Handing DVFS to Hardware : Using Power Capping to Control Software Performance

Dynamic voltage and frequency scaling (DVFS) has been the cornerstone of innumerable software approaches to meeting application performance requirements with minimal energy. However, recent trends in technology—e.g., moving voltage converters on chip—favor hardware control of DVFS, as hardware can both react faster to external events and perform fine-grained power management across a device. We respond to these trends with CoPPer, which instead uses hardware power capping to meet application performance requirements with high energy efficiency. We find that meeting performance requirements with power capping is more challenging than using DVFS because the relationship between power and performance is non-linear and has diminishing returns at high power values. CoPPer overcomes these difficulties by using adaptive control to approximate nonlinearities and a novel gain limit to avoid over-allocating power when it is no longer beneficial. We evaluate CoPPer with 20 parallel applications on a dual-socket, 32-core server system and compare it to both a simple linear DVFS controller and to an existing control-theoretic, model-driven software DVFS manager. CoPPer does not require a userspecified model or application pre-characterization, yet provides all the functionality of the sophisticated DVFS-based approach. Compared to DVFS, CoPPer’s gain limit reduces energy by 6% on average and by 12% for memory-bound applications. For high performance requirements, the energy savings are even greater: 8% on average and 18% for memory-bound applications.