Automated OS-level Device Runtime Power Management

Non-CPU devices on a modern system-on-a-chip (SoC), ranging from accelerators to I/O controllers, account for a significant portion of the chip area. It is therefore vital for system energy efficiency that idle devices can enter a low-power state while still meeting the performance expectation. This is called device runtime Power Management (PM) for which individual device drivers in commodity OSes are held responsible today. Based on the observations of existing drivers and their evolution, we consider it harmful to rely on drivers for device runtime PM. This paper identifies three pieces of information as essential to device runtime PM, and shows that they can be obtained without involving drivers, either by using a software-only approach, or more efficiently, by adding one register bit to each device. We thus suggest a structural change to the current Linux runtime PM framework, replacing the PM code in all applicable drivers with a single kernel module called the central PM agent. Experimental evaluations show that the central PM agent is just as effective as hand-tuned driver PM code. The paper also presents a tool called PowerAdvisor that simplifies driver PM efforts under the current Linux runtime PM framework. PowerAdvisor analyzes execution traces and suggests where to insert PM calls in driver source code. Despite being a best-effort tool, PowerAdvisor not only reproduces hand-tuned PM code from stock drivers, but also correctly suggests PM code never known before. Overall, our experience shows that it is promising to ultimately free driver developers from manual PM.