Scaling the Energy Proportionality Wall with KnightShift

Measuring energy proportionality accurately and understanding the reasons for disproportionality are critical first steps in designing future energy-efficient servers. This article presents two metrics-linear deviation and proportionality gap-that let system designers analyze and understand server energy consumption at various utilization levels. An analysis of published SPECpower results shows that energy proportionality improvements are not uniform across various server utilization levels. Even highly energy proportional servers suffer significantly at nonzero but low utilizations. To address the lack of energy proportionality at low utilization, the authors present KnightShift, a server-level heterogeneous server providing an active low-power mode. KnightShift is tightly coupled with a low-power compute node called Knight. Knight responds to low-utilization requests whereas the primary server responds only to high-utilization requests, enabling two energy-efficient operating regions. The authors evaluate KnightShift against a variety of real-world datacenter workloads using a combination of prototyping and simulation.