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App-Specific Processors to Fight Dark Silicon

A processor designed around Android’s most-used apps could significantly extend the lives of smart-phone batteries.

A processor etched with circuits tailored to the most widely used apps on Android phones could help extend the devices’ battery life. Researchers at the University of California, San Diego have created software that scans the operating system and a collection of the most popular apps and then generates a processor design tailored to their demands. The result can be 11 times more efficient than today’s typical general-purpose smart-phone chip, says Michael Taylor, who leads the GreenDroid project with colleague Steven Swanson.

Living Longer: Microprocessors designed around the most-used apps could make smart phones more energy-efficient.

“Chip design for mobile phones needs rethinking for two reasons,” says Taylor. “One is to improve their use of the limited energy available to a phone, and the other is to attack a problem called dark silicon, which is set to make conventional chip designs even less efficient.”

“Dark silicon” is a portion of a microchip that is left unused. Although uncommon today, dark silicon is expected to become necessary in two or three years, because engineers will be unable to reduce chips’ operating voltages any further to offset increases in power consumption and waste heat produced by smaller, faster chips.

Operating shrinking transistors with lower voltages was “traditionally the escape valve that enabled more computational power without more heat output,” says Taylor, “but now there is no place to go.” Operating voltages have crept close to a fundamental limit at which transistors cease to function practically. This means that soon, as transistors continue to get smaller, each generation of chips will be less efficient than the one before, he says. “If you kept using all of the chip, each generation would generate double the heat of the one before.” Keeping energy use constant will require switching on only certain parts of a chip at any one time.

Taylor and Swanson’s GreenDroid design sidesteps this by surrounding a processor’s main core—the part of a chip that executes instructions—with 120 smaller ones that each take care of one piece of code frequently needed by the apps used most on a phone. Each core’s circuits closely mimic the structure of the code on which they are based, making them up to 10,000 times more efficient than a general-purpose processor core performing the same task. “If you fill the chip with highly specialized cores, then the fraction of the chip that is lit up at one time can be the most energy efficient for that particular task,” Taylor says.

Rather than manually translating source code into processor cores, the UCSD team has developed software to do it. They record the computational demands of the Android OS when running popular apps for e-mail, maps, video, and the Web radio service Pandora, among others, and from that information, the software generates the GreenDroid chip design.

Because around 70 percent of that code is shared between multiple apps or parts of the OS, a GreenDroid’s specialized cores can handle much of a phone’s most energy-sapping work. Detailed simulations of a complete GreenDroid processor prove its superior efficiency, says Taylor. “We’re sending the first design off to be fabricated in June and have designed a board so we can plug it in, install Android and apps, and then benchmark against conventional designs,” he says.

Having a custom processor fabricated is extremely expensive and rare in academia. The chip will use transistors smaller than those currently on the market, with feature sizes as small as 28 nanometers. Processors with 32-nanometer features have only recently reached the market, and it is in the next generation, at 22 nanometers, that dark silicon is expected to become a serious challenge.

Kevin Skadron, a professor at the University of Virginia, says the UCSD strategy is a good fit with smart phones, because apps are tightly integrated with a smart phone’s OS. “They are wise to target Android,” he says, “because on a phone the OS is responsible for a huge amount of the work done by the processor. That means every user of every phone will benefit from their specialized cores.” Phones with GreenDroid-style processors can be expected to last longer than conventional phones with the same battery, or to have the same lifetime with a sleeker design, he says.

However, the specialized hardware of this approach has drawbacks that make it less useful for nonmobile devices, says Skadron. “It’s more challenging with a PC or server, because the operating system has less effect on what the processor does. The applications on top of that are most important, and they vary a lot more between users.”

That drawback of specialization could apply to phones, too—for example, if new apps emerge that are unlike those used to generate a GreenDroid design. “For mobile phones, we’re not too worried, because people replace them so quickly,” says Taylor. And because upgrades to apps and operating systems tend to be evolutionary rather than revolutionary, he says, it’s unlikely that many of a GreenDroid chip’s specialized cores would become completely useless during a smart phone’s short lifetime. Taylor and Swanson did add features to their design that allow slight tweaking of conservation cores to fit new code, but some upgrades will be too big for that. “If that happens, your phone wouldn’t stop working, but its energy efficiency would drop,” says Taylor, noting that this prospect probably wouldn’t trouble manufacturers too much anyway.

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