[slurm-users] ticking time bomb? launching too many jobs in parallel

[Guillaume Perrault Archambault]

Hi Paul,

Thanks a lot for your suggestion.

The cluster I'm using has thousands of users, so I'm doubtful the admins
will change this setting just for me. But I'll mention it to the support
team I'm working with.

I was hoping more for something that can be done on the user end.

Is there some way for the user to measure whether the scheduler is in RPC
saturation? And then if it is, I could make sure my script doesn't launch
too many jobs in parallel.

Sorry if my question is too vague, I don't understand the backend of the
SLURM scheduler too well, so my questions are using the limited terminology
of a user.

My concern is just to make sure that my scripts don't send out more
commands (simultaneously) than the scheduler can handle.

For example, as an extreme scenario, suppose a user forks off 1000 sbatch
commands in parallel, is that more than the scheduler can handle? As a
user, how can I know whether it is?

Regards,
Guillaume.



On Mon, Aug 26, 2019 at 10:15 AM Paul Edmon <pedmon at cfa.harvard.edu> wrote:

> We've hit this before due to RPC saturation.  I highly recommend using
> max_rpc_cnt and/or defer for scheduling.  That should help alleviate this
> problem.
>
> -Paul Edmon-
> On 8/26/19 2:12 AM, Guillaume Perrault Archambault wrote:
>
> Hello,
>
> I wrote a regression-testing toolkit to manage large numbers of SLURM jobs
> and their output (the toolkit can be found here
> <https://github.com/gobbedy/slurm_simulation_toolkit/> if anyone is
> interested).
>
> To make job launching faster, sbatch commands are forked, so that numerous
> jobs may be submitted in parallel.
>
> We (the cluster admin and myself) are concerned that this may cause
> unresponsiveness for other users.
>
> I cannot say for sure since I don't have visibility over all users of the
> cluster, but unresponsiveness doesn't seem to have occurred so far. That
> being said, the fact that it hasn't occurred yet doesn't mean it won't in
> the future. So I'm treating this as a ticking time bomb to be fixed asap.
>
> My questions are the following:
> 1) Does anyone have experience with large numbers of jobs submitted in
> parallel? What are the limits that can be hit? For example is there some
> hard limit on how many jobs a SLURM scheduler can handle before blacking
> out / slowing down?
> 2) Is there a way for me to find/measure/ping this resource limit?
> 3) How can I make sure I don't hit this resource limit?
>
> From what I've observed, parallel submission can improve submission time
> by a factor at least 10x. This can make a big difference in users'
> workflows.
>
> For that reason I would like to keep the option of launching jobs
> sequentially as a last resort.
>
> Thanks in advance.
>
> Regards,
> Guillaume.
>
>
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