System Support for Loosely Coupled Resources in Mobile Computing
Doctor of Philosophy
Modern mobile platforms are embracing not only heterogeneous but also loosely coupled computational resources. For instance, a smartphone usually incorporates multiple processor cores that have no hardware cache coherence. Loosely coupled resources allow a high degree of resource heterogeneity that can greatly improve system energy efficiency for a wide range of mobile workloads. However, loosely coupled resources create application programming difficulty: both resources and program state are distributed, which call for explicit communication for consistency. This difficulty is further exacerbated by the large numbers of mobile developers and mobile applications. In order to ease application programming over loosely coupled resources, in this thesis work we explore system support -- at both user level and OS level -- that bridges desirable programming abstractions with the underlying hardware. We study three loosely coupled architectures widely seen in mobile computing: i) a smartphone accompanied by wearable sensors, ii) a mobile device encompassing multiple processors that share no memory, and iii) a mobile System-on-Chip (SoC) with multiple cores sharing incoherent memory. In order to address the three architectures, this thesis contributes three closely related research projects. In project Dandelion, we propose a Remote Method Invocation scheme to hide communication details from application components that are synchronizing over wireless links. In project Reflex, we design an energy-efficient software Distributed Shared Memory (DSM) to automatically keep user state consistent; the DSM always employs a low-power processor to host shared memory objects in order to maximize sleep periods of high-power processors. In project K2, we identify and apply a shared-most OS model to construct a single OS image over loosely coupled processor cores. Following the shared-most model, high-level OS services (e.g., device drivers and file systems) are mostly unmodified with their state transparently kept consistent; low-level OS services (e.g., page allocator) are implemented as separate instances with independent state for the sake of minimum communication overhead. We report the research prototypes, our experiences in building them, and the experimental measurements. We discuss future directions, in particular how our principles in treating loosely coupled resources can be used for improving other key system aspects, such as scalability.
mobile computing; systems; heterogeneous architecture