linux-kernel - Re: CFS review

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Message-ID: <alpine.LFD.0.999.0708280929000.25853@woody.linux-foundation.org>
Date:	Tue, 28 Aug 2007 09:34:03 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Al Boldi <a1426z@...ab.com>
cc:	Ingo Molnar <mingo@...e.hu>, Peter Zijlstra <peterz@...radead.org>,
	Mike Galbraith <efault@....de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS review

On Tue, 28 Aug 2007, Al Boldi wrote:
> 
> I like your analysis, but how do you explain that these stalls vanish when 
> __update_curr is disabled?

It's entirely possible that what happens is that the X scheduling is just 
a slightly unstable system - which effectively would turn a small 
scheduling difference into a *huge* visible difference.

And the "small scheduling difference" might be as simple as "if the 
process slept for a while, we give it a bit more CPU time". And then you 
get into some unbalanced setup where the X scheduler makes it sleep even 
more, because it fills its buffers.

Or something. I can easily see two schedulers that are trying to 
*individually* be "fair", fighting it out in a way where the end result is 
not very good.

I do suspect it's probably a very interesting load, so I hope Ingo looks 
more at it, but I also suspect it's more than just the kernel scheduler.

		Linus
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