Hardware

Member debate: Is there such a thing as the Megahertz Myth?

Can a CPU that is rated half as fast as another actually accomplish tasks faster? It can, according to Apple, which calls the need for more megahertz a myth. Join this Member Debate and tell us if you think mega megahertz is more about hype than speed.


As the speeds of Intel Pentium 4 processors hit the 1.8-GHz mark, other processor manufacturers are downplaying the role that frequency plays in productivity.

A prime example of this can be found on the Apple Computer Web site, which features a video discussion by a top Apple engineer who calls the gigahertz war nothing more than the “Megahertz Myth.” Apple’s fastest CPUs claim to run at 867 MHz.

The question for debate is: Is processor frequency, or speed, an accurate representation of how productive a CPU can be?

Apple uses a processor made by Motorola called the PowerPC chip, and it argues that even though its chip frequencies are much lower than Intel’s, its computers actually accomplish tasks 30 percent or more faster than the fastest Intel chip.

You can check out the video at Apple, but you will need to install a copy of QuickTime player, if you haven’t already.

Apple’s argument
In the video, Apple’s senior vice president of hardware, Jon Rubinstein, focuses his argument on the relationship between frequency and pipeline length.

Rubinstein points out that the PowerPC G4 chip has only seven pipeline stages, compared to Intel’s Pentium 4 with its 20 pipeline stages. He uses a graphic to show how the same instruction sets would travel through these different length pipelines, with the result being that the instructions are displayed at the end.

The shorter pipeline produces more results quicker, but Rubinstein admits that the longer pipeline, at twice the frequency, would produce an end product at about the same time in a perfect world.

But the world is an imperfect place, he says, and in the pipeline there will be bubbles of no data where there are data dependencies that require previous data to execute. Even worse are branching instructions that cause the entire pipeline to clear. This is what Rubinstein calls the “pipeline tax,” and he says it occurs fairly frequently.

Because the longer pipeline flushes so much data out of the pipeline when it hits one of these data dependencies, it takes more time to refill the pipeline with new instructions, compared to the shorter pipeline that flushes and refills quicker.

After making this case, Rubinstein points out that Intel’s next processor, the Itanium, will have only 10 pipeline stages and a frequency of only 800 MHz.

Another detractor
Apple isn’t the only company to find fault with the push for more megahertz. Intel’s biggest competitor, AMD, makes a similar argument against the Pentium 4.

According to an AMD press release: “The 1.4-GHz AMD Athlon processor-based systems with Double Data Rate (DDR) memory continue to outperform 1.7-GHz Intel Pentium 4 processor-based systems by up to 40 percent on a variety of benchmarks, including cutting-edge multimedia, desktop publishing, digital imaging, and voice recognition applications.”

The same press release quotes Pat Moorhead, vice president of desktop and mobile marketing for AMD's Computation Products Group, as saying, "It is evident from competitive offerings that megahertz alone is no longer a clear indicator of processor performance.”

Join the debate!
Now it’s time for you to put your two cents into the discussion. Is megahertz the defining attribute of a processor? Does it depend on what types of tasks need to be performed? Tell us what you think in the discussion below or send us a note.

 

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