linux-kernel - Re: [patch] CFS scheduler, -v7

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Message-ID: <20070503074552.GA14960@elte.hu>
Date:	Thu, 3 May 2007 09:45:52 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Al Boldi <a1426z@...ab.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [patch] CFS scheduler, -v7


* Al Boldi <a1426z@...ab.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), and it gets 
rectified quickly once they get over a few reschedules?

> 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.

	Ingo
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