FoREST-mn: Runtime DVFS beyond communication slack

Dynamic Voltage and Frequency Scaling (DVFS) is commonly used to save energy in computing systems. However, when it comes to parallel programs, existing DVFS controllers only reduce frequency while or before waiting in blocking communications. As a consequence, energy savings are only possible for the program tasks out of the critical path and when the workload is imbalanced. We propose a new runtime DVFS controller, FoREST-mn. It allows to take advantage of both the low CPU usage of some program phases as well as communication slack to save more energy with parallel programs. The DVFS control then becomes more complex, but energy savings are even obtained when the workload is balanced. The resulting slowdown on programs is carefully controlled and constrained by a user-defined threshold. We implemented the presented strategies and evaluated it on 4 compute nodes totaling 64 cores. FoREST-mn is able to perform significant CPU energy savings on the NAS programs, up to 34 % on MG, while efficiently bounding the resulting slowdown.