A static scheduling algorithm for distributed hard real-time systems

A static scheduling algorithm is presented for off-line scheduling of tasks in distributed hard real-time systems. The tasks considered are instances of periodic jobs and have deadlines, resource requirements and precedence constraints. Tasks are divided into nonpreemptable blocks and all task characteristics are known a priori. The algorithm orders the tasks and iteratively schedules the tasks according to the order. Each task is scheduled globally by selecting a node to which it is assigned. Then, the task is scheduled locally by adding the task to the schedule of the selected node. Heuristics are used for both task ordering and node selection in order to guide the algorithm to a feasible schedule. Whenever local scheduling leads to an infeasible schedule, backtracking is used.Results of simulation studies of randomly generated task sets are presented. Although the scheduling problem is NP-hard, the results show that time performance is acceptable for off-line scheduling, except for extremely difficult task sets which make extensive use of the available resources.     

Task scheduling in distributed real-time systems

An approach to the flow shop scheduling of the computation process in distributed realtime systems is considered. This approach is based on the concept of a solvable class of systems for which simple optimal scheduling algorithms exist.     

Rolling-horizon scheduling for energy constrained distributed real-time embedded systems

Energy-efficient scheduling approaches are critical to battery driven real-time embedded systems. Traditional energy-aware scheduling schemes are mainly based on the individual task scheduling. Consequently, the scheduling space for each task is small, and the schedulability and energy saving are very limited, especially when the system is heavily loaded. To remedy this problem, we propose a novel rolling-horizon (RH) strategy that can be applied to any scheduling algorithm to improve schedulability. In addition, we develop a new energy-efficient adaptive scheduling algorithm (EASA) that can adaptively adjust supply voltages according to the system workload for energy efficiency. Both the RH strategy and EASA algorithm are combined to form our scheduling approach, RH-EASA. Experimental results show that in comparison with some typical traditional scheduling schemes, RH-EASA can achieve significant energy savings while meeting most task deadlines (namely, high schedulability) for distributed real-time embedded systems with dynamic workloads.     

Stable, distributed, real-time scheduling of flexiblemanufacturing/assembly/diassembly systems

The authors consider general flexible manufacturing/assembly/disassembly systems with the following features: (i) there are several types, each with given processing time requirements at a specified sequence of machines; (ii) each part type needs to be produced at a prespecified rate; (iii) parts may incur variable transportation delays when moving from one machine to another; (iv) set-up times are required whenever a machine changes from a production run of parts of one type to a run of another type; (v) some part types may also need assembly or disassembly; and (vi) a proportion of parts of a part type may require separate routing on exiting from a machine, for reasons including, but not limited to, poor quality. The authors exhibit a class of scheduling policies implementable in real time in a distributed way at the various machines, which ensure that the cumulative production of each part type trails the desired production by no more than a specific constant. The buffers of all the machines are guaranteed to be bounded, and the system can thus operate with finite buffer capacities     

面向分布式实时系统的安全驱动调度算法研究

针对现有实时调度算法无法适应动态安全需求的问题,构建了一种安全驱动调度模型,该模型从系统安全级别、系统安全服务和任务安全策略三个方面描述了实时系统的动态安全需求,并设计了一种基于安全驱动的实时任务调度器框架。以该模型和框架为基础,提出了一种安全驱动调度算法(Security Driven Scheduling Algorithm,SDSA)。从全局角度对新到达任务进行可调度性检查,并将可调度任务分配到合适的处理机上运行。按照系统安全级别来动态调整已分配到各处理机上实时任务的安全策略,使其达到安全性和可调度性的最优平衡。采用优先级抢占式策略对各实时任务进行调度。仿真结果表明,SDSA算法与其他同类算法相比,在系统动态安全需求的适应性、关键任务的可调度性以及安全防危能力等方面具有较好的表现。     

异构分布系统的实时任务轮转式容错调度算法

建立了一个异构分布式系统实时调度模型,对异构分布式系统中的任务及不同处理机资源进行了形式化描述.结合基版本/副版本技术,给出了用于异构分布式系统的实时任务轮转式容错调度算法.实例分析表明,该算法有效提高了异构处理机环境下的资源利用率以及整体计算性能.     

A non-preemptive scheduling algorithm for soft real-time systems

Real-time systems are often designed using preemptive scheduling and worst-case execution time estimates to guarantee the execution of high priority tasks. There is, however, an interest in exploring non-preemptive scheduling models for real-time systems, particularly for soft real-time multimedia applications. In this paper, we propose a new algorithm that uses multiple scheduling strategies for efficient non-preemptive scheduling of tasks. Our goal is to improve the success ratio of the well-known Earliest Deadline First (EDF) approach when the load on the system is very high and to improve the overall performance in both underloaded and overloaded conditions. Our approach, known as group-EDF (gEDF) is based on dynamic grouping of tasks with deadlines that are very close to each other, and using Shortest Job First (SJF) technique to schedule tasks within the group. We will present results comparing gEDF with other real-time algorithms including, EDF, Best-effort, and Guarantee, by using randomly generated tasks with varying execution times, release times, deadlines and tolerance to missing deadlines, under varying workloads. We believe that grouping tasks dynamically with similar deadlines and utilizing a secondary criteria, such as minimizing the total execution time (or other metrics such as power or resource availability) for scheduling tasks within a group, can lead to new and more efficient real-time scheduling algorithms.     

A pre-run-time scheduling algorithm for hard real-time systems

Process scheduling, an important issue in the design and maintenance of hard real-time systems, is discussed. A pre-run-time scheduling algorithm that addresses the problem of process sequencing is presented. The algorithm is designed for multiprocessor applications with preemptable processes having release times, computation times, deadlines and arbitrary precedence and exclusion constraints. The algorithm uses a branch-and-bound implicit enumeration technique to generate a feasible schedule for each processor. The set of feasible schedules ensures that the timing specifications of the processes are observed and that all the precedence and exclusion constraints between pairs of processes are satisfied. the algorithm was tested using a model derived from the F-18 mission computer operational flight program     

UM-RTCOM: An analyzable component model for real-time distributed systems

Component-based development is a key technology in the development of software for modern real-time systems. However, standard component models and tools are not suitable for this type of system, since they do not explicitly address real time, memory or cost constraints. This paper presents a new predictable component model for real-time systems (UM-RTCOM) together with a set of tools to support it. The environment allows new components to be developed which can then be assembled to build complete applications, including hardware interaction. The model includes support for real-time analysis at the component and application level. The analysis is achieved by combining component meta-information in the form of an abstract behaviour model and a method to measure worst-case execution times in the final platform. Additionally, we propose an implementation model based on RT-CORBA where the developer uses the UM-RTCOM components and a set of tools to map these elements to elements of the desired platform. In order to apply our proposals, we have used the model and tools in real applications specifically in the context of nuclear power plant simulators.     

A predictable real-time kernel for distributed multisensor systems

The authors present a real-time kernel developed to support a distributed multisensor system encountered in robotics applications. To ensure predictability, the kernel provides services with bounded worst-case execution times. In addition, the kernel allows the programmer to specify timing constraints for process execution and interprocess communication. The kernel uses these timing constraints both for scheduling processes and for scheduling communications. To illustrate the kernel, the authors describe a multisensor system being developed on their distributed real-time system. They present the measured performance of kernel primitives along with conclusions and remarks regarding distributed real-time systems     

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。     

Priority scheduling of distributed systems based on model checking

Priorities are used to control the execution of systems to meet given requirements for optimal use of resources, e.g., by using scheduling policies. For distributed systems it is hard to find efficient implementations for priorities; because they express constraints on global states, their implementation may incur considerable overhead. Our method is based on performing model checking for knowledge properties. It allows identifying where the local information of a process is sufficient to schedule the execution of a high priority transition. As a result of the model checking, the program is transformed to react upon the knowledge it has at each point. The transformed version has no priorities, and uses the gathered information and its knowledge to limit the enabledness of transitions so that it matches or approximates the original specification of priorities.     

Global cyclic scheduling: A method to guarantee the timing behavior of distributed real-time systems

The use of the global cyclic scheduling discipline in distributed real-time systems guarantees that the response time requirements of the environment are always met. The global scheduling discipline uses cyclic scheduling plans for its decisions. These plans are determined off-line. During the development phase of a real-time system deadlines are checked and the scheduling plans are generated. Software tools support the computation of the scheduling plans. In this paper we discuss the global cyclic scheduling discipline and present a software system to support the development of distributed real-time systems. Our software system consists of a specificaton tool for the real-time software and a schedule computation tool. Three different algorithms for the computation of the scheduling plans are presented.The research work was supported as an Ernst-von-Siemens scholarship of the Siemens AG.     

Language constructs for real-time distributed systems

The paper observes syntactic and semantic requirements for a language for programming real-time distributed systems. A proposal for language features that meet these requirements is offered, and the features are applied to an example.     

Virtual machines for distributed real-time systems

The steady increase in raw computing power of the processors commonly adopted for distributed real-time systems leads to the opportunity of hosting diverse classes of tasks on the same hardware, for example process control tasks, network protocol stacks and man–machine interfaces.This paper describes how virtualization techniques can be used to concurrently run multiple operating systems on the same physical machine, although they are kept fully separated from the security and execution timing points of view, and still have them exhibit acceptable real-time execution characteristics.With respect to competing approaches, the main advantages of this method are that it requires little or no modifications to the operating systems it hosts, along with a better modularity and clarity of design.     

异构分布式实时系统的容错优化调度算法

针对分布式实时系统,在分析了单处理调度算法的基础上,结合版本复制技术和首次适应方法,给出了一种容错调度算法。分析了算法的可调度性,给出任务的可调度性条件。在满足任务容错可调度的情况下,以提高处理器的利用率为目标,对基版本时限进行了优化,给出了基版本优化时限的求取算法。仿真结果表明,本文算法将可以得到比FTEDFFF和FTRMFF更高的处理器利用率。     

Monitoring distributed real-time systems

This paper emphasizes the power ofmonitoring of distributed real-time systems as a promising tool for both scientific work and practical purposes. Starting out from a number of well-known problems with today's (industrial) real-time systems, a classification of remedial monitoring applications is given. The most important features of a monitoring system suitable for such purposes are discussed and related to the current research into monitoring of (general) distributed systems. Finally, some of the resulting conceptual issues underlying our prototype VTA monitoring system—currently being under development at our department—are presented.