Michaël Richard

Allocating and scheduling tasks in multiple fieldbus real-time systems

We consider real-time systems connected via several fieldbuses. Validating such systems consists of proving that tasks meet their end-to-end deadlines. Tasks are scheduled on processors by fixed-priority schedulers. We propose an automatic method for allocating tasks on processors and assigning priorities to tasks so that every deadline is met. Allocation and scheduling are simultaneously achieved. We do not limit the search space to a specific priority rule (such as rate monotonic or deadline monotonic). Feasible schedules are validated by a holistic analysis. Numerical results of the method are lastly presented on a real-size application. Our tool will be a beneficial help to design real-time distributed systems.

On-line scheduling of real-time distributed computers with complex communication constraints

We consider the scheduling of periodic tasks running on distributed computers. Every execution of a task must meet its deadline. Response time analysis of the tasks is used to prove the schedulability of hard real-time distributed systems according to the on-line priority rules that schedule the processors and the network. Its main advantage is to take into account the precedence dependencies of the schedules of the tasks on the processors and the messages sent on the network(s). Past works have addressed the issue of tasks related by asynchronous communication constraints with the senders and the receivers working at the same rate. We study more general relations among tasks when the rates of dependent tasks are not equal. We call such relations generalized communication constraints. Usually distributed systems are scheduled using a synchronization protocol and an on-line scheduling algorithm by processor. We present a graph theoretical approach to this schedulability analysis. Our algorithm transforms complex communication relations into classical ones, so that the classical scheduling analysis can be fully applied. That transformation is independent of the architecture of the distributed systems and no assumption is made on the synchronization protocol considered.

Reducing the gap between design and scheduling

Model-based design techniques for real-time systems have limited real-time expressiveness, focusing their abilities to classic scheduling models (like the classic sporadic model) and to a reduced set of temporal analysis techniques (like the Rate Monotonic Analysis). Moreover, to perform analysis with the real-time scheduling theory, the system designers must check that their models are compliant with the assumptions of this theory. This article introduces an open meta-model, based on model-driven engineering, which aims to cover new real-time scheduling models and techniques. Therefore, it will be possible to connect several independent schedulability analysis tools, following closely the advances in real-time scheduling theory, dealing with a temporal model that will be covered by our meta-model. This connection can be done at different stages of the design (early for sensitivity analysis, at a later stage for temporal validation) to create a temporal model of the designed system, and to assist the designer who is not necessarily an expert in scheduling theory. This paper can be considered as an attempt to motivate the real-time community to have a taxonomy of real-time scheduling models, problems and analysis techniques.

Model Driven Timing Analysis for Real-Time Systems

This paper stresses the difficulty for a system designer to use an appropriate real-time task model for his system, and to choose the associated scheduling analysis tests/dimensioning techniques. We propose a model-based approach tackling this difficulty. We focus on the schedulability analysis tree used by our method in order to help the designer to dimension, and then to validate his system.

An efficient response-time analysis for real-time transactions with fixed priority assignment

In the general context of tasks with offsets (general transactions), only exponential methods are known to calculate the exact worst-case response time (WCRT) of a task. The known pseudo-polynomial techniques give an upper bound of the WCRT. In this paper, we present a new worst-case response-time analysis technique (mixed method) for transactions scheduled by fixed priorities, and executing on a uniprocessor system. This new analysis technique gives a better (i.e. lower) pseudo-polynomial upper bound of the WCRT. The main idea is to combine the principle of exact calculation and the principle of approximation calculation, in order to decrease the pessimism of WCRT analysis, thus allowing improving the upper bound of the response time provided while preserving a pseudo-polynomial complexity. Then we define the Accumulative Monotonic property on which a necessary condition of feasibility is discussed. We also propose, to speed up the exact and mixed analysis, the dominated candidate task concept that allows reducing significantly the number of critical instants to study in an analysis.

Towards a simple meta-model for complex real-time and embedded systems

We introduce an open meta-model that will be easily enriched to cover new real-time scheduling models and techniques. On the one hand, it will be possible to connect several independent schedulability analysis tools, following closely the advances in real-time scheduling theory, dealing with a temporal model that will be covered by our metamodel. On the other hand, we will use model transformation techniques in order to extract information from different design methodologies. This extraction can be done at different stages of the design (early for sensitivity analysis, at a later stage for temporal validation) to create a temporal model of the designed system, without the designer to be an expert in scheduling theory. We are working currently on the meta-model phase. The objective is to cover enough concepts to be able to represent a big part of the real-time scheduling models and problems, without introducing too much complexity. This paper uses the UML profile mechanism to describe such a meta-model.

A model-based process for the modelling and the analysis of avionic architectures

To design and analyse integrated modular avionics (IMA) architectures, this paper presents a model-based process with separation of concerns going from the business view to the scheduling analysis view. A pivot meta-model dedicated to schedulability analysis has been extended to support recent and new systems architectures as well as new analysis methods. This pivot meta-model represents the front-end of a design framework filling the gap between the system architect and the schedulability analysis expert. This is presented in this paper by showing how the design framework is used to handle IMA architecture, and how simple plugins were developed and connected to the framework. The plugins propose hierarchical schedulability analysis, as well as end-to-end response-time analysis for network architecture. This research is illustrated by discussing a case study.