Abstract
A multiple-processor system can potentially achieve higher energy savings than a single processor, because the reduced workload on each processor creates new opportunities for dynamic voltage scaling (DVS). However, as the cost of communication starts to match or surpass that of computation, many new challenges arise in making DVS effective in a distributed system under communication-intensive workload. This paper discusses implementation issues for supporting DVS on distributed embedded processors. We implemented and evaluated four distributed schemes: (1) DVS during I/O, (2) partitioning, (3) power-failure recovery, and (4) node rotation. We validated the results on a distributed embedded system with the Itsy pocket computers connected by serial links. Our experiments confirmed that a distributed system can create new DVS opportunities and achieve further energy savings. However, a surprising result was that aggregate energy savings do not translate directly into a longer battery life. In fact, the best partitioning scheme, which distributed the workload onto two nodes and enabled the most power-efficient CPU speeds at 30-50%, resulted in only 15% improvement in battery lifetime. Of the four techniques evaluated, node rotation showed the most measurable improvement to battery lifetime at 45% by balancing the discharge rates among the nodes.