Automatic schedule computation for distributed real-time systems using timed automata

Park, Young-Saeng (2008) Automatic schedule computation for distributed real-time systems using timed automata. Doctoral thesis, Northumbria University.

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Abstract

The time-triggered architecture is becoming accepted as a means of implementing scalable, safer and more reliable solutions for distributed real-time systems. In such systems, the execution of distributed software components and the communication of messages between them take place in a fixed pattern and are scheduled in advance within a given scheduling round by a global scheduling policy. The principal obstacle in the design of time-triggered systems is the difficulty of finding the static schedule for all resources which satisfies constraints on the activities within the scheduling round, such as the meeting of deadlines. The scheduler has to consider not only the requirements on each processor but also the global requirements of system-wide behaviour including messages transmitted on networks. Finding an efficient way of building an appropriate global schedule for a given system is a major research challenge. This thesis proposes a novel approach to designing time-triggered schedules which is radically different from existing mathematical methods or algorithms for schedule generation. It entails the construction of timed automata to model the arrival and execution of software tasks and inter-task message communication for a system; the behaviour of an entire distributed system is thus a parallel composition of these timed automata models. A job comprises a sequence of tasks and messages; this expresses a system-wide transaction which may be distributed over a system of processors and networks. The job is formalized by a timed automata based on the principle that a task or message can be modelled by finite states and a clock variable. Temporal logic properties are formed to express constraints on the behaviour of the system components such as precedence relationships between tasks and messages and adherence to deadlines. Schedules are computed by formally verifying that these properties hold for an evolution of the system; a successful schedule is simply a trace generated by the verifier, in this case the UPPAAL model-checking tool has been employed to perform the behaviour verification. This approach guarantees to generate a practical schedule if one exists and will fail to construct any schedule if none exists. A prototype toolset has been developed to automate the proposed approach to create of timed automata models, undertake the analysis, extract schedules from traces and visualize the generated schedules. Two case studies, one of a cruise control system, the other a manufacturing cell system, are presented to demonstrate the applicability and usability of the approach and the application of the toolset. Finally, further constraints are considered in order to yield schedules with limited jitter, increased efficiency and system-wide properties.

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