[slurm-users] Use a portion of resources already allocated for a script

[Michael Lamparski]

Hello all,

For years I've been looking for what I might consider to be the holy grail
of composable resource allocation in slurm jobs:

* A command that can be run inside of an sbatch script...
* ...which immediately and synchronously invokes another sbatch script
(which may or may not invoke mpirun in turn)...
* ...using a subset of the currently allocated resources.

This is the smallest unit of functionality that would compose well with
existing tools in UNIX for orchestration.  For instance, I could use xargs
as a semaphore to let each node work on one input at a time, and for a
given input I could have an arbitrarily complex python script decide
dynamically what computations to run.

Years of Google and manpage searches have continually failed me.

* salloc can synchronously run an sbatch script, but as far as I can tell,
it cannot make job steps, only jobs.
* srun can run sychronously and make job steps, but as far as I can tell,
it cannot call a script which calls mpirun (it insists on *replacing*
mpirun)
* Today I discovered that sbatch can also create job steps (albeit
awkwardly, via --jobid), and it can obviously run sbatch scripts... but as
far as I can tell, it cannot run synchronously!

One can't help but wonder whether this is a deliberate omission or just
criminal oversight!

Today I snapped and started working on a synchronous wrapper around sbatch;
the plan is to use --jobid=$SLURM_JOB_ID, find out the job step id
(somehow...), and then sattach to it.  I say this knowing it'll probably
make your skin crawl.  And I ask: What do you think I ought to do instead?

Michael
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