Messages in this thread | | | Date | Wed, 31 Aug 2005 06:51:31 +0200 | From | Willy Tarreau <> | Subject | Re: strange CPU speedups with SMP on Athlon 64 X2 |
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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 > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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