linux-kernel - Re: [patch] CFS scheduler, -v5

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Message-ID: <20070423034310.GA19845@elte.hu>
Date:	Mon, 23 Apr 2007 05:43:10 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Williams <pwil3058@...pond.net.au>,
	Thomas Gleixner <tglx@...utronix.de>, caglar@...dus.org.tr,
	Willy Tarreau <w@....eu>,
	Gene Heskett <gene.heskett@...il.com>, Mark Lord <lkml@....ca>,
	Ulrich Drepper <drepper@...hat.com>
Subject: Re: [patch] CFS scheduler, -v5


* Nick Piggin <npiggin@...e.de> wrote:

> > note that CFS's "granularity" value is not directly comparable to 
> > "timeslice length":
> 
> Right, but it does introduce the kbuild regression, [...]

Note that i increased the granularity from 1msec to 5msecs after your 
kbuild report, could you perhaps retest kbuild with the default settings 
of -v5?

> [...] and as we discussed, this will be only worse on newer CPUs with 
> bigger caches or less naturally context switchy workloads.

yeah - but they'll all be quad core, so the SMP timeslice multiplicator 
should do the trick. Most of the CFS testers use single-CPU systems.

> > (in -v6 i'll scale the granularity up a bit with the number of CPUs, 
> > like SD does. That should get the right result on larger SMP boxes 
> > too.)
> 
> I don't really like the scaling with SMP thing. The cache effects are 
> still going to be significant on small systems, and there are lots of 
> non-desktop users of those (eg. clusters).

CFS using clusters will want to tune the granularity up drastically 
anyway, to 1 second or more, to maximize throughput. I think a small 
default with a scale-up-on-SMP rule is pretty sane. We'll gather some 
more kbuild data and see what happens, ok?

> > while i agree it's a tad too finegrained still, I agree with Con's 
> > choice: rather err on the side of being too finegrained and lose 
> > some small amount of throughput on cache-intense workloads like 
> > compile jobs, than err on the side of being visibly too choppy for 
> > users on the desktop.
> 
> So cfs gets too choppy if you make the effective timeslice comparable 
> to mainline?

it doesnt in any test i do, but again, i'm erring on the side of it 
being more interactive.

	Ingo
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