Multicore_CPUs_Processor_Proliferation-IEEE_Spectrum -...

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2/15/11 1:51 PM Multicore CPUs: Processor Proliferation - IEEE Spectrum Page 1 of 4 Illustration: Frank Chimero SEMICONDUCTORS / PROCESSORS FEATURE Multicore CPUs: Processor Proliferation From multicore to many-core to hard-to-describe-in-a-single-word core By SAMUEL K. MOORE / JANUARY 2011 This is part of IEEE Spectrum's special report: Top 11 Technologies of the Decade Back in 1994, programmers figured that whatever code they wrote would run at least 50 percent faster on a 1995 machine and 50 percent faster still on a '96 system. Coding would continue as it always had, with instructions designed to be executed one after the other. But Kunle Olukotun, then a newly minted professor of electrical engineering at Stanford, saw that the party couldn't go on forever. The microprocessors of the day couldn't scale up as efficiently as you'd expect through the mere addition of ever more and ever faster transistors, the two things that Moore's Law provided. To solve that problem, Olukotun and his students designed the first general-purpose multicore CPU . This idea, more than any other in the past decade, is what has kept the semiconductor industry climbing the Moore's Law performance curve. Without multicore chips, the computing capability of everything from servers to netbooks would not be much better than it was a decade ago. Everyone's happy—except perhaps for the programmers, who must now write code with threads of instructions that must be executed together—in pairs, quartets, or even larger groupings. It's not that old, single-core CPUs weren't already doing some parallel processing. When Olukotun began his work, most microprocessors had a "superscalar" architecture. In the superscalar scheme, the CPU contained many replicated
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2/15/11 1:51 PM Multicore CPUs: Processor Proliferation - IEEE Spectrum Page 2 of 4 components, such as arithmetic units. Individual instructions would be parceled out to the waiting components. Scaling up such "instruction-level parallelism" meant building in more and more such components as the years rolled by. Olukotun argued that within a few more generations, it wasn't going to be worth the effort. You needed to provide a quadratic increase in resources for a linear increase in performance, he said, because of the complexity of the logic involved in parceling out and keeping track of all the instructions. If you combined that with the delays inherent in the mess of interconnects among all those parts, it seemed a losing proposition. Doug Burger and Stephen Keckler, both computer scientists at the University of Texas, Austin, put a finer point on it later in the decade, calculating that instead
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Multicore_CPUs_Processor_Proliferation-IEEE_Spectrum -...

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