linux-kernel - Re: [BUG] rebuild_sched_domains considered dangerous

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Message-ID: <20110309144636.3f0899d1@mschwide.boeblingen.de.ibm.com>
Date:	Wed, 9 Mar 2011 14:46:36 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
	Jesse Larrew <jlarrew@...ux.vnet.ibm.com>
Subject: Re: [BUG] rebuild_sched_domains considered dangerous

On Wed, 09 Mar 2011 14:33:56 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Wed, 2011-03-09 at 14:31 +0100, Martin Schwidefsky wrote:
> > > But if you don't also update the cpu->node memory mappings (which I
> > > think it near impossible) what good is it to change the scheduler
> > > topology?
> > 
> > The memory for the different LPARs is striped over all nodes (or books as we
> > call them). We heavily rely on the large shared cache between the books to hide
> > the different memory access latencies. 
> 
> Right, so effectively you don't have NUMA due to that striping. So why
> then change the CPU topology? Simply create a topology without NUMA and
> keep it static, that accurately reflects the memory topology.

Well the CPU topology can change due to different grouping of logical CPUs
dependent on which LPARs are activated. And we effectively do not have a
memory topology, only CPU. Its basically all about caches, we want to
reflect the distance between CPUs over the up to 4 cache levels.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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