Recent work has introduced memory system dynamic voltage and frequency scaling (DVFS), and has suggested that balanced scaling of both CPU and the memory system is the most promising approach for conserving energy in server systems. In this paper, we first demonstrate that CPU and memory system DVFS often conflict when performed independently by separate controllers. In response, we propose CoScale, the first method for effectively coordinating these mechanisms under performance constraints. CoScale relies on execution profiling of each core via (existing and new) performance counters, and models of core and memory performance and power consumption. CoScale explores the set of possible frequency settings in such a way that it efficiently minimizes the full-system energy consumption within the performance bound. Our results demonstrate that, by effectively coordinating CPU and memory power management, CoScale conserves a significant amount of system energy compared to existing approaches, while consistently remaining within the prescribed performance bounds. The results also show that CoScale conserves almost as much system energy as an offline, idealized approach.
