Re: [rfc][patch] sched: remove smpnice

From: Con Kolivas
Date: Mon Feb 13 2006 - 09:11:30 EST

On Friday 10 February 2006 18:01, Siddha, Suresh B wrote:
> On Tue, Feb 07, 2006 at 03:36:17PM -0800, Andrew Morton wrote:
> > Suresh, Martin, Ingo, Nick and Con: please drop everything, triple-check
> > and test this:
> >
> > From: Peter Williams <pwil3058@xxxxxxxxxxxxxx>
> >
> > This is a modified version of Con Kolivas's patch to add "nice" support
> > to load balancing across physical CPUs on SMP systems.
> I have couple of issues with this patch.
> a) on a lightly loaded system, this will result in higher priority job
> hopping around from one processor to another processor.. This is because of
> the code in find_busiest_group() which assumes that SCHED_LOAD_SCALE
> represents a unit process load and with nice_to_bias calculations this is
> no longer true(in the presence of non nice-0 tasks)
> My testing showed that 178.galgel in SPECfp2000 is down by ~10% when run
> with nice -20 on a 4P(8-way with HT) system compared to a nice-0 run.

While I voted to remove smp nice till this regression is fixed can I just
point out a mildly amusing catch 22 in this. Nice levels are _broken_ on smp
without smp nice, yet it's only by using nice levels that you get this
performance problem. I'm not arguing for changing our decision based on this.

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