On the Convergence of the Holistic Analysis for EDF Distributed Systems

Dynamic scheduling techniques, and EDF (Earliest Deadline First) in particular, have demonstrated their ability to increase the schedulability of real time systems compared to fixed-priority scheduling. In distributed systems, the scheduling policies of the processing nodes tend to be the same as in stand-alone systems and, although few EDF networks exist, it is foreseen that dynamic scheduling will gradually develop into real-time networks. There are some response time analysis techniques for EDF scheduled distributed systems, mostly derived from the holistic analysis developed by Spuri. The convergence of the holistic analysis in context of EDF distributed systems with shared resources had not been studied until now. There is a circular dependency between tasks’ release jitter values, response times and preemption level ceilings of shared resources. In this paper we present an extension of Spuri’s algorithm and we demonstrate that its iterative formulas are non-decreasing, even in the presence of shared resources. This result enables us to assert that the new algorithm converges towards a solution for the response times of the tasks and messages in a distributed system

Análisis de calidad de servicio de Middlewares asociados a la norma IEC61850

Máster en Computació

Análisis de planificabilidad basado en offsets en sistemas distribuidos de tiempo real con prioridades dinámicas

RESUMEN: La tesis doctoral está dedicada al análisis de planificabilidad en sistemas de tiempo real. Este análisis se realiza mediante técnicas de cálculo de tiempo de respuesta de peor caso. El trabajo está centrado en sistemas multiprocesador o distribuidos gobernados por eventos. El objetivo principal de la tesis es potenciar el uso de planificadores por prioridades dinámicas, concretamente planificadores EDF (Earliest Deadline First, Plazo más Cercano Primero). Habiéndose demostrado que la planificación EDF permite alcanzar cotas de utilización superiores a las de las técnicas de prioridades fijas en sistemas monoprocesador, el estudio y desarrollo de técnicas de análisis en sistemas distribuidos o multiprocesador es claramente inferior a los de la planificación por prioridades fijas. Para este tipo de sistemas las técnicas de análisis exactas son muy costosas y, por ello, las técnicas de análisis usuales son aproximadas. En esta tesis, se pretende mejorar las técnicas de análisis para planificación EDF reduciendo así el pesimismo de las técnicas actuales. Para conseguir este objetivo, en primer lugar se valida el método utilizado previamente y, posteriormente, se aplican mejoras sobre este para reducir el pesimismo del cálculo. Estas mejoras se obtienen adaptando métodos desarrollados para prioridades fijas a sistemas EDF. En concreto, los métodos utilizados para dicha adaptación son el modelo basado en intervalos de desfase (offsets) y la técnica que explota las relaciones de precedencia entre las tareas de un mismo flujo de tareas. El análisis de los desfases entre activaciones de las tareas (offsets) y de las relaciones de precedencia entre las tareas de un mismo flujo de tareas permiten reducir el pesimismo del análisis y con ello aumentar la utilización del sistema. Por último, se introduce una modificación al modelo de flujo de tareas o transacciones lineales (end-to-end flows) que permite ampliar el tipo de aplicaciones representables por el modelo de tareas. Esta modificación consiste en introducir secciones atómicas de tareas que permiten la representación de threads que se autosuspenden en una operación de espera o llamadas bloqueantes (típicamente llamadas bloqueantes a procedimientos remotos). Se han adaptado las técnicas de análisis EDF para este nuevo modelo.ABSTRACT: The thesis is dedicated to schedulability analysis in real-time systems. This analysis is carried out through worst case response time techniques and the targets are event-driven multiprocessor and distributed systems. The main purpose of this thesis is to enhance the use of dynamic priority schedulers, in particular those using EDF(Earliest Deadline First) scheduling algorithm, for distributed systems. Although it has been demonstrated that EDF scheduling can reach higher levels of utilization than fixed-priority scheduling techniques in uniprocessor systems, studies and techniques for distributed or multiprocessor systems in EDF than in fixed-priority scheduling. In these systems exact analysis techniques are too expensive and, thus, the usual techniques are approximate. Thus, the aim of this thesis is to improve analysis techniques for EDF scheduled systems decreasing the pessimism of the current techniques. To achieve this aim, the first step is to validate previously used methods. Then, some improvements have been implemented to reduce the pessimism of the calculation. These improvements have been achieved by adapting to EDF methods developed for fixed priorities. In particular, the methods used are the offset-based model and the technique that exploits precedence relations between tasks of the same end-to-end flow. The study of the offsets between releases of the different tasks and of the precedence relations between tasks of the same end-to-end flow allows reducing the pessimism of the analysis and, consequently, to increasing the overall utilization of the system. Finally, a modification in the end-to-end flow model has been introduced to represent atomic task sections. This modification allows us to expand the type of real-time applications representable by the task model. This new model allows us to represent threads that suspend themselves on delay operations or blocking calls (usually remote procedure calls). The EDF analysis techniques have been extended to cover this new model