Analysis of Clocked Schedules-High-Priority Tasks

Abstract

Clocked schedules are used in a variety of real-time systems to perform tasks in accordance with their delay requirements. A comprehensive overview of various clocked schedules can be found in the accompanying paper by Fredericks et al. For one model of a clocked schedule we can see approximate and exact analysis of the relevant performance measures in the papers by Fredericks et al. and by Ackroyd in this issue of the Journal. These results can also be used to evaluate the long-term delays in the other models. For extremely time-critical (high-priority) tasks, the probability of tasks not getting served in the scheduled interval and the short-term delay distribution are important performance measures that are sensitive to the detail structure of the clocked schedule. In this paper, we show that these performance measures can be obtained in terms of steady-state distribution of an embedded Markov chain. This steady-state distribution is calculated, exactly or approximately, for a number of models, and the results are used to compare, numerically, various scheduling mechanisms.