Probabilistic MicroChips

Krishna Palem Credit: Brent Humphreys

Multimedia • Krishna Palem on the importance of error.

Who: Krishna Palem, Rice University Definition: PCMOS is a microchip design technology that allows engineers to trade a small degree of accuracy in computation for substantial energy savings. Impact: In the short term, PCMOS designs could significantly increase battery life in mobile devices; in a decade, the theories behind PCMOS may need to be invoked if Moore’s Law is to continue to hold. Context: Palem and his collaborators have begun building test chips for specific applications; Palem is working on plans for startup companies to commercialize the technology.

Krishna Palem is a heretic. In the world of microchips, precision and perfection have always been imperative. Every step of the fabrication process involves testing and retesting and is aimed at ensuring that every chip calculates the exact answer every time. But Palem, a professor of computing at Rice ­University, believes that a little error can be a good thing.

Palem has developed a way for chips to use significantly less power in exchange for a small loss of precision. His concept carrie­s the daunting moniker "probabilistic complementary metal-oxide semi­conductor technology"--PCMOS for short. Palem's premise is that for many applications--in particular those like audio or video processing, where the final result isn't a number--maximum precision is ­unnecessary. Instead, chips could be designed to produce the correct answer sometimes, but only come close the rest of the time. Because the errors would be small, so would their effects: in essence, Palem believes that in computing, close enough is often good enough.

Every calculation done by a microchip depends on its transistors' registering either a 1 or a 0 as electrons flow through them in response to an applied voltage. But electron­s move constantly, producing electrical "noise." In order to overcome noise and ensure that their transistors register the correct values, most chips run at a relatively high voltage. Palem's idea is to lower the operating voltage of parts of a chip--specifically, the logic circuits that calculate the least significant bits, such as the 3 in the number 21,693. The resulting decrease in signal-to-noise ratio means those circuits would occasionally arrive at the wrong answer, but engineers can calculate the probability of getting the right answer for any specific voltage. "Relaxing the probability of correctness even a little bit can produce significant savings in energy," Palem says.

Within a few years, chips using such designs could boost battery life in mobile devices such as music players and cell phones. But in a decade or so, Palem's ideas could have a much larger impact. By then, silicon transistors will be so small that engineers won't be able to precisely control their behavior: the transistors will be inherently probabilistic. Palem's techniques could then become important to the continuation of Moore's Law, the exponential increase in transistor density--and thus in computing power--that has persisted for four decades.

When Palem began working on the idea around 2002, skepticism about the prin­ciples behind PCMOS was "pretty universal," he says. That changed in 2006. He and his students simulated a PCMOS circuit that would be part of a chip for processing video, such as streaming video in a cell phone, and compared it with the performance of existing chips. They presented the work at a technical conference, and in a show of hands, much of the audience couldn't discern any difference in picture quality.