ReAPER: Region Aware Power and Energy Regulator

Improving CPU power/energy efficiency without degrading performance requires an accurate application characterization. Rather than characterizing the application as a whole, we find that dividing the application into individual regions is much more effective. This fine-grained approach gives us the opportunity to save power/energy during memory-bound regions and MPI slack regions (time spent waiting on other processes) by lowering core frequency and during compute-bound regions by lowering uncore frequency. We propose an intuitive, lightweight, and portable algorithm for identifying these regions at runtime which relies only on the IPS (instructions per second) metric, rather than on performance counters that can differ across platforms. At the same time, we meet a user-specified level of acceptable performance degradation by adapting core and uncore frequencies to the application, achieving additional CPU power/energy savings. We evaluate our approach on the SPEC 2017 benchmarks and various MPI applications.