[Beowulf] performance tweaks and optimum memory configs for a Nehalem

David N. Lombard dnlombar at ichips.intel.com
Tue Aug 11 11:40:56 EDT 2009


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<jlb17 at duke.edu> 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.
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