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Computing in a Parallel Universe

Multicore chips could bring about the biggest change in computing since the microprocessor

Brian Hayes

The Helium-cooled Laptop

Although there's doubtless a multicore processor in my future (and yours), I'm not yet entirely convinced that massive parallelism is the direction computing will follow for decades to come. There could be further detours and deviations. There could be a U-turn.

The multicore design averts a power catastrophe, but it won't necessarily break through the memory bottle­neck. All of those cores crammed onto a single silicon chip have to compete for the same narrow channel to reach off-chip main memory. As the number of cores increases, contention for memory bandwidth may well be the factor that limits overall system performance.

In the present situation we have an abundance of transistors available but no clear idea of the best way to make use of them. Lots of little processors is one solution, but there are alternatives. One idea is to combine a single high-performance CPU and several gigabytes of main memory on the same sliver of silicon. This system-on-a-chip is an enticing possibility; it would have benefits in price, power and performance. But there are also impediments. For one thing, the steps in fabricating a CPU are different from those that create the highest-density memories, so it's not easy to put both kinds of devices on one chip. There are also institutional barriers: Semiconductor manufacturers tend to have expertise in microprocessors or in memories but not in both.

Finally, we haven't necessarily seen the last of the wicked-fast uniprocessor. The power and memory constraints that have lately driven chipmakers to multicore designs are not fundamental physical limits; they are merely hurdles that engineers have not yet learned to leap. New materials or new fabrication techniques could upset all our assumptions.

A year ago, IBM and Georgia Tech tested an experimental silicon-germanium chip at a clock rate of 500 gigahertz—more than a hundred times the speed of processors now on the market. Reaching that clock rate required cooling the device to 4 Kelvins, which might seem to rule it out as a practical technology. But which is harder: Writing reliable and efficient parallel software, or building a liquid-helium cooler for a laptop computer? I'm not sure I know the answer.

© Brian Hayes

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