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Date: Sat, 17 Apr 2010 21:18:12 +0300
From: Avi Kivity <avi@redhat.com>
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Subject: Re: hugepages will matter more in the future
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To: Arjan van de Ven <arjan@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>, Jason Garrett-Glaser <darkshikari@gmail.com>, Mike Galbraith <efault@gmx.de>, Andrea Arcangeli <aarcange@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, Pekka Enberg <penberg@cs.helsinki.fi>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nick Piggin <npiggin@suse.de>, Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>, Dave Hansen <dave@linux.vnet.ibm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Mike Travis <travis@sgi.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Christoph Lameter <cl@linux-foundation.org>, Chris Wright <chrisw@sous-sol.org>, bpicco@redhat.com, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>, "Michael S. Tsirkin" <mst@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Johannes Weiner <hannes@cmpxchg.org>, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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On 04/17/2010 06:12 PM, Arjan van de Ven wrote:
> On Mon, 12 Apr 2010 14:29:58 +0300
> Avi Kivity<avi@redhat.com>  wrote:
>    
>> Pointer chasing defeats OoO.  The cpu is limited in the amount of
>> speculation it can do.
>>      
> Pointer chasing defeats the CPU cache as well.
>    

True.

> As long as the CPU cache contains, mostly, the page tables for all the
> data in the cache, applications that try to work good with a cache
> don't notice too much. Sure, once you start doing pointer chasing cache
> misses things suck. they do very much so.
>    

Correct.  We're trying to reduce suckage from 2 cache misses per access 
(3 for virt), to 1 cache miss per access.  We're also freeing up space 
in the cache for data.

Saying the application already sucks isn't helping anything.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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