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

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Message-ID: <m1zloptrwt.fsf@frodo.ebiederm.org>
Date:	Thu, 10 Jul 2008 13:17:54 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Mike Travis <travis@....com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Ingo Molnar <mingo@...e.hu>,
	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

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.

Eric

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