linux-kernel - Re: [announce] CFS-devel, performance improvements

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Date:	Fri, 14 Sep 2007 14:04:28 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>
Subject: Re: [announce] CFS-devel, performance improvements

Hi,

On Thu, 13 Sep 2007, Peter Zijlstra wrote:

> On Thu, 2007-09-13 at 18:50 +0200, Roman Zippel wrote:
> 
> > I never claimed to understand every detail of CFS, I can _guess_ what 
> > _might_ have been intended, but from that it's impossible to know for 
> > certain how important they are. Let's take this patch fragment:
> > 
> 
> 	delta_fair = se->delta_fair_sleep;
> 
> we slept that much
> 
> > -       /*
> > -        * Fix up delta_fair with the effect of us running
> > -        * during the whole sleep period:
> > -        */
> > -       if (sched_feat(SLEEPER_AVG))
> > -               delta_fair = div64_likely32((u64)delta_fair * load,
> > -                                               load + se->load.weight);
> 
> if we would have ran we would not have been removed from the rq and the
> weight would have been: rq_weight + weight
> 
> so compensate for us having been removed from the rq by scaling the
> delta with: rq_weight/(rq_weight + weight)
> 
> > -       delta_fair = calc_weighted(delta_fair, se);
> 
> scale for nice levels

AFAICT the compensation part is already done by the scaling part, without 
the load part it largely mirrors what __update_stats_wait_end() does, i.e. 
it gets the same time as other tasks, which have been on the rq.

bye, Roman
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