Interactive jobs

Sometimes you might want to test or debug a calculation interactively, but running interactively on the login node is discouraged and not an option.

Requesting an interactive job

Instead of running on a login node, you can ask the queue system to allocate compute resources for you, and once assigned, you can run commands interactively for as long as requested. The examples below are for devel jobs, but the procedure also holds for the other job types except optimist jobs.

On Saga:

$ salloc --ntasks=1 --mem-per-cpu=4G --time=00:30:00 --qos=devel --account=YourAccount

On Fram or Betzy:

$ salloc --nodes=1 --time=00:30:00 --qos=devel --account=YourAccount

This will allocate resources, and start a shell on a compute node. When you are done, simply exit the shell (exit, logout or ^D) to end the job.

The arguments to salloc (or srun) could be any arguments you would have given to sbatch when submitting a non-interactive job. However, --qos=devel is probably a good idea to avoid waiting too long in the queue.

Note that interactive jobs stop when you log out from the login node, so unless you have very long days in office (or elsewhere, for that matter), specifying more than 6-8 hours runtime is not very useful. An alternative is to start the job in a tmux session (see below).

Graphical user interface in interactive jobs

It is possible to run X commands, i.e., programs with a graphical user interface (GUI), in interactive jobs. This allows you to get graphical output back from your job running on a login node. (Note that currently, this has not been activated on Betzy.)

First, you must make sure that you have turned on X forwarding when logging in to the cluster. With ssh from a Linux or MacOS machine, you do this with the -Y flag, e.g.:

$ ssh -Y


$ ssh -Y

Check that the X forwarding works by running a graphical command like xeyes and verify that it sets up a window. (Note that due to network latency, it can take a long time to set up a window.)

To be able to run X commands in interactive jobs, add the argument --x11 (note the lowercase x) to salloc, like this:

On Saga:

$ salloc --ntasks=1 --mem-per-cpu=4G --time=00:30:00 --qos=devel --account=YourAccount --x11

On Fram:

$ salloc --nodes=1 --time=00:30:00 --qos=devel --account=YourAccount --x11

Running the shell or a command on the login node

For some applications (see for instance TotalView), it is preferrable to have the shell or a command running on the login node instead of on the compute node(s).

This can be achieved by just adding bash or the command to the end of the salloc command line, i.e.,

$ salloc <options> bash


$ salloc <options> <command>

Note that the shell will be running on the login node. That means that you must start all calculations with srun or mpirun or equivalent, to make sure they run on the allocated compute node(s).

Keeping interactive jobs alive

Interactive jobs stop when you disconnect from the login node either by choice or by internet connection problems. To keep a job alive you can use a terminal multiplexer like tmux.

tmux allows you to run processes as usual in your standard bash shell

You start tmux on the login node before you get a interactive Slurm session with srun and then do all the work in it. In case of a disconnect you simply reconnect to the login node and attach to the tmux session again by typing:

$ tmux attach

Or in case you have multiple session running:

$ tmux list-session
$ tmux attach -t SESSION_NUMBER

As long as the tmux session is not closed or terminated (e.g. by a server restart) your session should continue. One problem with our systems is that the tmux session is bound to the particular login server you get connected to. So if you start a tmux session on login-1 on SAGA and next time you get randomly connected to login-2 you first have to connect to login-1 again by:

$ ssh login-1

To log out a tmux session without closing it you have to press Ctrl-B (that the Ctrl key and simultaneously “b”, which is the standard tmux prefix) and then “d” (without the quotation marks). To close a session just close the bash session with either Ctrl-D or type exit. You can get a list of all tmux commands by Ctrl-B and the ? (question mark). See also this page for a short tutorial of tmux. Otherwise working inside of a tmux session is almost the same as a normal bash session.