Select Vs Nodes difference


#1

Hi Team,

what is the difference between select and nodes.
In an HPC environment, when we are using the " select=1:ncpus=2" jobs are entering in queue states And reason is " Not enough free nodes available"
But, when we replace the resource with “nodes=1:ppn=2” jobs are running fine.

Can you please provide information on this behavior.

Thank You
Atul Yadav


#2

Hello Atul, those jobs should be almost exactly the same as far as the scheduler is concerned. The -lnodes syntax gets translated by PBS into select (and place) statement equivalent. The job’s schedselect attribute is what the scheduler actually looks at when considering a job to run, and the -lnodes=1:ppn=2" job should have ended up as “schedselect = 1:ncpus=2:mpiprocs=2”. The mpiprocs resource is not actually scheduled upon (it just impacts how the $PBS_NODEFILE gets written), so it is really should be the same as far as the scheduler is concerned.

The jobs would also differ in their place resource (-lselect would have defaulted to free, -lnodes to scatter), but since these are just single chunk jobs that should not have mattered.

Please share the full “qstat -f” output for one of each kind of the jobs you mention above, with the command run as root on the PBS server host (or as a different PBS manager, since schedselect does not show up to normal users in qstat -f output).

Thanks.