Reducing the number of preemptions in fixed priority scheduling

Fixed priority scheduling (FPS) has been widely studied and used in a number of applications, mostly due to its flexibility, simple run-time mechanism and small overhead. However, preemption related overhead in FPS may cause undesired high processor utilization, high energy consumption, or, in some cases, even infeasibility. In this paper, we propose a method to reduce the number of preemptions in legacy FPS systems consisting of tasks with priorities, periods and offsets. Unlike other approaches, our algorithm does not require modification of the basic FPS mechanism. Our method analyzes offline a set of periodic tasks scheduled by FPS, detects the maximum number of preemptions that can occur at run-time, and reassigns task attributes such that the tasks are schedulable by the same scheduling mechanism, while achieving a significantly lower number of preemptions. In some cases, there is a cost to pay for a lower number of preemptions in terms of increased amount of tasks and/or reduced task execution flexibility. Our method provides for the ability to choose a user-defined number of preemptions with respect to the cost to pay.