Design And Implementation Of A Hierarchical Scheduling Model Of Operating System On WSN Nodes

Wireless sensor network (WSN) is composed of a large number of sensor nodes with the capability of sensing, computing and communicating. It becomes increasingly popular and attractive due to its wide use. In fact, WSN related technology has become one of the hottest research directions of computer secience nowadays. In the rescent five years, great progresses have been made in research of operating system on WSN nodes. Several operating systems have been developed, such as TinyOS, Mantis OS, SOS and Contiki. As one of the core components of operating system, the scheduling system provides definition of schedulable objects, appropriate scheduling algorithm and concurrency control between these objects. It has a great impact on the efficiency of the entire operating system.So far in the field of operating system for WSN nodes, there are two types of scheduling system architecture: the event-driven single threaded system and the multithreaded system. The former one is adopted in TinyOS. It can achieve high concurrency at a very low memory overhead, thus fits into simple application environments in early days. However, scheduling systems of this type do have internal drawbacks, such as blocking due to long-running tasks (the producer-consumer problem) and poor real-time performance, which make it inappropriate for complex application evironments. In contrast, multithreaded operating systems for WSN nodes such as Mantis OS introduce preemption between threads to solve the above problems. However, as this system requires one stack space per thread, stack consumption would be unacceptable for low-end nodes which only run in simple application environments. How to implement a scheduling system which is suitable for different applications has become one of the most important research points.In this thesis, scheduling systems of most popular operating systems on WSN nodes are analyzed. Based on that, we designed and implemented an event-driven based, multithreaded hierarchical scheduling system. The event-driven system and the multithreading system compose the first hierarchy, while the second and the third hierarchy are composed of the primary scheduler and the child scheduler of the multithreading system respectively. Three types of scheduling objects are defined by the system: events, tasks and child schedulers. Events are scheduled by the event-driven system while tasks and child schedulers are scheduled by themultithreading system. With hierarchical scheduling, our system enables stack sharing between tasks in the same child scheduler while introducing preemption between child schedulers. Meanwhile, with the implementation of concurrency control between events and tasks, we introduce the software interrupt masking to shorten the interrupt disable time, decrease the interrupt response time and improve system concurrency. Moreover, upon this hierarchy framework, we defined the task model of WSN applications, as well as a hierarchical scheduling system construction algorithm (HSC), which is able to construct a multithreaded hierarchical system that meet the real-time and low-cost demands of the task sets in deferent application environment. Finally, measurement has been designed to prove the efficiency of the hierarchical scheduling system.