RS: [Beowulf] Sempron compile optimization

[Andrew M.A. Cater]

On Mon, Nov 28, 2005 at 09:20:24PM -0800, Greg Lindahl wrote:
> On Tue, Nov 29, 2005 at 03:40:04PM +1100, Chris Samuel wrote:
> 
> > Don't get me wrong, I've been able to build 64-bit versions of NAMD and 
> > various other codes on SLES9 on PPC64 successfully, but I am suprised that 
> > it's not fully native 64-bit.  I certainly was under the impression it was.
> 
> That depends on what "native" means. SGI shipped 64-bit OSes where
> most of the binaries were 32-bit. The only reason Opteron OSes are
> 64-bit is because of the extra registers in 64-bit mode, meaning that
> most programs are faster as 64-bit programs. Our compiler is a 32-bit
> app because it uess a lot of pointers, and so the speed hit of fewer
> registers is more than made up with less cache busting.
> 
Same goes for Red Hat Enterprise 4 - bits and pieces of 32 bit.
Debian's AMD64 port is 64 bit clean to the extent that it doesn't
have any 32 bit libraries [ :) or :( depending on point of view ].
Happily using it at work as a testbed for all sorts of stuff.
So, of course, is the ia64 port.

Most people's view is that full 64 bit is a waste of space and resources
so they're content to limp on with a "half and half" because it's easy
and they don't have to recompile all the apps. The reasoning is that
the bits that matter are probably only the kernel (accessing large
memory), libc and a couple of other bits. 64 bit to serve large
database space probably doesn't care that the maths routines are all
64 bit internally.

In this instance, the HPC community is at odds with the rest of the
Linux world - for some of us, pure 64 bit is the only way to go.
It's certainly the only way to go if you have to support a mixed
suite of Itanium and Opteron or Itanium and Cray, say - it solves
all kinds of weirdness and leaves programmers with all their hair
intact. This is, however, a weird requirement in itself :)

PPC comes from a 32 bit heritage - obviously they haen't quite
shaken it off yet and there is no huge commercial pressure to 
do so.

HTH,

Andy