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Interactive Job Sessions§

For an interactive session, you reserve some compute nodes via the scheduler and then are logged in live, just like on the login nodes. These can be used for live visualisation, software debugging, or to work up a script to run your program without having to submit each attempt separately to the queue and wait for it to complete.

Requesting Access§

You will be granted an interactive shell after running a command that checks with the scheduler whether the resources you wish to use in your tests/analysis are available. Interactive sessions are requested using the qrsh command. It typically takes the form:

qrsh -pe mpi 8 -l mem=512M,h_rt=2:00:00 -now no

In this example you are asking to run eight parallel processes within an MPI environment, 512MB RAM per process, for a period of two hours.

All job types we support on the system are supported via an interactive session (see our examples section). Likewise, all qsub options are supported like regular job submission with the difference that with qrsh they must be given at the command line, and not with any job script (or via -@).

In addition the -now option is useful when a cluster is busy. By default qrsh and qlogin jobs will run on the next scheduling cycle or give up. The -now no option tells it to keep waiting until it gets scheduled. Pressing Ctrl+C (i.e. the control key and the C key at the same time) will safely cancel the request if it doesn't seem to be able to get you a session.

More resources can be found here:

Interactive X sessions§

You can get an interactive X session from the head node of the job back to the login node. The way to do this is to run the qrsh command in the following generic fashion:

qrsh <options> <command> <arguments to <command>>

Where <command> is either a command to launch an X terminal like Xterm or Mrxvt or a GUI application like XMGrace or GaussView.

To make effective use of the X forwarding you will need to have logged in to the login node with ssh -X or some equivalent method. Here is an example of how you can get a X terminal session with the qrsh command:

qrsh -l mem=512M,h_rt=0:30:0 \
   "/shared/ucl/apps/mrxvt/0.5.4/bin/mrxvt -title 'User Test Node'"

Working on the nodes§

If you want to run a command on one of your other allocated nodes, you can use a standard ssh command from the interactive session: 

ssh <hostname> <command> [args]

to access other nodes within your allocation. Note that you are not able to ssh directly from the login node.

In the above, <hostname> can be obtained by inspecting the file $TMPDIR/machines.

GPU test nodes§

You can also run GPU jobs interactively simply by adding the -l gpu=1 or -l gpu=2 options to the qrsh command as normal.

For more information, please contact us on rc-support@ucl.ac.uk