Poster Abstract : Interface-based design approach for analysis of mode changes in distributed real-time systems

Schedulability analysis of adaptive real-time systems has been a well-known problem. It is relevant for many practical systems that have different operating modes. It is not a trivial to solve problem because schedulability needs to be shown not only in the individual operating modes, but also during the transitions between the modes. Many approaches have been proposed to address this problem but most of them are restricted to systems with fixed priority scheduling and periodic task activations. Recently, we have proposed an approach based on Real-time calculus that can perform schedulability analysis of systems with fixed and dynamic priorities such as EDF. It is also applicable to arbitrarily complex task activations. However, it is restricted to single processor systems. In this work, we propose an interfacebased design approach based on Real-time interfaces which extends our previous work and makes it applicable to open dynamic environments and distributed systems. I. PROBLEM STATEMENT Adaptive real-time systems that change their operating mode during runtime have been in use for a long time in the aviation and the automotive industries. Performance and schedulability analysis techniques have been successfully developed for these areas. However, emerging application domains such as software defined radios in modern mobile phones and scalable video coding exhibit complex activations of tasks that require more sophisticated real-time analysis methods. Such methods are not trivial because adaptive systems require that they meet real-time constraints not only in the individual operating modes, but also during the transitions between modes. A system may require a mode change because it needs to enter an emergency state, it needs to adapt to changes in the environment, or it needs to reduce its resource usage. A mode change may involve changes in the set of executing tasks, changes in the parameters of tasks e.g. execution times and deadlines, or changes in the activation patterns of tasks e.g. periodic, periodic with jitter, sporadic, etc. In general, mode changes can lead to violations in the realtime constraints of a system. For example, if a task is triggered by an event stream, immediately switching from one stream to another can cause missed deadlines because bursts of events from the two streams may appear close to the switching time and lead to transient overloads of the system. Such overloads are usually overcome by introducing a delay between the two streams. The work presented in this paper was partially supported by the National Competence Center in Research on Mobile Information and Communication Systems (NCCR-MICS), a center supported by the Swiss National Science Foundation under grant number 5005-67322. II. RELATED WORK Schedulability of mode changes has been addressed previously, see e.g. [1]. Analysis methods for mode changes on single processor systems with rate-monotonic and deadlinemonotonic scheduling have been introduced in [2] and [3], respectively. Later in [4], transition delays have been included in the model which help to avoid overload situations. However, all these approaches are limited to a strictly periodic task activation scheme. An approach proposed recently in [5] overcomes this limitation. It models event streams with three parameters: period, jitter, and minimum interarrival distance between events. Even though this model captures more complex activation patterns, it still describes only a limited set of event streams. In [5] the authors identify problems with the mode change analysis of an example distributed system however, a detailed analysis procedure for the general distributed case is missing. In [6], [7], we have proposed an approach that can deal with fixed priority as well as with dynamic priority scheduling policies such as earliest deadline first (EDF). It uses the concepts of arrival curves α and service curves β from Network and Real-time calculus [8], [9] to model arbitrarily complex task activation patterns and resource availabilities. The method is component-based which means that the schedulability of the system is derived from the individual properties of the components. It can be used to analyze the schedulability of a mode change in a system given the parameters for the different modes and the parameters of the system. In order to deal with system overloads during a mode transition, the method can analyze schedulability of the system with a delay between the streams from the old and the new modes, and it can search for the minimum delay δ ≥ 0 that will make a mode transition schedulable, i.e. no deadlines will be violated in the old mode (mode I), the new mode (mode II), and during the transition