Linux's Processor Affinity API, Refined: Shifting Real-Time Tasks Towards Higher Schedulability

Abstract

Virtually all major real-time operating systems such as QNX, VxWorks, LynxOS, and most real-time variants of Linux expose processor affinity APIs to restrict task migrations. Initially motivated by throughput and isolation reasons, the ability to flexibly control migrations on a per-task basis has also proved to be useful from a schedulability perspective. However, as the motivation to use processor affinities is highly application-specific, the two interests can conflict, i.e., The fixed, user-specified processor affinities chosen for non-schedulability reasons can actually limit any possible gains in schedulability. This paper specifically addresses the scenario where processor affinities are given as input, and investigates the following question: while maintaining API compatibility (i.e., Without changing the interface exposed to the programmer), is it possible to improve schedulability beyond what Linux and Linux-like systems currently offer, without violating the original affinity restrictions? To answer this question, we explore the similarities between priority-based scheduling with processor affinities and the assignment problem with seniority and job priority constraints, studied previously by Caron et al. In an operations-research context, to derive a more generic model of migrations. Based on vertex-weighted bipartite matchings, the proposed model exploits the idea of shifting high-priority tasks among processors in their affinity set, in order to accommodate lower-priority tasks that have more constrained processor affinities. The proposed approach is analyzed with a novel shifting-aware schedulability analysis based on linear programming. An empirical evaluation in terms of schedulability shows shifting to be effective, although performance naturally degrades if migration overheads are high.