Uma contribuição a programação e escalonamento de sistemas distribuidos de tempo-real

AUTOR(ES)
DATA DE PUBLICAÇÃO

1993

RESUMO

A real-time system should meet the timing requirements imposed by the application it implements, i.e., it should be predictable. A real-time system should also be easy to reconfigure in order to adapt to the changes that are likely to occur during its life time, i .e, it should be flexible. The main objective of this thesis is to contribute to the design of distributed real-time systems development environments. Particularly, it is intended to show that the production of flexible and yet predictable systems is feasible. To reach this objective it is proposed a programming model, supported by two programming languages (LPM-RC and LCM-RC) integrated to a scheduling strategy. In the proposed model, applications are constructed combining reusable modules of software, leading to highly reconfigurable systems. Modules temporal restrictions (periodicity and deadline) are not attributes of the modules themselves, but instead depend on the context in which they are used (the applications). Modules can share resources with the aid of servers. Servers do not have their own timing requirements, but inherit their clients timing constraints. A client-server interaction may involve the execution of a single service or the execution of a sequence of services in mutual exclusion from other clients. The objective of the scheduling strategy integrated to the languages is to try to meet applications timing constraints. The scheduling strategy is structured into three levels: 1) off-line scheduling of periodic modules, 2) time driven dynamic scheduling of aperiodic modules, and 3) dynamic scheduling of modules without timing constraints, when there are no periodic or aperiodic modules running. As part of the validation of the scheduling strategy, it is shown how to extract from a program the information needed for its scheduling, and proposed an algorithm to schedule periodic modules. This algorithm is based on an off-line heuristic algorithm available in the literature, extended to support the peculiarities of the proposed mode!. The extended algorithm, searchs for a schedule that meets the timing, resource and placement constraints of periodic modules. Besides that, the algorithm also tries to balance the system load, in the time and space domains, in order to improve its fault tolerance and, particularly, the schedulability of aperiodic modules. The off-line scheduling algorithm was evaluated applying to it sets of periodic modules with a wide range of time related characteristics. The conducted experiments aimed at determine the success rates and computational costs of the algorithm, using different heuristics. Specific experiments were also conducted to investigate the effectiveness and cost of the load balance strategies, as well as its influence on the success rate of the algorithm in finding feasible schedules

ASSUNTO(S)

algoritmos programação em tempo-real linguagem de programação (computadores)

ACESSO AO ARTIGO

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