Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr_running

Mathieu Desnoyers
2010-09-13T20:56:31Z

* Mathieu Desnoyers (mathieu.desnoyers@efficios.com) wrote:
> * Ingo Molnar (mingo@elte.hu) wrote:
> >
> > * Mike Galbraith <efault@gmx.de> wrote:
> >
> > > On Sun, 2010-09-12 at 14:16 -0400, Mathieu Desnoyers wrote:
> >
> > > > Or am I missing your point ?
> > >
> > > Yes and no. I'm pondering the parent, but by the same token, the
> > > vfork child shouldn't be penalized either.
> > >
> > > Does your latency go down drastically if you turn START_DEBIT off?
> > > Seems like it should. Perhaps START_DEBIT should not start a task
> > > further right than rightmost. I've done that before.
> > >
> > > maximum latency: 19221.5 µs
> > > average latency: 5159.0 µs
> > > missed timer events: 0
> > >
> > > maximum latency: 43901.0 µs
> > > average latency: 8430.1 µs
> > > missed timer events: 0
> > >
> > > Turning it off here cut latency roughly in half (i've piddled vfork
> > > though, but not completely). Limiting child placement to no further
> > > right than rightmost should help quite a bit.
> >
> > Very interesting observation. Mathieu, mind testing Mike's suggestion
> > with wakeup-latency.c?
>
> Sure. this is with the smaller min_granularity:
>
> With START_DEBIT:
>
> maximum latency: 21111.1 µs
> average latency: 4188.2 µs
> missed timer events: 0
>
> Without:
>
> maximum latency: 6670.2 µs
> average latency: 1586.0 µs
> missed timer events: 0
>
> So yes, as expected, it makes a huge difference. This is because SIGEV_THREAD
> creates a new thread each time the timer fires, and newly created tasks are put
> at the end of the runqueue with START_DEBIT.
>
> However, removing START_DEBIT makes my Xorg feel less responsive (again, just my
> own impression). We might need a more suitable way to deal with forks than just
> putting the newly forked task at the end of the spread, but just putting it at
> the beginning of the spread does not seem to do well neither.
>
> One idea: we could temporarily tweak the nice value of both the parent and the
> child after a fork to a lower nice value, but only apply this for their first
> slice after the fork. The goal behind this is that their respective vruntime
> will increment faster in the first slice after the fork, so a fork bomb
> (worse-case) will end up running with a very very low nice level. With this
> measure in place, START_DEBIT might not be needed. Thoughts ?

A small note: Steven made me realize that when I say "low nice level" here, I
actually mean "high nice values". Less is more when we talk about nice levels.

Thanks,

Mathieu

>
> Thanks,
>
> Mathieu
>
> >
> > Thanks,
> >
> > Ingo
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com

--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.