Supporting real-time distributed computer-controlled systems with multi-hop P-NET networks

Fieldbus communication networks aim to interconnect sensors, actuators and controllers within process control applications. Therefore, they constitute the foundation upon which real-time distributed computer-controlled systems can be implemented. P-NET is a fieldbus communication standard, which uses a virtual token-passing medium-access-control mechanism. In this paper pre-run-time schedulability conditions for supporting real-time traffic with P-NET networks are established. Essentially, formulae to evaluate the upper bound of the end-to-end communication delay in P-NET messages are provided. Using this upper bound, a feasibility test is then provided to check the timing requirements for accessing remote process variables. This paper also shows how P-NET network segmentation can significantly reduce the end-to-end communication delays for messages with stringent timing requirements.     

Engineering a hard real-time system: From theory to practice

More and more programmers find their software being used in performance critical applications. Unfortunately, they have limited techniques at their disposal to help guarantee this particular aspect of their programs. There has been considerable activity in recent years on developing analysis techniques for hard real-time systems. Inevitably these techniques make simplifying assumptions so as to reduce the complexity of the problem to be solved. For example hard real-time schedulability analysis techniques often assume that the timing properties of the underlying kernel can be accounted for by incorporating extra execution time into the application tasks. Furthermore, they assume that the application task structure is very simple and uniform. This paper considers the implications of using these techniques in the analysis of a typical single processor application, the attitude and orbital control system (AOCS) for the Olympus satellite. The paper outlines a common approach for estimating the response times for tasks, and then extends the scheduling equations so that they can be used in the engineering of realistic real-time systems.     

匿名通信系统中时间攻击模型研究

时间攻击是指对手研究信息穿过系统的时间来找出信息发送者和接收者之间的相关性,对支持交互式的低延时匿名通信系统形成一种巨大威胁。论文提出了一种基于重路由技术的匿名通信系统和针对该系统的时间攻击模型,理论分析和计算数据表明这种时间攻击方式可以有效地破坏匿名通信系统中信息发送者和接收者之间的连接性,但这种时间攻击的成功率受到信息到达率和节点延迟时间以及重路由路长等因素的影响。     

A rectangle bin packing optimization approach to the signal scheduling problem in the FlexRay static segment

As FlexRay communication protocol is extensively used in distributed real-time applications on vehicles, signal scheduling in FlexRay network becomes a critical issue to ensure the safe and efficient operation of time-critical applications. In this study, we propose a rectangle bin packing optimization approach to schedule communication signals with timing constraints into the FlexRay static segment at minimum bandwidth cost. The proposed approach, which is based on integer linear programming (ILP), supports both the slot assignment mechanisms provided by the latest version of the FlexRay specification, namely, the single sender slot multiplexing, and multiple sender slot multiplexing mechanisms. Extensive experiments on a synthetic and an automotive X-by-wire system case study demonstrate that the proposed approach has a well optimized performance.     

Design optimization for security- and safety-critical distributed real-time applications

In this paper, we are interested in the design of real-time applications with security, safety, timing, and energy requirements. The applications are scheduled with cyclic scheduling, and are mapped on distributed heterogeneous architectures. Cryptographic services are deployed to satisfy security requirements on confidentiality of messages, task replication is used to enhance system reliability, and dynamic voltage and frequency scaling is used for energy efficiency of tasks. It is challenging to address these factors simultaneously, e.g., better security protections need more computing resources and consume more energy, while lower voltages and frequencies may impair schedulability and security, and also lead to reliability degradation. We introduce a vulnerability based method to quantify the security performance of communications on distributed systems. We then focus on determining the appropriate security measures for messages, the voltage and frequency levels for tasks, and the schedule tables such that the security and reliability requirements are satisfied, the application is schedulable, and the energy consumption is minimized. We propose a Tabu Search based metaheuristic to solve this problem. Extensive experiments and a real-life application are conducted to evaluate the proposed techniques.     

Schedulability analysis and efficient scheduling of rate constrained messages in the TTEthernet protocol

Over time, cyber‐physical systems are becoming mixed criticality systems. As the complexity and the size of these systems grow, computation/communication resources should be more efficient than with traditional systems. TTEthernet is a communication infrastructure that enables the use of a single physical communication infrastructure for distributed mixed criticality applications while providing timely determinism. TTEthernet distinguishes between two traffic categories: the standard event triggered and the time triggered. The latter, for which higher priority is granted, is subject to strong timing guarantees because of strict periodicity constraint that fixes start‐time cycles of time‐triggered messages. In addition, event‐triggered traffic includes rate‐constrained messages that are of lower priority and have a minimum time interval between their transmission. The paper proposes both an on‐line efficient scheduling algorithm and a necessary and sufficient schedulability condition based on the worst case response time computation for rate‐constrained messages while taking into account time‐triggered messages transmission. Copyright © 2017 John Wiley & Sons, Ltd.     

Conventional and early token release scheduling models for the IEEE 802.5 token ring

We develop analyticalscheduling models for both the original IEEE 802.5 token ring protocol and a recent extension to the original protocol that allows early token release (ETR). A scheduling model is an abstraction that supports reasoning about the timing correctness of a given set of real-time messages scheduled on the network. Scheduling analysis of the original IEEE 802.5 token ring protocol has previously been discussed in Strosnider and Marchok (1989) and Pleinevaux (1992) in the context of improving responsiveness of soft deadline aperiodic messages. In contrast, this paper develops schedulability conditions for arbitrary periodic message sets. The main contributions of this work are: Scheduling models for both the original protocol and ETR protocol; comparison for maximum achievable utilizations for the two protocols; comparison between the original protocol and ETR from a schedulability viewpoint. We also demonstrate the utility of our scheduling models to select network operating parameters such as maximum packet size, and to quantify effects of parameters such as the number of stations, and network size on schedulability.     

Scheduler Modeling Based on the Controller Synthesis Paradigm

The controller synthesis paradigm provides a general framework for scheduling real-time applications. Schedulers can be considered as controllers of the applications; they restrict their behavior so that given scheduling requirements are met. We study a modeling methodology based on the controller synthesis paradigm. The methodology allows to get a correctly scheduled system from timed models of its processes in an incremental manner, by application of composability results which simplify schedulability analysis. It consists in restricting successively the system to be scheduled by application of constraints defined from scheduling requirements. The latter are a conjunction of schedulability requirements that express timing properties of the processes and policy requirements about resource management. The presented methodology allows a unified view of scheduling theory and approaches based on timing analysis of models of real-time applications.     

多处理器实时系统可调度性分析的UPPAAL模型

随着多处理器实时系统在安全性攸关系统中的广泛应用,保证这类系统的正确性成为一项重要的工作.可调度性是实时系统正确性的一项关键性质.它表示系统必须满足的一些时间要求.传统的可调度性分析方法结论保守或者不完备,为了避免这些方法的缺陷,提出使用模型检测的方法来实现可调度性分析.提出了一个用于多处理器实时系统可调度性分析的模板,将与系统可调度性相关的部分包括实时任务、运行平台和调度管理模块都用时间自动机建模,并使用UPPAAL验证可调度的性质是否总被满足.符号化模型检测方法被用于推断可调度性,但是由于秒表触发的近似机制,符号化模型检测方法不能用于证明系统不可调度.作为补充,统计模型检测方法被用于估算系统不可调度的概率,并在系统不可调度时生成反例.此外,在系统可调度时,通过统计模型检测方法获取一些性能相关的信息.     

Sensitivity analysis for fixed-priority real-time systems

At early stages in the design of real-time embedded applications, the timing attributes of the computational activities are often incompletely specified or subject to changes. Later in the development cycle, schedulability analysis can be used to check the feasibility of the task set. However, the knowledge of the worst-case response times of tasks is often not sufficient to precisely determine the actions that would correct a non-schedulable design. In these situations, sensitivity analysis provides useful information for changing the implementation, by giving a measure of those computation times that must be reduced to achieve feasibility, or those that can be increased in case of a product extension, or providing the range of feasible periods for selecting the proper task activation rates. In this work, we exploit the concept of feasibility region to propose a faster and more concise solution to the sensitivity analysis problem with respect to existing techniques based on binary search. Furthermore, we show how the formalization of other problems in the feasibility domain, such as managing overloads through elastic scheduling, can be extended to the exact analysis.     

Efficient online schedulability tests for real-time systems

Many computer systems, such as those for open system environments or multimedia services, need an efficient schedulability test for online admission control of new jobs. Although various polynomial time schedulability tests have been proposed, they often fail to decide the schedulability of the system precisely when the system is heavily loaded. On the other hand, most precise schedulability tests proposed to date have a high complexity and may not be suitable for online tests. We present new efficient online schedulability tests for both the periodic process model [C. L. Liu et al., (1973)] and the multiframe process model [A. K. Mok et al., (1997)] in uniprocessor environments. The schedulability tests are shown to be more precise and efficient than any existing polynomial-time schedulability tests. Moreover, the tests can be done incrementally as each new task arrives at the system. Our proposed tests can also be used for the multiframe model where a task may have different computation times in different periods. We show the performance of the proposed schedulability tests in several simulation experiments.     

Task automata: Schedulability,decidability and undecidability

We present a model, task automata, for real time systems with non-uniformly recurring computation tasks. It is an extended version of timed automata with asynchronous processes that are computation tasks generated (or triggered) by timed events. Compared with classical task models for real time systems, task automata may be used to describe tasks (1) that are generated non-deterministically according to timing constraints in timed automata, (2) that may have interval execution times representing the best case and the worst case execution times, and (3) whose completion times may influence the releases of task instances. We generalize the classical notion of schedulability to task automata. A task automaton is schedulable if there exists a scheduling strategy such that all possible sequences of events generated by the automaton are schedulable in the sense that all associated tasks can be computed within their deadlines. Our first technical result is that the schedulability for a given scheduling strategy can be checked algorithmically for the class of task automata when the best case and the worst case execution times of tasks are equal. The proof is based on a decidable class of suspension automata: timed automata with bounded subtraction in which clocks may be updated by subtractions within a bounded zone. We shall also study the borderline between decidable and undecidable cases. Our second technical result shows that the schedulability checking problem will be undecidable if the following three conditions hold: (1) the execution times of tasks are intervals, (2) the precise finishing time of a task instance may influence new task releases, and (3) a task is allowed to preempt another running task.     

A framework for mapping periodic real-time applications onmulticomputers

This short paper presents a framework for periodic execution of task-flow graphs that enables schedulability analysis of the communication requirements. The analysis performs the steps of segmenting messages, assigning the segments to specific links and time intervals, and ordering them within the intervals to generate node switching schedules that provide contention-free message routing at run-time. The analysis is also used to integrate task allocation with message routing using a contention-based objective function. Usefulness of the proposed scheme in ensuring guaranteed communication performance is demonstrated by an appropriate example     

Compositional schedulability analysis of real-time systems using time Petri nets

This paper presents an approach to the schedulability analysis of real-time systems modeled in time Petri nets by separating timing properties from other behavioral properties. The analysis of behavioral properties is conducted based on the reachability graph of the underlying Petri net, whereas timing constraints are checked in terms of absolute and relative firing domains. If a specific task execution is schedulable, we calculate the time span of the task execution, and pinpoint nonschedulable transitions to help adjust timing constraints. A technique for compositional timing analysis is also proposed to deal with complex task sequences, which not only improves efficiency but also facilitates the discussion of the reachability issue with regard to schedulability. We identified a class of well-structured time Petri nets such that their reachability can be easily analyzed.     

Securing FlexRay-based in-vehicle networks

In this work, we propose defense techniques against message spoofing attacks in FlexRay networks. For this purpose, we explore how to leverage the slot- and channel-based FlexRay communication for ensuring the authenticity of safety-critical in-vehicle traffic. In particular, we suggest to split authentication tags across two physical independent channels, allowing for their concurrent transmission. This eventually leads to a higher degree of fault-tolerance and security. We put a special focus on the efficient management and distribution of cryptographic keys by using reversed hash chains. They allow us to derive new and forward-secure keys at low cost during operational runtime. Our evaluation consists of both a discussion and a practical implementation on an exemplary network. Assuming constrained hardware resources, we conclude that authentication for groups of time slots is the most promising approach in terms of timing and computational overhead.     

一种基于延迟抢先的消息最大响应时间算法

分布式实时系统是实时系统的一个重要研究方向，有着广泛的应用背景，消息最大响应时间的定量分析是该研究方向中的一个关键问题，在深入分析国内外相关研究的基础上，针对采用TDMA协议的分布实时系统提出了一种基于延迟抢先的消息最大响应时间算法，解决了现有消息最大响应时间算法存在的正确性问题，对消息的最大响应时间做出完整、精确的分析。     

一种基于简化任务模型的消息最大发送时间分析算法

分布式实时系统是实时系统的一个重要研究方向，有着广泛的应用背景。消息最大发送时间的定量分析是该研究方向中的一个关键问题。本文针对分布式实时系统中的一种简化任务模型，对原有分析算法进行了改进，提出了一种更加精确的消息最大发送时间分析算法。     

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.     

Delay composition in preemptive and non-preemptive real-time pipelines

Uniprocessor schedulability theory made great strides, in part, due to the simplicity of composing the delay of a job from the execution times of higher-priority jobs that preempt it. In this paper, we bound the end-to-end delay of a job in a multistage pipeline as a function of job execution times on different stages under preemptive as well as non-preemptive scheduling. We show that the end-to-end delay is bounded by that of a single virtual “bottleneck” stage plus a small additive component. This contribution effectively transforms the pipeline into a single stage system. The wealth of schedulability analysis techniques derived for uniprocessors can then be applied to decide the schedulability of the pipeline. The transformation does not require imposing artificial per-stage deadlines, but rather models the pipeline as a whole and uses the end-to-end deadlines directly in the single-stage analysis. It also does not make assumptions on job arrival patterns or periodicity and thus can be applied to periodic and aperiodic tasks alike. We show through simulations that this approach outperforms previous pipeline schedulability tests except for very short pipelines or when deadlines are sufficiently large. The reason lies in the way we account for execution overlap among stages. We discuss how previous approaches account for overlap and point out interesting differences that lead to different performance advantages in different cases. Further, we also show that in certain cases non-preemptive scheduling can result in higher system utilization than preemptive scheduling in pipelined systems. We hope that the pipeline delay composition rule, derived in this paper, may be a step towards a general schedulability analysis foundation for large distributed systems.

基于时间自动机的CAN网络可调度性分析

CAN网络已在汽车、航空、过程控制等实时网络化控制系统中获得了广泛应用。它能够传输周期性和非周期性两种实时报文，所有报文必须是可调度的，即在截止期前完成传输。本文提出了一种基于时间自动机的CAN网络非周期性实时报文的严格可调度性方法。