The cluster is running SLURM's default scheduler which is very (maybe too) simple.

• first-in first-out queue.
• Simple round-robin node selection.

The scheduler does not check which nodes are busy and try to avoid them.

This has an advantage in that it tends to leave some nodes empty for people who want a whole node.

You can use the -w option to select a specific node. Actually it asks for “at least” the nodes in the node list you specify. So a command like:

sbatch -n 20 -w node2 my_script

Would get you some cores on node2 and some on another node (since there are only 16 cores total on node2). If there were no cores free on node2 the job would be queued until some became available.

Note that using the -w option with multiple nodes is not a way of queueing jobs on just those nodes: it will actually allocate codes across all nodes you specify and run the job on just the first on them.

You can use the -x option to avoid specific nodes. A list of node names looks like this:

node[1-4,7,11]

Read as “nodes 1 to 4, 7 and 11” i.e. 1,2,3,4,7,11.

You can use -c 16 to request all cores on a (standard) node.

You can use the –exclusive option to ask for exclusive access to all the nodes your job is allocated. This is especially useful if you have a program which attempts to use all the cores it finds. Please only use it if you need it.