[Beowulf] Win64 Clusters!!!!!!!!!!!!

[Mark Hahn]

> performance boost when going to 64 bit in most situations. Core 2 chips
> suffer a small penalty when going 64 bit. I suspect this is due to the

Core2 has some micro-optimizations which are only enabled for 32b mode,
which is an interesting choice.  AMD tends to be biased towards HPC
or at least serverish stuff, which is 64b-oriented...

> cache hit rate suffering which, with a traditional north bridge, would
> have proportionally more effect.

I can't think of negative effects of x86_64 other than possible 
higher memory overhead for apps that use lots of pointers.
compilers are a typical example, but perhaps some sparse matrix stuff, too? 
code density in x86_64 mode is not much different in mcmodel=small
(for instance, it's not the case that every offset in branch and memory 
reference instructions become giant...)

it'll be most interesting to see how AMD has optimized Barcelona...