🚀 go-pugleaf

In article <35825@lll-winken.LLNL.GOV>, brooks@maddog.llnl.gov writes:
> Fact number 1:
> The best of the microprocessors now EXCEED supercomputers for scalar
> performance and the performance of microprocessors is not yet stagnant.
> On scalar codes, commodity microprocessors ARE the fastest machines at
> any price and custom cpu architectures are doomed in this market.

Alas, I believe you have been sucked into the MIPS=Performance falacy.  There
is *not* a simple relationship between something as basic as scalar performance
and something as complex as overall application (or even routine) performance.

Case in point:  The R2000 chipset implemented on the R/120 (mentioned by others
in this conversation) has, by all measures *excellent* scalar performance.  One
would benchmark it at about 12-14 times a microVAX.  However, in real-world,
doing-useful-work, not-just-simply-benchmarking situations, one finds that
actual performance (i.e., performance in very simple routines with very simple
algorithms doing simple floating point operations) is about 1/2 that expected.

Why?  Because memory bandwidth is *not* as good on a R2000 as it is on other
machines, even machines with considerably "slower" processors.  There are
several components to this, the most important being the cache implementation
on an R/120.  Other implementations using the R2000/R3000/Rx000 chipsets might
well do much better, but only with considerable effort and cost, both of which
mean that those "better" implementations will begin to approach the price/
performance of the "big" machines that you argue will be killed by the
price/performance of commodity microprocessors.

I think you are to a degree correct, but one must always tailor such
generalities with a dose of real-world applications.  I didn't, and I got bit
to the tune of a fine bottle of wine. :-(

Phil

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Philip A. Naecker                            Consulting Software Engineer
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