How to rapidly prototype a real-time scheduler

Journal ArticleImplementing a new scheduling algorithm in an OS kernel is often an important step in scheduling research because it permits evaluation of the algorithm's performance on real workloads. However, developing a new scheduler is not a trivial task because it requires sophisticated programming skills and a deep knowledge of kernel internals. In this paper we show how to use the HLS scheduling framework to develop new schedulers in a user-level simulator, where advanced debugging tools can be used to achieve a high level of robustness before the scheduler is converted to a loadable kernel module simply by recompiling it. Besides facilitating debugging and porting, the HLS abstraction has the benefit of bringing the programming model very close to what, in our experience, scheduler developers want

Implementing Multi-Periodic Critical Systems: from Design to Code Generation

This article presents a complete scheme for the development of Critical Embedded Systems with Multiple Real-Time Constraints. The system is programmed with a language that extends the synchronous approach with high-level real-time primitives. It enables to assemble in a modular and hierarchical manner several locally mono-periodic synchronous systems into a globally multi-periodic synchronous system. It also allows to specify flow latency constraints. A program is translated into a set of real-time tasks. The generated code (\C\ code) can be executed on a simple real-time platform with a dynamic-priority scheduler (EDF). The compilation process (each algorithm of the process, not the compiler itself) is formally proved correct, meaning that the generated code respects the real-time semantics of the original program (respect of periods, deadlines, release dates and precedences) as well as its functional semantics (respect of variable consumption).Comment: 15 pages, published in Workshop on Formal Methods for Aerospace (FMA'09), part of Formal Methods Week 2009

Rate Monotonic vs. EDF: Judgment Day

Since the first results published in 1973 by Liu and Layland on the Rate Monotonic (RM) and Earliest Deadline First (EDF) algorithms, a lot of progress has been made in the schedulability analysis of periodic task sets. Unfortunately, many misconceptions still exist about the properties of these two scheduling methods, which usually tend to favor RMmore than EDF. Typical wrong statements often heard in technical conferences and even in research papers claim that RM is easier to analyze than EDF, it introduces less runtime overhead, it is more predictable in overload conditions, and causes less jitter in task execution. Since the above statements are either wrong, or not precise, it is time to clarify these issues in a systematic fashion, because the use of EDF allows a better exploitation of the available resources and significantly improves system’s performance. This paper comparesRMagainstEDFunder several aspects, using existing theoretical results, specific simulation experiments, or simple counterexamples to show that many common beliefs are either false or only restricted to specific situations

Marco de referencia para la definición de planificadores a nivel de usuario en sistemas de tiempo-real

Después de más de 25 años de intensa investigación, la planificación de sistemas de tiempo-real ha mostrado una transición que va desde una infraestructura basada en ejecutivos cíclicos, a modelos de planificación más flexibles, tales como planificación basada en prioridades estáticas y dinámicas, planificación de tareas no críticas, y planificación con retroalimentación, por nombrar algunas. A pesar de lo anterior, actualmente tan sólo unas cuantas políticas de planificación están disponibles para la implementación de sistemas de tiempo-real. Por ejemplo, la mayoría de los sistemas operativos de tiempo-real existentes proporcionan únicamente planificación basada en prioridades fijas. Sin embargo, no todos los requerimientos de las aplicaciones de tiempo-real pueden ser satisfactoriamente atendidos utilizando exclusivamente planificación estática. Existen sistemas constituidos por tareas críticas y no críticas que son planificados de mejor manera utilizando planificación basada en prioridades dinámicas. Además, se ha demostrado que la planificación dinámica permite una mayor utilización de los recursos del sistema. En años recientes, algunos autores han publicado diferentes esquemas para integrar nuevas políticas de planificación a un sistema operativo. Algunos de ellos proponen que los nuevos servicios de planificación se implementen a nivel de usuario, evitando así que la estructura interna del sistema operativo tenga que ser modificada, y ofreciendo la oportunidad de implementar y probar muchos de los resultados generados por el trabajo de investigación en el área de planificación de sistemas de tiempo-real. De entre los trabajos relacionados publicados a la fecha, destaca el Modelo para la Planificación Definida por el Usuario, propuesto por Mario Aldea y Michael González-Harbour. El modelo presenta una Interfaz para Programas de Aplicación (API) que permite crear y utilizar planificadores a nivel de usuario de manera compatible con el modelo de planificaciónDíaz Ramírez, A. (2006). Marco de referencia para la definición de planificadores a nivel de usuario en sistemas de tiempo-real [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1934Palanci

A framework for flexible scheduling in real-time middleware

The traditional vehicle for the deployment of a real-time system has been a real-time operating system (RTOS). In recent years another programming approach has increasingly found its way into the real-time systems domain: the use of middleware. Examples are the so called pervasive systems (embedded, interactive but not mobile), and ubiquitous systems (embedded, interactive and mobile), e.g. hand-held devices. These tend to be dynamic systems that often exhibit a need for flexible scheduling because of their operating requirement; or their execution environment. Thus, today there is a true need in many realtime applications for more flexible scheduling than what is currently the stateof- prac'tice. By flexible scheduling we mean the ability of the program execution platform to provide a range of scheduling policies, all the way from hard real-time to soft real-time policies, from which an application can choose one most suited to its needs. Furthermore, some applications may need to be scheduled by one policy while others may need a different policy, e.g. fi'Ced priority or earliest deadline first (EDF) for hard real-time tasks, least slack time first (LST) or shortest remaining time for soft real-time tasks. It would be difficult for the middleware to expect this functionality from the RTOS. This would require a fine balance to be struck in the RTOS between flexibility and usability, and many years will probably pass until such approaches become mainstream and usable. 'This thesis maintains that this flexibility can be introduced into the middleware. It presents a viable solution to introducing flexible scheduling in real-time program execution middleware in the form of a flexible scheduling framework. Such a framework allows use of the same program execution middleware for a variety of applications - soft, firm and hard. In particular, the framework allows different scheduling policies to co-exist in the system and their tasks to share common resources. The thesis describes tlle framework's protocol, examines the different types of scheduling policies that can be supported, tests its correctness through the use of a model checker and evaluates the proposed framework by measuring its execution cost overhead. The framework is deemed appropriate for the types of real-time applications that need the services of flexible scheduling