基于Pfair的分布式实时调度策略Linux下实现

任务调度策略是嵌入式分布式实时系统关键问题之一,以Pfair公平调度为代表的全局调度技术是当前研究的热点,调度方法要在实际中得到应用,需要与具体的操作系统相结合.分析了分布式实时系统的调度理论,比较研究了几种Pfair算法,通过修改Linux内核的数据结构和调度函数,初步实现了Pfait的PD2算法,实验证明达到了预期的实验结果.     

分布式实时系统任务与消息综合调度算法研究

分布式实时系统作为一种典型的实时系统应用受到了关注，目前提出了许多分布式实时系统的调度策略。但是，绝大多数策略往往忽略了全局任务调度时的消息调度。本文提出一种综合任务调度和消息调度的算法，提供了一种实时系统调度的新思路。     

多处理器EPDFPfair算法的可调度性判定

针对多处理器实时调度中的最早伪时限优先(EPDF)Pfair算法,分析了EPDF算法在M个处理器平台上的可调度利用率约束,根据基于利用率的充分可调度性判定,提出了一种改进的可调度性判定方法。这种方法可以得到更多的可调度任务集,从而使得满足判定的强实时系统和使用tie-breaking规则困难的动态任务系统的调度有较小的开销。实验结果表明,改进的可调度性判定方法增加了判为可调度的任务集数量,具有较好的性能。     

基于启发式算法降低比例公平调度开销策略

在多处理器系统中已经证明了比例公平(proportion fair, Pfair)算法是调度周期任务最优的全局调度算法。然而在该算法的最坏执行情况下,任务在每个调度时刻均产生切换或迁移,导致系统开销过大。针对这一问题,对Pfair算法进行深入研究后发现,任务的分配过程是一个重要原因。基于此,提出基于启发式算法的模拟退火比例公平(simulated annealing-proportion fair, SA-Pfair)调度算法,即在Pfair算法做出调度决策后,用启发式算法将任务分配给处理器,以弥补原算法的不足。最后,采用LITMUS-RT平台对SA-Pfair算法和以此为基础设计的调度器进行仿真。结果表明,新算法在一定程度上减少了任务的切换次数以及50%以上的任务迁移总量,且能够有效地降低调度过程中的系统开销。     

可靠性代价驱动的实时任务调度算法

1 概述分布式系统越来越广泛地用于重要的实时系统应用程序中,关键问题在于必须保证每个任务在其截止时间之前完成。在许多实时调度算法中调度性是需要最大化的功能目标之一。为了使实时调度算法更实用,必须考虑任务优先权限制。文[11]中提出将离线分析和在线保证结合使用的方案。文[12]提出了一个分布式实时系统中的最佳任务调度算法。上述算法都是为同构分布式系统设计的,均假定系统中的处理器都是一样的,所以不能直接应用于异构分布式系     

SLABOT分布式调度算法分析

本文介绍了一个适合实时系统的分布式调度算法——SLABOT算法。并对算法在理论上,模拟结果上和效率上进行分析讨论。     

分布式实时系统动态任务调度的一个有效算法

本文提供了一个分布式实时系统动态任务调度的改进了的招标算法.着重讨论了有保证调度算法.提出了最小调度优先,最早启动与最迟启动等三种基本调度策略,指出了设计这类算法的一般原则,提供了几个基本的算法实例.     

一种分布式实时系统任务调度算法的设计

结合分布式系统和实时系统的特点,分析了分布式系统任务调度算法和实时系统任务调度算法,为了能够较好地实现系统的并行性能、实时任务的调度性能以及网络的负载平衡,提出一种将分布式系统任务调度算法和实时系统任务调度算法想结合的算法,采用层次式调度算法以及动态权值的轮转调度算法和速率单调调度算法相结合,在队列权值固定的实验基础上,采用随机改变队列权值的算法,实验证明该随机改变队列权值的算法能够更好地调度任务.     

分布式实时系统中的预测调度算法

对于分布式实时系统中的周期性任务,人们提出了一系列静态分配调度算法,有效地解决了各种特定条件下的任务分配和调度问题.这些算法的主要特点是,它们均要求被调度任务的特征参数为已知条件.然而在很多实时系统中,周期性任务的运行时间或任务数量常常是一些具有一定规律的随机过程,因而上述静态算法的效能将受到限制.在分析了特定应用背景中的处理流程之后,抽象得到两类随机任务模型,针对这两类模型介绍了在分布式实时系统中已经得到应用的静态分配调度算法SAA(static allocation algorithms),进而提出了多任务分配调度的预测算法PAA(predicting allocation algorithm).它根据周期性任务执行时间或子任务数量的统计特性,实现任务参量的合理预测和多任务的动态调度,以提高系统的实时性能.仿真结果表明,对于两类任务模型,PAA算法与SAA算法相比,在任务完成时间、负载均衡度、系统响应时间及任务夭折率等多方面均有显著改善.     

强实时系统的调度

实时系统的一个重要研究领域是调度 ,实时任务能否在规定的时限内完成依赖于调度算法的好坏。本文给出了当前强实时系统的主要调度思想和模型 ,并对各算法的特点进行了评述 ,对强实时系统的设计和论证具有重要意义。     

An optimal boundary fair scheduling algorithm for multiprocessor real-time systems

With the emergence of multicore processors, the research on multiprocessor real-time scheduling has caught more researchers’ attention recently. Although the topic has been studied for decades, it is still an evolving research field with many open problems. In this work, focusing on periodic real-time tasks with quantum-based computation requirements and implicit deadlines, we propose a novel optimal scheduling algorithm, namely boundary fair (Bfair), which can achieve full system utilization as the well-known Pfair scheduling algorithms. However, different from Pfair algorithms that make scheduling decisions and enforce proportional progress (i.e., fairness) for all tasks at each and every time unit, Bfair makes scheduling decisions and enforces fairness to tasks only at tasks’ period boundaries (i.e., deadlines of periodic tasks). The correctness of the Bfair algorithm to meet the deadlines of all tasks’ instances is formally proved and its performance is evaluated through extensive simulations. The results show that, compared to that of Pfair algorithms, Bfair can significantly reduce the number of scheduling points (by up to 94%) and the overhead of Bfair at each scheduling point is comparable to that of the most efficient Pfair algorithm (i.e., PD²). Moreover, by aggregating the time allocation of tasks for the time interval between consecutive period boundaries, the resulting Bfair schedule can dramatically reduce the number of required context switches and task migrations (as much as 82% and 85%, respectively) when compared to those of Pfair schedules, which in turn reduces the run-time overhead of the system.     

Improved conditions for bounded tardiness under EPDF Pfair multiprocessor scheduling

The earliest-pseudo-deadline-first (EPDF) Pfair algorithm is more efficient than other known Pfair scheduling algorithms, but is not optimal for scheduling recurrent real-time task systems on more than two identical processors. Although not optimal, EPDF may be preferable for real-time systems instantiated on less-powerful platforms, those with soft timing constraints, or those whose task composition can change at run-time. In prior work, Srinivasan and Anderson established a sufficient per-task utilization restriction for ensuring a tardiness of at most q quanta, where $q?1$, under EPDF. They also conjectured that under this algorithm, a tardiness bound of one quantum applies to task systems that are not subject to any restriction other than the obvious restrictions, namely, that the total system utilization not exceed the available processing capacity and per-task utilizations not exceed 1.0. In this paper, we present counterexamples that show that their conjecture is false and present sufficient per-task utilization restrictions that are more liberal than theirs. For ensuring a tardiness bound of one quantum, our restriction presents an improvement of 50% over the previous one.     

Supporting lock-free synchronization in Pfair-scheduled real-time systems

We consider the use of lock-free techniques for implementing shared objects in real-time Pfair-scheduled multiprocessor systems. Lock-free objects are more economical than locking techniques when implementing relatively simple objects such as buffers, stacks, queues, and lists. However, the use of such objects on real-time multiprocessors is generally considered impractical due to the need for analytical real-time guarantees. In this paper, we explain how the quantum-based nature of Pfair scheduling enables the effective use of such objects on real-time multiprocessors and present analysis specific to Pfair-scheduled systems. In addition, we show that analytical improvements can be obtained by using such objects in conjunction with group-based scheduling techniques. In this approach, a group of tasks is scheduled as a single entity (called a supertask in the Pfair literature). Such grouping prevents tasks from executing simultaneously, and hence from executing in parallel. Consequently, grouping tasks can improve the worst-case scenario with respect to object contention. Grouping also enables the use of less costly uniprocessor algorithms when all tasks sharing an object reside within the same group. We illustrate these optimizations with a case study that focuses on shared queues. Finally, we present and experimentally evaluate a simple heuristic for grouping tasks in order to reduce object contention. Though the analysis presented herein focuses specifically on Pfair-scheduled systems, the observations and techniques should be applicable to other quantum-scheduled systems as well.     

A new fair scheduling algorithm for periodic tasks on multiprocessors

We present a new scheduling algorithm, called PL that is work-conserving and in terms of schedulability, optimal on multiprocessors for a synchronous periodic task set. The PL algorithm is a laxity based algorithm and ensures execution of a task with approximate proportional fairness at each task's period. Existing optimal algorithms on multiprocessors may cause excessive scheduling decisions and preemptions or may not be applied in a discrete environment. The proposed algorithm can be applied in a discrete environment and reduce the number of scheduling decisions and preemptions compared with a Pfair algorithm.     

Optimal rate-based scheduling on multiprocessors

The PD² Pfair/ERfair scheduling algorithm is the most efficient known algorithm for optimally scheduling periodic tasks on multiprocessors. In this paper, we prove that PD² is also optimal for scheduling “rate-based” tasks whose processing steps may be highly jittered. The rate-based task model we consider generalizes the widely-studied sporadic task model.     

实时性分布嵌入式系统设计

分布嵌入式系统的应用通常具有实时性要求，其实时性需要从任务调度、通信协议、调用接口和嵌入式操作系统四个方面得到支持。采用两阶段调度策略既可以保证调用的透明性，又可以简化任务调度算法；基于优先级的线路竞争算法可以改进以太网使其符合实时性要求；采用微型Java虚拟机作为分布式中间件可以为系统调用接口提供一个统一的支持平台。     

基于Petri网的分布式实时调度模型研究

Petri网作为一种可视化的规格语言,越来越多地用于实时系统的评估和分析。该文提出了一种基于Petri网的分布式实时系统模型,并对该模型中的局部调度器和消息调度器进行了描述。根据该模型可以开发相应的分布式实时调度模拟器,这样就可以在系统的开发初期,利用模拟器来验证在给定的局部调度策略和消息调度策略下,系统任务的时间约束是否能够得到有效的保障。同时该模型还可以很容易地转化为系统的快速原型。