A hierarchical multiprocessor bandwidth reservation scheme with timing guarantees

A multiprocessor scheduling scheme is presented for supporting hierarchical containers that encapsulate sporadic soft and hard real-time tasks. In this scheme, each container is allocated a specified bandwidth, which it uses to schedule its children (some of which may also be containers). This scheme is novel in that, with only soft real-time tasks, no utilization loss is incurred when provisioning containers, even in arbitrarily deep hierarchies. Presented experiments show that the proposed scheme performs well compared to conventional real-time scheduling techniques that do not provide container isolation

Planificación global en sistemas multiprocesador de tiempo real

Esta tesis afronta el problema de la planificación de sistemas de tiempo real utilizando sistemas multiprocesador con memoria compartida. Según laliteratura este problema es NP-Hard. En las aplicaciones de sistemas de tiempo real se imponen unos plazos temporales para la realización de las tareas. Así, lo importante es obtener los resultados a tiempo y no lo es tanto el obtener un rendimiento alto en promedio. La solución al problematradicionalmente ha consistido en repartir las tareas en tiempo de diseño y tratar a losprocesadores como monoprocesadores aislados. La solución alternativa, la planificación global del multiprocesador, tiene una teoría poco desarrollada. Los límites de utilización del sistema con garantías para losplazos son muy bajos, del orden del 50%, y la capacidad sobrante difícilmente se puede usar para dar servicio a las tareas aperiódicas. Así, el objetivoprincipal de la tesis es la planificación global con garantías de los plazos y con buen servicio a las tareas aperiódicas, llegando a usar el 100% de la capacidad de proceso. Primero se estudiaron cuatro posibilidades de distribución: estática o dinámica según las tareas, periódicas o aperiódicas. Para ello se trató el servicioa las tareas aperiódicas con dos métodos distintos: con servidores y sin servidores. En las distribuciones dinámicas, con el método de los servidoresse encontraron dificultades en su dimensionado y en las garantías de los plazos. Los métodos sin servidores probados fueron los planificadores Slack Stealing y Total Bandwidth. Ambos solo se pudieron adaptar para la planificación estática de las tareas periódicas. Las simulaciones mostraron que laplanificación local con Slack Stealing y un distribuidor de las tareas aperiódicas tipo Next-Fit proporcionan los mejores tiempos de respuesta medios para las tareas aperiódicas. Sin embargo, cuando las cargas son muy altas su tiempo de respuesta se dispara. Todos los métodos ensayados hasta elmomento quedaron desestimados para la planificación global. En segundo lugar se adaptó a la planificación global el algoritmo Dual Priority. Primero se analizaron sus características en monoprocesadores y se realizaron diversas mejoras. El algoritmo depende del cálculo off-line del peor tiempo de respuesta de las tareas periódicas y la fórmula paracalcularlos en monoprocesadores no es válida para multiprocesadores. Así, se analizaron tres métodos para su cálculo: un método analítico, unmétodo con simulación y un método con un algoritmo. El primero obtiene valores demasiado pesimistas; el segundo obtiene valores más ajustados pero en ocasiones son demasiado optimistas; el tercero es un método aproximado y obtiene valores tanto optimistas como pesimistas. Así, estemétodo no garantiza los plazos y no se puede usar en sistemas de tiempo real estrictos. En sistemas laxos, con una monitorización on-liney un ajuste dinámico de las promociones, el número de plazos incumplidos es muy bajo y el tiempo de repuesta de las tareas aperiódicas es excelente. Finalmente, se presenta una solución híbrida entre el repartimiento estático de las tareas periódicas y la planificación global. En tiempo de diseño, sereparten las tareas periódicas entre los procesadores y se calculan las promociones para la planificación local. En tiempo de ejecución las tareasperiódicas se pueden ejecutar en cualquier procesador hasta el instante de su promoción, instante en el que deben migrar a su procesador. Así segarantizan los plazos y se permite un cierto grado de balanceo dinámico de la carga. La flexibilidad conferida por las promociones de las tareas y el balanceo de la carga se utiliza para (i) admitir tareas periódicas que de otra forma no serian planificables, (ii) servir tareas aperiódicas y (iii) servirtareas aperiódicas con plazo o esporádicas. Para los tres casos se diseñaron y analizaron distintos métodos de distribución de las tareas periódicas en tiempo de diseño. También se diseño un método para reducir el número de migraciones. Las simulaciones mostraron que con este método se puedenconseguir cargas con solo tareas periódicas muy cercanas al 100%, lejos del 50% de la teoría de la planificación global. Las simulaciones con tareasaperiódicas mostraron que su tiempo de repuesta medio es muy bueno. Se diseño un test de aceptación de las tareas esporádicas, de forma que si una tarea es aceptada entonces su plazo queda garantizado. El porcentaje de aceptación obtenido en los experimentos fue superior al 80%.Finalmente, se diseñó un método de distribución de las tareas periódicas pre-rutime capaz de facilitar en tiempo de ejecución la aceptación de un alto porcentaje de tareas esporádicas y mantener un buen nivel de servicio medio para las tareas aperiódicas.This thesis takes into consideration the problem of real-time systems scheduling using shared memory multiprocessor systems. According to the literature, this problem is NP-Hard. In real-time systems applications some time limits are imposed to tasks termination. Therefore, the really important thing is to get results on time and it is not so important to achieve high average performances. The solution to the problem traditionally has been to partition the tasks at design time and treat processors as isolated uniprocessors. The alternative solution, the global scheduling, has an undeveloped theory. The limit on the system utilization with deadlines guarantees is very low, around 50%, and spare capacity can hardly be used to service aperiodic tasks. Thus, the main goal of this thesis is to develop global scheduling techniques providing deadlines guarantees and achieving good service for aperiodic tasks, being able to use 100% of the processing capacity.First of all, we explored four possibilities of distribution: static or dynamic depending on the tasks, periodic or aperiodic. We tried to schedule aperiodic tasks with two different methods: with servers and without servers. In dynamic distributions, with the method of servers were found difficulties in its size and guarantees for deadlines. The methods without servers were The Slack Stealing and The Total Bandwidth. Both were adapted only for scheduling the static case. The simulations showed that the local scheduling with Slack Stealing and an allocation of aperiodic tasks kind Next-Fit provides the best mean average response time for the aperiodic tasks. However, when the load is very high response time increases. All methods tested so far were dismissed for the global scheduling.Secondly the Dual Priority algorithm was adapted to global scheduling. First we discussed its characteristics in uniprocessors and various improvements were made. The algorithm depends on the off-line calculation of the worst case response time for the task and the formula to compute them in uniprocessors is not valid for multiprocessors. We have analyzed three methods for its calculation: an analytical method, a simulation method and an algorithmic method. The former gets too pessimistic values, the second gets adjusted values but are sometimes too optimistic, and the third is a method that obtains approximate values. Thus, this method does not guarantee deadlines and may not be used in hard real-time systems. However, it is very suitable for soft real-time systems. In these systems, using an on-line monitoring and dynamic adjustment of promotions, the number of missed deadlines is very low and the response time of aperiodic tasks is excellent.Finally, we present a hybrid solution between static task allocation and global scheduling. At design time, is performed the distribution of periodic tasks among processors and their promotions are calculated for local scheduling. At runtime, the task can be run on any processor until the moment of its promotion, when it has to migrate to its processor. This will ensure deadlines and allowing a certain degree of dynamic load balancing. The flexibility provided by task promotions and load balancing is used (i) to admit task that would otherwise not be scheduled, (ii) to serve aperiodic tasks and (iii) to serve aperiodic tasks with deadlines or sporadic tasks. For the three cases were designed and analyzed various methods of task distribution at design time. We also designed a method to reduce the number of migrations. The simulations showed that this method can achieve with only periodic task loads very close to 100%, far from the 50% of the global scheduling theory. The simulations showed that aperiodic tasks average response time is very good. We designed an acceptance test for sporadic tasks, hence, if a task is accepted then its deadline is guaranteed. The acceptance rate obtained in the experiments was over 80%. Finally, we devised a pre-rutime distribution method of periodic tasks that is able to provide at run time a high acceptance ratio for sporadic tasks and maintain a good level of service for aperiodic task

Scheduling and locking in multiprocessor real-time operating systems

With the widespread adoption of multicore architectures, multiprocessors are now a standard deployment platform for (soft) real-time applications. This dissertation addresses two questions fundamental to the design of multicore-ready real-time operating systems: (1) Which scheduling policies offer the greatest flexibility in satisfying temporal constraints; and (2) which locking algorithms should be used to avoid unpredictable delays? With regard to Question 1, LITMUSRT, a real-time extension of the Linux kernel, is presented and its design is discussed in detail. Notably, LITMUSRT implements link-based scheduling, a novel approach to controlling blocking due to non-preemptive sections. Each implemented scheduler (22 configurations in total) is evaluated under consideration of overheads on a 24-core Intel Xeon platform. The experiments show that partitioned earliest-deadline first (EDF) scheduling is generally preferable in a hard real-time setting, whereas global and clustered EDF scheduling are effective in a soft real-time setting. With regard to Question 2, real-time locking protocols are required to ensure that the maximum delay due to priority inversion can be bounded a priori. Several spinlock- and semaphore-based multiprocessor real-time locking protocols for mutual exclusion (mutex), reader-writer (RW) exclusion, and k-exclusion are proposed and analyzed. A new category of RW locks suited to worst-case analysis, termed phase-fair locks, is proposed and three efficient phase-fair spinlock implementations are provided (one with few atomic operations, one with low space requirements, and one with constant RMR complexity). Maximum priority-inversion blocking is proposed as a natural complexity measure for semaphore protocols. It is shown that there are two classes of schedulability analysis, namely suspension-oblivious and suspension-aware analysis, that yield two different lower bounds on blocking. Five asymptotically optimal locking protocols are designed and analyzed: a family of mutex, RW, and k-exclusion protocols for global, partitioned, and clustered scheduling that are asymptotically optimal in the suspension-oblivious case, and a mutex protocol for partitioned scheduling that is asymptotically optimal in the suspension-aware case. A LITMUSRT-based empirical evaluation is presented that shows these protocols to be practical