The capacity exchange protocol

This paper proposes a new strategy to integrate shared resources and precedence constraints among real-time tasks, assuming no precise information on critical sections and computation times is available. The concept of bandwidth inheritance is combined with a capacity sharing and stealing mechanism to efficiently exchange bandwidth among tasks to minimise the degree of deviation from the ideal system’s behaviour caused by inter-application blocking. The proposed Capacity Exchange Protocol (CXP) is simpler than other proposed solutions for sharing resources in open real-time systems since it does not attempt to return the inherited capacity in the same exact amount to blocked servers. This loss of optimality is worth the reduced complexity as the protocol’s behaviour nevertheless tends to be fair and outperforms the previous solutions in highly dynamic scenarios as demonstrated by extensive simulations. A formal analysis of CXP is presented and the conditions under which it is possible to guarantee hard real-time tasks are discussed

Scheduling of hard real-time systems using non-conventional techniques

[EN] In the scheduling of hard real time systems it is key to find a temporal plan in which tasks can be executed before the deadline expires. Different types of scheduling algorithms can be used to achieve this goal. In addition to finding a feasible plan, it is often beneficial, out of all existing feasible plans, to find one that minimises some time parameter of the system. There are many scheduling algorithms that are able to find a correct scheduling of the set of tasks, but it is not so common to find algorithms that optimise other parameters to improve the performance of the system in terms of stability, power consumption, etc. Existing heuristics can improve the performance but without ensuring an optimal result. In this work we explore unconventional scheduling techniques based on integer linear programming to solve scheduling in mono processor systems with the objective of minimising the worst-case response time of the system tasks and the assignment of tasks to processors in multiprocessor systems with the objective of minimising the interference produced by the access to common hardware resources.[ES] En la planificación de sistemas de tiempo real crí­tico es clave encontrar un plan temporal en el que las tareas pueden ejecutarse antes de que venza el plazo establecido. Para lograr este objetivo se pueden utilizar diferentes tipos de algoritmos de planificación. Además de encontrar un plan factible, muchas veces es beneficioso, de todos los planes factibles existentes, encontrar aquel que minimiza algún parámetro temporal del sistema. Existen muchos algoritmos de planificación que son capaces de encontrar una planificación correcta del conjunto de tareas. Pero no es tan común encontrar algoritmos que optimicen otros parámetros para mejorar el funcionamiento del sistemas en términos de estabilidad, consumo de potencia, etc. Los heurí­sticos existentes puede mejorar el comportamiento pero sin asegurar un resultado óptimo. En este trabajo exploramos las técnicas de planificación no convencionales basadas en programación lineal entera para resolver la planificación en sistemas mono procesador con el objetivo de minimizar el peor tiempo de respuesta y el cambio de contexto de las tareas del sistema y la asignación de tareas a procesadores en sistemas multiprocesador con el objetivo de minimizar la interferencia producida por el acceso a recursos hardware comunes.Esta publicacion es parte del proyecto de I+D+i PLEC2021-007609 financiado por MCIN/ AEI/10.13039/501100011033 y por “Unión Europea NextGenerationEU / PRTR”.Balbastre, P.; Aceituno, JM.; Guasque, A.; Blanes, JF.; Crespo, A.; Poza, JL. (2022). Planificación de sistemas de tiempo real crí­tico mediante técnicas no convencionales. Revista Iberoamericana de Automática e Informática industrial. 19(4):369-379. https://doi.org/10.4995/riai.2022.1714836937919

Escalonar sistemas de tempo-real de alta críticalidade

Cyclic executives are used to schedule safety-critical real-time systems because of their determinism, simplicity, and efficiency. One major challenge of the cyclic executive model is to produce the cyclic scheduling timetable. This problem is related to the bin-packing problem [34] and is NP-Hard in the strong sense. Unnecessary context switches within the scheduling table can introduce significant overhead; in IMA (Integrated Modular Avionics), cache-related overheads can increase task execution times up to 33% [18]. Developed in the context of the Software Engineering Master’s Degree at ISEP, the Polytechnic Institute of Engineering in Porto Portugal, this thesis contains two contributions to the scheduling literature. The first is a precise and exact approach to computing the slack of a job set that is schedule policy independent. The method introduces several operations to update and maintain the slack at runtime, ensuring the slack of all jobs is valid and coherent. The second contribution is the definition of a state-of-the-art preemptive scheduling algorithm focused on minimizing the number of system preemptions for real-time safety-critical applications within a reasonable amount of time. Both contributions have been implemented and extensively tested in scala. Experimental results suggest our scheduling algorithm has similar non-preemptive schedulability ratio than Chain Window RM [69], yet lower ratio in high utilizations than Chain Window EDF [69] and BB-Moore [68]. For ask sets that failed to be scheduled non-preemptively, 98-99% of all jobs are scheduled without preemptions. Considering the fact that our scheduler is preemptive, being able to compete with non-preemptive schedulers is an excellent result indeed. In terms of execution time, our proposal is multiple orders of magnitude faster than the aforementioned algorithms. Both contributions of this work are planned to be presented at future conferences such as RTSS@Work and RTAS