linux-kernel - Re: [PATCH] [33/58] x86_64: Avoid too many remote cpu references due to /proc/stat

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Message-ID: <20070719105532.GS3801@stusta.de>
Date:	Thu, 19 Jul 2007 12:55:32 +0200
From:	Adrian Bunk <bunk@...sta.de>
To:	Andi Kleen <ak@...e.de>
Cc:	Christoph Hellwig <hch@...radead.org>, kiran@...lex86.org,
	patches@...-64.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] [33/58] x86_64: Avoid too many remote cpu references
	due to /proc/stat

On Thu, Jul 19, 2007 at 12:41:28PM +0200, Andi Kleen wrote:
> On Thursday 19 July 2007 12:21:49 Christoph Hellwig wrote:
> > On Thu, Jul 19, 2007 at 11:55:19AM +0200, Andi Kleen wrote:
> > > 
> > > From: Ravikiran G Thirumalai <kiran@...lex86.org>
> > > Too many remote cpu references due to /proc/stat.
> > > 
> > > On x86_64, with newer kernel versions, kstat_irqs is a bit of a problem.
> > > On every call to kstat_irqs, the process brings in per-cpu data from all
> > > online cpus.  Doing this for NR_IRQS, which is now 256 + 32 * NR_CPUS
> > > results in (256+32*63) * 63 remote cpu references on a 64 cpu config.
> > > /proc/stat is parsed by common commands like top, who etc, causing
> > > lots of cacheline transfers
> > > 
> > > This statistic seems useless. Other 'big iron' arches disable this.
> > > Can we disable computing/reporting this statistic?  This piece of
> > > statistic is not human readable on x86_64 anymore,
> > > 
> > > If not, can we optimize computing this statistic so as to avoid
> > > too many remote references (patch to follow)
> > 
> > If we disable this on x86_64 we should just kill it completely for consistency.
> 
> I guess it's fine on UP only architectures.  I will change it to !CONFIG_SMP
> unless someone complains.

Making it depending on the kernel configuration will only cause 
surprises for users. And if you really need the data you can get 
it from /proc/interrupts.

> -Andi

cu
Adrian

-- 

       "Is there not promise of rain?" Ling Tan asked suddenly out
        of the darkness. There had been need of rain for many days.
       "Only a promise," Lao Er said.
                                       Pearl S. Buck - Dragon Seed

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