linux-kernel - Re: Volanomark slows by 80% under CFS

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Date:	Fri, 27 Jul 2007 22:47:21 -0400
From:	Rik van Riel <riel@...hat.com>
To:	tim.c.chen@...ux.intel.com
CC:	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: Volanomark slows by 80% under CFS

Tim Chen wrote:
> Ingo,
> 
> Volanomark slows by 80% with CFS scheduler on 2.6.23-rc1.  
> Benchmark was run on a 2 socket Core2 machine.
> 
> The change in scheduler treatment of sched_yield 
> could play a part in changing Volanomark behavior.
> In CFS, sched_yield is implemented
> by dequeueing and requeueing a process .  The time a process 
> has spent running probably reduced the the cpu time due it 
> by only a bit. The process could get re-queued pretty close
> to head of the queue, and may get scheduled again pretty
> quickly if there is still a lot of cpu time due.  

I wonder if this explains the 30% drop in top performance
seen with the MySQL sysbench benchmark when the scheduler
changed to CFS...

See http://people.freebsd.org/~jeff/sysbench.png

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