linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Message-ID: <20080710202453.GA7534@elte.hu>
Date:	Thu, 10 Jul 2008 22:24:53 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Mike Travis <travis@....com>, "H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses


* Eric W. Biederman <ebiederm@...ssion.com> wrote:

> Mike Travis <travis@....com> writes:
> 
> 
> > The biggest growth came from moving all the xxx[NR_CPUS] arrays into 
> > the per cpu area.  So you free up a huge amount of unused memory 
> > when the NR_CPUS count starts getting into the ozone layer.  4k now, 
> > 16k real soon now, ??? future?
> 
> Hmm.  Do you know how big a role kernel_stat plays.
> 
> It is a per cpu structure that is sized via NR_IRQS.  NR_IRQS is by 
> NR_CPUS. So ultimately the amount of memory take up is 
> NR_CPUS*NR_CPUS*32 or so.
> 
> I have a patch I wrote long ago, that addresses that specific nasty 
> configuration by moving the per cpu irq counters into pointer 
> available from struct irq_desc.
> 
> The next step which I did not get to (but is interesting from a 
> scaling perspective) was to start dynamically allocating the irq 
> structures.

/me willing to test & babysit any test-patch in that area ...

this is a big problem and it's getting worse quadratically ;-)

	Ingo
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