Andy J. Wellings

Applying new scheduling theory to static priority pre-emptive scheduling

The paper presents exact schedulability analyses for real-time systems scheduled at runtime 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) to 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 relevant 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.

An extendible approach for analyzing fixed priority hard real-time tasks

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.

Calculating controller area network (can) message response times

Abstract Controller Area Network (CAN) is a well-designed communications bus for sending and receiving short real-time control messages at speeds of up to 1Mbit/sec. One of the perceived drawbacks to CAN has been the inability to bound accurately the worst-case response time of a given message (i.e., the longest time between queueing the message and the message arriving at the destination processors). This paper presents an analysis to bound such latencies. A benchmark is used to illustrate the application of this analysis.

Hard Real-Time Scheduling: The Deadline-Monotonic Approach

Abstract The scheduling of processes to meet deadlines is a difficult problem often simplified by placing severe restrictions upon the timing characteristics of individual processes. One restriction often introduced is that processes must have deadline equal to period. This paper investigates schedulability tests for sets of periodic processes whose deadlines are permitted to be less than their period. Such a relaxation enables sporadic processes to be directly incorporated without alteration to the process model. Following an introduction oudining the constraints associated with existing scheduling approaches and associated schedulability tests, the deadline-monotonic approach is introduced. New schedulability tests are derived which vary in computational complexity. The tests are shown to be directly applicable to the scheduling of sporadic processes.

Fixed priority pre-emptive scheduling: an historical perspective

From its roots in job-shop scheduling, research into fixed priority pre-emptive scheduling theory has progressed from the artificial constraints and simplistic assumptions used in early work to a sufficient level of maturity that it is being increasingly used in the implementation of real-time systems. It is therefore appropriate that within this special issue we provide an historical perspective on the development of fixed priority pre-emptive scheduling.

Allocating hard real-time tasks: an NP-hard problem made easy

A distributed hard real time system can be composed from a number of communicating tasks. One of the difficulties with building such systems is the problem of where to place the tasks. In general there are PT ways of allocating T tasks to P processors, and the problem of finding an optimal feasible allocation (where all tasks meet physical and timing constraints) is known to be NP-Hard. This paper describes an approach to solving the task allocation problem using a technique known as simulated annealing. It also defines a distributed hard real-time architecture and presents new analysis which enables timing requirements to be guaranteed.

Analysis of hard real-time communications

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.

Adding instruction cache effect to schedulability analysis of preemptive real-time systems

Cache memories are commonly avoided in real time systems because of their unpredictable behavior. Recently, some research has been done to obtain tighter bounds on the worst case execution time (WCET) of cached programs. These techniques usually assume a non preemptive underlying system. However, some techniques can be applied to allow the use of caches in preemptive systems. The paper describes how to incorporate the effect of instruction cache to the Response Time schedulability Analysis (RTA). RTA is an efficient analysis for preemptive fixed priority schedulers. We also compare through simulations the results of such an approach to both cache partitioning (increase of the cache predictability by assigning private cache partitions to tasks) and CRMA (Cached RMA: cache effect is incorporated in the utilization based rate monotonic schedulability analysis). The results show that the cached version of RTA (CRTA) clearly outperforms CRMA, however the partitioning scheme may be better depending on the system configuration. The obtained results bound the applicability domain for each method for a variety of hardware and workload configurations. The results can be used as design guidelines.

Effective analysis for engineering real-time fixed priority schedulers

There has been considerable activity in recent years in developing analytical techniques for hard real-time systems. Inevitably these techniques make simplifying assumptions so as to reduce the complexity of the problem to be solved. Unfortunately this leads to a gap between theory and engineering practice. The paper presents new analysis that enables the costs of the scheduler (clock overheads, queue manipulations and release delays) to be factored into the standard equations for calculating worst-case response times. As well as predicting the true behavior of realistic systems, the analysis also allows free parameters, such as clock interrupt rate, to be determined. >

HRT-HOOD: a structured design method for hard real-time systems

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 analyzed for their timing properties. In this article 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 practice, which in effect treats these topics as performance issues. To illustrate our approach we present a simple case study of a Mine Drainage Control System, and show how it can be designed using the abstractions presented in the article.