The Energy Efficiency Of Modern Multicore Systems

Motivated by the proliferation of both homogeneous and heterogeneous shared-memory multicore systems with large core counts, and by the energy usage issue faced by computing today, we extend Amdahl's and Gustafson's laws for speedup to estimate the energy savings of a multicore system compared to the system using a single core. To define energy savings, we introduce two key parameters, (i) the active power fraction (APF) of a core representing the ratio between core's average active power and the power of the idle system, and (ii) the inter-core speedup (ICS) depicting the difference in speed among different types of cores in heterogeneous multicores. We show that energy savings are achievable, but they rapidly plateau on large core counts and are affected by system's APF such that a lower APF value leads to higher energy savings. However, on low core counts, energy savings are affected by both the APF and the sequential fraction such that for workloads with large sequential fraction, energy savings are small, regardless of system's APF. We validate our analytical models for energy savings with seven applications covering a wide range of sequential fractions, on two homogeneous server systems with 48 cores representing both traditional brawny x86/64 and emerging wimpy ARM server nodes, and on one heterogeneous system representing the emerging ARM big.LITTLE architecture.