Releasing jobs and performing scheduling decisions in real-time operating systems (RTOSes) is often realized within tick interrupts. In each tick interrupt, a set of tasks that are waiting to release new jobs, namely the waiting set, is inspected to determine the jobs that should be released at this tick. Such a waiting set is sorted whenever a job has finished, by which the release process can be efficiently achieved. However, the overhead of sorting can vary vastly depending on the task set, which has to be taken into account in the worst-case timing analysis. Moreover, the tick interrupt in common practices is backed by a single hardware timer that is configured to trigger interrupts, either with a fixed period or reconfigured to the next release time (so-called one-shot timer). Since not necessarily at every interrupt a job will be released, several tick interrupts might be redundant. For the one-shot timers, the reconfiguration during runtime also incurs overheads at a variable interval. To reduce such variability and amount of overhead, in this work, we propose LazyTick which partitions the task set and distributes the subsets over multiple timers. Specifically, we propose two job release procedures with constant operations overhead—one for harmonic task sets and one for non-harmonic task sets. We implemented the support for multiple hardware timers in FreeRTOS and conducted intensive experimental evaluations. The evaluation shows that LazyTick can reduce the variability in overhead by up to ≈ 5 × in peak and ≈ 3 × on average in comparison to the default implementation. Additionally, the combined overhead of the job release process is reduced by up to ≈ 6.1 × in peak and ≈ 3.6 × on average.
Heider et al. (Wed,) studied this question.