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

[Mark Hahn]

> Is it a bad mistake to configure a Nehalem (2 sockets quad core giving
> a total of 8 cores; E5520) with 16 GB RAM (4 DIMMs of 4GB each)? I

there's no ambiguity here: unpopulated channels decrease bandwidth
and/or concurrency.  (does anyone know whether nehalem can "ungang"
memory channels like opteron can?  it would be fascinating to see 
benchmarks showing a benefit to higher memory concurrency for a 
manycore workload...)

> Our code requirements dictate 2 GB / core is enough. Should I be
> paying for the additional RAM to make it 24 GB?

ram is, historically and relatively, cheap.  otoh, can your code get 
by with 1.5G/core?  actually, I tend to see some association with 
smallish memory footprints (2G/core is definitely not large) with 
cache-friendliness.  this would argue that the higher bandwidth may 
not make much difference to your code...

regards, mark hahn.