On Mon, Aug 10, 2009 at 01:02:51PM -0700, Rahul Nabar wrote: > On Mon, Aug 10, 2009 at 2:09 PM, Joshua Baker-LePain<[email protected]> wrote: > > Well, as there are only 8 "real" cores, running a computationally intensive > > process across 16 should *definitely* do worse than across 8.
<YMMV> Some workloads will benefit materially from SMT, some are neutral, and some will degrade. For those that degrade, simply not oversubscribing the physical cores will get best performance. </YMMV> > > However, it's > > not so surprising that you're seeing peak performance with 2-4 threads. > > Nehalem can actually overclock itself when only some of the cores are busy > > -- it's called Turbo Mode. That *could* be what you're seeing. > > That could very well be it! Is there any way to test if the CPU has > overclocked itself? There's an application note on the subect at: <http://download.intel.com/design/processor/applnots/320354.pdf> Be aware this document is very technical, talking about MSRs & performance counters. > Or can I turn the "turbo mode" off and check? That would work, but... Alternately, take a look at <http://software.intel.com/en-us/articles/using-enhanced-intel-speedstep-features-in-hpc-clusters/> -- David N. Lombard, Intel, Irvine, CA I do not speak for Intel Corporation; all comments are strictly my own. _______________________________________________ Beowulf mailing list, [email protected] sponsored by Penguin Computing To change your subscription (digest mode or unsubscribe) visit http://www.beowulf.org/mailman/listinfo/beowulf
