Re: [PATCH] proc: Add RLIMIT_RTTIME to /proc/<pid>/limits

["Michael Kerrisk" <mtk.manpages@googlemail.com>]

On Fri, Apr 11, 2008 at 11:01 AM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Fri, 2008-04-11 at 10:56 +0200, Michael Kerrisk wrote:
> > On Thu, Feb 28, 2008 at 5:21 PM, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> > >
> > > On Thu, 2008-02-28 at 16:12 +0100, Michael Kerrisk wrote:
> > > > Peter,
> > > >
> > > > Could you please provide some text describing RLIMIT_RTTIMEfor the
> > > > getrlimit.2 man page.
> > >
> > > The rlimit sets a timeout in [us] for SCHED_RR and SCHED_FIFO tasks.
> > > This time is measured between sleeps, so a schedule in RR or a
> > > preemption in either is not a sleep - the task needs to be dequeued and
> > > enqueued for the timer to reset.
> > >
> > > Upon reaching the cur limit we start giving SIGXCPU every second, upon
> > > reaching the hard limit we give SIGKILL - matching RLIMIT_CPU.
> > >
> > > Time is measured in tick granularity (for now).
> >
> > So I have another question: why is the granularity of this rlimit
> > microseconds?  On the one hand, specifying limits down at the
> > microsecond level seems (to my naive eye) unlikely to be useful.  (But
> > perhaps I have missed a thread where this was explained.)  On the
> > other hand, it means that on 32-bit the largest time limit we can set
> > is ~4000 seconds, and I wonder if there are scenarios where it might
> > be useful to have larger limits than that.
> >
> > Why not, for example, have a granularity of milliseconds?
>
> The us scale seemed the best fit in that it allows sub-ms granularity
> while still allowing for quite long periods too. I'd preferred ns scale
> as that is what we use throughout the scheduler where possible - but
> that seemed too restrictive at the high end.
>
> No real hard arguments either way.

I'm curious: what scenarios require sub-millisecond timeouts?



-- 
Michael Kerrisk
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