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Subjectstrange CPU speedups with SMP on Athlon 64 X2
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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