linux-kernel - Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64

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Message-ID: <Pine.LNX.4.64.0703161356190.6204@schroedinger.engr.sgi.com>
Date:	Fri, 16 Mar 2007 13:59:21 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	David Miller <davem@...emloft.net>
cc:	andi@...stfloor.org, rostedt@...dmis.org,
	torvalds@...ux-foundation.org, mbligh@...igh.org, mingo@...e.hu,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	chrisw@...s-sol.org, rusty@...tcorp.com.au, glommer@...il.com
Subject: Re: [PATCH 00/18] Make common x86 arch area for i386 and x86_64 -
 Take 2

On Fri, 16 Mar 2007, David Miller wrote:

> From: Christoph Lameter <clameter@....com>
> Date: Fri, 16 Mar 2007 13:52:18 -0700 (PDT)
> 
> > Virtual mmap allows holes in the same way as page tables do.
> 
> I don't want to take expensive TLB misses to lookup a page.

Ummm. You are missing key details please look at linux-mm. We will use 
huge tlb entries for the mmap and its optional.

> TLB misses are extremely expensive on some platforms, that's why
> I try to make all the critical structures go through the 4MB
> locked TLB entry for the kernel image, or failing that the
> 4MB linear mappings at PAGE_OFFSET on sparc64.

Exactly. That is why 16M TLB entries speed up the kernel by 
4-5% using vmmemmap on ia64. Basically we end up with one TLB per node 
for the vmemmap which always stays in the cache. No additional lookups.
Without vmemmap we still need a 16M TLB for the 1-1 kernel mapping. This 
jsut cuts out the crud from sparsemem.


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