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

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Date:	Thu, 10 Jul 2008 14:26:17 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Mike Travis <travis@....com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Christoph Lameter <cl@...ux-foundation.org> writes:

> H. Peter Anvin wrote:
>
>> And how much is that, especially on *small* systems?
>
> i386?
>
> i386 uses 4K mappings. There are just a few cpus supported, there is scarcity of
> ZONE_NORMAL memory so the per cpu areas really cannot get that big. See the
> cpu_alloc patchsets for i386.

i386 is fundamentally resource constrained.  However x86_32 should support a
strict superset of the machines the x86_64 kernel supports.

Because it is resource constrained in the lowmem zone you should not
be able to bring up all of the cpus on a huge cpu box.   But you should still
be able to boot and run the kernel.  So for percpu data we have effectively
same size constraints.

Eric
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