Abstract: This paper examines a fundamental problem in the theory of real-time scheduling, that of scheduling a set of periodic or sporadic tasks on a uniprocessor without preemption and without inserted idle time. We exhibit a necessary and sufficient set of conditions C for a set of periodic or sporadic tasks to be schedulable for arbitrary release times of the tasks. We then show that any set of periodic or sporadic tasks that satisfies conditions C can be scheduled with an earliest deadline first (EDF) scheduling algorithm. We also address the question of schedulability of a set of tasks with specified release times. For sets of sporadic tasks with specified release times, we show that the conditions C are again necessary and sufficient for schedulability. However, for sets of periodic tasks with specified release times, the conditions C, while sufficient, are not necessary. In fact, we show that determining whether a set of periodic tasks with specified release times is schedulable is intractable (i.e., NP-hard in the strong sense). Moreover, we show that the existence of a universal algorithm for scheduling periodic tasks with specified release times would imply that P = NP. 1.

Most structured design methods claim to address the needs of hard real-time systems. However, few contain abstractions which directly relate to common hard real-time activities, such as periodic or sporadic processes. Furthermore, the methods do not constrain the designer to produce systems which can be analysed for their timing properties. In this paper we present a structured design method called HRT-HOOD (Hard Real-Time Hierarchical Object Oriented Design). HRT-HOOD is an extension of HOOD, and includes object types which enable common hard real-time abstractions to be represented. The method is presented in the context of a hard real-time system life cycle, which enables issues of timeliness and dependability to be addressed much earlier on in the development process. We argue that this will enable dependable real-time systems to be engineered in a more cost effective manner than the current practise, which in effect treats these topics as performance issues. To illustrate our appr...

Optimal scheduling of real-time tasks on multiprocessor systems is known to be computationally intractable for large task sets. Any practical scheduling algorithm for assigning realtime tasks to a multiprocessor system presents a trade-off between its computational complexity and its performance. In this study, new schedulability conditions are presented for homogeneous multiprocessor systems where individual processors execute the rate-monotonic scheduling algorithm. The conditions are used to develop new strategies for assigning real-time tasks to processors. The performance of the new strategies is shown to be significantly better than suggested by the existing literature. Under the realistic assumption that the load of each real-time task is small compared to the processing speed of each processor, it is shown that the processors can be almost fully utilized.

The preemptive scheduling of systems of periodic tasks on a platform comprised of several identical multiprocessors is considered. A scheduling algorithm is proposed for static-priority scheduling of such systems; this algorithm is a simple extension of the uniprocessor ratemonotonic scheduling algorithm. It is proven that this algorithm successfully schedules any periodic task system with a worst-case utilization no more than a third the capacity of the multiprocessor platform; for the special case of harmonic periodic task systems, the algorithm is proven to successfully schedule any system with a worst-case utilization of no more than half the platform capacity.

This paper discusses the addition of so-called time offsets to task sets dispatched according to fixed priorities. The motivation for this work is two-fold: firstly, direct expression of time offsets is a useful structuring approach for designing complex hard real-time systems. Secondly, analysis directly addressing time offsets can be very much less pessimistic than extant analysis. In this report we extend our current fixed priority schedulability analysis, and then present two major worked examples, illustrating the approach. 1. INTRODUCTION Previous work has addressed the problem of determining the worst-case timing behaviour of tasks dispatched according to fixed priority scheduling [11, 10]. Much of this work has been aimed at determining the worst-case case response time of a given task; of course, the worst-case response time is, by definition, the response time of the task in the worst-case scheduling scenario. So far, in all these previous pieces of work, tasks have been ass...

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This paper concerns the problem of scheduling sets of preemptable, periodic tasks on multiple resources. We consider a task model that allows arbitrary mixes of fixed and migratable tasks, and prove the existence of an optimal pfair scheduler in this model. Fixed tasks must always be scheduled on a given resource, while migratable tasks can be scheduled on different resources at different times. A pfair scheduler produces a periodic schedule in which the times each task is allocated a processor are approximately evenly spread throughout its period. This work extends work of Baruah et al, who proved a similar result for systems in which all tasks are migratable.

Recent results in the application of... this paper. The review takes the form of an analysis of the problems presented by different application requirements and characteristics. Issues covered include uniprocessor and multiprocessor systems, periodic and aperiodic processes, static and dynamic algorithms, transient overloads and resource usage. Protocols that limit and reduce blocking are discussed. Considerations are also given to scheduling Ada tasks.

Each processor in a uniform multiprocessor machine is characterized by a speed or computing capacity, with the interpretation that a job executing on a processor with speed s for t time units completes (s t) units of execution. The on-line scheduling of hard-real-time systems, in which all jobs must complete by specified deadlines, on uniform multiprocessor machines is considered. It is known that online algorithms tend to perform very poorly in scheduling such hard-real-time systems on multiprocessors; resourceaugmentation techniques are presented here that permit online algorithms to perform better than may be expected given the inherent limitations. Results derived here are applied to the scheduling of periodic task systems on uniform multiprocessor machines. 1.

This thesis develops the Sporadic Server (SS) algorithm for scheduling aperiodic tasks in real-time systems. The SS algorithm is an extension of the rate monotonic algorithm which was designed to schedule periodic tasks. This thesis demonstrates that the SS algorithm is able to guarantee deadlines for hard-deadline aperiodic tasks and provide good responsiveness for soft-deadline aperiodic tasks while avoiding the schedulability penalty and implementation complexity of previous aperiodic service algorithms. It is also proven that the aperiodic servers created by the SS algorithm can be treated as equivalently-sized periodic tasks when assessing schedulability. This allows all the scheduling theories developed for the rate monotonic algorithm to be used to schedule aperiodic tasks. For scheduling aperiodic and periodic tasks that share data, this thesis defines the interactions and schedulability impact of using the SS algorithm with the priority inheritance protocols. For scheduling ha...