Task-dependent processor shutdown for hard real-time systems

Abstract

Mobile devices relying on batteries can save energy by using lowpower modes of their processors. In a hard real-time environment, one has to prove the real-time feasibility and thereby to guarantee that energy saving methods do not violate real-time constraints. Besides the processor’s unavailability during low-power mode, the transition to and from the mode consumes energy and time. This work introduces a task-dependent policy for mode switching and compares it to procrastination techniques from the literature. The low-power interval is placed between occurrences of one task of the task set such that low-power mode and instances of this task do not overlap. Optimisation of task to depend on, duration of low-power, and shutdown rate is done with the help of a hard real-time test to provide feasible results. The used test provides analysis for preemptible, deadline scheduled task sets. Tasks are allowed to have periodic, periodic with jitter, sporadic, or other behaviour regarding their occurrence. Thus, this work extends the applicability of processor shutdown to such environments. And since parameters are determined off-line, apart form a programmable real-time clock, no power consuming extra circuitry is needed. However, the method comes with a slight modification of the task set.