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Message-ID: <439CF3B1.4050803@yahoo.com.au>
Date: Mon, 12 Dec 2005 14:51:13 +1100
From: Nick Piggin <nickpiggin@yahoo.com.au>
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To: Marcelo Tosatti <marcelo.tosatti@cyclades.com>
CC: Andi Kleen <ak@suse.de>, Christoph Lameter <clameter@sgi.com>,
       linux-kernel@vger.kernel.org, Hugh Dickins <hugh@veritas.com>,
       linux-mm@kvack.org
Subject: Re: [RFC 3/6] Make nr_pagecache a per zone counter
References: <20051210005440.3887.34478.sendpatchset@schroedinger.engr.sgi.com> <20051210005456.3887.94412.sendpatchset@schroedinger.engr.sgi.com> <20051211183241.GD4267@dmt.cnet> <20051211194840.GU11190@wotan.suse.de> <20051211204943.GA4375@dmt.cnet>
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Marcelo Tosatti wrote:
> On Sun, Dec 11, 2005 at 08:48:40PM +0100, Andi Kleen wrote:
> 
>>>By the way, why does nr_pagecache needs to be an atomic variable on UP systems?
>>
>>At least on X86 UP atomic doesn't use the LOCK prefix and is thus quite
>>cheap. I would expect other architectures who care about UP performance
>>(= not IA64) to be similar.
> 
> 
> But in practice the variable does not need to be an atomic type for UP, but
> simply a word, since stores are atomic on UP systems, no?
> 
> Several arches seem to use additional atomicity instructions on 
> atomic functions:
> 

Yeah, this is to protect from interrupts and is common to most
load store architectures. It is possible we could have
atomic_xxx_irq / atomic_xxx_irqsave functions for these, however
I think nobody has yet demostrated the improvements outweigh the
complexity that would be added.

-- 
SUSE Labs, Novell Inc.

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