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Interactive Jobs

This page describes how to request resources for interactive jobs and how to use the allocated resources when working interactively.

There are various use cases for requesting interactive resources, e.g.

  • debugging: launching a job, tweaking the setup (e.g. compile options), launching job again, tweaking, …
  • interactive interfaces: inspecting a node, or when launching a GUI

Resource allocation with salloc

salloc creates a SLURM job allocation with the specified (of default) resources, including CPUs/GPUs, memory, walltime. Here 4 tasks on 1 node with 2GB (8GB total) and 1h is requested:`

bash-4.2$ hostname
bash-4.2$ salloc --nodes=1 --ntasks-per-node=4 --mem-per-cpu=2G --time=01:00:00
salloc: Pending job allocation 63752579
salloc: job 63752579 queued and waiting for resources
salloc: job 63752579 has been allocated resources
salloc: Granted job allocation 63752579

After submitting the salloc command the terminal will be blocked until the job gets granted. Then the session still persists on the login node submit01. Only when using srun commands are executed on the requested compute node. The task send with srun can run immediately, since the resources are allocated already.

Interactive shell session

An interactive shell session on a compute node can be established using

bash-4.2$ srun --pty bash 
srun: job 9173384 queued and waiting for resources
srun: job 9173384 has been allocated resources
bash-4.2$ hostname
The command is blocking until the resources are granted. The session then is established directly on the first compute node.


Please release resources immediately if not needed anymore, by using exit, Ctrl-d or scancel $SLURM_JOB_ID

Graphical Interfaces using X11 Forwarding


X11 forwarding is always going to be extremely slow. A command-line based approach is always preferred. In some situations, port-forwarding can be used to provide a graphical interface without the limitations of X11.


  • You must login to UBELIX with X11 forwarding enabled: ssh -Y <user> . Make this the default with ForwardX11 yes in ~/.ssh/config .
    • Password-less communication between all nodes within UBELIX. In order to make this possible, generate a new SSH key (without passphrase) on the login node (submit) and add the public key to ~/.ssh/authorized_keys :
      ssh-keygen -t rsa -b 4096 cat ~/.ssh/ >> ~/.ssh/authorized_keys
  • A X-Server on your local workstation, e.g.
    • MAC: Xquartz (X11 no longer included in macOS)
    • Windows: MobaXTerm or Xming

DO NOT reuse an existing SSH key for this purpose, i.e. do not copy an existing private key from your local machine to UBELIX.

With all the requirements in place can now submit an interactive job and export an X11 display on the allocated compute node, e.g:

srun -n 1 --x11 gnuplot ex_gnuplot.p

You can also use X11 forwarding with non interactive jobs adding the option

#SBATCH --x11
in your job script and using again srun --x11 to launch your application.