SAN JOSE, Calif. – Chip designers face a growing problem of dark silicon they cannot use due to rising power leakage. A new class of fine-grained, application-specific cores can help reclaim lost die area and create more efficient processors, said a University of California researcher developing a prototype Android processor that uses the approach.
"With each process node, the percent of a chip you can actively switch drops exponentially," said Nathan Goulding, a graduate student at UC San Diego in a paper at Hot Chips. "The utilization wall is here and scaling theory says it will just get worse," he said.
An experimental 45-nm block had 2.8 times less useable die area than a similar block in a 90nm part when handling the same function in a similar power envelop, he reported. The answer, Goulding said, is to create a variety of application-specific cores that could deliver eight- to eleven-fold energy savings compared to using a general-purpose CPU.
UCSD researchers are building the Green Droid application processor to prove out their concepts. They have already identified and designed--using their own automated tools--21 cores that handle energy intensive jobs specific to a Google Android handset.
The group's so-called Conservation cores run tasks that may consist of a handful of instructions or loops. They range from running some aspect of Android's Dalvik virtual machine to processing key code for Linux or memory management functions.
Goulding claims his team has put in 7mm2 of silicon the capability to run 43,000 fundamental instructions that cover 95 percent of the most energy intensive tasks of an Android handset. The cores are managed by a central MIPS processor, a CPU chosen simply because the university had access to the core.
One of the most interesting aspects of the project is the automated tool the group created to generate its cores. It consists of a compiler that condenses a group of instructions into a higher level task and a code generator that spits out the Verilog to run the job in silicon.
"We wanted to control the whole process of creating the cores," said Goulding.
The application-specific cores don't have the MIPS CPU's overhead of fetching general purpose instructions and managing low-level registers. However, much work remains.
The team has yet to determine how to keep a MIPS host in a low power state while the accelerator cores work and how to arbitrate access to shared memory. Although the Green Droid design is still in an early state, its concepts stirred plenty of questions at the conference.
"I haven’t seen anything in accelerator cores at this granularity before," said Marc Tremblay, a veteran processor designer who recently left the former Sun Microsystems for a position at Microsoft.
UCSD graduate student Nathan Goulding presents his concepts of application accelerators