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

Guillaume Perrault Archambault gperr050 at uottawa.ca
Fri Aug 30 18:58:05 UTC 2019

Hi Paul,

Thanks for your pointers.

I'll looking into QOS and MCS after my paper deadline (Sept 5). Re QOS, as
expressed to Peter in the reply I just now sent, I wonder if it the QOS of
a job can be change while it's pending (submitted but not yet running).


On Fri, Aug 30, 2019 at 10:24 AM Paul Edmon <pedmon at cfa.harvard.edu> wrote:

> A QoS is probably your best bet.  Another variant might be MCS, which
> you can use to help reduce resource fragmentation.  For limits though
> QoS will be your best bet.
> -Paul Edmon-
> On 8/30/19 7:33 AM, Steven Dick wrote:
> > It would still be possible to use job arrays in this situation, it's
> > just slightly messy.
> > So the way a job array works is that you submit a single script, and
> > that script is provided an integer for each subjob.  The integer is in
> > a range, with a possible step (default=1).
> >
> > To run the situation you describe, you would have to predetermine how
> > many of each test you want to run (i.e., you coudln't dynamically
> > change the number of jobs that run within one array)., and a master
> > script would map the integer range to the job that was to be started.
> >
> > The most trivial way to do it would be to put the list of regressions
> > in a text file and the master script would index it by line number and
> > then run the appropriate command.
> > A more complex way would be to do some math (a divide?) to get the
> > script name and subindex (modulus?) for each regression.
> >
> > Both of these would require some semi-advanced scripting, but nothing
> > that couldn't be cut and pasted with some trivial modifications for
> > each job set.
> >
> > As to the unavailability of the admin ...
> > An alternate approach that would require the admin's help would be to
> > come up with a small set of alocations (e.g., 40 gpus, 80 gpus, 100
> > gpus, etc.) and make a QOS for each one with a gpu limit (e.g.,
> > maxtrespu=gpu=40 ) Then the user would assign that QOS to the job when
> > starting it to set the overall allocation for all the jobs.  The admin
> > woudln't need to tweak this except once, you just pick which tweak to
> > use.
> >
> > On Fri, Aug 30, 2019 at 2:36 AM Guillaume Perrault Archambault
> > <gperr050 at uottawa.ca> wrote:
> >> Hi Steven,
> >>
> >> Thanks for taking the time to reply to my post.
> >>
> >> Setting a limit on the number of jobs for a single array isn't
> sufficient because regression-tests need to launch multiple arrays, and I
> would need a job limit that would take effect over all launched jobs.
> >>
> >> It's very possible I'm not understand something. I'll lay out a very
> specific example in the hopes you can correct me if I've gone wrong
> somewhere.
> >>
> >> Let's take the small cluster with 140 GPUs and no fairshare as an
> example, because it's easier for me to explain.
> >>
> >> The users, who all know each other personally and interact via chat,
> decide on a daily basis how many jobs each user can run at a time.
> >>
> >> Let's say today is Sunday (hypothetically). Nobody is actively
> developing today, except that user 1 has 10 jobs running for the entire
> weekend. That leaves 130 GPUs unused.
> >>
> >> User 2, whose jobs all run on 1 GPU decides to run a regression test.
> The regression test comprises of 9 different scripts each run 40 times, for
> a grand total of 360 jobs. The duration of the scripts vary from 1 and 5
> hours to complete, and the jobs take on average 4 hours to complete.
> >>
> >> User 2 gets the user group's approval (via chat) to use 90 GPUs (so
> that 40 GPUs will remain for anyone else wanting to work that day).
> >>
> >> The problem I'm trying to solve is this: how do I ensure that user 2
> launches his 360 jobs in such a way that 90 jobs are in the run state
> consistently until the regression test is finished?
> >>
> >> Keep in mind that:
> >>
> >> limiting each job array to 10 jobs is inefficient: when the first job
> array finishes (long before the last one), only 80 GPUs will be used, and
> so on as other arrays finish
> >> the admin is not available, he cannot be asked to set a hard limit of
> 90 jobs for user 2 just for today
> >>
> >> I would be happy to use job arrays if they allow me to set an
> overarching job limit across multiple arrays. Perhaps this is doable.
> Admttedly I'm working on a paper to be submitted in a few days, so I don't
> have time to test jobs arrays thoroughly, but I will try out job arrays
> more thoroughly once I've submitted my paper (ie after sept 5).
> >>
> >> My solution, for now, is to not use job arrays. Instead, I launch each
> job individually, and I use singleton (by launching all jobs with the same
> 90 unique names) to ensure that exactly 90 jobs are run at a time (in this
> case, corresponding to 90 GPUs in use).
> >>
> >> Side note: the unavailability of the admin might sound contrived by
> picking Sunday as an example, but it's in fact very typical. The admin is
> not available:
> >>
> >> on weekends (the present example)
> >> at any time outside of 9am to 5pm (keep in mind, this is a cluster used
> by students in different time zones)
> >> any time he is on vacation
> >> anytime the he is looking after his many other responsibilities.
> Constantly setting user limits that change on a daily basis would be too
> much too ask.
> >>
> >>
> >> I'd be happy if you corrected my misunderstandings, especially if you
> could show me how to set a job limit that takes effect over multiple job
> arrays.
> >>
> >> I may have very glaring oversights as I don't necessarily have a big
> picture view of things (I've never been an admin, most notably), so feel
> free to poke holes at the way I've constructed things.
> >>
> >> Regards,
> >> Guillaume.
> >>
> >>
> >> On Fri, Aug 30, 2019 at 1:22 AM Steven Dick <kg4ydw at gmail.com> wrote:
> >>> This makes no sense and seems backwards to me.
> >>>
> >>> When you submit an array job, you can specify how many jobs from the
> >>> array you want to run at once.
> >>> So, an administrator can create a QOS that explicitly limits the user.
> >>> However, you keep saying that they probably won't modify the system
> >>> for just you...
> >>>
> >>> That seems to me to be the perfect case to use array jobs and tell it
> >>> how many elements of the array to run at once.
> >>> You're not using array jobs for exactly the wrong reason.
> >>>
> >>> On Tue, Aug 27, 2019 at 1:19 PM Guillaume Perrault Archambault
> >>> <gperr050 at uottawa.ca> wrote:
> >>>> The reason I don't use job arrays is to be able limit the number of
> jobs per users
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