linux-kernel - Re: CFS review

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Message-Id: <200708280823.53663.a1426z@gawab.com>
Date:	Tue, 28 Aug 2007 08:23:53 +0300
From:	Al Boldi <a1426z@...ab.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
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

Linus Torvalds wrote:
> On Tue, 28 Aug 2007, Al Boldi wrote:
> > No need for framebuffer.  All you need is X using the X.org vesa-driver.
> > Then start gears like this:
> >
> >   # gears & gears & gears &
> >
> > Then lay them out side by side to see the periodic stallings for ~10sec.
>
> I don't think this is a good test.
>
> Why?
>
> If you're not using direct rendering, what you have is the X server doing
> all the rendering, which in turn means that what you are testing is quite
> possibly not so much about the *kernel* scheduling, but about *X-server*
> scheduling!
>
> I'm sure the kernel scheduler has an impact, but what's more likely to be
> going on is that you're seeing effects that are indirect, and not
> necessarily at all even "good".
>
> For example, if the X server is the scheduling point, it's entirely
> possible that it ends up showing effects that are more due to the queueing
> of the X command stream than due to the scheduler - and that those
> stalls are simply due to *that*.
>
> One thing to try is to run the X connection in synchronous mode, which
> minimizes queueing issues. I don't know if gears has a flag to turn on
> synchronous X messaging, though. Many X programs take the "[+-]sync" flag
> to turn on synchronous mode, iirc.

I like your analysis, but how do you explain that these stalls vanish when 
__update_curr is disabled?


Thanks!

--
Al

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