Unifying DVFS and offlining in mobile multicores

Energy efficiency is a primary design criterion of the modern smartphone due to limitations in battery capacity. Multi-core processors are now commonplace in these devices, which adds a new dimension, the number cores used, to energy management. In this paper we investigate how the mechanisms of frequency scaling and core offlining interact, and how to use them to reduce energy consumption. We find surprising differences in the characteristics of latest-generation smartphones, specifically in the importance of static power. This implies that policies that work well on one processor can lead to poor results on another. We propose a simple policy that integrates core offlining with frequency scaling and implement it in a Linux-based frequency governor called medusa. We show that, despite its simplicity, medusa obtains energy savings that are as good or better than governors presently shipping on the studied phones and approaches the static optimal setting.