ARM brews Big.Little tech for less greedy smartphones

ARM has introduced Big.Little technology, which twins a low-powered chip with a high-powered one in order to extend battery life on phones and tablets without sacrificing processing power.

The system, announced on Wednesday, provides a recipe for chaining chips of differing architectures together via an interconnect to maximise power efficiency without having to concede on computational oomph. The Cambridge chip designer said it has applied Big.Little to couple up to four dual-core Cortex-A7 processors with up to four dual-core Cortex-A15 processors, via a CCI-400 Coherent Interconnect.

It allows a device to power down to just an A7 core when performing trivial computing tasks, such as syncing data or browsing simple websites, and to transition to an A15 processor when performing a more difficult task, such as playing a game or encoding a video.

"There's an increasing demand for what we'd call low-level types of tasks," Tom Cronk, deputy manager of ARM's processor division, said at press launch. "It's becoming increasingly difficult to serve that within a single-processor implementation."

Scale down power use

The upshot is Big.Little means that high-end smartphones will have the ability to scale down their power use to that of a feature phone, if the application is simple enough.

There's an increasing demand for what we'd call low-level types of tasks. It's becoming increasingly difficult to serve that within a single-processor implementation.

– Tom Cronk, ARM

"The combination of these [processors] would deliver, over the use-cycle of a phone, a 70-percent energy saving over today's dual-core Cortex A9-type systems," Cronk said. "It's invisible to software... the OS doesn't have to have a sense of what processor it's running on at a time, nor do the applications."

Though the system only allows one type of chip to work at any one time — either A7 or A15, but not both — Cronk said ARM is working on technologies that could allow multiple applications to be run across the chips in parallel. If this comes to fruition, an application could use an A15 core to edit a photo while an A7 devoted itself to syncing data, though this would drain the battery faster.

The technology, which was introduced alongside the Cortex-A7 processor, is expected to make it into phones in a couple of years' time. 

In concept, Big.Little is "very similar" to the low-powered companion core system within Nvidia's ARM-based Kal-El processor, or to Qualcomm's asynchronous symmetric multiprocessing (aSMP) technology that has made it into its ARM-based Snapdragon chips, Cronk said.

However, ARM's technology allows for two processors with different architectures to sit alongside one another, he noted. Nvidia uses the same chip but tuned for power as opposed to performance at the process level, while Qualcomm just varies the voltage on the core, he said.