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AltiVec vs. Pentium III

A Little Bad Math Goes a Long Way

Dan Knight - 2000.11.21

If there's one thing that gets my goat, it's bad math and statistics.

In Gauging the Gigaflops Gap, Don Granberry of ZDNet tries to explain how the G4 with AltiVec is superior to the Pentium III. It's actually a fairly accessible explanation, but the numbers are flawed.

Granberry uses the following table to compare a 700 MHz PIII with a 500 MHz G4:

It's a nice table. It's clearly labeled. The facts are clearly established. But there's one piece of misleading data: CPU speed doesn't mean a thing when you're looking at how many clock cycles are used.

MHz for MHz, the G4 is 3.68 times more efficient at these particular functions (or, as Granberry puts it, the G4 does the same work in 27% as many processor cycles).

This wouldn't be a problem, except that later in the article he links clock speed to this. Which wouldn't be a problem, except that he doesn't apply the same clock speed to both processors.

Multiplying 3.68 by 700 MHz, Granberry states that a 2.6 GHz Pentium III would match the performance of a G4/500 on these tests - but it just isn't so.

The problem is the misleading data (remember how story problems would sometimes introduce numbers that were not necessary to solve the problem?) brought him to the wrong number. No matter how precise the math, if you're multiplying the wrong number, you get the wrong result.

Instead of 2,576 MHz, he should have multiplied 3.68 by 500 MHz, resulting in a Pentium III running at 1,840 MHz to match the G4/500's performance on these particular functions.

Of course, Apple is now shipping the Power Mac G4/500 in a dual-processor configuration, so it would take 3,680 Pentium III MHz to match it. For the sake of convenience, let's say a quad-processor PIII/900 system would match the dual-processor G4/500 on these vector functions.

Whether this means much in the real world is another question entirely, but at least we have the math right.

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