From: Al Boldi <a1426z@gawab.com>
To: Ingo Molnar <mingo@elte.hu>
Subject: Re: [patch] CFS scheduler, -v7
Date: Thu, 3 May 2007 11:42:02 +0300
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Ingo Molnar wrote:
> * Al Boldi <a1426z@gawab.com> wrote:
> > > i'm pleased to announce release -v7 of the CFS scheduler patchset.
> > > (The main goal of CFS is to implement "desktop scheduling" with as
> > > high quality as technically possible.)
> > >
> > >
> > > As usual, any sort of feedback, bugreport, fix and suggestion is
> > > more than welcome,
> >
> > This one seems on par with SD, [...]
>
> excellent :-)
>
> > [...] but there are still some nice issues.
> >
> > Try running 3 chew.c's, then renicing one to -10, starves others for
> > some seconds while switching prio-level.  Now renice it back to 10, it
> > starves for up to 45sec.
>
> ok - to make sure i understood you correctly: does this starvation only
> occur right when you renice it (when switching prio levels),

Yes.

> and it gets
> rectified quickly once they get over a few reschedules?

Well, depending on nice level this delay may be more than 45sec.

And, in cfs-v8 there is an additional repeating latency blip akin to an 
expiry when running procs at different nice levels.  chew.c shows this 
clearly.

> > Also, nice levels are only effective on every other step; ie:
> >  ... -3/-2 , -1/0 , 1/2 ... yields only 20 instead of 40 prio-levels.
>
> yeah - this is a first-approximation thing.
>
> Some background: in the upstream scheduler (and in SD) nice levels are
> linearly scaled, while in CFS they are exponentially scaled. I did this
> because i believe exponential is more logical: regardless of which nice
> level a task uses, if it goes +2 nice levels up then it will halve its
> "fair CPU share". So for example the CPU consumption delta between nice
> 0 and nice +10 is 1/32 - and so is the delta between -5 and +5, -10 and
> -5, etc. This makes nice levels _alot_ more potent than upstream's
> linear approach.

Actually, I think 1/32 for +10 is a bit to strong.  Introducing a scalefactor 
tunable may be useful.

Also, don't you think it reasonable to lower-bound the timeslices?


Thanks!

--
Al

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