Knightshift: Scaling the energy proportionality wall through server-level heterogeneity

Server energy proportionality has been improving over the past sev-eral years. Many components in a system, such as CPU, memory and disk, have been achieving good energy proportionality behav-ior. Using a wide range of server power data from the published SPECpower data we show that the overall system energy proportion-ality has reached 80%. We present two novel metrics, linear deviation and proportionality gap, that provide insights into accurately quan-tifying energy proportionality. Using these metrics we show that energy proportionality improvements are not uniform across various server utilization levels. In particular, the energy proportionality of even a highly proportional server suffers significantly at non-zero but low utilizations. We propose to tackle the lack of energy pro-portionality at low utilization using server-level heterogeneity. We present KnightShift, a server-level heterogeneous server architec-ture that introduces an active low power mode, through the addition of a tightly-coupled compute node called the Knight, enabling two energy-efficient operating regions. We evaluated KnightShift against a variety of real-world datacenter workloads using a combination of prototyping and simulation, showing up to 75 % energy savings with tail latency bounded by the latency of the Knight and up to 14% improvement to Performance per TCO dollar spent. 1