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SubjectRe: strange CPU speedups with SMP on Athlon 64 X2
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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