[Beowulf] advantages of this particular 64-bit chip

Greg Lindahl lindahl at pathscale.com
Mon Mar 1 20:57:48 EST 2004

On Mon, Mar 01, 2004 at 07:37:11PM -0500, Mark Hahn wrote:

> bah.  buying chips based on their address register width makes 
> about as much sense as buying based on clock.  yes, some people have 
> good reason to be excited about 64b hitting the mass market.  but 
> that number is quite small - how many machines do you have with 
> >4 GB per cpu?

Don't forget that "64 bits", in this case means "wider GPRs, and twice
as many, plus a better ABI." These are substantial wins on many codes,
even on machines with small memories. Bignums are a well known example,
but there are far more general-purpose examples.

For example, with the PathScale compilers on the Opteron, we find that
only 1 of the SPECfp benchmarks and 3 of the SPECint benchmarks run
faster in 32-bit mode than 64-bit mode -- keeping in mind that 64-bit
mode features longer instructions and bigger pointers and longs. (This
is our alpha 32-bit mode vs. our beta 64-bit mode, so this answer will
change a little by the time both are production quality.)

So yes, there's a reason to buy Opteron and IA32e chips beyond the
address width: more bang for the buck.

-- greg

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