Re: strange CPU speedups with SMP on Athlon 64 X2

[Willy Tarreau <willy@w.ods.org>]

Hi,

stupid question : isn't it possible that your motherboard does some sort of
overclocking when it detects high cpu usage (bus activity, etc...) ? It
should not be easy to check (rdtsc every second ?), but you might want to
explore such a possibility.

Regards,
willy

On Tue, Aug 30, 2005 at 12:16:04PM -0700, Nathan Becker wrote:
> Hi,
> 
> I'm having a strange problem when I benchmark some of my physics 
> simulation code on my new Athlon 64 X2 4800 machine.  It occurs on all 
> current kernels that I have tested including 2.6.12.5 and 2.6.13.
> 
> If I run my benchmark single threaded, so that one of the two CPU cores 
> is just idling then the calculation goes pretty fast.  But if I load both 
> CPU cores simultaneously but with INDEPENDENT calculations, then each 
> calculation runs about 12-15% faster than when running alone.  I have 
> found this to be always reproducible.  There is no disk access involved 
> in the calculation and RAM usage is fairly minimal so this is not caused 
> by caching. Also, if I compile the kernel to disable SMP then the machine 
> runs a single calculation at the same speed as when running alone when 
> SMP is enabled.
> 
> I am aware of the timing issues on these machines (especially since I 
> reported the bug http://bugzilla.kernel.org/show_bug.cgi?id=5105 ). 
> However, I double-checked my benchmark with a stop-watch, so this is 
> independent of something strange happening in the timer.
> 
> I also checked the cpufreq governor and according to the logs, my CPU is 
> holding steady at the maximum setting of 2.4GHz.  I set the governor to 
> "performance" mode which should prevent unintended downclocking.
> 
> I would be happy to post my exact C source that I use to do the 
> benchmark, but I wanted to get some feedback first in case I'm just doing 
> something stupid.  Also, since I'm not subscribed to this list, please cc 
> me directly regarding this topic.
> 
> Thanks very much,
> 
> Nathan
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