linux-kernel - Re: 2.6.19-rc1: x86_64 slowdown in lmbench's fork

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Message-ID: <m1lkmsxwk7.fsf@ebiederm.dsl.xmission.com>
Date:	Fri, 03 Nov 2006 10:35:36 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	tim.c.chen@...ux.intel.com
Cc:	Adrian Bunk <bunk@...sta.de>, linux-kernel@...r.kernel.org,
	ak@...e.de, discuss@...-64.org
Subject: Re: 2.6.19-rc1: x86_64 slowdown in lmbench's fork

Tim Chen <tim.c.chen@...ux.intel.com> writes:

> On Fri, 2006-11-03 at 03:11 +0100, Adrian Bunk wrote:
>
>> 
>> What's your CONFIG_NR_CPUS setting that you are seeing such a big
>> regression?
>> 
>
> CONFIG_NR_CPUS is set to 8.  

Ugh.  This simply changes NR_IRQS from 256 to 512.  Changing
the size of data from 1K to 2K.

So unless there is some other array that is sized by NR_IRQs
in the context switch path which could account for this in
other ways.  It looks like you just got unlucky.

The only hypothesis that I can seem to come up with is that maybe
you are getting an extra tlb now that you didn't use to.  
I think the per cpu area is covered by huge pages but maybe not.

Eric
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