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

Rahul Nabar rpnabar at gmail.com
Sat Aug 8 19:50:40 EDT 2009


On Fri, Aug 7, 2009 at 7:58 PM, Gus Correa<gus at ldeo.columbia.edu> wrote:
> Some people are reporting good results when using the
> Nehalem hypethreading feature (activated on the BIOS).
> When the code permits, this virtually doubles the number
> of cores on Nehalems.
> That feature works very well on IBM PPC-6 processors
> (IBM calls it "simultaneous multi-threading" SMT, IIRR),
> and scales by a factor of >1.5, at least with the atmospheric
> model I tried.


Thanks for all the useful comments, Gus!  Hyperthreading is confusing
the hell out of me. I expected to see 8 cores in cat /proc/cpuinfo Now
I see 16. (This means I must have left hyperthreading on I guess; I
ought to go to the server room; reboot and check the BIOS)

This is confusing my benchmarking too. Let's say I ran an MPI job with
-np 4. If there was no other job on this machine would hyperthreading
bring the other CPUs into play as well?

The reason I ask is this: I have noticed that a single 4 core job is
slower than two 4 core jobs run simultanously. This seems puzzling to
me.

-- 
Rahul
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