linux-kernel - Re: [ck] Re: [REPORT] cfs-v6-rc2 vs sd-0.46 vs 2.6.21-rc7

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Message-ID: <20070427065204.GA31708@elte.hu>
Date:	Fri, 27 Apr 2007 08:52:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Con Kolivas <kernel@...ivas.org>
Cc:	ck@....kolivas.org, Michael Gerdau <mgd@...hnosis.de>,
	Nick Piggin <npiggin@...e.de>,
	Bill Davidsen <davidsen@....com>,
	Juliusz Chroboczek <jch@....jussieu.fr>,
	Mike Galbraith <efault@....de>, linux-kernel@...r.kernel.org,
	William Lee Irwin III <wli@...omorphy.com>,
	Peter Williams <pwil3058@...pond.net.au>,
	Gene Heskett <gene.heskett@...il.com>,
	Willy Tarreau <w@....eu>, Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [ck] Re: [REPORT] cfs-v6-rc2 vs sd-0.46 vs 2.6.21-rc7


* Con Kolivas <kernel@...ivas.org> wrote:

> > as a summary: i think your numbers demonstrate it nicely that the
> > shorter 'timeslice length' that both CFS and SD utilizes does not have a
> > measurable negative impact on your workload. To measure the total impact
> > of 'timeslicing' you might want to try the exact same workload with a
> > much higher 'timeslice length' of say 400 msecs, via:
> >
> >     echo 400000000 > /proc/sys/kernel/sched_granularity_ns  # on CFS
> >     echo 400 > /proc/sys/kernel/rr_interval                 # on SD
> 
> I thought that the effective "timeslice" on CFS was double the 
> sched_granularity_ns so wouldn't this make the effective timeslice 
> double that of what you're setting SD to? [...]

The two settings are not really comparable. The "effective timeslice is 
the double of the granularity" thing i mentioned before is really a 
special-case: only true for a really undisturbed 100% CPU-using 
_two-task_ workload, if and only if the workload would not reschedule 
otherwise, but that is clearly not the case here: and if you look at the 
vmstat output provided by Michael you'll see that all 3 schedulers 
rescheduled this workload at around 1000/sec or 1 msec per scheduling 
atom. (But i'd agree that to be on the safe side the context-switch rate 
has to be monitored and if it seems too high on SD, the rr_interval 
should be increased.)

> [...] Anyway the difference between 400 and 800ms timeslices is 
> unlikely to be significant so I don't mind.

even on a totally idle system there's at least a 10 Hz 'background 
sound' of various activities, so any setting above 100 msecs rarely has 
any effect.

	Ingo
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