MobileHub: No Programmer Effort for Power Efficiency with Sensor Hub

Abstract

T oday’s smartphones provide a rich sensing platform that developers have used in tens of thousands of mobile applications. Many of these applications require continuous sensing for tasks ranging from simple step counting to more complex fall detection, sleep apnea diagnoses, dangerous driver monitoring and others. Unfortunately, continuous sensing applications are power-hungry. Interestingly, it is neither the sensors nor the computation that make these applications battery drainers. Instead, the main processor needs to be powered on frequently to collect sensor samples, in turn increasing the power consumption [12, 9, 13]. Hardware manufacturers recognize that supporting low-power continuous sensing is crucial. To this end, companies are embedding a low power microcontroller called a sensor hub in their smartphones [11, 10, 2]. Th e sensor hub continuously collects sensor data keeping the higher power main processor idle. In practice, however, sensor hubs fail to deliver on their power-effi ciency promise. Th e problem is in the diffi culty in programming them. For example, to leverage the sensor hub for a fall detection app, the developer not only needs to write the main application, but also needs to program the sensor hub to sense and notify the main application when a fall is detected. Two approaches have been used to make it easier for developers to program a sensor hub: APIs and hardware SDKs. In the API approach [4, 1], a set of important sensor inference functions are exported via high level APIs to the app developers. Th e problem is that the APIs only support a set of predefi ned events or activities such as step counting. Today, a fall detection application cannot use any of the existing APIs to leverage the sensor hub. It is possible that sensor hub APIs will stabilize, but this is unlikely to happen for many years. Consider how location APIs have evolved since the Java Location API (JSR 179) was introduced in 2003. Sensor hubs themselves have regularly been part of phones since 2011, but it is only in 2014 that a small set of sensor APIs are aligning around common functionality. In the meanwhile, ambitious sensing applications such as BeWell [3] cannot leverage the sensor hub for power effi ciency. In the hardware SDK approach, the developer is provided with specialized tools to directly access the sensor hub. For example, TI provides a proprietary TivaWare Sensor Library [5] to allow developers access to functionality not exposed by soft ware APIs. However, MobileHub: No Programmer Effort for Power Efficiency with Sensor Hub Haichen Shen and David Wetherall University of Washington Aruna Balasubramanian Stony Brook University Anthony LaMarca Intel