linux-kernel - Re: [REPORT] cfs-v5 vs sd-0.46

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Date:	Tue, 24 Apr 2007 10:23:06 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Michael Gerdau <mgd@...hnosis.de>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Nick Piggin <npiggin@...e.de>,
	Gene Heskett <gene.heskett@...il.com>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Mike Galbraith <efault@....de>,
	Peter Williams <pwil3058@...pond.net.au>,
	ck list <ck@....kolivas.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	William Lee Irwin III <wli@...omorphy.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Bill Davidsen <davidsen@....com>, Willy Tarreau <w@....eu>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [REPORT] cfs-v5 vs sd-0.46


* Michael Gerdau <mgd@...hnosis.de> wrote:

> > so to be totally 'fair' and get the same rescheduling 'granularity' 
> > you should probably lower CFS's sched_granularity_ns to 2 msecs.
> 
> I'll change default nice in cfs to -10.
> 
> I'm also happy to adjust /proc/sys/kernel/sched_granularity_ns to 
> 2msec. However checking /proc/sys/kernel/rr_interval reveals it is 16 
> (msec) on my system.

ah, yeah - there due to the SMP rule in SD:

       rr_interval *= 1 + ilog2(num_online_cpus());

and you have a 2-CPU system, so you get 8msec*2 == 16 msecs default 
interval. I find this a neat solution and i have talked to Con about 
this already and i'll adopt Con's idea in CFS too. Nevertheless, despite 
the settings, SD seems to be rescheduling every 6-7 msecs, while CFS 
reschedules only every 13 msecs.

Here i'm assuming that the vmstats are directly comparable: that your 
number-crunchers behave the same during the full runtime - is that 
correct? (If not then the vmstat result should be run at roughly the 
same type of "stage" of the workload, on all the schedulers.)

	Ingo
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