The paper presents exact schedulability analyses for real-time systems scheduled at run-time with a static priority pre-emptive dispatcher. The tasks to be scheduled are allowed to experience internal blocking (from other tasks with which they share resources) and (with certain restrictions) release jitter — such as waiting for a message to arrive. The analysis presented is more general than that previously published, and subsumes, for example, techniques based on the Rate Monotonic approach. In addition to presenting the theory, an existing avionics case study is described and analysed. The predictions that follow from this analysis are seen to be in close agreement with the behaviour exhibited during simulation studies. 1.

As the real-time computing industry moves away from static cyclic executive-based scheduling towards more flexible process-based scheduling, so it is important for current scheduling analysis techniques to advance and to address more realistic application areas. This paper extends the current analysis associated with static priority pre-emptive based scheduling; in particular it derives analysis for tasks with arbitrary deadlines that may suffer release jitter due to being dispatched by a tick driven scheduler. We also consider bursty sporadic activities, where tasks arrive sporadically but then execute periodically for some bounded time. The paper illustrates how a window-based analysis technique can be used to find the worst-case response time of a task set, and shows that the technique can be easily extended to cope with realistic and complex task characteristics. 1. INTRODUCTION One commonly proposed way of constructing a hard real-time system is to build the system from a number...

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...

Scheduling theories for fixed priority scheduling are now sufficiently mature that a genuine engineering approach to the construction of hard real-time systems is possible. In this paper we review recent advances. A flexible computational model is adopted that can accommodate periodic and sporadic activities, different levels of criticality, process interaction and blocking, cooperative scheduling (deferred preemption), release jitter, precedence constrained processes, arbitrary deadlines, deadlines associated with specific events (rather than the end of a task's execution) and offsets. Scheduling tests for these different application characteristics are described. This model can be supported by structured, object oriented or formal development methods. The paper also considers the issues involved in producing safe and predictable kernels to support this computational model.

We propose a technique for analyzing cache-related preemption delays of tasks that cause unpredictable variation in task execution time in the context of fixed-priority preemptive scheduling. The proposed technique consists of two steps. The first step performs a per-task analysis to estimate cache-related preemption cost for each execution point in a given task. The second step computes the worst case response time of each task that includes the cache-related preemption delay using a response time equation and a linear programming technique. This step takes as its input the preemption cost information of tasks obtained in the first step. This paper also compares the proposed approach with previous approaches. The results show that the proposed approach gives a prediction of the worst case cache-related preemption delay that is up to 60% tighter than the best of predictions obtained from the previous approaches. Index Terms--- real-time system, fixed-priority scheduling, cache memory,...

Building distributed embedded systems from scratch is not cost-effective. Instead, designing and building these systems by using domain specific components has promise. However, in using components, the most difficult issues are ensuring that hidden dependencies won't cause failures and that non-functional properties such as real-time performance are being met. We have built the VEST toolkit whose aim is to provide a rich set of dependency checks based on the concept of aspects to support distributed embedded system development via components. We describe the toolkit and its novelty. We also use VEST on two case studies of a CORBA-based middleware for avionics. Data collected shows that VEST can significantly reduce the time it takes to build a distributed real-time embedded system by over 50%.

In a distributed hard real-time system, communications between tasks on different processors must occur in bounded time. The inevitable communication delay is composed of both the delay in transmitting a message on the communications media, and also the delay in delivering the data to the destination task. This paper derives schedulability analysis bounding the media access delay and the delivery delay. Two access protocols are considered: a simple timed token passing approach, and a real-time priority broadcast bus. A simple delivery approach is considered where the arrival of a message generates an interrupt --- the so-called `on demand' approach. 1. INTRODUCTION A hard real-time system is often composed from a number of periodic and sporadic tasks which communicate their results by passing messages; in a distributed system these messages are sent between processors across a communications device. In order to guarantee that the timing requirements of all tasks are met, the communica...

Traditional multiprocessor real-time scheduling partitions a task set and applies uniprocessor scheduling on each processor. For architectures where the penalty of migration is low, such as uniform-memory access shared-memory multiprocessors, the non-partitioned method becomes a viable alternative. By allowing a task to resume on another processor than the task was preempted on, some task sets can be scheduled where the partitioned method fails. We address fixed-priority scheduling of periodically arriving tasks on Ñ equally powerful processors having a non-partitioned ready queue. We propose a new priorityassignment scheme for the non-partitioned method. Using an extensive simulation study, we show that the priorityassignment scheme has equivalent performance to the best existing partitioning algorithms, and outperforms existing fixed-priority assignment schemes for the non-partitioned method. We also propose a dispatcher for the nonpartitioned method which reduces the number of preemptions to levels below the best partitioning schemes.

One common way of constructing hard real-time systems is to use a number of periodic and sporadic tasks assigned static priorities and dispatched at run-time according to the preemptive priority scheduling algorithm. Most analysis for such systems attempts to find the worst-case response time for each task by assuming that the worst-case scheduling point is when all tasks in the system are released simultaneously. Often, however, a given set of hard real-time tasks will have offset constraints: a number of tasks sharing the same periodic behaviour will be constrained to execute at fixed offsets in time relative to each other. In this situation the assumption of a simultaneous release of all tasks can lead to pessimistic scheduling results. In this paper we derive good response time bounds for tasks with offset information, giving an optimal priority ordering algorithm. 1.

In many distributed real-time systems, the workload can be modeled as a set of periodic tasks, each of which consists of a chain of subtasks executing on different processors. Synchronization protocols are used to govern the release of subtasks so that the precedence constraints among subtasks are satisfied and the schedulability of the resultant system is analyzable. When different protocols are used , tasks can have different worst-case and average end-to-end response times. This paper focuses on distributed real-time systems that contain independent, periodic tasks scheduled by fixed-priority scheduling algorithms. It describes three synchronization protocols together with algorithms to analyze the schedulability of the system when these protocols are used. Simulation was conducted to compare the performance of these protocols with respect to the worst-case and average case end-to-end response times. The simulation experiment and the performance of the protocols are described. 1 Int...