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

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Date:	Thu, 26 Jul 2007 23:31:54 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Tong Li <tong.n.li@...el.com>
Cc:	linux-kernel@...r.kernel.org, Chris Snook <csnook@...hat.com>
Subject: Re: [RFC] scheduler: improve SMP fairness in CFS


* Tong Li <tong.n.li@...el.com> wrote:

> > you need to measure it over longer periods of time. Its not worth 
> > balancing for such a thing in any high-frequency manner. (we'd trash 
> > the cache constantly migrating tasks back and forth.)
> 
> I have some data below, but before that, I'd like to say, at the same 
> load balancing rate, my proposed approach would allow us to have 
> fairness on the order of seconds. I'm less concerned about trashing 
> the cache. The important thing is to have a knob that allow users to 
> trade off fairness and performance based on their needs. [...]

such a knob already exists to a certain degree, but i havent tested its 
full effects on SMP fairness yet. If you pull my scheduler tree:

  git://git.kernel.org/pub/scm/linux/kernel/git/mingo/linux-2.6-sched.git

and if you enable CONFIG_SCHED_DEBUG, then all the sched-domain 
parameters become runtime tunable under /proc/sys/cpu*.

Could you try to increase the cross-CPU rebalancing frequency and see 
how it impacts the precision of your measurement? Tune 'min_interval' 
and 'max_interval' down to increase the frequency of rebalancing.

	Ingo
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