Dynamic scheduling of hard real-time tasks and real-time threads

The authors investigate the dynamic scheduling of tasks with well-defined timing constraints. They present a dynamic uniprocessor scheduling algorithm with an O(n log n) worst-case complexity. The preemptive scheduling performed by the algorithm is shown to be of higher efficiency than that of other known algorithms. Furthermore, tasks may be related by precedence constraints, and they may have arbitrary deadlines and start times (which need not equal their arrival times). An experimental evaluation of the algorithm compares its average case behavior to the worst case. An analytic model used for explanation of the experimental results is validated with actual system measurements. The dynamic scheduling algorithm is the basis of a real-time multiprocessor operating system kernel developed in conjunction with this research. Specifically, this algorithm is used at the lowest, threads-based layer of the kernel whenever threads are created     

Low-Power Design for Real-Time Systems

Real-time Systems often are located in the special environments where the power consumption is a big concern. Upon presence of timing constraints, the low power design on the real-time systems has significant impact on the performance as well as the schedulability of the systems. The system developers are facing the challenges for reducing the power consumption and meeting the timing constraints in the real-time systems.This paper represents one of few attempts to address the issue of the low power design on real-time systems. We present two power reduction methods: one is at the software compilation level and the other at the operating system level. Given a real-time program, an inter-instruction power reduction technique is proposed to transform the program to another one with lower power consumption. In addition, a scheduling algorithm for real-time operating systems is proposed to reschedule real-time programs when the execution time of the programs is changed. Therefore, the proposed scheduling algorithm works together with the proposed power reduction technique to make sure all programs meet their deadlines and to improve the system schedulability. We also evaluate the performance of the proposed inter-instruction reduction method by comparing it with the cold scheduling algorithm and show that the proposed method outperforms the cold scheduling algorithm and reduces more energy power.     

RTC: Language support for real-time concurrency

This paper presents a model and language constructs for expressing timing and concurrency requirements in distributed real-time programs. Our approach combines an abstract data type paradigm for the specification of shared resources and a distributed transaction-based paradigm for the specification of application processes. Resources provide abstract views of shared system entities, such as devices and data structures. Each resource has a state and defines a set ofactions that can be invoked by processes to examine or change its state. A resource also specifies scheduling constraints on the execution of its actions to ensure its consistency. Processes access resources by invoking actions and by expressing precedence, execution and timing constraints on action invocations. The implementation of our language constructs and the use of this system to control the simulation of a distributed robotics application is also described.This work is supported in part by the following grants: ARO DAAG-29-84-k-0061, ONR N000014-89-J-1131, and NSF CCR90-14621.     

时间Petri网的可调度分析及在FMS中的应用

在实时系统中,检查任务执行的计划是否满足要求的时间约束称为可调度分析.通过把时间特性与其他行为特性分离,提出了一种以时间Petri网建模的实时系统调度分析方法.如果特定任务的执行是可调度的,则可以计算任务执行的时间跨度,否则确定出不可调度的变迁以便于调整时间约束和纠正设计错误.提出了一种通过把复杂的任务序列分解成一些子序列来进行可调度性分析的综合时序分析技术,它不仅提高了效率,也有助于关于调度的可达性问题的讨论.讨论了柔性制造系统FMS中的车间装配子系统的可调度性.     

基于软件容错的动态实时调度算法

在硬实时系统中，由于任务超时完成将会导致灾难性后果，因而硬实时系统具有严格的时间及可靠性限制条件．目前实时容错调度算法大多针对硬件的容错，很少考虑软件运行的故障．提出了一种类似EDF的软件容错的动态实时调度算法PKSA(Probng-step Algorithm)，本算法在任务执行过程中，通过若干试探性检测步骤，提高了任务可执行性的预测，尽可能地避免了任务早期的失败对后续任务的影响，因此提高了任务的完成率，并同时有效地减少了浪费的CPU时间片．通过实验测试．同目前所知的同类算法相比，具有更佳的调度性能-调度成本比.     

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。     

基于对象分布式实时系统约束的一致性研究

在分布式实时系统中,时间约束规格的一致性是解决任务分配和调度等关键问题的必要前提。该文给出了一种基于对象分布式实时系统调度的通用模型,并对该模型进行了形式化描述。该模型克服了以往模型不能在应用系统的逻辑和功能部件上描述系统实时约束的不足,允许从方法和活动上描述所需的约束,降低了单一约束描述的繁杂程度。为了解决使用该模型进行约束规格的一致性问题,该文给出了绝对时间约束、相对时间约束、一致性约束以及相对时间约束和一致性约束之间的一致性判定的必要条件。     

支持绿色异构计算的能效感知调度模型与算法

异构调度可使大规模计算系统采用并行方式聚合广域分布的各种资源以提高性能.传统调度目标追求高性能而忽视高效能,远不能适应绿色计算科学发展新要求.因此,在理论上,一方面基于对动态频率和电压等系统参数的精细表述及有效量化,建立面向协同异构计算且易于复用的能效感知云调度模型;另一方面,提出并实现适于超计算机混合体系的多学科背景的元启发式多目标全局优化算法.从技术上解决了面向不同环境目标的云调度实施条件界定及其调度指标（时间、能效）实时变化描述等问题.大量仿真实验结果表明：与3个元启发式云调度器相比,该方法在能效及可扩展等方面优势明显;对于高维实例,整体性能改善分别达到8%,12%和14%.     

Task behavior monitoring for adaptive real-time communication

Real-time distributed systems include communicating tasks that interact via message-passing. In such systems the timely delivery of messages is essential for meeting task timing constraints. Consequently, in addition to task execution times, message delivery times must also be constrained. In order to minimize the number of failures to meet timing constraints message communication protocols, in addition to task scheduling algorithms, play a crucial role. A legitimate question to ask is whether making such protocols adaptive to run-time system and environment status can significantly improve system performance. Consequently, a rum-time monitoring approach to adaptive real-time distributed systems is proposed; the work focuses on an investigation of adaptive message communication protocols and corresponding run-time support mechanisms. Simulation is used to obtain performance results. It is concluded that although improvement is obtained it ,ay not be significant enough to offset the increased overhead and requirement for task information.     

Scheduling multiple task graphs with end-to-end deadlines in distributed real-time systems utilizing imprecise computations

In order to meet the inherent need of real-time applications for high quality results within strict timing constraints, the employment of effective scheduling techniques is crucial in distributed real-time systems. In this paper, we evaluate by simulation the performance of strategies for the dynamic scheduling of composite jobs in a homogeneous distributed real-time system. Each job that arrives in the system is a directed acyclic graph of component tasks and has an end-to-end deadline. For each scheduling policy, we provide an alternative version which allows imprecise computations, taking into account the effects of input error on the processing time of the component tasks of a job. The simulation results show that the alternative versions of the algorithms outperform their respective counterparts. To our knowledge, an imprecise computations approach for the dynamic scheduling of multiple task graphs with end-to-end deadlines and input error has never been discussed in the literature before.     

基于对象的分布式实时系统调度模型研究

为了解决分布式实时系统有关分配和调度等问题，给出并用形式化方法描述了一种基于对象分布式实时系统调度的通用模型。该模型包括表示时限的绝对时间约束，表示周期属性的周期约束，表示各种前趋关系和同步要求的相对时间约束以及保证资源使用一致性的一致性约束，此外该模型克服了以往模型不能在应用系统的逻辑和功能部件上描述系统实时的约束的不足，允许从方法和活动上描述所需的约束，降低了单一约束描述的繁杂程度，为了能够使用现有调度算法进行任务调度，讨论了约束转换的问题，给出了高层约束到底层约束的转换规则和相应的转换算法。     

Scheduling on-demand broadcast with timing constraints

In a distributed system, broadcasting is an efficient way to dispense data in certain highly dynamic environments. While there are several well-known on-line broadcast scheduling strategies that minimize wait time, there has been little research that considers on-demand broadcasting with timing constraints. One application which could benefit from a strategy for on-demand broadcast with timing constraints is a real-time database system. Scheduling strategies are needed in real-time databases that identify which data item to broadcast next in order to minimize missed deadlines. The scheduling decisions required in a real-time broadcast system allow the system to be modeled as a Markov Decision Process (MDP). In this paper, we analyze the MDP model and determine that finding an optimal solution is a hard problem in PSPACE. We propose a scheduling approach, called Aggregated Critical Requests (ACR), which is based on the MDP formulation and present two algorithms based on this approach. ACR is designed for timely delivery of data to clients in order to maximize the reward by minimizing the deadlines missed. Results from trace-driven experiments indicate the ACR approach provides a flexible strategy that can outperform existing strategies under a variety of factors.     

通用调度框架的改进及其在Linux中的实现

在嵌入式Linux实时系统中,要求内核对不同时问约束的任务采用不同的调度算法.但目前Linux内核采用单一的实时调度模式,不能灵活地执行多种调度算法,也就无法满足实时系统中实时任务的时间约束.引入了一种能够在Linux内核调度中执行多种调度算法的框架,即通用调度框架(GSF),并改进了其中的多算法调用机制,从而更好地在Linux内核中实现GSF.     

Scheduling for reactive real-time systems

In designing Chinook, a hardware-software cosynthesis system for reactive real-time controllers, the impact of timing constraints on software scheduling has been a central concern. By dividing constraints into two levels, corresponding to low-level interactions with device interfaces and high-level real-time response and rate requirements, we have developed solutions tailored to each aspect. These scheduling techniques enable Chinook to map a high-level specification onto a specified collection of processors and peripheral devices while respecting performance requirements     

实时异构系统中的积极复制容错调度算法

在设计实时异构系统中的容错调度算法时,既要考虑到实时性的约束,又要最大化系统的可靠性.此外,异构系统中的并行应用调度问题已经被证明了是NP完全问题.现有的容错调度算法大多采用复制技术来提升系统的可靠性,但是任务的多次执行会导致应用执行时间变长,系统实时性下降.为此,提出了一个基于积极复制技术的容错调度算法,该算法连续的复制任务集中对当前系统实时性影响最小的任务,然后将任务集中的所有任务调度至最早完成的处理器,用以在满足实时性约束的同时,提升系统的可靠性.实验表明,相比于同样着眼于实时异构系统的DB-FTSA算法,该算法在实时性约束严格的情况下,可靠性有较大提升.     

An Analysis of Totally Clairvoyant Scheduling

Traditional scheduling models assume that the execution time of a job in a periodic job-set is constant in every instance of its execution. This assumption does not hold in real-time systems wherein job execution time is known to vary. A second feature of traditional models is their lack of expressiveness, in that constraints more complex than precedence constraints (for instance, relative timing constraints) cannot be modeled. Thirdly, the schedulability of a real-time system depends upon the degree of clairvoyance afforded to the dispatcher. In this paper, we shall discuss Totally Clairvoyant Scheduling, as modeled within the E-T-C scheduling framework (Subramani, 2002). We show that this instantiation of the scheduling framework captures the central issues in a real-time flow-shop scheduling problem and devise a polynomial time sequential algorithm for the same. The design of the polynomial time algorithm involves the development of a new technique, which we term Mutable Dynamic Programming. We expect that this technique will find applications in other areas of system design, such as Validation and Software Verification. We also introduce an error-minimizing performance metric called Violation Degree and establish that optimizing this metric in a Totally Clairvoyant Scheduling System is NP-Hard.This research is supported in part by the Air Force Office of Scientific Research under Grant F49620-02-1-0043.     

一种面向嵌入式实时软件的需求建模语言

针对嵌入式实时系统复杂动态交互行为和严格实时的领域特征,提出了一种软件需求规约语言RTRSM。该语言以扩充的层次并发有穷状态机HCA为核心,以支持合成的模板为基本组成单元．利用转换有效期和事件预定机制来描述时间限制,既具有较强的时间限制描述能力,又能自然而直接地支持交互行为的建模,可执行且具有良好的形式语义。给出了该语言的形式化语法,举例说明了其时间描述机制,并通过执行步算法和基于HCA项的结构化操作规则定义了该语言的形式化操作语义。     

RFRTOS:基于Linux的实时操作系统

随着实时应用领域的逐渐扩大,不仅传统嵌入式系统的需求日益紧迫,而且这种情况也渗透到桌面环境.为了满足国内用户在这方面的广泛需求,基于红旗Linux操作系统,开发了RFRTOS实时操作系统.主要从实时调度策略、内核抢占机制、细粒度定时器几方面讨论了RFRTOS的实时支持.实验结果表明,所作改进有效地提高了Linux的调度精度,满足了软实时方面的需求.     

Processing real-time transactions in a replicated database system

A database system supporting a real-time application has to provide real-time information to the executing transactions. Each real-time transaction is associated with a timing constraint, typically in the form of a deadline. It is difficult to satisfy all timing constraints due to the consistency requirements of the underlying database. In scheduling the transactions it is aimed to process as many transactions as possible within their deadlines. Replicated database systems possess desirable features for real-time applications, such as a high level of data availability, and potentially improved response time for queries. On the other hand, multiple copy updates lead to a considerable overhead due to the communication required among the data sites holding the copies. In this paper, we investigate the impact of storing multiple copies of data on satisfying the timing constraints of real-time transactions. A detailed performance model of a distributed database system is employed in evaluating the effects of various workload parameters and design alternatives on the system performance. The performance is expressed in terms of the fraction of satisfied transaction deadlines. A comparison of several real-time concurrency control protocols, which are based on different approaches in involving timing constraints of transactions in scheduling, is also provided in performance experiments. Recommended by: A. ElmagarmidThis work was initiated while the author was at the Computer Science Department, University of Illinois at Urbana-Champaign.     

A sequential real-time refinement calculus

We present a comprehensive refinement calculus for the development of sequential, real-time programs from real-time specifications. A specification may include not only execution time limits, but also requirements on the behaviour of outputs over the duration of the execution of the program. The approach allows refinement steps that separate timing constraints and functional requirements. New rules are provided for handling timing constraints, but the refinement of components implementing functional requirements is essentially the same as in the standard refinement calculus. The product of the refinement process is a program in the target programming language extended with timing deadline directives. The extended language is a machine-independent, real-time programming language. To provide valid machine code for a particular model of machine, the machine code produced by a compiler must be analysed to guarantee that it meets the specified timing deadlines. Received: 27 September 1997 / 13 June 2000