The forgotten 'uncore': on the energy-efficiency of heterogeneous cores

Heterogeneous multicore processors (HMPs), consisting of cores with different performance/power characteristics, have been proposed to deliver higher energy efficiency than symmetric multicores. This paper investigates the opportunities and limitations in using HMPs to gain energy-efficiency. Unlike previous work focused on server systems, we focus on the client workloads typically seen in modern end-user devices. Further, beyond considering core power usage, we also consider the 'uncore' subsystem shared by all cores, which in modern platforms, is an increasingly important contributor to total SoC power. Experimental evaluations use client applications and usage scenarios seen on mobile devices and a unique testbed comprised of heterogeneous cores, with results that highlight the need for uncore-awareness and uncore scalability to maximize intended efficiency gains from heterogeneous cores.