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

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Date:	Thu, 19 Jul 2007 11:21:49 +0100
From:	Christoph Hellwig <hch@...radead.org>
To:	Andi Kleen <ak@...e.de>
Cc:	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 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.

> -#if !defined(CONFIG_PPC64) && !defined(CONFIG_ALPHA) && !defined(CONFIG_IA64)
> +#if !defined(CONFIG_PPC64) && !defined(CONFIG_ALPHA) && !defined(CONFIG_IA64) \
> +					&& !defined(CONFIG_X86_64)
>  	for (i = 0; i < NR_IRQS; i++)
>  		seq_printf(p, " %u", kstat_irqs(i));
>  #endif
-
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