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The demands placed on smart phones by marathon sessions of texting, streaming video and surfing the Web require that they have blazing-fast processors while, at the same time, be able to disburse the heat these processors generate. A team of engineers is proposing something of a counterintuitive model to designing smart phones in the future—one that has processors alternately powering up and then cooling down, more like sprinters than long-distance runners.
Heat dissipation has become a major limitation to the computational power of processors used in smart phones, where there is no room for a fan or other type of cooling system. Only a fraction of the transistors on a smart phone's processor can safely operate at a time. The transistors that are shut down are known as "dark silicon." According to the researchers, more transistors will only lead to more dark silicon because improvements in the processors' ability to dissipate heat are lagging.
University of Pennsylvania and University of Michigan researchers suggest a "computational sprinting" approach to power usage. A processor could run at, for example, 16 times its sustainable rate for about half a second and then throttle back so that the processor can cool down—until the next time a surge is needed. The researchers presented the findings of their computational sprinting feasibility studies (pdf) yesterday at the 18th International Symposium on High Performance Computer Architecture in New Orleans.
Many interactive applications on smart phones depend on short bursts of computational demand punctuated by long idle periods waiting for user input. Computational sprinting would activate otherwise powered-down processor cores (dark silicon) for subsecond bursts of intense parallel computation in response to such sporadic user activity. To handle the heat pulse from sprinting, the researchers propose encasing the processor in a phase-change material—something like candle wax—that would absorb heat by melting during the sprint, then slowly dissipate it by hardening while the device is at rest.
Most of the heat generated by mobile phones comes from the processor, as opposed to the battery, says Thomas Wenisch, a study author and an assistant professor of computer science and engineering at the University of Michigan. "All of the power drawn by the processor turns into heat," he says. "Only a fraction drawn from the battery is lost as heat within the battery due to battery inefficiency (internal resistance in the battery)."
Next up for the researchers is constructing a prototype, focusing on the thermal and material issues, says Milo Martin, another study author and an associate professor in the department of computer and information sciences at the University of Pennsylvania's School of Engineering and Applied Science. "We are also looking at additional applications, mostly in the mobile space but also in other domains, such as servers," he adds.
The researchers also plan to investigate what impact sprinting would have on application design. "This would dictate how programmers should exploit sprints and then deal with the fact that the phone might need to rest, and its impact on end-user satisfaction," Martin says.