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

[Mikhail Kuzminsky]

In message from Rahul Nabar <rpnabar at gmail.com> (Sun, 9 Aug 2009 
22:42:25 -0500):
>(a) I am seeing strange scaling behaviours with Nehlem cores. eg A
>specific DFT (Density Functional Theory) code we use is maxing out
>performance at 2, 4 cpus instead of 8. i.e. runs on 8 cores are
>actually slower than 2 and 4 cores (depending on setup)

If this results are for HyperThreading "ON", it may be not too strange 
because of "virtual cores" competition.

But if this results are for switched off Hyperthreading - it's 
strange.
I have usual good DFT scaling w/number of cores on G03 - about in 7 
times for 8 cores.

Mikhail Kuzminsky
Computer Assistance to Chemical Research Center
Zelinsky Institute of Organic Chemistry RAS
Moscow