Model-based implementation of real-time applications

Abstract

Correct and efficient implementation of general real-time applications remains by far an open problem. A key issue is meeting timing constraints whose satisfaction depends on features of the execution platform, in particular its speed. Existing rigorous implementation techniques are applicable to specific classes of systems e.g. with periodic tasks, time deterministic systems. We present a general model-based implementation method for real-time systems based on the use of two models. An abstract model representing the behavior of real-time software as a timed automaton. The latter describes user-defined platform-independent timing constraints. Its transitions are timeless and correspond to the execution of statements of the real-time software. A physical model representing the behavior of the real-time software running on a given platform. It is obtained by assigning execution times to the transitions of the abstract model. A necessary condition for implementability is time-safety, that is, any (timed) execution sequence of the physical model is also an execution sequence of the abstract model. Time-safety simply means that the platform is fast enough to meet the timing requirements. As execution times of actions are not known exactly, time-safety is checked for worst-case execution times of actions by making an assumption of time-robustness: time-safety is preserved when speed of the execution platform increases. We show that as a rule, physical models are not time-robust and show that time-determinism is a sufficient condition for time-robustness. For given real-time software and execution platform corresponding to a time-robust model, we define an Execution Engine that coordinates the execution of the application software so as to meet its timing constraints. Furthermore, in case of non-robustness, the Execution Engine can detect violations of time-safety and stop execution. We have implemented the Execution Engine for BIP programs with real-time constraints. We have validated the implementation method for an adaptive MPEG video encoder. Experimental results reveal the existence of timing anomalies seriously degrading performance for increasing platform execution speed.