Combined Scheduling of Time-Triggered and Priority-Based Task Sets in Ravenscar

Time-triggered and priority-based are the two major approaches for scheduling real-time systems. Both have their own advantages and drawbacks and none is superior in the general case. While time-triggered schedules excel at determinism and jitter control, they are hard to design and lack flexibility. Priority-based scheduling, on the other hand, keeps the logical and timing aspects of real-time applications conveniently separated from each other, at the cost of indeterminism and larger input and output jitter for all but the highest-priority tasks. In a previous paper, we presented a model and a related Ada implementation to support the combined execution of time-triggered and priority-based task sets, aiming to obtain the best of both worlds. This paper presents continuation of that work in two directions. One is the extension of the original model to support more behavioural patterns; the other is providing a Ravenscar implementation, targeting high-integrity systems. We conclude that Ravenscar is expressive enough to support most of the patterns in the original full-Ada version, and those that require forbidden features (such as dynamic priorities) are not out of reach if the time-triggered scheduler is implemented at the runtime level.