Building flexible real-time systems using the Flex language

The design and implementation of a real-time programming language called Flex, which is a derivative of C++, are presented. It is shown how different types of timing requirements might be expressed and enforced in Flex, how they might be fulfilled in a flexible way using different program models, and how the programming environment can help in making binding and scheduling decisions. The timing constraint primitives in Flex are easy to use yet powerful enough to define both independent and relative timing constraints. Program models like imprecise computation and performance polymorphism can carry out flexible real-time programs. In addition, programmers can use a performance measurement tool that produces statistically correct timing models to predict the expected execution time of a program and to help make binding decisions. A real-time programming environment is also presented     

Meet real-time requirements of parallel programs and maximallyutilize system resources

The time-cost behavior of a program directly depends on its execution environment (i.e., the number of other programs in the system). We present techniques to analyze the time-cost behavior of programs that include the effect of their execution environment. It is assumed the underlying system has a shared memory architecture. The paper shows that one can analytically relate a parallel program's time cost to its execution environment. A direct benefit of the work is to let us meet the real-time requirement of a parallel program and utilize maximally a multiprocessor system's resource     

Modeling and evaluating the feasibility of timing constraints under different real-time scheduling algorithms

The objective of the work described here is to provide a software tool to assist real-time system specifiers and designers to predict, at an early stage of the development process, the timing behavior of the system being developed. The timing behavior of the system is dependent on the scheduler which is the central component of any real-time system. Our tool (Simulation of Real-Time systems (SRT)) is used to model the timing aspects of a real-time system and to simulate the system with a particular scheduling policy so as to predict its behavior. This paper will present an overview of the capabilities of the system which is implemented using an object-oriented programming language (Smalltalk-80).This work has been jointly funded by the Science and Engineering Research Council, UK and British Aerospace, Warton, UK.     

基于Flex技术的建筑设备集成监测报警系统的设计

提出一种基于Flex的建筑设备集成监测报警的可视化技术,即运用OPC、RS485/JBus、数据库和网页抓取解析等技术采集各类建筑设备运行状况和运行参数,并采用基于Flex编程技术开发的Flash图形与动画来展示这些采集来的工控敬据,将不易理解的工控数据表现成直观形象的仪表盘等图形,或者由数据变化产生的动画;在实现对各建筑设备现场系统监测数据的实时采集与集成监测的基础上,建立设备故障与异常信息的自动报警系统;应用结果表明,该系统运行稳定可靠,故障报警响应时间短,界面友好直观,具有一定的推广价值.     

Analysis and verification of real-time systems using quantitative symbolic algorithms

The task of checking if a computer system satisfies its timing specifications is extremely important. These systems are often used in critical applications where failure to meet a deadline can have serious or even fatal consequences. This paper presents an efficient method for performing this verification task. In the proposed method a real-time system is modeled by a state-transition graph represented by binary decision diagrams. Efficient symbolic algorithms exhaustively explore the state space to determine whether the system satisfies a given specification. In addition, our approach computes quantitative timing information such as minimum and maximum time delays between given events. These results provide insight into the behavior of the system and assist in the determination of its temporal correctness. The technique evaluates how well the system works or how seriously it fails, as opposed to only whether it works or not. Based on these techniques a verification tool called Verus has been constructed. It has been used in the verification of several industrial real-time systems such as the robotics system described below. This demonstrates that the method proposed is efficient enough to be used in real-world designs. The examples verified show how the information produced can assist in designing more efficient and reliable real-time systems.     

FIex在开发Web系统客户端应用探究与实现

本文以一具体的资源监控系统为例,对系统涉及到的关键技术进行了简要的讨论,分析了Flex技术优势,重点介绍了如何使用Hex技术开发该Web监控系统客户端部分,很好的解决了B／S模式中界面友好交互、无刷新实时监控数据走势图等难点。     

Timing constraint Petri nets and their application toschedulability analysis of real-time system specifications

We present timing constraint Petri nets (or TCPN's for short) and describe how to use them to model a real-time system specification and determine whether the specification is schedulable with respect to imposed timing constraints. The strength of TCPN's over other time-related Petri nets is in the modeling and analysis of conflict structures. Schedulability analysis is conducted in three steps: specification modeling, reachability simulation, and timing analysis. First, we model a real-time system by transforming its system specification along with its imposed timing constraints into a TCPN; we call this net N_s. Then we simulate the reachability of N_s to verify whether a marking, M_n, is reachable from an initial marking, M_o. It is important to note that a reachable marking in Petri nets is not necessarily reachable in TCPN's due to the imposed timing constraints, Therefore, in the timing analysis step, a reachable marking M_n, found in the reachability simulation step is analyzed to verify whether M_n, is reachable with the timing constraints. M_n is said to be reachable in the TCPN's if and only if we can find at least one firing sequence σ so that all transitions in σ are strongly schedulable with respect to M_o under the timing constraints. If such M_n can be found, then we can assert that the specification is schedulable under the imposed timing constraints, otherwise the system specification needs to be modified or the timing constraints need to be relaxed. We also present a synthesis method for determining the best approximation of the earliest fire beginning time (EFBT) and the latest fire ending time (LFET) of each strongly schedulable transition     

A reference string sampling method with dynamically changing parameters

Wittneben proposed a technique which determines the structural characteristics of a program's memory referencing behavior based only on a sampling of the complete reference string. This method controls the cost of the measurement by adjusting the sampling rate while simultaneously attempting to determine accurately the referencing behavior as reflected in its working set measurements. Wittneben's method is controlled by parameters with statically determined values. The aim of this paper is to present a modified sampling method in which sampling parameters are updated dynamically according to the actual program referencing behavior. The updating process is controlled by the phase-transition structure of a program. Furthermore, working set measurements of the program are based on modified principles which take into account the most probable sources of errors made during the sampling process. Experiments conducted with synthetic and real reference strings seem to demonstrate the superiority of the modified method in comparison with the technique originally formulated by Wittneben.     

Rate-monotonic analysis for real-time industrial computing

Issues of real-time resource management are pervasive throughout industrial computing. The underlying physical processes of many industrial computing applications impose explicit timing requirements on the tasks processed by the computer system. These timing requirements are an integral part of the correctness and safety of a real-time system. It is tempting to think that speed (for example, processor speeds or higher communication bandwidths) is the sole ingredient in meeting system timing requirements, but speed alone is not enough. Proper resource-management techniques also must be used to prevent, for example, situations in which long, low priority tasks block higher priority tasks with short deadlines. One guiding principle in real-time system resource management is predictability, the ability to determine for a given set of tasks whether the system will be able to meet all of the timing requirements of those tasks. Predictability calls for the development of scheduling models and analytic techniques to determine whether or not a real-time system can meet its timing requirements. The author illustrates an analysis methodology, rate monotonic analysis, for managing real-time requirements in a distributed industrial computing situation. The illustration is based on a comprehensive robotics example drawn from a typical industrial application     

Admission control for a responsive distributed middleware using decision trees to model run-time parameters

The software in modern systems has become too complex to make accurate predictions about their performance under different configurations. Real-time or even responsiveness requirements cannot be met because it is not possible to perform admission control for new or changing tasks if we cannot tell how their execution affects the other tasks already running. Previously, we proposed a resource-allocation middleware that manages the execution of tasks in a complex distributed system with real-time requirements. The middleware behavior can be modeled depending on the configuration of the tasks running, so that the performance of any given configuration can be calculated. This makes it possible to have admission control in such a system, but the model requires knowledge of run-time parameters. We propose the utilization of machine-learning algorithms to obtain the model parameters, and be able to predict the system performance under any configuration, so that we can provide a full admission control mechanism for complex software systems. In this paper, we present such an admission control mechanism, we measure its accuracy in estimating the parameters of the model, and we evaluate its performance to determine its suitability for a real-time or responsive system.     

基于Flex和BlazeDS推技术实现WEB方式实时监控系统

监控系统逐步向B／S框架结构^[1]转变,但由于HTTP协议固有的缺陷,使WEB模式在工业监控实时领域应用一直不尽人意,AJAX技术出现虽然解决了异步通讯和页面刷新问题^[2],但其依然使用HTTP请求拉方式（Pull）并没有真正解决实时问题。Flex技术和BlazeDS^[3]技术的出现,结合消息服务技术实现的推技术（Push）,解决了实时难题,彻底实现了Web实时监控工作模式。     

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.     

基于UML实时扩展的系统模型设计方法

时扩展的系统建模方法为实时系统的设计带来了全系统时间资源进行分析,定义了时间模型,包括时间为时间需求提供相应的服务。将该扩展方法应用于模型突出了对平台实时性的分析,满足了对时间的系统模型设计提供了新的解决方法。     

Combining static worst-case timing analysis and program proof

This paper describes SPATS—a new toolset for the development of safety-critical and hard real-time systems. SPATS integrates the analysis traditionally offered by program proof and static timing analysis tools through analysis of program basic-path graphs. This paper concentrates on SPATS' facilities for high-level static timing analysis and analysis of worst-case stack usage. The integration of timing analysis and program proof allows timing analysis to be performed where worst-case execution time (WCET) depends on a program's input data, and allows timing annotations to be formally verified. The approach is developed and illustrated with a worked example. The implementation and experimental application of SPATS to realistic industrial case-studies are also described. We conclude that SPATS offers a novel new approach to static timing analysis, offers several new analyses not seen in previous systems, and can be implemented in a useful and efficient toolset.This work was completed while Rod Chapman was with the Dependable Computing Systems Centre at the University of York.     

Discovering and exploiting program phases

Understanding program behavior is at the foundation of computer architecture and program optimization. Many programs have wildly different behavior on even the largest of scales (that is, over the program's complete execution). During one part of the execution, a program can be completely memory bound; in another, it can repeatedly stall on branch mispredicts. Average statistics gathered about a program might not accurately picture where the real problems lie. This realization has ramifications for many architecture and compiler techniques, from how to best schedule threads on a multithreaded machine, to feedback-directed optimizations, power management, and the simulation and test of architectures. Taking advantage of time-varying behavior requires a set of automated analytic tools and hardware techniques that can discover similarities and changes in program behavior on the largest of time scales. The challenge in building such tools is that during a program's lifetime it can execute billions or trillions of instructions. How can high-level behavior be extracted from this sea of instructions? Some programs change behavior drastically, switching between periods of high and low performance, yet system design and optimization typically focus on average system behavior. It is argued that instead of assuming average behavior, it is now time to model and optimize phase-based program behavior.     

Rate monotonic analysis for real-time systems

Rate monotonic analysis (RMA), a mathematical approach that helps ensure that a real-time system meets its performance requirements, is discussed. It does so through a collection of quantitative methods and algorithms that let engineers understand, analyze, and predict the timing behavior of their designs. An application of RMA to an air traffic control system is described     

交会对接多航天器联合闭环测试模式设计与验证

在载人航天领域,多航天器交会对接技术是研制的关键和难题,闭环测试系统设计的重要性尤为突出,用于交会对接的多航天器联合闭环测试的设计与实施,实现了多航天器间的一体化实时动态同步电测,实现了多航天器及其测试系统的时序同步,以及敏感器及其模拟器和机电系统的动力学模型同步动态联合实时驱动的同步控制,采用了多航天器一体化实时精确控制的自动化测试,以及多航天器间多通道大回路信息流的实时统一管理。解决飞行任务阶段,多航天器交互状态的验证问题,在地面真实呈现交会对接飞行的全过程,达到多船器联合电测的目的。     

面向软件系统的指令时序建模与分析方法

软件指令时序调度是实时软件系统运行的基本问题,如何识别出实时系统中的关键任务,并对其进行及时调度和处理则是解决这一基本问题的关键。应用企业过程建模原理,构造了实时系统的软件过程时序模型,并基于SADT模型,提出了一个基于CPU时间和带宽分配的指令活动模型TB-SADT。引入合成数的概念以求解不确定性软件过程关键路径。针对软件过程模型的控制结构,考虑CPU时间和通信时间的分配,探讨模型中各活动之间时序调度问题。