Re: [asa] NASA - Climate Simulation Computer Becomes More Powerful

On Aug 30, 2009, at 12:44 PM, Bill Powers wrote:

> I agree and remember much that you recount.
> But to say that with 64-bit machines roundoff is not an issue is
> clearly false. At LANL we have had 64-bit machines since the 60s,
> although some still remember the shift from 32-bit. Those who
> remember that shift, also remember the lessons learned from roundoff
> in nonlinear systems. For a long time now we have been clamoring for
> 128-bit machines because studies of numerical noise, when they are
> done, indicate that they are biting us in the rear end now. With
> the advent of 3D codes and their eventual inclusion of the full
> spectrum of physics packages, cycles increase wildly, making
> numerical noise the hidden demon that everyone is afraid to examine.
>
> There were some machines that could implement 128-bit machines via
> software. Although this slowed codes down to a crawl, some had the
> guts to see what would result. We also had the capability with some
> machines to make numerical noise experiments by controlling the
> number of bits for real computations. From these results, certain
> extrapolated results might be derived.
>
> Still I know of very few codes, certainly at the national labs, that
> seriously undertook these kinds of studies.
>
> bill
>

64-bit is a bit a misnomer here. The Xeon 51xx (Opteron 23xx) series
introduced 128-bit SSE units. Then came Harpertown and then Nehelem
which is in the new NASA computer. The Xeon 5570 (I am assuming they
are using this Nehelem processor) is "only" three times faster than
the Xeon 54xx machines (Harpertown). Nehelem is faster at
multiplication than the Opteron while the latter is faster at
division. This trade-off was chosen since multiplication was more
common.

Bottom line: these boxes do 128-bit SIMD hardware floating point
really, really fast, and have a 64-bit memory address space. This
combination is what is making the folks at NASA drool.

Rich Blinne
Member ASA

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Received on Sun Aug 30 15:57:24 2009