Exploiting Process Variability in Voltage/Frequency Control

Abstract

Fine-grained dynamic voltage/frequency scaling (DVFS) is an important tool in managing the balance between power and performance in chip-multiprocessors. Although manufacturing process variations are giving rise to significant core-to-core variations in power and performance, traditional DVFS controllers are unaware of these variations. Exploiting the different power profiles of the cores can significantly improve energy efficiency. Process variations do not significantly affect dynamic power, so less-leaky processing units are more energy-efficient than their leakier counterparts at a given supply voltage and frequency. Taking advantage of this observation, three existing DVFS control algorithms are modified to shift work from inefficient, leaky processing units to efficient, less leaky ones, maintaining performance while reducing total power consumption. This work-shifting is carried out both between dies in a given speed bin and between voltage/frequency islands on a given die. The gains enabled by incorporating variability-awareness into the three DVFS algorithms are demonstrated on both multithreaded and multiprogrammed workloads. For a baseline 16-core design with per-core voltage/frequency islands (VFIs) and a 4×4 mesh on-chip network, the aggregate power per squared throughput (power/throughput <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> or P/T <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) over all fabricated dies is reduced by 9.2%, 5.7%, and 7.7% for the three controllers. Chip multiprocessor designs using other VFI granularities and network topologies are also examined.