linux-kernel - Re: Ten percent test

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Date:	Tue, 10 Apr 2007 07:23:32 -0400
From:	Ed Tomlinson <edt@....ca>
To:	Mike Galbraith <efault@....de>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Con Kolivas <kernel@...ivas.org>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	ck list <ck@....kolivas.org>
Subject: Re: Ten percent test

On Monday 09 April 2007 22:39, Mike Galbraith wrote:
> On Mon, 2007-04-09 at 07:38 +0200, Mike Galbraith wrote:
> 
> > I don't think you can have very much effect on latency using nice with
> > SD once the CPU is fully utilized.  See below.
> > 
> > /*
> >  * This contains a bitmap for each dynamic priority level with empty slots
> >  * for the valid priorities each different nice level can have. It allows
> >  * us to stagger the slots where differing priorities run in a way that
> >  * keeps latency differences between different nice levels at a minimum.
> >  * ie, where 0 means a slot for that priority, priority running from left to
> >  * right:
> >  * nice -20 0000000000000000000000000000000000000000
> >  * nice -10 1001000100100010001001000100010010001000
> >  * nice   0 0101010101010101010101010101010101010101
> >  * nice   5 1101011010110101101011010110101101011011
> >  * nice  10 0110111011011101110110111011101101110111
> >  * nice  15 0111110111111011111101111101111110111111
> >  * nice  19 1111111111111111111011111111111111111111
> >  */
> > 
> > Nice allocates bandwidth, but as long as the CPU is busy, tasks always
> > proceed downward in priority until they hit the expired array.  That's
> > the design.
> 
> There's another aspect of this that may require some thought - kernel
> threads.  As load increases, so does rotation length.  Would you really
> want CPU hogs routinely preempting house-keepers under load?

SD has a schedule batch nice level.  This is good for tasks that want lots
of cpu when they can get it.  If you overload your cpu I expect the box
to slow down - including kernel threads.  If really required they can be
started with a higher priority...

Ed
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