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

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Date:	Mon, 23 Apr 2007 04:55:53 +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:

> > the biggest user-visible change in -v5 are various interactivity 
> > improvements (especially under higher load) to fix reported 
> > regressions, and an improved way of handling nice levels. There's 
> > also a new sys_sched_yield_to() syscall implementation for i686 and 
> > x86_64.
> > 
> > All known regressions have been fixed. (knock on wood)
> 
> I think the granularity is still much too low. Why not increase it to 
> something more reasonable as a default?

note that CFS's "granularity" value is not directly comparable to 
"timeslice length":

> [ Note: while CFS's default preemption granularity is currently set to
>   5 msecs, this value does not directly transform into timeslices: for 
>   example two CPU-intense tasks will have effective timeslices of 10 
>   msecs with this setting. ]

also, i just checked SD: 0.46 defaults to 8 msecs rr_interval (on 1 CPU 
systems), which is lower than the 10 msecs effective timeslice length 
CVS-v5 achieves on two CPU-bound tasks.

(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.)

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.

	Ingo
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