Energy-Agility: A New Grid-centric Metric for Evaluating System Performance

Energy-efficiency has long been considered a “first class” metric for evaluating computer system performance. A high energy-efficiency translates into more work done per unit of energy, which increases the operating time for battery-powered systems and lowers the energy bill for conventional grid-powered systems. Unfortunately, optimizing solely for energy-efficiency leads systems designers to make two im-plicit assumptions about how the electric grid generates en-ergy: i) that all energy is created equal and ii) that it is avail-able in unlimited quantities at any time. In reality, all en-ergy is not created equal—its cost and carbon footprint vary substantially over time—and, as the penetration of intermit-tent renewable energy sources in the grid increases, it may not be available in unlimited quantities at any time. Thus, we propose a new grid-centric performance metric, called energy-agility, that accounts for the assumptions above. We argue that grid-friendly computer systems are better judged by their energy-agility, rather than their energy-efficiency. 1