On Modelling and Analysis of Dynamic Reconfiguration of Dependable Real-Time Systems

This paper motivates the need for a formalism for the modelling and analysis of dynamic reconfiguration of dependable real-time systems. We present requirements that the formalism must meet, and use these to evaluate well established formalisms and two process algebras that we have been developing, namely, Webpi and CCSdp. A simple case study is developed to illustrate the modelling power of these two formalisms. The paper shows how Webpi and CCSdp represent a significant step forward in modelling adaptive and dependable real-time systems.Comment: Presented and published at DEPEND 201

ReTiF: A declarative real-time scheduling framework for POSIX systems

This paper proposes a novel framework providing a declarative interface to access real-time process scheduling services available in an operating system kernel. The main idea is to let applications declare their temporal requirements or characteristics without knowing exactly which underlying scheduling algorithms are offered by the system. The proposed framework can adequately handle such a set of heterogeneous requirements configuring the platform and partitioning the requests among the available multitude of cores, so to exploit the various scheduling disciplines that are available in the kernel, matching application requirements in the best possible way. The framework is realized with a modular architecture in which different plugins handle independently certain real-time scheduling features. The architecture is designed to make its behavior customization easier and enhance the support for other operating systems by introducing and configuring additional plugins

Mode Changes in Priority Pre-emptively Scheduled Systems

software load in any mode is lower than the total software load. However, the auto-stability software is needed in all three modes, and needs to run continuously without missing a deadline --- a mode change must not cause a glitch where the software fails to meet its timing requirements (e.g. a task missing a deadline or release being delayed). In addition to planned mode changes there are changes that may occur sporadically: abnormal events, such as a processor failure, might require the system functions to degrade in a graceful manner (existing functions might be deleted, or be degraded to use fewer resources, to accommodate functions from the failed processor). Similarly, the failure of a piece of controlled equipment may cause a mode change: in our hypothetical commercial airliner there may be many sensor and actuator failure patterns, with many different controller algorithms that could be used, depending on the sensors and actuators remaining. After an actuator failure (say), the..

Fixed-Priority Scheduling Algorithms with Multiple Objectives in Hard Real-Time Systems

In the context ofFixed-Priority Scheduling in Real-Time Systems, we investigate scheduling mechanisms for supporting systems where, in addition to timing constraints, their performance with respect to additional QoS requirements must be improved. This'type of situation may occur when the worst-case res~urce requirements of all or some running tasks cannot be simultaneously met due to task contention. . Solutions to these problems have been proposed in the context of both fixed-priority and dynamic-priority scheduling. In fixed-priority scheduling, the typical approach is to artificially modify the attributes or structure of tasks, and/or usually require non-standard run-time support. In dynamic-priority scheduling approaches, utility functions are employed to make scheduling decisions with the objective of maximising the utility. The main difficulties with these approaches are the inability to formulate and model appropriately utility functions for each task, and the inability to guarantee hard deadlines without executing computationally costly algorithms. In this thesis we propose a different approach. Firstly, we introduce the concept of relative importance among tasks as a new metric for expressing QoS requirements. The meaning of this importance relationship is to express that in a schedule it i~ desirable to run a task in preference to other ones. This model is more intuitive and less restrictive than traditional utility-based app~oaches. Secondly, we formulate a scheduling problem in terms of finding a feasible assignment of fixed priorities that maximises the new QoS metric, and propose the DI and DI+ algorithms that find optimal solutions. By extensive simulation, we show that the new QoS metric combined with the DI algorithm outperforms the rate monotonic priority algorithm in several practical problems such as minimising jitter, minimising the number of preemptions or minimising the latency. In addition, our approach outperforms EDF in several scenarios

Meta-razonamiento en Agentes con Restricciones Temporales Críticas

El paradigma de agentes/sistemas multi-agente es uno de lo smodelos computacionales de mayor relevancia de los últimos tiempos, habiendo dado lugar a múltiples investigaciones y aplicaciones concretas. Este modelo computacional tiene como objetivo la construcción de sistemas que se enfrenten a situaciones mostrando ciertas características propias de un ser humano, tales como inteligencia, reactividad, pro-actividad,... De entre todas las variedades de tipos de agente que se pueden definir, el trabajo realizado se centra en aquellos agentes que deben trabajar en un entorno con restricciones temporales críticas, es decir, donde existen ciertos problemas a los que el agente debe dar respuesta antes de que pase un determinado tiempo o las consecuencias serán catastróficas. En un agente de este tipo es fundamental tratar de conseguir un uso óptimo del tiempo de procesador, recurso más importante en esta clase de sistemas. Es por esto que resulta relevante ocnseguir que dicho agente sea capaz de dedicar su timpo de procesador a aquello que sea necesario de acuerdo a la situación en la que se encuentre. Para conseguir esta adaptación es fundamental que el agente sea capaz de razonar sobre su propio proceso de razonamiento, es decir, meta-razonar, siempre teniendo en cuenta que este proceso de metarazonamiento va a consumir también tiempo de procesador. De esta manera, el objetivo de este trabajo es el estudio de las capacidades necesarias para poder incorporar la habilidad de meta-razonar a un agente con restricciones temporales críticas, así como la incorporación a una arquitectura de agente concreta, la de agente ARTIS. Después del estudio comentado, se llegó a la conclusión de que para poder incorporar la habilidad de meta-razonamiento a un agente con restricciones temporales críticas era necesario incluir al agente las siguientes capacidades: detección de situaciones significativas, adaptar su comportamiento, adaptar el proceso de razonamiento del agente teniendo en ..Carrascosa Casamayor, C. (2004). Meta-razonamiento en Agentes con Restricciones Temporales Críticas [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/2670Palanci

Extensiones a los métodos de planificación de sistemas de tiempo real críticos basados en prioridades

Los sistemas de tiempo real tienen un papel cada vez más importante en nuestra sociedad. Constituyen un componente fundamental de los sistemas de control, que a su vez forman parte de diversos sistemas de ingeniería básicos en actividades industriales, militares, de comunicaciones, espaciales y médicas. La planificación de recursos es un problema fundamental en la realización de sistemas de tiempo real. Su objetivo es asignar los recursos disponibles a las tareas de forma que éstas cumplan sus restricciones temporales. Durante bastante tiempo, el estado de la técnica en relación con los métodos de planificación ha sido rudimentario. En la actualidad, los métodos de planificación basados en prioridades han alcanzado un nivel de madurez suficiente para su aplicación en entornos industriales. Sin embargo, hay cuestiones abiertas que pueden dificultar su utilización. El objetivo principal de esta tesis es estudiar los métodos de planificación basados en prioridades, detectar las cuestiones abiertas y desarrollar protocolos, directrices y esquemas de realización práctica que faciliten su empleo en sistemas industriales. Una cuestión abierta es la carencia de esquemas de realización de algunos protocolos con núcleos normalizados. El resultado ha sido el desarrollo de esquemas de realización de tareas periódicas y esporádicas de tiempo real, con detección de fallos de temporización, comunicación entre tareas, cambio de modo de ejecución del sistema y tratamiento de fallos mediante grupos de recuperación. Los esquemas se han codificado en Ada 9X y se proporcionan directrices para analizar la planificabilidad de un sistema desarrollado con esta base. Un resultado adicional ha sido la identificación de la funcionalidad mínima necesaria para desarrollar sistemas de tiempo real con las características enumeradas. La capacidad de adaptación a los cambios del entorno es una característica deseable de los sistemas de tiempo real. Si estos cambios no estaban previstos en la fase de diseño o si hay módulos erróneos, es necesario modificar o incluir algunas tareas. La actualización del sistema se suele realizar estáticamente y su instalación se lleva a cabo después de parar su ejecución. Sin embargo, hay sistemas cuyo funcionamiento no se puede detener sin producir daños materiales o económicos. Una alternativa es diseñar el sistema como un conjunto de unidades que se pueden reemplazar, sin interferir con la ejecución de otras unidades. Para tal fin, se ha desarrollado un protocolo de reemplazamiento dinámico para sistemas de tiempo real crítico y se ha comprobado su compatibilidad con los métodos de planificación basados en prioridades. Finalmente se ha desarrollado un esquema de realización práctica del protocolo.---ABSTRACT---Real-time systems are very important now a days. They have become a relevant issue in the design of control systems, which are a basic component of several engineering systems in industrial, telecommunications, military, spatial and medical applications. Resource scheduling is a central issue in the development of real-time systems. Its purpose is to assign the available resources to the tasks, in such a way that their deadlines are met. Historically, hand-crafted techniques were used to develop real-time systems. Recently, the priority-based scheduling methods have reached a sufficient maturity level to be feasible its extensive use in industrial applications. However, there are some open questions that may decrease its potential usefulness. The main goal of this thesis is to study the priority-based scheduling methods, to identify the remaining open questions and to develop protocols, implementation templates and guidelines that will make more feasible its use in industrial applications. One open question is the lack of implementation schemes, based on commercial realtime kernels, of some of the protocols. POSIX and Ada 9X has served to identify the services usually available. A set of implementation templates for periodic and sporadic tasks have been developed with provisión for timing failure detection, intertask coraraunication, change of the execution mode and failure handling based on recovery groups. Those templates have been coded in Ada 9X. A set of guidelines for checking the schedulability of a system based on them are also provided. An additional result of this work is the identification of the minimal functionality required to develop real-time systems based on priority scheduling methods, with the above characteristics. A desirable feature of real-time systems is their capacity to adapt to changes in the environment, that cannot be entirely predicted during the design, or to misbehaving software modules. The traditional maintenance techniques are performed by stopping the whole system, installing the new application and finally resuming the system execution. However this approach cannot be applied to non-stop systems. An alternative is to design the system as a set of software units that can be dynamically replaced within its operative environment. With this goal in mind, a dynamic replacement protocol for hard real-time systems has been defined. Its compatibility with priority-based scheduling methods has been proved. Finally, a execution témplate of the protocol has been implemented