Asymmetry-Aware Work-Stealing Runtimes.

Amdahl's law provides architects a compelling reason to introduce system asymmetry to optimize for both serial and parallel regions of execution. Asymmetry in a multicore processor can arise statically (e.g., from core microarchitecture) or dynamically (e.g., applying dynamic voltage/frequency scaling). Work stealing is an increasingly popular approach to task distribution that elegantly balances task-based parallelism across multiple worker threads. In this paper, we propose asymmetry-aware work-stealing (AAWS) runtimes, which are carefully designed to exploit both the static and dynamic asymmetry in modern systems. AAWS runtimes use three key hardware/software techniques: work-pacing, work-sprinting, and work-mugging. Work-pacing and work-sprinting are novel techniques that combine a marginal-utility-based approach with integrated voltage regulators to improve performance and energy efficiency in high- and low-parallel regions. Work-mugging is a previously proposed technique that enables a waiting big core to preemptively migrate work from a busy little core. We propose a simple implementation of work-mugging based on lightweight user-level interrupts. We use a vertically integrated research methodology spanning software, architecture, and VLSI to make the case that holistically combining static asymmetry, dynamic asymmetry, and work-stealing runtimes can improve both performance and energy efficiency in future multicore systems.