1 Introduction
Continuous mobile sensing applications have been increasingly emerging, for instance, trajectory logging [44], dust level monitor [25], interaction monitor [17], [31], group-aware ads and resource planning [41], and calorie monitor [33]. These applications provide useful services to mobile users while running in the background, not requiring any explicit user intervention. However, many users are still reluctant to run such applications; these applications incur significant energy consumption and take up computational resources, potentially disrupting other common uses of the smartphones.