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Date: Thu, 2 Aug 2007 23:38:00 -0500
From: Matt Mackall <mpm@selenic.com>
To: Arjan van de Ven <arjan@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Roman Zippel <zippel@linux-m68k.org>,
       Mike Galbraith <efault@gmx.de>,
       Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org
Subject: Re: CFS review
Message-ID: <20070803043800.GE11115@waste.org>
References: <20070711214649.GK14435@v2.random> <alpine.LFD.0.999.0707111500210.20061@woody.linux-foundation.org> <Pine.LNX.4.64.0707130415520.1818@scrub.home> <1184302024.6709.11.camel@Homer.simpson.net> <Pine.LNX.4.64.0707131728070.1817@scrub.home> <1184389456.6632.13.camel@Homer.simpson.net> <Pine.LNX.4.64.0708010439050.1817@scrub.home> <20070801112229.GA11710@elte.hu> <20070803030436.GO11166@waste.org> <1186113467.3996.0.camel@laptopd505.fenrus.org>
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On Thu, Aug 02, 2007 at 08:57:47PM -0700, Arjan van de Ven wrote:
> On Thu, 2007-08-02 at 22:04 -0500, Matt Mackall wrote:
> > On Wed, Aug 01, 2007 at 01:22:29PM +0200, Ingo Molnar wrote:
> > > 
> > > * Roman Zippel <zippel@linux-m68k.org> wrote:
> > > 
> > > > [...] e.g. in this example there are three tasks that run only for 
> > > > about 1ms every 3ms, but they get far more time than should have 
> > > > gotten fairly:
> > > > 
> > > >  4544 roman     20   0  1796  520  432 S 32.1  0.4   0:21.08 lt
> > > >  4545 roman     20   0  1796  344  256 R 32.1  0.3   0:21.07 lt
> > > >  4546 roman     20   0  1796  344  256 R 31.7  0.3   0:21.07 lt
> > > >  4547 roman     20   0  1532  272  216 R  3.3  0.2   0:01.94 l
> > > 
> > > Mike and me have managed to reproduce similarly looking 'top' output, 
> > > but it takes some effort: we had to deliberately run a non-TSC 
> > > sched_clock(), CONFIG_HZ=100, !CONFIG_NO_HZ and !CONFIG_HIGH_RES_TIMERS.
> > 
> > ..which is pretty much the state of play for lots of non-x86 hardware.
> 
> question is if it's significantly worse than before. With a 100 or
> 1000Hz timer, you can't expect perfect fairness just due to the
> extremely rough measurement of time spent...

Indeed. I'm just pointing out that not having TSC, fast HZ, no-HZ
mode, or high-res timers should not be treated as an unusual
circumstance. That's a PC-centric view.

-- 
Mathematics is the supreme nostalgia of our time.