linux-kernel - Re: About CPU's Load Balance and CFS functions

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Message-ID: <20090908065519.GC6505@elte.hu>
Date:	Tue, 8 Sep 2009 08:55:19 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	lookeylam <lookeylam@...il.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: About CPU's Load Balance and CFS functions


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, 2009-09-07 at 16:14 +0800, lookeylam wrote:
> > Hello:
> >          I am  not sure this  is the right maillist to ask this
> > question. I just have a try.
> >          I have a test on Dell 1950 with 8 cpus on board for testing
> > the apache by ab command. And I find that in 
> >          linux 2.6.18. The processes forked by apache are not well
> > distributed on these 8 cpus.
> >          linux 2.6.23 is a little better than 2.6.18, but still some
> > cpus are running busy and some cpus remains idle.
> >          While in  2.6.30, these 8 cpus are well used and the
> > percentage of each cpu is nearly the same. And when I 
> >          start the control group with cpuset type with
> > sched_relax_domain_level( with value 3,4,5). The result of ab is 50ms
> > better than test results without control group. 
> >         
> >          I attribute this situation to to load_balance but not CFS,
> > because CFS is just a scheduler for orgnizing the process inside one
> > cpu, while load_balance is the main character to control the process
> > and load between different cpus.
> >          But when i give out this conclusion, I confuse about the
> > differences of these three kernels of load_balance.
> >          
> >          My questions are the above conclusion  is right or not? How
> > would these situation happen and why? I read the code of the kernel
> > but I am still not sure.
> 
> load-balancing is generally considered part of the scheduler as a 
> whole, while CFS is indeed the cpu scheduler, it and the 
> load-balancer are related because they do have to work together.
> 
> Now, in the past 3+years the load-balancer has undergone 
> significant changes too -- and we're now again poking at it, .32 
> will likely have quite radical changes to the whole load balancer.
> 
> The sched_relax_domain_level knob is one that controls one of the 
> coupling mechanisms, namely wake on idle, that is, we try and push 
> newly woken tasks away to idle cpus. The level you put in there is 
> related to the sched_domain level.
> 
> Normally we don't try and push newly woken tasks too far away, 
> because that'll increase the remote access penalty for related 
> tasks, but some workloads have lots of very short running 
> unrelated tasks which do benefit from this.
> 
> Anyway, I would suggest you keep an eye out for scheduler patches 
> if you're interested in this, all the scheduler development 
> happens in -tip.

Which can be tested via:

  http://people.redhat.com/mingo/tip.git/README

	Ingo
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