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.

This paper addresses the problem of jointly scheduling tasks with both hard and soft time constraints. We present a new analysis which builds upon previous research into slack stealing algorithms. Our analysis determines the maximum processing time which may be stolen from hard deadline periodic or sporadic tasks, without jeopardising their timing constraints. It extends to tasks with characteristics such as synchronisation, release jitter and stochastic execution times, as well as forming the basis for a family of optimal and approximate slack stealing algorithms.

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.

Testing is an important part of any software development project, and can typically surpass more than half of the development cost. For safety-critical computer based systems, testing is even more important due to stringent reliability and safety requirements. However, most safety-critical computer based systems are real-time systems, and the majority of current testing and debugging techniques have been developed for sequential (non real-time) programs. These techniques are not directly applicable to real-time systems, since they disregard issues of timing and concurrency. This means that existing techniques for reproducible testing and debugging cannot be used. Reproducibility is essential for regression testing and cyclic debugging, where the same test cases are run repeatedly with the intention of verifying modified program code or to track down errors. The current trend of consumer and industrial applications goes from single microcontrollers to sets of distributed micro-controllers, which are even more challenging than handling real-time per-see, since multiple loci of observation and control additionally must be considered. In this thesis we try to remedy these problems by presenting an integrated approach to monitoring, testing, and debugging of distributed real-time systems. For monitoring

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...

EMBEDDED COMPUTER SYSTEMS are now everywhere: from alarm clocks to PDAs, from mobile phones to cars, almost all the devices we use are controlled by embedded computer systems. An important class of embedded computer systems is that of real-time systems, which have to fulfill strict timing requirements. As realtime systems become more complex, they are often implemented using distributed heterogeneous architectures. The main objective of this thesis is to develop analysis and synthesis methods for communication-intensive heterogeneous hard real-time systems. The systems are heterogeneous not only in terms of platforms and communication protocols, but also in terms of scheduling policies. Regarding this last aspect, in this thesis we consider time-driven systems, event-driven systems, and a combination of both, called multi-cluster systems. The analysis takes into

We show that previous algorithmic and scheduling work concerning the use of lock-free objects in hard real-time systems can be extended to support real-time transactions on memory-resident data. Using our approach, transactions are not susceptible to priority inversion or deadlock, do not require complicated mechanisms for data-logging or for rolling back aborted transactions, and are implemented as library routines that require no special kernel support. 1 Introduction In most real-time database systems, conventional mechanisms suchaslocks, timestamps, and serialization graphs are used for concurrency control. The main problem when using any of these mechanisms is that of handling con#icting operations. If an operation of a transaction creates a con#ict, then one of two strategies may be used: either that transaction may be blocked, or one or more of the transactions involved in the con- #ict may be aborted. When using con#ict resolution schemes that employ blocking, deadlockisakey ...

The incorporation of unbounded components (i.e. software modules that cannot be analysed to produce realistic worst case execution times) into hard real-time applications has been recognised as a key issue for the next generation of systems. We present an architectural model that caters for the three main approaches to integrating unbounded components --- imprecise computation, sieve functions and multiple versions. This architectural model is feasible because it is supported by schedulability tests that will guarantee the bounded tasks. These test are defined in the paper. A simple computational model that uses preemptive priority-based dispatching is required. The wide-spread use of techniques such as imprecise computation will only happen if they are integrated into standard software engineering methods. We therefore show how the techniques can be realised in Ada 9X. ############### + This work is supported, in part, by the UK Information Engineering Advanced Technology Programme, ...

Current feasibility tests for the static-priority scheduling of periodic task systems run in pseudo-polynomial time. We present a fully polynomial-time approximation scheme (FPTAS) for feasibility analysis in static-priority systems where each task’s relative deadline is constrained to be at most its period. This test is an approximation with respect to the amount of processor capacity that must be “sacrificed ” for the test to become exact. We show that an arbitrary level of accuracy, ɛ, may be chosen for the approximation scheme, and present a run-time bound that is polynomial in terms of ɛ and the number of tasks, n. Keywords: Real-time scheduling; Uniprocessor systems; Static-priority systems; Feasibility analysis; Approximation algorithms.

In this paper, we present a new strategy for providing flexibility in hard real-time systems. This approach, based on dual priorities, retains the offline guarantees afforded to crucial tasks by fixed priority scheduling. Further, it provides an efficient method of responsively scheduling soft tasks and a means of providing online guarantees for tasks with firm deadlines. An effective O (n ) acceptance test is derived in the paper. Supported by the analysis, acceptance tests and mechanisms described, the dual priority approach provides a basis for combining the benefits of guaranteeing hard requirements with the flexibility inherent in the besteffort paradigm. 1. Introduction The requirement to support dynamic, adaptive and intelligent behaviour whilst also providing the 100% guarantees needed by crucial hard real-time services, has been identified as one of the key challenges presented by the next generation of real-time systems [8]. In this paper, we present a minimally dynamic sch...