linux-kernel - Re: [RFC patch] CFS fix place entity spread issue (v2)

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Date:	Sun, 18 Apr 2010 18:59:06 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <greg@...ah.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jarkko Nikula <jhnikula@...il.com>,
	Tony Lindgren <tony@...mide.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC patch] CFS fix place entity spread issue (v2)

* Linus Torvalds (torvalds@...ux-foundation.org) wrote:
> 
> 
> On Sun, 18 Apr 2010, Mathieu Desnoyers wrote:
> >
> > CFS fix place entity spread issue (v2)
> > 
> > Huge CFS vruntime spread (18 minutes) has been observed with LTTng while simply
> > running Xorg on a uniprocessor machine. Detailed explanation in my ELC2010
> > presentation at:
> 
> Hmm. I tested this patch with my favourite non-scientific desktop load 
> test: do web browsing while doing a kernel compile with "make -j16" (after 
> doing a "git clean -dqfx" and "ccache -C" to get rid of old object files 
> and ccache).
> 
> This is on a dual-core (with SMT, so 4 threads) Core i5, so "make -j16" 
> overcommits the CPU's quite a bit.
> 
> And quite frankly, I think your patch makes things much worse. I don't 
> have numbers, but it felt much choppier and slower to do scrolling in 
> firefox o moving windows around while the load average is 20+.
> 
> My testload is in no way objective, nor necessarily any good, but it's the 
> one I happen to use, and while it's subjective, I think the difference was 
> pretty clear and not good.
> 
> 			Linus

Degradation of this particular workload is expected, because the nature of the
large CFS spread issue favors Xorg: it gets litterally minutes of available
vruntime, while most other threads (e.g. kernel build, audio playing, etc..)
suffer from this.

I'll try to play a bit with this (I got plenty of time to kill at the airport
today, missed my connecting flight back from San Francisco). Ideally, we might
want to try to find a way to ensure that place_entity() is kind to sleepers, but
without putting them before min_vruntime. I don't have any SMP machine with Xorg
handy, but I'll try to push my UP laptop a bit more heavily with a kernel build.

If we are lucky enough, we might just have to twist a knob or two to make Xorg
more responsive.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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