不可抢占式EDF调度算法的可调度性分析

现有的不可抢占式EDF调度算法的可调度性分析判定条件限定实时任务的截止期必须等于其周期,限制了它的使用范围。论文突破这一限制,提出了更具一般性的可调度性分析判定充要条件。通过对可调度性判定充要条件的分析,提出了基于不可抢占式EDF调度算法的周期性实时系统可调度性分析算法。     

非抢占式EDF算法下周期性任务的最小相对截止期计算*

现有的求解周期性任务最小相对截止期的方法均假定任务集是采取抢占式EDF调度算法,并不适用于当任务为基于非抢占式EDF调度算法的场合,如实时通信领域。在分析了非抢占式EDF调度算法的可调度性判定条件基础上,提出了基于非抢占式EDF调度算法下周期性任务最小相对截止期的计算算法。算法通过逐渐增加任务的相对截止期直到使任务集变为可调度的方式,实现某个任务相对截止期的最小化。仿真实验表明该算法具有较好的计算复杂度。     

EDF调度算法可调度性分析方法的改进研究

任务集的可调度性分析是实时系统研究和应用的关键问题。针对抢占式与不可抢占式EDF(earliest deadline first)调度算法, 分别给出了实时任务集新的可调度性测试条件, 针对任务集为可调度时可以实现快速判定。通过与已有的EDF算法的可调度性判定充要条件相结合, 提出了改进的抢占式与不可抢占式EDF算法的可调度性分析方法。仿真实验表明, 相对现有EDF算法的可调度性分析方法, 所提出的方法能有效提高算法性能。     

主从结构交换式以太网中实时消息调度研究

为使交换式以太网能满足实时通信的要求,针对FTT SE网络调度模型,提出了一种同时适用于周期性和非周期性实时消息的链路可调度性判定方法。在证明了消息链路调度优化问题MLSOP为NP complete的同时,针对周期性实时消息的链路调度优化给出了启发式算法LSHA。最后,对于周期性和非周期性实时消息分别设计了基于EDF的调度算法。仿真实验表明,在提高网络链路带宽利用率和减小消息平均延时方面,该算法均较FTT SE有明显的优势。     

CAN总线中改进的EDF调度算法可调度性分析

针对于CAN总线的调度问题,因现有的平均分区EDF调度算法在对于优先级反转问题上收效甚微,从而导致消息缺乏一定的可调度性,故提出一种改进的基于幂函数分区的EDF算法;同时借助量化误差的概念,对该调度算法进行可调度性分析,充分论证了在该调度算法下,消息可调度的判定条件;采用CANoe平台进行实验仿真,对比平均分区EDF调度算法和双幂函数分区EDF调度算法,经试验测试验证了双幂函数分区EDF调度算法的可行性和优越性,改善了消息的最坏响应时间,提高了CAN网络通讯的实时性。     

基于EDF调度策略的端到端实时系统可调度性分析算法

端到端实时任务调度模型可用于描述许多分布式实时系统.提出一种基于EDF调度策略的端到端实时任务调度模型,给出了端到端实时系统的可调度性判定条件,并提出其可调度性分析算法,该可调度性判定条件及可调度性分析算法适用于采用非连续工作型同步协议和连续工作型同步协议控制下的端到端实时系统.与固定优先级的端到端实时任务调度模型及其算法相比,基于EDF调度策略的端到端实时任务调度模型和算法更加简单和易于实现,仿真结果也表明具有较高的性能.     

基于RM与EDF的实时混合调度算法研究

通过对实时系统中静态调度算法RM和动态调度算法EDF的研究与分析,针对两种调度算法在实际应用中的问题,提出了一种基于阈值δ的混合调度算法,将RM与EDF调度算法相结合,并从数学角度描述了混合调度算法的可调度性与实时任务的周期、执行时间等属性之间的关系,给出了混合调度算法可调度性的充分必要条件。最后用实验验证了混合调度算法的有效性。     

实时中间件动态调度算法的研究及应用

文章建立了实时中间件OPC服务器的实时调度模型,设计了处理混合任务的动态调度算法(基于EDF)和实现方式,分析了算法的可调度性和非周期任务的响应性能及参数设计,并给出了相应的调度结果。     

COTS Switch工业以太网实时调度算法

针对基于商用现成组件技术（Commercial Off-The-Shelf,COTS）的交换式以太网不能满足工业数据通信的实时性要求问题,给出了改进的FTT（Flexible Time-Triggered communication paradigm）网络调度模型,提出了新的周期性实时消息链路可调度性优化判定方法,在此基础上设计了一种基于EDF（Earliest Deadline First）的实时调度算法。算法通过对网络消息流量进行有效控制防止交换机缓冲区溢出,同时充分发挥网络在同一时间存在多个并行传输链路的优势,提高了网络实时性。仿真实验表明该算法在提高网络的聚合带宽、减小消息的平均等待延时和丢失率方面均有明显的优势。     

基于最优虚拟截止日期多处理器混合时序调度

为提高混合临界系统实时调度有效性,提出基于最优虚拟截止日期的多处理器混合时序调度算法.将现有非抢占最早截止时间可调度性测试算法推广到混合临界多处理器系统,引入时序保证技术,确保系统在两个不同临界值间过渡;将所提可调度性测试扩展到混合临界系统,利用系统级截止期缩减参数控制,设计最优虚拟截止日期分配策略.仿真结果表明,采用最优虚拟截止时间分配策略可调度性测试可发现大量额外可调度任务集,实现混合临界多处理器非抢占调度性能提升.     

A priority-driven flow control mechanism for real-time traffic inmultiprocessor networks

Real-time applications when mapped to distributed memory multiprocessors produce periodic messages with an associated deadline and priority. Real-time messages may be hard or soft deadline. Real-time extensions to wormhole routing (WR) with multiple virtual channels (VCs) and priority-based physical link arbitration and VC allocation have been proposed in the literature. With a fixed number of VCs/link, a message can face an unbounded priority inversion, rendering the global priority ineffective. In this paper, we propose a new flow control mechanism called Preemptive Pipelined Circuit Switching for Real-Time messages (PPCS-RT) to reduce the priority inversion problem. For the proposed model, with some architectural support, we present an off-line approach to compute delivery guarantees of hard deadline real-time messages. We also perform a comparison of real-time WR and PPCS-RT in terms of performance with soft deadline traffic. The overall miss ratio percentage is over 30 percent higher for WR than PPCS-RT with one VC/link at high traffic loads. Finally, we compare the architectural complexity of a PPCS-RT router and other real-time routers     

Network access protocol for hard real-time communication systems

Distributed hard real-time systems are characterized by communication messages associated with timing constraints, typically in the form of deadlines. A message should be received at the destination before its deadline expires. Carrier sense multiple access with collision detection (CSMA/CD) appears to be one of the most common communication network access schemes that can be used in distributed hard real-time systems. In this paper, we propose a new real-time network access protocol which is based on the CSMA/CD scheme. The protocol classifies the messages into two classes as ‘critical’ and ‘noncritical’ messages. The messages close to their deadlines are considered to be critical. A critical message is given the right to access the network by preempting a noncritical message in transmission. Extensive simulation experiments have been conducted to evaluate the performance of the protocol. It is shown that the protocol can provide considerable improvement over the virtual time CSMA/CD protocol proposed for hard real-time communication by Zhao et al.¹.     

A probabilistic approach to estimate the mean waiting times in the earliest deadline first polling

In queueing system, the mean waiting times of messages are important measures to characterize the quality of service (QoS) under various requirements. In a time-critical system, message transactions which cannot meet deadline constraints might lead to catastrophic consequences. Currently, the waiting time estimations using the first-come-first-served (FCFS) and priority (PRI) strategies are already well developed. However, in the case of multi-queue dynamic environments, these quantities are more difficult to analyze due to multiple classes of messages are considered. In this paper, we aim to consider a polling system consisting of a number of parallel infinite-capacity single-server queues. We propose a probabilistic approach to derive the waiting times for different classes of messages by using non-preemptive earliest deadline first (EDF) polling policy. The resulting formula can also lead to the FCFS polling and PRI polling by altering the relative deadlines. Moreover, the bounds of waiting times are discussed. The accuracy of the proposed algorithm is established by comparisons with simulation results. The runtime results are in very good convergence with the theoretical predictions made by our formulas, in terms of prediction accuracies of waiting times and untimely service ratios of messages under various scenarios and timing constraints.     

Robust Adaptive Metrics for Deadline Assignment in Distributed Hard Real-Time Systems

Distributed real-time applications usually consist of several component tasks and must be completed by its end-to-end (E-T-E) deadline. As long as the E-T-E deadline of an application is met, the strategy used for dividing it up for component tasks does not affect the application itself. One would therefore like to slice each application E-T-E deadline and assign the slices to component tasks so as to maximize the schedulability of the component tasks, and hence the application. Distribution of the E-T-E deadline over component tasks is a difficult and important problem since there exists a circular dependency between deadline distribution and task assignment. We propose a new deadline-distribution scheme which has two major improvements over the best scheme known to date. It can distribute task deadlines prior to task assignment and relies on new adaptive metrics that yield significantly better performance in the presence of high resource contention. The deadline-distribution problem is formulated for distributed hard real-time systems with relaxed locality constraints, where schedulability analysis must be performed at pre-run-time, and only a subset of the tasks are constrained by pre-assignment to specific processors. Although it is applicable to any scheduling policy, the proposed deadline-distribution scheme is evaluated for a non-preemptive, time-driven scheduling policy. Using extensive simulations, we show that the proposed adaptive metrics deliver much better performance (in terms of success ratio and maximum task lateness) than their non-adaptive counterparts. In particular, the simulation results indicate that, for small systems, the adaptive metrics can improve the success ratio by as much as an order of magnitude. Moreover, the new adaptive metrics are found to exhibit very robust performance over a large variety of application and architecture scenarios.     

主从交换式以太网中跨多Switch传输消息实时调度研究

针对FTT-SE协议在单Master多交换机的网络扩展结构中存在的消息跨多Switch传输调度问题,给出了消息在每个基本调度周期内到达各交换机输出端口时间的计算方法,提出了单EC内的消息可调度性判定算法,并对算法的可行性进行了证明。在此基础上,设计了基于EDF的消息实时调度算法和准入控制算法。通过确定消息在每个基本调度周期内到达各交换机输出端口时间,所提出的调度算法能针对COTS交换机输出端口的FCFS消息传输机制,实现对单EC内消息传输的精确控制和调度。相对已有的调度算法,仿真实验表明,所提出的算法能更有效地利用网络带宽,提高了主从交换式以太网通信的实时性。     

Aperiodic task scheduling for Hard-Real-Time systems

A real-time system consists of both aperiodic and periodic tasks. Periodic tasks have regular arrival times and hard deadlines. Aperiodic tasks have irregular arrival times and either soft or hard deadlines. In this article, we present a new algorithm, the Sporadic Server algorithm, which greatly improves response times for soft deadline aperiodic tasks and can guarantee hard deadlines for both periodic and aperiodic tasks. The operation of the Sporadic Server algorithm, its performance, and schedulability analysis are discussed and compared with previously published aperiodic service algorithms.     

Periodic Gossiping in Commuted Networks

In this paper we deal with global communication schemes such as broadcasting or multicasting, in a packet (sub)network in which each node participates in the same distributed application. We want the communication scheme to be performed periodically as often as possible and with some bandwidth guarantee. Such periodicity properties are needed, for example, by multimedia and grid computation applications. We consider commuted switches in the network, i.e., each outgoing link of the switch is assigned to at most one incoming link. Each switch avoids intermediate buffering. Given a network G and a global communication scheme S, an algorithm performing S in G consists in commuting each switch and in specifying to each emitting node the infinite sequence of steps at which it will send a message. The period of this algorithm is the greatest number of steps between two messages emitted by a node. The window of the algorithm is the maximal number of steps needed by a message to reach its destination(s). We first give a general definition of the model of network we consider and of the global communication schemes we study, i.e., the many-to-all scheme where many nodes have one message to send to all the other ones. One well-known case of the many-to-all scheme we especially focus on is gossiping. We present a general way of periodically performing these communication schemes, by covering the graph by substructures called octopuses. Then we show that optimizing the period and the windoware two different problems.We end by giving some examples of applications in the torus network.     

A Feasibility Decision Algorithm for Rate Monotonic and Deadline Monotonic Scheduling

Rate monotonic and deadline monotonic scheduling are commonly used for periodic real-time task systems. This paper discusses a feasibility decision for a given real-time task system when the system is scheduled by rate monotonic and deadline monotonic scheduling. The time complexity of existing feasibility decision algorithms depends on both the number of tasks and maximum periods or deadlines when the periods and deadlines are integers. This paper presents a new necessary and sufficient condition for a given task system to be feasible and proposes a new feasibility decision algorithm based on that condition. The time complexity of this algorithm depends solely on the number of tasks. This condition can also be applied as a sufficient condition for a task system using priority inheritance protocols to be feasible with rate monotonic and deadline monotonic scheduling.