Session Summary

This position paper follows from a previous proposal to integrate a time-triggered scheduler in a priority- based, preemptive scheduler such as that supported by Ada's task dispatching policy FIFO Within Priorities . The resulting combined scheduling carries the advantages of both time-triggered and priority-based scheduling, and helps mitigating their drawbacks. The paper presents a system model for the time-triggered subsystem that extends the original proposal, and describes a Ravenscar implementation of the scheduler at the run-time system level, in the form of a new package Ada.Dispatching.TTS. Multiple programming patterns can be implemented on top of this scheduler. With respect to the previously proposed full-Ada implementation, only patterns that implied the use of asynchronous transfer of control have been excluded. On the other hand, the extension of the original model enables new patterns, not supported in our previous proposal, using the new types of continuation and optional slots. We hold that bringing the time-triggered paradigm to Ravenscar is both feasible and convenient for the High-Integrity and Embedded application domains.