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Final Report of the COST-247 Action

A first experiment with ET-LOTOS: The specification of real-time schedulers

Christian Hernalsteen

Universite Libre de Bruxelles
Departement d'Informatique, CP212
Bl. du Triomphe
Brussels, Belgium
tel: +32 2 650 50 42
fax: +32 2 650 56 09


Three different real-time schedulers have been specified using the formal description language ET-LOTOS. The first one handles independent periodic tasks characterized by their period, execution time and deadline. The scheduler is cyclic; it considers the system every "delta" time units. The second one handles the same kind of tasks but in an event driven way. The scheduler is woken up by two kinds of events: the begining of a new task occurence and the end of the current executing task. The last scheduler is also event-driven but considers dependent tasks. The scheduled tasks are represented by a sequence of independent execution-time and interaction points which are represented by semaphores. These three schedulers are preemptive and schedule tasks based on a static priority policy. Problems of deadlocks and priority inversion are not resolved in these schedulers.

For each of these schedulers, an ET-LOTOS specification is given for the scheduler and for the tasks. The tasks must be defined since they interact with the scheduler for the CPU allocation. A task can be in one of the following states: running, preempted or waiting for its next occurence. Each of these states is represented by an ET-LOTOS process. The specification has been done in a stepwise way, where a prelimary description is done and then modified several times to resolve some apparent timing problems. These different specification steps are presented in the paper in order to show what kind of problems the specifier must be aware of when using a timed specification description language like ET-LOTOS. Indeed some new problems can arise with this language that never occur with non timed specification languages.

The produced specifications are not very large but use all the ET-LOTOS operators and give a good intuition of the expressive power of the language.

This presentation has been given during the COST-247 6th Management Committee Meeting (Budapest, Hungary, October 26-27, 1995).

COST-247 Working Group(s): 1

This Page was prepared by Mark Jorgensen.

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