A Review of Priority Assignment in Real-Time Systems

It is over 40 years since the first seminal work on priority assignment for real-time systems using fixed priority scheduling. Since then, huge progress has been made in the field of real-time scheduling with more complex models and schedulability analysis techniques developed to better represent and analyse real systems. This tutorial style review provides an in-depth assessment of priority assignment techniques for hard real-time systems scheduled using fixed priorities. It examines the role and importance of priority in fixed priority scheduling in all of its guises, including: preemptive and non-pre-emptive scheduling; covering single- and multi-processor systems, and networks. A categorisation of optimal priority assignment techniques is given, along with the conditions on their applicability. We examine the extension of these techniques via sensitivity analysis to form robust priority assignment policies that can be used even when there is only partial information available about the system. The review covers priority assignment in a wide variety of settings including: mixed-criticality systems, systems with deferred pre-emption, and probabilistic real-time systems with worstcase execution times described by random variables. It concludes with a discussion of open problems in the area of priority assignment

Parametric Schedulability Analysis of Fixed Priority Real-Time Distributed Systems

Parametric analysis is a powerful tool for designing modern embedded systems, because it permits to explore the space of design parameters, and to check the robustness of the system with respect to variations of some uncontrollable variable. In this paper, we address the problem of parametric schedulability analysis of distributed real-time systems scheduled by fixed priority. In particular, we propose two different approaches to parametric analysis: the first one is a novel technique based on classical schedulability analysis, whereas the second approach is based on model checking of Parametric Timed Automata (PTA). The proposed analytic method extends existing sensitivity analysis for single processors to the case of a distributed system, supporting preemptive and non-preemptive scheduling, jitters and unconstrained deadlines. Parametric Timed Automata are used to model all possible behaviours of a distributed system, and therefore it is a necessary and sufficient analysis. Both techniques have been implemented in two software tools, and they have been compared with classical holistic analysis on two meaningful test cases. The results show that the analytic method provides results similar to classical holistic analysis in a very efficient way, whereas the PTA approach is slower but covers the entire space of solutions.Comment: Submitted to ECRTS 2013 (http://ecrts.eit.uni-kl.de/ecrts13

On the effectiveness of cache partitioning in hard real-time systems

In hard real-time systems, cache partitioning is often suggested as a means of increasing the predictability of caches in pre-emptively scheduled systems: when a task is assigned its own cache partition, inter-task cache eviction is avoided, and timing verification is reduced to the standard worst-case execution time analysis used in non-pre-emptive systems. The downside of cache partitioning is the potential increase in execution times. In this paper, we evaluate cache partitioning for hard real-time systems in terms of overall schedulability. To this end, we examine the sensitivity of (i) task execution times and (ii) pre-emption costs to the size of the cache partition allocated and present a cache partitioning algorithm that is optimal with respect to taskset schedulability. We also devise an alternative algorithm which primarily optimises schedulability but also minimises processor utilization. We evaluate the performance of cache partitioning compared to state-of-the-art pre-emption cost analysis based on benchmark code and on a large number of synthetic tasksets with both fixed priority and EDF scheduling. This allows us to derive general conclusions about the usability of cache partitioning and identify taskset and system parameters that influence the relative effectiveness of cache partitioning. We also examine the improvement in processor utilization obtained using an alternative cache partitioning algorithm, and the tradeoff in terms of increased analysis time

Mixed Criticality on Multi-cores Accounting for Resource Stress and Resource Sensitivity

The most significant trend in real-time systems design in recent years has been the adoption of multi-core processors and the accompanying integration of functionality with different criticality levels onto the same hardware platform. This paper integrates mixed criticality aspects and assurances within a multi-core system model. It bounds cross-core contention and interference by considering the impact on task execution times due to the stress on shared hardware resources caused by co-runners, and each task’s sensitivity to that resource stress. Schedulability analysis is derived for four mixed criticality scheduling schemes based on partitioned fixed priority preemptive scheduling. Each scheme provides robust timing guarantees for high criticality tasks, ensuring that their timing constraints cannot be jeopardized by the behavior or misbehavior of low criticality tasks

A Lazy Bailout Approach for Dual-Criticality Systems on Uniprocessor Platforms

© 2019 by the authors. Licensee MDPI, Basel, Switzerland.A challenge in the design of cyber-physical systems is to integrate the scheduling of tasks of different criticality, while still providing service guarantees for the higher critical tasks in case of resource-shortages caused by faults. While standard real-time scheduling is agnostic to the criticality of tasks, the scheduling of tasks with different criticalities is called mixed-criticality scheduling. In this paper we present the Lazy Bailout Protocol (LBP), a mixed-criticality scheduling method where low-criticality jobs overrunning their time budget cannot threaten the timeliness of high-criticality jobs while at the same time the method tries to complete as many low-criticality jobs as possible. The key principle of LBP is instead of immediately abandoning low-criticality jobs when a high-criticality job overruns its optimistic WCET estimate, to put them in a low-priority queue for later execution. To compare mixed-criticality scheduling methods we introduce a formal quality criterion for mixed-criticality scheduling, which, above all else, compares schedulability of high-criticality jobs and only afterwards the schedulability of low-criticality jobs. Based on this criterion we prove that LBP behaves better than the original {\em Bailout Protocol} (BP). We show that LBP can be further improved by slack time exploitation and by gain time collection at runtime, resulting in LBPSG. We also show that these improvements of LBP perform better than the analogous improvements based on BP.Peer reviewedFinal Published versio