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.