On the Interplay between Global DVFS and Scheduling Tasks with Precedence Constraints

Many multicore processors are capable of decreasing the voltage and clock frequency to save energy at the cost of an increased delay. While a large part of the theory oriented literature focuses on local dynamic voltage and frequency scaling (local DVFS), where every core's voltage and clock frequency can be set separately, this article presents an in-depth theoretical study of the more commonly available global DVFS that makes such changes for the entire chip. This article shows how to choose the optimal clock frequencies that minimize the energy for global DVFS, and it discusses the relationship between scheduling and optimal global DVFS. Formulas are given to find this optimum under time constraints, including proofs thereof. The problem of simultaneously choosing clock frequencies and a schedule that together minimize the energy consumption is discussed, and based on this a scheduling criterion is derived that implicitly assigns frequencies and minimizes energy consumption. Furthermore, this article studies the effectivity of a large class of scheduling algorithms with regard to the derived criterion, and a bound on the maximal relative deviation is given. Simulations show that with our techniques an energy reduction of 30% can be achieved with respect to state-of-the-art research.