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。     

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。     

交换以太网中实时消息截止期划分算法研究

为解决交换式以太网中实时消息在发送及接收链路上的截止期划分问题,首先给出了非抢占式EDF调度算法的可调度性分析方法,并分析了消息的提前释放对非抢占式EDF调度算法可调度性的影响,在此基础上提出了新的截止期划分算法(MDPS)。算法通过计算可满足调度条件时消息在发送链路及接收链路截止期应该划分的下限,并结合截止期的松驰量实现实时消息在发送及接收链路上的截止期划分。算法实验比较和分析表明,该算法在提高网络聚合带宽、减小消息的错失率方面明显优于现有的ADPS算法。     

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。     

抢占阈值调度算法的分析与研究

本文详细论述了应用于静态优先级实时系统的抢占阈值调度算法。描述了算法实现和任务集合可调度性判定公式的推导,分析了算法的性能特点,阐述了抢占阈值调度是静态优先级嵌入式实时系统开发中调度算法的合适选择。     

多处理器硬实时系统的抢占阈值调度研究

在实时系统中,抢占在提高系统灵活性的同时带来额外的系统开销,特别在多处理器平台上抢占导致的作业迁移会造成相当大的性能下降,减少不必要的抢占是硬实时系统研究的重要方向.抢占阈值调度是处于抢占调度和不可抢占调度之间的一种混合调度方法,在保持调度能力的基础上限制抢占.基于截止期分析建立了多处理器硬实时系统抢占阈值调度的可调度性判定条件,针对抢占阈值调度提出一种改进的优先级分配算法OPA-MLL,并建立了抢占阈值分配(preemption threshold assignment, PTA)算法.仿真结果表明,采用OPA-MLL算法和PTA算法分别给任务集分配优先级和抢占阈值时,可调度任务集比率明显提高,同时能最大程度限制抢占次数.     

基于改进型统一调度算法改善任务集的可调度性

实时系统要求任务在最差情况下能在其截止时间前获得结果,若超过了其截止时间,也会认为是错误的行为,所以改进任务可调度性分析、提高任务集可调度性尤其重要。统一调度能结合固定优先级调度的优点,防止不必要的抢占,降低资源额外销耗,能够提高任务集合的可调度性;但其任务的可调度性分析方法过于粗糙,影响任务最差响应时间分析的结果,降低了任务集的可调度性。针对存在的问题,基于统一调度,增加任务运行阶段数,重新建立任务模型,并提出通过分配任务抢占阈值、调整运行阶段的抢占阈值与长度,优化任务可容忍阻塞,改善任务集可调度性的算法。最后,实验表明,与统一调度算法及其他算法相比,所提出的调度算法能够有效改善任务集的可调度性。     

多处理器固定优先级算法的可调度性分析

针对多处理器实时调度中的固定优先级(FP)调度算法,提出了一种改进的可调度性判定方法。引入Baruah的最早截止期优先(EDF)窗口分析框架,将高优先级任务带入作业的最大数量限定为m-1(m为处理器个数),进而对任务的干涉上界进行重新界定,并由此得到一个更加紧密的可调度性判定充分条件。仿真实验结果表明,该方法增加了通过判定任务集的数量,体现出更优的可调度判定性能。     

应用于实时系统的DMS算法可调度性分析

固定优先级任务的可调度性判定是实时系统调度理论研究的核心问题之一。本文提出了一种可行的DMS可调度性判定方法——确切性判定方法（precised schedulability test algorithln,简称PSTA）,利用DMS调度的充要条件,保证任何任务集均可被判定,并且判定结果是确切的。首先给出了DMS调度模型,介绍了可调度性判定的基本思想,然后进一步通过实验提出并证明了PSTA相关的定理。     

Schedulability Analysis of Global Scheduling Algorithms on Multiprocessor Platforms

This paper addresses the schedulability problem of periodic and sporadic real-time task sets with constrained deadlines preemptively scheduled on a multiprocessor platform composed by identical processors. We assume that a global work-conserving scheduler is used and migration from one processor to another is allowed during a task lifetime. First, a general method to derive schedulability conditions for multiprocessor real-time systems will be presented. The analysis will be applied to two typical scheduling algorithms: earliest deadline first (EDF) and fixed priority (FP). Then, the derived schedulability conditions will be tightened, refining the analysis with a simple and effective technique that significantly improves the percentage of accepted task sets. The effectiveness of the proposed test is shown through an extensive set of synthetic experiments.     

Priority-Driven Scheduling of Periodic Task Systems on Multiprocessors

The scheduling of systems of periodic tasks upon multiprocessor platforms is considered. Utilization-based conditions are derived for determining whether a periodic task system meets all deadlines when scheduled using the earliest deadline first scheduling algorithm (EDF) upon a given multiprocessor platform. A new priority-driven algorithm is proposed for scheduling periodic task systems upon multiprocessor platforms: this algorithm is shown to successfully schedule some task systems for which EDF may fail to meet all deadlines.     

A New Approach for Scheduling of Parallelizable Tasks in Real-Time Multiprocessor Systems

In a parallelizable task model, a task can be parallelized and the component tasks can be executed concurrently on multiple processors. We use this parallelism in tasks to meet their deadlines and also obtain better processor utilisation compared to non-parallelized tasks. Non-preemptive parallelizable task scheduling combines the advantages of higher schedulability and lower scheduling overhead offered by the preemptive and non-preemptive task scheduling models, respectively. We propose a new approach to maximize the benefits from task parallelization. It involves checking the schedulability of periodic tasks (if necessary, by parallelizing them) off-line and run-time scheduling of the schedulable periodic tasks together with dynamically arriving aperiodic tasks. To avoid the run-time anomaly that may occur when the actual computation time of a task is less than its worst case computation time, we propose efficient run-time mechanisms.We have carried out extensive simulation to study the effectiveness of the proposed approach by comparing the schedulability offered by it with that of dynamic scheduling using Earliest Deadline First (EDF), and by comparing its storage efficiency with that of the static table-driven approach. We found that the schedulability offered by parallelizable task scheduling is always higher than that of the EDF algorithm for a wide variety of task parameters and the storage overhead incurred by it is less than 3.6% of the static table-driven approach even under heavy task loads.     

Improved multiprocessor global schedulability analysis

A new technique was recently introduced by Bonifaci et al. for the analysis of real-time systems scheduled on multiprocessor platforms by the global Earliest Deadline First (EDF) scheduling algorithm. In this paper, this technique is generalized so that it is applicable to the schedulability analysis of real-time systems scheduled on multiprocessor platforms by any work-conserving algorithm. The resulting analysis technique is applied to obtain a new sufficient global Deadline Monotonic (DM) schedulability test. It is shown that this new test is quantitatively superior to pre-existing DM schedulability analysis tests; in addition, the degree of its deviation from any hypothetical optimal scheduler (that may be clairvoyant) is quantitatively bounded. A new global EDF schedulability test is also proposed here that builds on the results of Bonifaci et al. This new test is shown to be less pessimistic and more widely applicable than the earlier result was, while retaining the strong theoretical properties of the earlier result.     

An efficient approach for the multiprocessor non-preemptive strictly periodic task scheduling problem

Strict periodicity constraint is of great importance since it concerns some hard real-time systems where missing deadlines leads to catastrophic situations. However, the problem of schedulability analysis for non-preemptive strictly periodic tasks on a multiprocessor platform is even more intractable than the one with the common periodicity. In order to implement such systems, designers need effective tools based on fast and near-optimal solutions.This paper presents a schedulability analysis which results mainly in a, two versions, task assignment and start-time calculation algorithm. The first one targets the harmonic task periods case while the second one targets the non-harmonic task periods case. Each version is based on a sufficient uniprocessor schedulability test. In addition, for the non-harmonic case which is the most intractable, the uniprocessor sufficient schedulability test uses the strictly periodic task utilization factor. This factor stands for the fraction of time spent to execute a task while its strict periodicity and the ones of the already scheduled tasks are met. As a result, an efficient and easily implementable scheduling algorithm is proposed which begins by assigning tasks to processors then attributes a start-time to every task in such a way that strict periodicity and deadline constraints are met. The effectiveness of the proposed scheduling algorithm, in both versions, has been shown by a performance evaluation and comparisons with an optimal and a similar suboptimal solution.     

一种改进的RM可调度性判定算法

固定优先级任务可调度性判定是实时系统调度理论研究的核心问题之一.目前已有的各种判定方法可归结为两大类:多项式时间调度判定和确切性判定.多项式时间调度判定通常采用调度充分条件来进行,为此,许多理想条件下基于RM(rate monotonic)调度算法的CPU利用率最小上界被提了出来.确切性判定利用RM调度的充要条件,保证任何任务集均可被判定,并且判定结果是确切的.但是由于时间复杂度较差,确切性判定方法难以实现在线分析.提出了一种改进的RM可调度性判定方法(improved schedulability test algorithm,简称ISTA).首先介绍了任务调度空间这一概念,并提出了二叉树表示,然后进一步提出了相关的剪枝理论.在此基础上,研究了任务之间可调度性的相关性及其对判定任务集可调度性的影响,提出并证明了相关的定理.最后基于提出的定理,给出了一种改进的伪多项式时间可调度性判定算法,并与已有的判定方法进行了比较.仿真结果表明,该算法平均性能作为任务集内任务个数的函数具有显著提高.     

EDZL scheduling analysis

A schedulability test is derived for the global Earliest Deadline Zero Laxity (EDZL) scheduling algorithm on a platform with multiple identical processors. The test is sufficient, but not necessary, to guarantee that a system of independent sporadic tasks with arbitrary deadlines will be successfully scheduled, with no missed deadlines, by the multiprocessor EDZL algorithm. Global EDZL is known to be at least as effective as global Earliest-Deadline-First (EDF) in scheduling task sets to meet deadlines. It is shown, by testing on large numbers of pseudo-randomly generated task sets, that the combination of EDZL and the new schedulability test is able to guarantee that far more task sets meet deadlines than the combination of EDF and known EDF schedulability tests. In the second part of the paper, an improved version of the EDZL-schedulability test is presented. This new algorithm is able to efficiently exploit information on the slack values of interfering tasks, to iteratively refine the estimation of the interference a task can be subjected to. This iterative algorithm is shown to have better performance than the initial test, in terms of schedulable task sets detected.