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

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Message-ID: <48764CB4.8080207@zytor.com>
Date:	Thu, 10 Jul 2008 13:53:56 -0400
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	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

Christoph Lameter wrote:
> 
> There will be no additional overhead since the memory already mapped 1-1 using 2MB TLBs and we want to use the same for the percpu areas. This is similar to the vmemmap solution.
> 

THAT sounds strange.  If you're using dedicated virtual maps (which is 
what you're responding to here) then you will *always* have additional 
TLB pressure.  Furthermore, if you use 2 MB pages, you:

a) can only allocate full 2 MB pages, which is expensive for the static 
users and difficult for the dynamic users;

b) increase pressure in the relatively small 2 MB TLB.

	-hpa
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