Why is 64 bit faster. RE: [Beowulf] Win64 Clusters!!!!!!!!!!!!

Mark Hahn hahn at mcmaster.ca
Thu Apr 12 07:47:59 EDT 2007


> 1.) Why is a 64 bit cpu faster?  I had assumed the main benefit was the
> memory that could be addressed, obviously a bad assumption.

being able to address more memory is indeed critical for some codes.
certainly not all; in fact, the larger pointers hurt some codes.
64b mode also enables a lot more registers, of which the compiler can 
make very good use.  and some codes like the extra register width.

> 2.) If you install the 32 bit version of an os, say linux, instead of
> the 64 bit version and then run 32 bit apps, do you get the speedup?

no.  in any case, I doubt there's any reason to install 32b linux,
since a 64b kernel _can_ support 32b processes (which actually see a 
somewhat bigger practical address space...)
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