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

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Date:	Thu, 13 Sep 2007 19:06:05 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Roman Zippel <zippel@...ux-m68k.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Mike Galbraith <efault@....de>
Subject: Re: [announce] CFS-devel, performance improvements

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



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