linux-kernel - Re: CFS review

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Date:	Tue, 28 Aug 2007 18:45:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Al Boldi <a1426z@...ab.com>, 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 <torvalds@...ux-foundation.org> wrote:

> 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.

i think it's because disabling __update_curr() in essence removes the 
ability of scheduler to preempt tasks - that hack in essence results in 
a non-scheduler. Hence the gears + X pair of tasks becomes a synchronous 
pair of tasks in essence - and thus gears cannot "overload" X.

Normally gears + X is an asynchronous pair of tasks, with gears (or 
xperf, or devel versions of firefox, etc.) not being throttled at all 
and thus being able to overload/spam the X server with requests. (And we 
generally want to _reward_ asynchronity and want to allow tasks to 
overlap each other and we want each task to go as fast and as parallel 
as it can.)

Eventually X's built-in "bad, abusive client" throttling code kicks in, 
which, AFAIK is pretty crude and might yield to such artifacts. But ... 
it would be nice for an X person to confirm - and in any case i'll try 
Al's workload - i thought i had a reproducer but i barked up the wrong 
tree :-) My laptop doesnt run with the vesa driver, so i have no easy 
reproducer for now.

( also, it would be nice if Al could try rc4 plus my latest scheduler
  tree as well - just on the odd chance that something got fixed
  meanwhile. In particular Mike's sleeper-bonus-limit fix could be
  related. )

	Ingo
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