linux-kernel - Re: [RFC] scheduler: improve SMP fairness in CFS

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Date:	Wed, 25 Jul 2007 14:15:48 -0700
From:	"Li, Tong N" <tong.n.li@...el.com>
To:	Chris Snook <csnook@...hat.com>
Cc:	Chris Friesen <cfriesen@...tel.com>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC] scheduler: improve SMP fairness in CFS

On Wed, 2007-07-25 at 16:55 -0400, Chris Snook wrote:
> Chris Friesen wrote:
> > Ingo Molnar wrote:
> > 
> >> the 3s is the problem: change that to 60s! We no way want to 
> >> over-migrate for SMP fairness, the change i did gives us reasonable 
> >> long-term SMP fairness without the need for high-rate rebalancing.
> > 
> > Actually, I do have requirements from our engineering guys for 
> > short-term fairness.  They'd actually like decent fairness over even 
> > shorter intervals...1 second would be nice, 2 is acceptable.
> > 
> > They are willing to trade off random peak performance for predictability.
> > 
> > Chris
> > 
> 
> The sysctls for CFS have nanosecond resolution.  They default to 
> millisecond-order values, but you can set them much lower.  See sched_fair.c for 
> the knobs and their explanations.
> 
> 	-- Chris

This is incorrect. Those knobs control local-CPU fairness granularity
but have no control over fairness across CPUs.

I'll do some benchmarking as Ingo suggested.

  tong
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