A Realistic Model of Real-Time Systems for Efficient Scheduling

In order to take semantical aspects into account for the scheduling problem and obtain scheduling results for a wide class of systems, we extend the Petri net scheduling approach for real-time systems. Our study focuses on tasks with conditional statements. Classical approaches consider only the worst case execution time that occurs in the different branches of conditional statements and don't take the semantic of the tasks into account. We show that this pessimistic model can lead to wrong conclusions for scheduling. We extend the task model with conditional statements, and the notion of schedule is replaced by the notion of scheduling tree. We propose then a model approach using Petri nets in order to explicitly take conditional instructions and their semantics into account.

I. Introduction

A characteristic of real-time systems is that they must satisfy time constraints coming from the criticity of certain actions that allow them to interact with the environment of the monitored processes [1]. Time constraints can be soft or strict. For strict constraints, dysfunctionings occur when these constraints are not satisfied, which may have unacceptable consequences e.g loss of human lives.