Re: [patch] CFS scheduler, -v12

Bill Huey
2007-05-18T01:02:33Z

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