In this paper, we describe an integrated environment to assist the development of hard real-time applications. It includes an interactive graphic interface which allows the user to describe the application requirements according to three hierarchical levels: the application level, the component level, and the object level. The development model we propose is based on an iterative process in which the real-time scheduling support is considered since the beginning of the design phases. Our graphic environment integrates several tools to analyse, test, and simulate the real-time application under development. In particular, the tools we have implemented are: a Design Tool, to describe the structure of the application, a Schedulability Analyser Tool (SAT), to verify off-line the feasibility of the schedule of a critical task set, a Scheduling Simulator, to test the average behaviour of the application, and a Maximum Execution Time (MET) estimator to bound the worst case duration of each task.

We present a comparative study among scheduling algorithms which use different priority assignments and different guarantee mechanisms to improve the performance of a real-time system during overload conditions. In order to enhance the quality of service, we assume that tasks are characterized not only by a deadline, but also by an importance value. The performance of the scheduling algorithm is then evaluated by computing the cumulative value gained on a task set, i.e. the sum of the values of those tasks that completed by their deadline. The purpose of this simulation study was twofold. Firstly, we wanted to discover which priority assignment is able to achieve the best performance in overload conditions. Secondly, we were interested in understanding how the pessimistic assumptions made in the guarantee test affect the performance of the scheduling algorithms, and how much a reclaiming mechanism can compensate this degradation. Simulation results show that, without any admission control, value-density scheduling performs best. Simple admission control based on worst case estimates of the load worsen the performance of all value based algorithms. EDF scheduling performs best if admission control is used along with a reclaiming mechanism that takes advantage of early completions. Finally, scheduling by deadline before overload and by value during overload works best in most practical conditions.

When hard periodic and firm aperiodic tasks are jointly scheduled in the same system, the processor workload can vary according to the arrival times of aperiodic requests. In order to guarantee the schedulability of the periodic task set, in overload conditions some aperiodic tasks must be rejected. In this paper we propose a technique that, in overload conditions, adds robustness to the joint scheduling of periodic and aperiodic tasks in systems with dynamic priorities. Our technique is based on an aperiodic server, called total bandwidth server, already proven effective in a previous work. Here the algorithm is first extended to efficiently handle firm aperiodic tasks and then integrated with a robust guarantee mechanism that allows to achieve graceful degradation in case of transient overloads. Extensive simulations show that the proposed new algorithm is effective in all workload conditions.

We present four new on-line algorithms for servicing soft aperiodic requests in real-time systems, where a set of hard periodic tasks is scheduled using the Earliest Deadline First (EDF) algorithm. All the proposed solutions can achieve full processor utilization and enhance aperiodic responsiveness, still guaranteeing the execution of the periodic tasks. Operation of the algorithms, performance, schedulability analysis, and implementation complexity are discussed and compared with classical alternative solutions, such as background and polling service. Extensive simulations show that algorithms with contained run-time overhead present nearly optimal responsiveness. A valuable contribution of this work is to provide the real-time system designer with a wide range of practical solutions which allow to balance efficiency against implementation complexity.>

Formal results for precedence constrained, real-time scheduling of unit time tasks are extended to arbitrary timed tasks with preemption. An exact characterisation of the EDF-like schedulers that can be used to transparently enforce precedence constraints among tasks is shown. These extended results are then integrated with a well-known protocol that handles real-time scheduling of tasks with shared resources, but does not consider precedence constraints. This results in schedulability formulas for task sets which allow preemption, shared resources, and precedence constraints, and a practical algorithm for many real-time uniprocessor systems.>