Real-time Scheduling of Mixed-critical Workloads upon Platforms with Uncertainties

ZHISHAN GUO : Real-Time Scheduling of Mixed-Critical Workloads upon Platforms with Uncertainties (Under the direction of Sanjoy K. Baruah) In designing safety-critical real-time systems, there is an emerging trend in moving towards mixed-criticality (MC), where functionalities with different degrees of importance (i.e., criticality) are implemented upon a shared platform. Since 2007, there has been a large amount of research in MC scheduling, most of which considers the Vestal Model. In this model, all kinds of uncertainties in the system are characterized into the workloads by assuming multiple worst-case execution time (WCET) estimations for each execution (of a piece of code). However, uncertainties of estimations may arise from different aspects (instead of WCET only), especially upon more widely used commercial-off-the-shelf (COTS) hardware that typically provides good average-case performance rather than worst-case guarantees. This dissertation addresses two questions fundamental to the modeling and analyzing of such MC real-time systems: (i) Can Vestal model be used to describe all kinds of uncertainties at no significant analytical capacity loss? (ii) If not, can new mechanisms be developed with better performances over existing ones (in MC scheduling theory), under certain assumptions? To answer these questions, we first investigate the Vestal model carefully. We propose a new algorithm (named LE-EDF) which dominates state-of-the-art schedulers for MC job scheduling. We also improve the understanding of certain existing algorithms by proving a better (and even optimal) speedup bound. We have found that by introducing the probabilistic WCET workload model into MC scheduling, the uncertain behaviors can be better characterized comparing to Vestal model in the sense of schedulability ratio via experiments.