[Beowulf] CCL:Experiences with 64 bits AMD processors (fwd from marc@klingon.uab.es)

[James Cownie]

> Recompiling your software in 64bit mode (which is the default, you
> can specify -m32 if you insist on 32bit executables) will make your
> software run faster:

... Architectural reasons elided ...

I think this is somewhat over-confident. 

Consider, for instance, that pointer intensive code sees an effective
cache which is only half as big when running in 64 bit mode vs 32 bit
mode. Halving the cache can certainly have a bad effect on performance
:-(

Note, too, that for SPEC submissions on some of the other machines
which can execute in both 32 and 64 bit modes the vendors have chosen
to submit some results compiled in each mode to get the best overall
performance. (The Alpha springs to mind here).

Of course the architectural changes between 32 and 64 bit modes on the
AMD64 processors may mean that this is not worth it, and the extra
registers gain you more than you lose, but the outright "Recompiling
in 64bit mode will make your software run faster" is certainly false.

As ever you need to test your specific codes with specific compilers.

-- 
-- Jim
--
James Cownie	<jcownie at etnus.com>
Etnus, LLC.     +44 117 9071438
http://www.etnus.com