linux-kernel - Re: nr_cpu_ids incorrect on AMD Quad-Core Opteron 8380

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Message-Id: <AD1C7BB3-C3E6-47EC-809F-4CE3C049D92C@cs.otago.ac.nz>
Date:	Mon, 1 Feb 2010 11:59:44 +1300
From:	zhiyi <zhuang@...otago.ac.nz>
To:	Borislav Petkov <petkovbb@...glemail.com>
Cc:	linux-kernel@...r.kernel.org, Zhiyi Huang <hzy@...otago.ac.nz>,
	jkmair@...otago.ac.nz
Subject: Re: nr_cpu_ids incorrect on AMD Quad-Core Opteron 8380

Hi Boris,
Thanks for your answer.
However, your answer doesn't fully resolve my puzzle.
My module worked for a kernel 2.6.29 with CONFIG_NR_CPUS=64 on my dual  
quad-core AMD Opteron. The value of nr_cpu_ids I got from the same  
module was correct (8). It seems nr_cpu_ids has nothing to do with  
CONFIG_NR_CPUS.
Cheers,
Zhiyi

On 31/01/2010, at 6:18 AM, Borislav Petkov wrote:

> On Thu, Jan 28, 2010 at 12:16:44PM +1300, zhiyi wrote:
>> Hi,
>> I have just found the value of nr_cpu_ids is 32, instead of 16 on my
>> new Dell PowerEdge R905 which has 16 cpus (4 quad-core Opteron
>> 8380). However, /proc/cpuinfo displays the right number (16).
>> I have searched the archive but found no related subject.  Is there
>> any patch already available to fix the problem?
>> I am using 2.6.31-4 and verified this incorrect value (32) using a
>> module.
>
> nr_cpu_ids is set to CONFIG_NR_CPUS on SMP kernels and denotes the max
> CPUs supported by your kernel. This is not a bug but a configurable
> option in the kernel for saving memory.
>
> You can still set CONFIG_NR_CPUS in "-> Processor type and features"  
> to
> the max number of cores N you have on your machine if you want to save
> approx 8*N KB. But guessing from your quadsocket configuration, memory
> shouldn't be that scarce on that machine to go the trouble :).
>
> -- 
> Regards/Gruss,
>    Boris.
>

=======================
Zhiyi Huang
Dept of Computer Science
University of Otago
Email: hzy@...otago.ac.nz
Phone: 0064-3-4795680
Fax: 0064-3-4798529

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