strange CPU speedups with SMP on Athlon 64 X2

From: Nathan Becker (nbecker_at_physics.ucsb.edu)
Date: 08/30/05

  • Next message: Holger Kiehl: "Re: Where is the performance bottleneck?" 
    Date:	Tue, 30 Aug 2005 12:16:04 -0700 (PDT)
To: linux-kernel@vger.kernel.org

    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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