linux-kernel - SD renice recommendation was: Re: [REPORT] cfs-v4 vs sd-0.44

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Date:	Thu, 26 Apr 2007 10:51:11 +1000
From:	Con Kolivas <kernel@...ivas.org>
To:	Ingo Molnar <mingo@...e.hu>, ck@....kolivas.org
Cc:	Peter Williams <pwil3058@...pond.net.au>,
	Arjan van de Ven <arjan@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	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: SD renice recommendation was: Re: [REPORT] cfs-v4 vs sd-0.44

On Tuesday 24 April 2007 16:36, Ingo Molnar wrote:

> 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)

Nice 0 as a default for X, but if renicing, nice -10 as the lower limit for X 
on SD. The reason for that on SD is that the priority of freshly woken up 
tasks (ie not fully cpu bound) for both nice 0 and nice -10 will still be the 
same at PRIO 1 (see the prio_matrix). Therefore, there will _not_ be 
preemption of the nice 0 task and a context switch _unless_ it is already cpu 
bound and has consumed a certain number of cycles and has been demoted. 
Contrary to popular belief, it is not universal that a less niced task will 
preempt its more niced counterpart and depends entirely on implementation of 
nice. Yes it is true that context switch rate will go up with a reniced X 
because the conditions that lead to preemption are more likely to be met, but 
it is definitely not every single wakeup of the reniced X.

Alas, again, I am forced to spend as little time as possible at the pc for my 
health, so expect _very few_ responses via email from me. Luckily SD is in 
pretty fine shape with version 0.46.

-- 
-ck
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