[Discuss] FPU differences based on different chips.

[bogstad at pobox.com (Bill Bogstad)]

On Fri, Aug 31, 2012 at 4:18 PM, Jerry Feldman <gaf at blu.org> wrote:

> We've got a situation where on a single monte-carlo simulation, there is a
> difference between a result using a Xeon E5530 2.4G and a E5570 2.93G. In
> gleaning out some details, the E5530 is a VMware ESX where I think the
> E5570is a physical machine.  I've suggested that the guy run another test
> on a system on another VMware server (with an E7 - 8870).  In this product
> the engineer is testing, there are many different simulations, but only one
> is showing a difference (about 5 or 6 decimal places. Additionally, there
> is a random number generator involved. We've also got a few different
> versions of RHEL (5.2, 5.4, and 5.8).
>

It's not clear from your email if the same simulation has been run more
then once on the same machine and if you
saw differences in that case.

And the fact that a random number generator is involved is an incredibly
huge red flag.   Is there a way to run the simulations with a pseudo-random
number generator with a known initial seed?   If only for testing purposes
(like this), it is often helpful when something like this comes up.

Bill Bogstad