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Date:	Thu, 17 May 2007 18:01:11 -0700
From:	Bill Huey (hui) <billh@...ppy.monkey.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	William Lee Irwin III <wli@...omorphy.com>,
	"Bill Huey (hui)" <billh@...ppy.monkey.org>
Subject: Re: [patch] CFS scheduler, -v12

On Thu, May 17, 2007 at 05:18:41PM -0700, Bill Huey wrote:
> On Sun, May 13, 2007 at 05:38:53PM +0200, Ingo Molnar wrote:
> > Even a simple 3D app like glxgears does a sys_sched_yield() for every 
> > frame it generates (!) on certain 3D cards, which in essence punishes 
> > any scheduler that implements sys_sched_yield() in a sane manner. This 
> > interaction of CFS's yield implementation with this user-space bug could 
> > be the main reason why some testers reported SD to be handling 3D games 
> > better than CFS. (SD uses a yield implementation similar to the vanilla 
> > scheduler.)
> > 
> > So i've added a yield workaround to -v12, which makes it work similar to 
> > how the vanilla scheduler and SD does it. (Xorg has been notified and 
> > this bug should be fixed there too. This took some time to debug because 
> > the 3D driver i'm using for testing does not use sys_sched_yield().) The 
> > workaround is activated by default so -v12 should work 'out of the box'.
> 
> This is an incorrect analysis. OpenGL has the ability to "yield" after
> every frame specifically for SGI IRIX (React/Pro) frame scheduler (driven
> by the system vertical retrace interrupt) so that it can free up CPU
> resources for other tasks to run. The problem here is that the yield
> behavior is treated generally instead of specifically to a particular
> proportion scheduler policy.
> 
> The correct solution is for the app to use a directed yield and a policy
> that can directly support it so that OpenGL can guaratee a frame rate
> governed by CPU bandwidth allocated by the scheduler.
> 
> Will is working on such a mechanism now.

Follow up:

http://techpubs.sgi.com/library/tpl/cgi-bin/getdoc.cgi/0650/bks/SGI_Developer/books/REACT_PG/sgi_html/ch04.html

bill

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