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From: Erich Focht <efocht@ess.nec.de>
To: "Martin J. Bligh" <mbligh@aracnet.com>,
       Michael Hohnbaum <hohnbaum@us.ibm.com>, landley@trommello.org
Subject: Re: Crunch time -- the musical.  (2.5 merge candidate list 1.5)
Date: Fri, 25 Oct 2002 10:15:46 +0200
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On Friday 25 October 2002 00:38, Martin J. Bligh wrote:
> > The situation is really funny: Everybody seems to agree that the design
> > ideas in my NUMA aproach are sane and exactly what we want to have on
> > a NUMA platform in the end. But instead of concentrating on tuning the
> > parameters for the many different NUMA platforms and reshaping this
> > aproach to make it acceptable, IBM concentrates on a very much stripped
> > down aproach.
>
> From my point of view, the reason for focussing on this was that
> your scheduler degraded the performance on my machine, rather than
> boosting it. Half of that was the more complex stuff you added on
> top ... it's a lot easier to start with something simple that works
> and build on it, than fix something that's complex and doesn't work
> well.

You're talking about one of the first 2.5 versions of the patch. It
changed a lot since then, thanks to your feedback, too.

> I still haven't been able to get your scheduler to boot for about
> the last month without crashing the system. Andrew says he has it
> booting somehow on 2.5.44-mm4, so I'll steal his kernel tommorow and
> see how it looks. If the numbers look good for doing boring things
> like kernel compile, SDET, etc, I'm happy.

I thought this problem is well understood! For some reasons independent of
my patch you have to boot your machines with the "notsc" option. This
leaves the cache_decay_ticks variable initialized to zero which my patch
doesn't like. I'm trying to deal with this inside the patch but there is
still a small window when the variable is zero. In my opinion this needs
to be fixed somewhere in arch/i386/kernel/smpboot.c. Booting a machine
with cache_decay_ticks=0 is pure nonsense, as it switches off cache
affinity which you absolutely need! So even if "notsc" is a legal option,
it should be fixed such that it doesn't leave your machine without cache
affinity. That would anyway give you a falsified behavior of the O(1)
scheduler.

Erich


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