linux-kernel - Re: bogomips discrepancy on Intel Core2 Quad CPU

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Message-ID: <46E32A5D.7080009@xs4all.nl>
Date:	Sun, 09 Sep 2007 01:03:57 +0200
From:	Bauke Jan Douma <bjdouma@...all.nl>
To:	Bill Davidsen <davidsen@....com>
CC:	linux-kernel@...r.kernel.org
Subject: Re: bogomips discrepancy on Intel Core2 Quad CPU

Bill Davidsen wrote on 03-09-07 16:47:
> Bauke Jan Douma wrote:
>> $> uname -a
>> Linux skyscraper 2.6.22.5 #7 SMP PREEMPT Sun Sep 2 12:12:25 CEST 2007 
>> i686 GNU/Linux
>>
>> $> cat /proc/cpuinfo | grep bogomips
>> bogomips    : 4813.46
>> bogomips    : 4810.91
>> bogomips    : 4810.91
>> bogomips    : 10583.94
>>
>> The latter seems way off base.
>> Prod me for more info.
>>
> I see this occasionally on a dual, speedstep (or similar) finds a way to 
> throttle down the cores under light load. I suspect that if you load the 
> system:
> 
>   for n in 1 2 3 4; do
>     nice -19 bash -c 'while true; do a=$RANDOM; done' &
>   done
> 
> Then you should see your bogomips rise on all cores.

Sorry for now getting back on this sooner.  Other matters forced it to
the backburner.

I don't have Speedstep enabled on this machine's kernel (as a matter
of fact, the entire cpufreq Kconfig subtree is unchecked). Needless to
say running your bash script doesn't change the values in any way.

I'd say the 4800-something bogomips value is the correct one (for a
2.4GHz CPU -- at least that's what I recall on single-CPU machines,
mostly bogomips being approx. twice the rated CPU speed), and the
10000-something is the one off base.

Btw., the 10000 can be something like 12000-odd after a machine reboot,
and readout on a different CPU.  The rest are all ca. 4810. Odd indeed.

bjd

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