srun and salloc. Both of these commands take options similar to sbatch.

Using salloc

Using salloc, you allocate resources and spawn a shell used to execute parallel tasks launched with srun. For example, you can allocate 2 nodes for 30 minutes with the command

$ salloc --nodes=2 --account=<project> --partition=<partition> --time=00:30:00
salloc: Granted job allocation 123456
salloc: Waiting for resource configuration

Once the allocation is made, this command will start a shell on the login node. You can start parallel execution on the allocated nodes with srun.

$ srun --ntasks=32 --cpus-per-task=8 ./mpi_openmp_application

After the execution of your application ended, the allocation can be terminated by exiting the shell (exit).

When using salloc, a shell is spawned on the login node. If you want to obtain a shell on the first allocated compute node you can use srun --pty.

$ srun --cpu_bind=none --nodes=2 --pty bash -i

If you want to use an application with a GUI, you can use the --x11 flag with srun to enable X11 forwarding.

Using srun

For simple interactive session, you can use srun with no prior allocation. In this scenario, srun will first create a resource allocation in which to run the job. For example, to allocate 1 node for 30 minutes and spawn a shell

$ srun --account=<project> --partition=<partition> --time=00:30:00 --nodes=1 --pty bash

Using srun to check running jobs

Currently, ssh'ing to compute nodes is not allowed, but the srun command can be used to check in on a running job in the cluster. In this case, you need to give the job ID and possibly also the specific name of a compute node to srun.

This starts a shell where you can run any command on the first allocated node in a specific job:

$ srun --overlap --pty --jobid=<jobid> $SHELL

By default, you will be connected to the master node of your job which is the first node in your allocation and the one on which your batch script is executed.

If your job spans multiple nodes and you need to connect to a specific compute node, you can achieve this by adding the -w nid00XXXX option.

$ srun --overlap --pty --jobid=<jobid> -w nid00XXXX $SHELL

where nid00XXXX is the hostname of the node you wish to access. To obtain the list of allocated nodes for a specific job with ID <jobid>, you can utilize the following command:

$ sacct --noheader -X -P -oNodeList --jobs=<jobid>

GPUs resource specification

Depending on how you have specified the GPU resources allocated to your job, you may encounter the following error when attempting to attach to a running job:

srun: error: Unable to create step for job 4203072: Invalid generic 
              resource (gres) specification

This error arises when you specify the number of GPUs using the --gpus=<numgpus> option, i.e., the total number of GPUs to allocate to your job. This value is inherited from the job and when creating the interactive job step and lead to an error if there are fewer available GPUs on the node you try to access than what you specified with --gpus.

If you encounter the aforementioned error message, you have to specify a number of GPUs corresponding to the actual number of GPUs allocated on the node you try to access:

srun --overlap --pty --jobid=<jobid> --gpus=<numgpus> $SHELL