Techniques for Enhancing the Temporal Predictability of Real-Time Embedded Systems Employing a Time-Triggered Software Architecture.

摘要

This thesis is concerned with the design and implementation of single-processor embedded real-time systems with highly predictable behaviour and strict constraints on resource usage. The main aim of this research is to identify the sources of unpredictable behaviour in such systems – exhibited as timing jitter - when a time-triggered pre-emptive task scheduling approach is adopted, and then provide software based techniques to enhance their temporal predictability. The thesis provides a review of related previous work on predictable real-time task scheduling, as well as resource-access control methods for maintaining predictable real-time system behaviour through the prevention of priority inversion and other related problems. The design and implementation of the time-triggered hybrid (TTH), time-triggered rate-monotonic (TTRM), and time-triggered deadline-monotonic (TTDM) task schedulers is discussed in detail as they provide the most predictable behaviour within the category of pre-emptive task schedulers. For that reason, they will be used as the software platforms in the experimental part of this research. Two novel software techniques for enhancing the temporal predictability in systems utilising time-triggered schedulers are introduced. The first software technique presented is a resource-access control protocol named Timed Resource-Access Protocol (TRAP). This protocol is designed to avoid the problems of priority inversion, chained blocking and deadlocks while coercing system tasks to exhibit timing predictability that is proportional to their significance in the system. This appears in the decreasing levels of task finishing jitter as the significance of tasks in the system increases. The second technique is named Planned Pre-emption (PP). This technique is aimed at eliminating the scheduling unpredictability due to variable timer interrupt service time in time-triggered scheduling systems. The impact of this technique appears in the considerable reduction in scheduler task release jitter.\udFinally, the thesis is concluded by a discussion and a summary of the work presented.