DLB: A Novel Real-time QoS Control Mechanism for Multimedia Transmission

This paper presents a new QoS guarantee scheme called R-(m,k)-firm (Relaxed-(m,k)-firm) which provides the guarantee on transmission delay of at least m out of any k consecutive packets (m´k). It has several advantages: (1) during network congestion, packets are dropped according to the (m,k) model rather than uncontrollably as the case of TD and RED, avoiding thus undesirable long consecutive packet drops; (2) it allows to admit more real-time flows than the traditional over-provisioning approach. A new mechanism, called DLB (Double Leaks Bucket) is also proposed for dropping a proportion of packets of a flow or of aggregated-flows in case of network congestion while still guaranteeing the R-(m,k)-firm constraint. The sufficient condition for this guarantee is given for configuring the DLB parameters. It is easy to implement DLB in the actual IntServ and Diffserv architectures (by simply replacing the actual leaky bucket by DLB) for providing respectively per flow and per class (m,k) guarantee, or event per flow R-(m,k)-firm guarantee in Diffserv

Dynamic Window-Constrained Scheduling for Real-Time Media Streaming

This paper describes an algorithm for scheduling packets in real-time multimedia data streams. Common to these classes of data streams are service constraints in terms of bandwidth and delay. However, it is typical for real-time multimedia streams to tolerate bounded delay variations and, in some cases, finite losses of packets. We have therefore developed a scheduling algorithm that assumes streams have window-constraints on groups of consecutive packet deadlines. A window-constraint defines the number of packet deadlines that can be missed in a window of deadlines for consecutive packets in a stream. Our algorithm, called Dynamic Window-Constrained Scheduling (DWCS), attempts to guarantee no more than x out of a window of y deadlines are missed for consecutive packets in real-time and multimedia streams. Using DWCS, the delay of service to real-time streams is bounded even when the scheduler is overloaded. Moreover, DWCS is capable of ensuring independent delay bounds on streams, while at the same time guaranteeing minimum bandwidth utilizations over tunable and finite windows of time. We show the conditions under which the total demand for link bandwidth by a set of real-time (i.e., window-constrained) streams can exceed 100% and still ensure all window-constraints are met. In fact, we show how it is possible to guarantee worst-case per-stream bandwidth and delay constraints while utilizing all available link capacity. Finally, we show how best-effort packets can be serviced with fast response time, in the presence of window-constrained traffic

Garantir la qualité de service temps réel : ordonnancement et gestion de files d'attente

National audienceL'objectif de ce document est de présenter la problématique de la garantie de qualité de service temps réel dans les réseaux à commutation de paquets. Le problème central dans ce type de réseaux est la gestion des files d'attente des paquets dans des noeuds de commutation (commutateurs et routeurs). En considérant un modèle général de noeuds de commutation où plusieurs files d'attente partagent un même lien de sortie, nous discutons sur l'ordonnancement de paquets, présentons des techniques d'évaluation de bornes de temps de réponse et indiquons les limites de ces techniques. Pour des applications temps réel souple tolérant le rejet de paquets en cas de surcharge (par exemple la transmission de flux multimédia sur Internet), nous présentons aussi des solutions efficaces basées sur le modèle (m,k)-firm

Explorando descartes de ativações periódicas para provimento de qualidade de serviço em redes IEEE 802.15.4

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Engenharia de Automação e SistemasAtualmente, tem se considerado cada vez mais o uso de redes sem fios em sistemas de automação industrial e controle de processos. Sistemas de Controle via Rede são exemplos de aplicações em que busca-se implantar em ambientes industriais. Para estes tipos de aplicações, o suporte à serviços de comunicação tempo real é uma das maiores necessidades. Tradicionalmente, aplicações de tempo real industriais assumem que nenhuma amostra será perdida durante os ciclos de controle. Contudo, alguns estudos realizados nos últimos anos indicam que os efeitos dos descartes de mensagens frente ao desempenho dos sistemas de controle podem ser compensados com emprego de outras técnicas. O padrão IEEE 802.15.4 têm se mostrado uma solução atrativa para uma vasta gama de aplicações no campo das Redes de Sensores sem Fios e mais recentemente no campo da instrumentação sem fios. Sendo assim, esta tese tem por objetivo principal propor abordagens de provimento de Qualidade de Serviço (QoS) em redes IEEE 802.15.4 para aplicações de tempo real que tolerem perdas de deadline. Primeiramente, um mecanismo de priorização de tráfego para dispositivos de tempo real, durante períodos de acesso com contenção (CAP) foi proposto. Este mecanismo, denominado DDBP, oferece de forma descentralizada aos dispositivos que desejam realizar suas transmissões durante o CAP, priorização de acesso ao meio de comunicação de acordo com a distância para falha dos dispositivos. Posteriormente, duas abordagens de escalonamento durante períodos de acesso sem contenção (CFP) foram propostas: abordagem SDBP, a qual oferece garantias de atendimento de deadline às aplicações, através de um teste de escalonabilidade determinístico e uma heurística on-line de priorização de mensagens; e abordagem (m,k)-spin, a qual apresenta uma análise de escalonabilidade baseada no conceito de período de nível de ocupação. As abordagens propostas voltadas para o escalonamento durante o CFP oferecem garantias temporais para as aplicações com restrições de QoS, modeladas de acordo com o modelo de tarefas (m,k)-firm. Dispositivos que não conseguem realizar suas transmissões durante períodos livres de contenção, poderão realizar suas transmissões utilizando a abordagem de priorização de tráfego tempo real desenvolvida para o CAP.In the last few years, the use of wireless networks has been increasingly seen in industrial automation and process control. Network Control Systems are examples of driving applications in industrial environments. For these types of applications, support for real-time communications services is one of the major requirements. In traditional approaches, many of these real-time applications assume no data losses during the control cycle. However some more recent studies indicate that the effect of control messages discards upon the performance of the control systems may be significantly encompassed with the adoption of other techniques. The IEEE 802.15.4 protocol is an attractive solution for a wide range of applications in the field of Wireless Sensors Networks and more recently in the field of wireless instrumentation. Thus, this thesis aims to propose approaches to providing Quality of Service (QoS) in IEEE 802.15.4 for real-time applications that tolerate deadline misses. First, a mechanism for prioritizing traffic for real time devices during Contention Access Periods (CAP) was proposed. This mechanism, called DDBP offers in a decentralized way, priority access in accordance to the distance to device failure. Subsequently, two approaches to scheduling during Contention Free Periods (CFP) were proposed: SDBP approach which provides deadlines guarantees by the adoption of a deterministic scheduling test and an on-line prioritization heuristic; and (m,k)-spin approach, which presents a schedulability analysis based on the busy period concept. The proposed approaches applied to the CFP scheduling provide temporal guarantees for applications with QoS constraints, modelled according to the (m,k)-firm task model. Devices that cannot perform their transmissions during contention free periods, can try to perform their transmissions using the real-time traffic prioritization approach developed for the CAP

Soft real-time scheduling on multiprocessors

The design of real-time systems is being impacted by two trends. First, tightly-coupled multiprocessor platforms are becoming quite common. This is evidenced by the availability of affordable symmetric shared-memory multiprocessors and the emergence of multicore architectures. Second, there is an increase in the number of real-time systems that require only soft real-time guarantees and have workloads that necessitate a multiprocessor. Examples of such systems include some tracking, signal-processing, and multimedia systems. Due to the above trends, cost-effective multiprocessor-based soft real-time system designs are of growing importance. Most prior research on real-time scheduling on multiprocessors has focused only on hard real-time systems. In a hard real-time system, no deadline may ever be missed. To meet such stringent timing requirements, all known theoretically optimal scheduling algorithms tend to preempt process threads and migrate them across processors frequently, and also impose certain other restrictions. Hence, the overheads of such algorithms can significantly reduce the amount of useful work that is accomplished and limit their practical implementation. On the other hand, non-optimal algorithms that are more practical suffer from the drawback that their validation tests require workload restrictions that can approach roughly 50% of the available processing capacity. Thus, for soft real-time systems, which can tolerate occasional or bounded deadline misses, and hence, allow for a tradeoff between timeliness and improved processor utilization, the existing scheduling algorithms or their validation tests can be overkill. The thesis of this dissertation is: Processor utilization can be improved on multiprocessors while providing non-trivial soft real-time guarantees for different soft real-time applications, whose preemption and migration overheads can span different ranges and whose tolerances to tardiness are different, by designing new algorithms, simplifying optimal algorithms, and developing new validation tests. The above thesis is established by developing validation tests that are sufficient to provide soft real-time guarantees under non-optimal (but more practical) algorithms, designing and analyzing a new restricted-migration scheduling algorithm, determining the guarantees on timeliness that can be provided when some limiting restrictions of known optimal algorithms are relaxed, and quantifying the benefits of the proposed mechanisms through simulations. First, we show that both preemptive and non-preemptive global earliest-deadline-first(EDF) scheduling can guarantee bounded tardiness (that is, lateness) to every recurrent real-time task system while requiring no restriction on the workload (except that it not exceed the available processing capacity). The tardiness bounds that we derive can be used to devise validation tests for soft real-time systems that are EDF-scheduled. Though overheads due to migrations and other factors are lower under EDF (than under known optimal algorithms), task migrations are still unrestricted. This may be unappealing for some applications, but if migrations are forbidden entirely, then bounded tardiness cannot always be guaranteed. Hence, we consider providing an acceptable middle path between unrestricted-migration and no-migration algorithms, and as a second result, present a new algorithm that restricts, but does not eliminate, migrations. We also determine bounds on tardiness that can be guaranteed under this algorithm. Finally, we consider a more efficient but non-optimal variant of an optimal class of algorithms called Pfair scheduling algorithms. We show that under this variant, called earliest- pseudo-deadline-first (EPDF) scheduling, significantly more liberal restrictions on workloads than previously known are sufficient for ensuring a specified tardiness bound. We also show that bounded tardiness can be guaranteed if some limiting restrictions of optimal Pfair algorithms are relaxed. The algorithms considered in this dissertation differ in the tardiness bounds guaranteed and overheads imposed. Simulation studies show that these algorithms can guarantee bounded tardiness for a significant percentage of task sets that are not schedulable in a hard real-time sense. Furthermore, for each algorithm, conditions exist in which it may be the preferred choice

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