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

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Message-ID: <20070423032215.GC25162@wotan.suse.de>
Date:	Mon, 23 Apr 2007 05:22:15 +0200
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
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

On Mon, Apr 23, 2007 at 04:55:53AM +0200, Ingo Molnar wrote:
> 
> * 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":

Right, but it does introduce the kbuild regression, and as we
discussed, this will be only worse on newer CPUs with bigger
caches or less naturally context switchy workloads.

> > [ 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.

This is about an order of magnitude more than the current scheduler, so
I still think it is too small.

> (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).

> 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?

My approach is completely the opposite. For testing, I prefer to make
the timeslice as large as possible so any problems or regressions are
really noticable and will be reported; it can be scaled back to be
smaller once those kinks are ironed out.
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