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TR10: Probabilistic Chips

Krishna Palem thinks a little uncertainty in chips could extend battery life in mobile devices–and maybe the duration of Moore’s Law, too.
February 19, 2008
Krishna PalemKrishna Palem
Krishna PalemBrent Humphreys

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.

Applications where the limits of human perception reduce the need for precision are perfectly suited to PCMOS designs, Palem says. In cell phones, laptop computers, and other mobile devices, graphics and sound processing consume a significant proportion of the battery power; Palem believes that PCMOS chips might increase battery life as much as tenfold without compromising the user’s experience.

PCMOS also has obvious applications in fields that employ probabilistic approaches, such as cryptography and machine learning. Algorithms used in these fields are typically designed to arrive quickly at an approximate answer. Since PCMOS chips do the same thing, they could achieve in hardware what must be done with software today–with a significant gain in both energy efficiency and speed. Palem envisions devices that use one or more PCMOS coprocessors to handle specialized tasks, such as encryption, while a traditional chip assists with other computing chores.

Palem and his team have already built and started testing a cryptography engine. They are also designing a graphics engine and a chip that people could use to adjust the power consumption and performance of their cell phones: consumers might choose high video or call quality and consume more power or choose lower quality and save the battery. Palem is discussing plans for one or more startup companies to commercialize such PCMOS chips. Companies could launch as early as next year, and products might be available in three or four years.

As silicon transistors become smaller, basic physics means they will become less reliable, says Shekhar Borkar, director of Intel’s Microprocessor Tech­nology Lab. “So what you’re looking at is having a probability of getting the result you wanted,” he says. In addition to developing hardware designs, Palem has created a probabilistic analogue to the Boolean algebra that is at the core of computational logic circuits; it is this probabilistic logic that Borkar believes could keep Moore’s Law on track. Though he says that much work remains to be done, Borkar says Palem’s research “has a very vast applicability in any digital electronics.”

If Palem’s research plays out the way the optimists believe it will, he may have the rebel’s ultimate satisfaction: seeing his heresy become dogma.