Some Results in Rate Monotonic Scheduling with Priority Promotion

Abstract—Rate monotonic (RM) scheduling algorithm cannot guarantee schedulability of highly utilized tasks in all cases. In this paper, we increase schedulability of RM by assigning fixed set of priority promotions for each task. We present an algorithm to assign promotion times (PT) and the new priority at each PT. Each priority promotion occurs after a constant relative time from the release of an instance of a task. This technique can be implemented using a container at the operating system’s side which sets a timer interrupt for the priority promotion. Our priority promotion algorithm is offline and it promotes priorities only at the latest release of the higher priority tasks before deadline of the first instance of each task. In our solution, the maximum number of promotions is bounded to the number of higher priority tasks with smaller periods. The performance of our solution approach has been evaluated by extensive sets of simulations. According to our experimental results, a) our solution dominates RM, i.e., it guarantees schedulability whenever the task set is RM-feasible while it improves the schedulability in the task sets which are not RM-feasible, b) it has a significantly higher schedulablity ratio than RM, and c) even in cases where it cannot guarantee the schedulability, it has a low ratio of missed jobs.