linux-kernel - Re: [patch] CFS scheduler, -v12

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Date:	Wed, 16 May 2007 12:04:08 +1000
From:	Peter Williams <pwil3058@...pond.net.au>
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>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>
Subject: Re: [patch] CFS scheduler, -v12

Ingo Molnar wrote:
> i'm pleased to announce release -v12 of the CFS scheduler patchset.
> 
> The CFS patch against v2.6.22-rc1, v2.6.21.1 or v2.6.20.10 can be 
> downloaded from the usual place:
>   
>     http://people.redhat.com/mingo/cfs-scheduler/
> 
> -v12 fixes the '3D bug' that caused trivial latencies in 3D games: it 
> turns out that the problem was not resulting out of any core quality of 
> CFS, it was caused by 3D userspace growing dependent on the current 
> inefficiency of the vanilla scheduler's sys_sched_yield() 
> implementation, and CFS's "make yield work well" changes broke it.
> 
> 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'.
> 
> Mike Galbraith has fixed a bug related to nice levels - the fix should 
> make negative nice levels more potent again.
> 
> Changes since -v10:
> 
>  - nice level calculation fixes (Mike Galbraith)
> 
>  - load-balancing improvements (this should fix the SMP performance 
>    problem reported by Michael Gerdau)
> 
>  - remove the sched_sleep_history_max tunable.
> 
>  - more debugging fields.
> 
>  - various cleanups, fixlets and code reorganization
> 
> As usual, any sort of feedback, bugreport, fix and suggestion is more 
> than welcome,

Load balancing appears to be badly broken in this version.  When I 
started 4 hard spinners on my 2 CPU machine one ended up on one CPU and 
the other 3 on the other CPU and they stayed there.

Peter
-- 
Peter Williams                                   pwil3058@...pond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce
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