The key to ARM Holdings plc's "big-little" dual core scheme is the ability to switch as quickly and seamlessly as possible between processing on the Cortex-A15 MPCore and the new Cortex-A7 core, according to Peter Greenhalgh, an ARM engineer who served as the lead designer on the A7.
"We've designed them so that you can transition really seamlessly and quickly and got the software to do it in a seamless way," Greenhalgh said in an interview last week in San Francisco.
ARM (Cambridge, England) last week rolled out the power efficient A7 core and also described the big-little scheme, where A7 is implemented alongside ARM's high-end A15 as part of a heterogeneous power-driven multicore strategy. Big-little enables a smartphone to rapidly switch from using one core or the other, depending on the task load, to ensure optimal power efficiency. The Cortex-A7, on its own, will enable sub-$100 entry level smartphones, according to ARM.
The biggest challenge associated with the design project, according to Greenhalgh, was to devise a scheme that would enable implementing big-little to run existing software. But, he said, smartphones have for years had various operating points, and it was possible for the ARM team to leverage the existing power infrastructure framework to determine when the handset should switch from one core to the other.
"You've already got all of the software on the OS which is able to save operating points, all you do is that when you get to the lowest level on the A15, invoke the switching software," Greenhalgh said. "The key is having both processors with identical instruction sets so the same things will run on them and you can switch back and forth very quickly."
According to Mike Inglis, executive vice president and general manager of ARM's processor division, a typical smartphone can leverage the A7 core for most applications, such as phone calls, data access and casual gaming, but switch to the A15 core for more demanding applications such as HD gaming and rich web services.
ARM says the A7 provides up to 70 percent power savings compared to the A15 on common workloads. But, according to Greenhalgh, that actual power savings is highly dependent on the application being used. "If you are just running games, it won't be [70 percent] better," he said. But in a typical workflow, involving gaming, listening to audio flies, running GPS navigation and other applications, the 70 percent savings is achievable, he said.
But, Greenhalgh added, 70 percent power savings in the processor won't translate into 70 percent improvement in battery life. A processor typically comprises only about 30 percent of a smartphone's total power budget.
Greenhalgh said big-little is similar in some respects to the approach that ARM licensee Nvidia Corp. has undertaken with its next-generation Tegra processor, codenamed Kal-El. He described Nvidia's work as "a good first step," but said that big-little takes the approach further.
"We find it very interesting to see what Nvidia has done," Greenhalgh said. "It's fantastic, and it validates big-little. You can use all of those techniques and you can lay the microarchitecture on their as well."
Greenhalgh said the many licensees in the ARM ecosystem bolster the innovation on ARM processors by building upon them. "Everyone has their own take," Greenhalgh said. "You still allow partners to differentiate in power and performance."
Greenhalgh, who has worked at ARM for 10 years, said the A7 project was similar to other design projects he has worked on in the past. He declined to describe the size of the team involved in the project.
Later this week, Greenhalgh will present a paper at ARM TechCon in Santa Clara, Calif., which will discuss the big-little concept. The presentation is scheduled for 3:30 p.m. on Wednesday, Oct. 26.
The A& and ARM's big-little concept have generated a good deal of excitement. But Greenhalgh has already moved on to his next project, which he described as exciting but could not talk much about.