Avaliação experimental do uso de estatística dos valores extremos na determinação de limites de tempos de execução para projeto de sistemas de tempo real

Abstract

Real-time systems (RTS) are those whose actions are subject to time constraints, defined in terms of execution time deadlines. A hard RTS is usually designed to never miss a deadline, as in the case of avionics and space systems. To guarantee this it is essential to know the maximum time that each task takes to be executed, a parameter known as Worst Case Execution Time (WCET). Estimating the WCET of a task is not simple and, in the case of modern architectures, it is subject to unpredictable influences caused by different hardware and software elements, which prevent the derivation of an absolute WCET value. One of the most used techniques in these cases is the Measurement- Based Probabilistic Timing Analysis (MBPTA) method based on the Extreme Values Theory (EVT) statistical area, for estimating worst-case probability distributions, called Probabilistic WCET (pWCET) . The present work aims to carry out an experimental analysis of the use of EVT via MBPTA for pWCET inference, presenting the process of applying the technique in a real RPi environment and exposing the challenges and failures encountered during this process; a point rarely discussed in the literature. The results indicate that EVT is effective but does not always produce adequate models and consistent pWCET results in a real modern architecture. Thus, the main contribution of this work is to offer an alert to the community, which may need alternative methods for temporal analysis. This work is part of the Kepler project, a cooperation between IC-UFBA and INRIA-Paris.