Message-ID: <43EC7473.20109@cosmosbay.com>
Date: Fri, 10 Feb 2006 12:09:39 +0100
From: Eric Dumazet <dada1@cosmosbay.com>
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To: Andrew Morton <akpm@osdl.org>
CC: Andi Kleen <ak@muc.de>, ashok.raj@intel.com, ntl@pobox.com,
       riel@redhat.com, linux-kernel@vger.kernel.org, torvalds@osdl.org,
       mingo@elte.hu, 76306.1226@compuserve.com, wli@holomorphy.com,
       heiko.carstens@de.ibm.com, pj@sgi.com
Subject: Re: [PATCH] percpu data: only iterate over possible CPUs
References: <20060209160808.GL18730@localhost.localdomain>	<20060209090321.A9380@unix-os.sc.intel.com>	<20060209100429.03f0b1c3.akpm@osdl.org>	<200602101102.25437.ak@muc.de> <20060210024222.67db06f3.akpm@osdl.org>
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Andrew Morton a ?crit :
> Andi Kleen <ak@muc.de> wrote:
>> On Thursday 09 February 2006 19:04, Andrew Morton wrote:
>>> Ashok Raj <ashok.raj@intel.com> wrote:
>>>> The problem was with ACPI just simply looking at the namespace doesnt
>>>>  exactly give us an idea of how many processors are possible in this platform.
>>> We need to fix this asap - the performance penalty for HOTPLUG_CPU=y,
>>> NR_CPUS=lots will be appreciable.
>> What is this performance penalty exactly? 
> 
> All those for_each_cpu() loops will hit NR_CPUS cachelines instead of
> hweight(cpu_possible_map) cachelines.

You mean NR_CPUS bits, mostly all included in a single cacheline, and even in 
a single long word :) for most cases (NR_CPUS <= 32 or 64)


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