linux-kernel - Re: [REPORT] cfs-v4 vs sd-0.44

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Message-ID: <20070424063633.GA17257@elte.hu>
Date:	Tue, 24 Apr 2007 08:36:33 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Williams <pwil3058@...pond.net.au>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Con Kolivas <kernel@...ivas.org>, ck list <ck@....kolivas.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	William Lee Irwin III <wli@...omorphy.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Gene Heskett <gene.heskett@...il.com>
Subject: Re: [REPORT] cfs-v4 vs sd-0.44


* Peter Williams <pwil3058@...pond.net.au> wrote:

> > The cases are fundamentally different in behavior, because in the 
> > first case, X hardly consumes the time it would get in any scheme, 
> > while in the second case X really is CPU bound and will happily 
> > consume any CPU time it can get.
> 
> Which still doesn't justify an elaborate "points" sharing scheme. 
> Whichever way you look at that that's just another way of giving X 
> more CPU bandwidth and there are simpler ways to give X more CPU if it 
> needs it.  However, I think there's something seriously wrong if it 
> needs the -19 nice that I've heard mentioned.

Gene has done some testing under CFS with X reniced to +10 and the 
desktop still worked smoothly for him. So CFS does not 'need' a reniced 
X. There are simply advantages to negative nice levels: for example 
screen refreshes are smoother on any scheduler i tried. BUT, there is a 
caveat: on non-CFS schedulers i tried X is much more prone to get into 
'overscheduling' scenarios that visibly hurt X's performance, while on 
CFS there's a max of 1000-1500 context switches a second at nice -10. 
(which, considering the cost of a context switch is well under 1% 
overhead.)

So, my point is, the nice level of X for desktop users should not be set 
lower than a low limit suggested by that particular scheduler's author. 
That limit is scheduler-specific. Con i think recommends a nice level of 
-1 for X when using SD [Con, can you confirm?], while my tests show that 
if you want you can go as low as -10 under CFS, without any bad 
side-effects. (-19 was a bit too much)

> [...]  You might as well just run it as a real time process.

hm, that would be a bad idea under any scheduler (including CFS), 
because real time processes can starve other processes indefinitely.

	Ingo
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