An optimal fixed-priority assignment algorithm for supporting fault-tolerant hard real-time systems

The main contribution of this paper is twofold. First, we present an appropriate schedulability analysis, based on response time analysis, for supporting fault-tolerant hard real-time systems. We consider systems that make use of error-recovery techniques to carry out fault tolerance. Second, we propose a new priority assignment algorithm which can be used, together with the schedulability analysis, to improve system fault resilience. These achievements come from the observation that traditional priority assignment policies may no longer be appropriate when faults are being considered. The proposed schedulability analysis takes into account the fact that the recoveries of tasks may be executed at higher priority levels. This characteristic is very important since, after an error, a task certainly has a shorter period of time to meet its deadline. The proposed priority assignment algorithm, which uses some properties of the analysis, is very efficient. We show that the method used to find out an appropriate priority assignment reduces the search space from O(n!) to O(n/sup 2/), where n is the number of task recovery procedures. Also, we show that the priority assignment algorithm is optimal in the sense that the fault resilience of task sets is maximized as for the proposed analysis. The effectiveness of the proposed approach is evaluated by simulation

Reasoning About the Reliability of Multi-version, Diverse Real-Time Systems

This paper is concerned with the development of reliable real-time systems for use in high integrity applications. It advocates the use of diverse replicated channels, but does not require the dependencies between the channels to be evaluated. Rather it develops and extends the approach of Little wood and Rush by (for general systems) by investigating a two channel system in which one channel, A, is produced to a high level of reliability (i.e. has a very low failure rate), while the other, B, employs various forms of static analysis to sustain an argument that it is perfect (i.e. it will never miss a deadline). The first channel is fully functional, the second contains a more restricted computational model and contains only the critical computations. Potential dependencies between the channels (and their verification) are evaluated in terms of aleatory and epistemic uncertainty. At the aleatory level the events ''A fails" and ''B is imperfect" are independent. Moreover, unlike the general case, independence at the epistemic level is also proposed for common forms of implementation and analysis for real-time systems and their temporal requirements (deadlines). As a result, a systematic approach is advocated that can be applied in a real engineering context to produce highly reliable real-time systems, and to support numerical claims about the level of reliability achieved

Quantitative Verification: Formal Guarantees for Timeliness, Reliability and Performance

Computerised systems appear in almost all aspects of our daily lives, often in safety-critical scenarios such as embedded control systems in cars and aircraft or medical devices such as pacemakers and sensors. We are thus increasingly reliant on these systems working correctly, despite often operating in unpredictable or unreliable environments. Designers of such devices need ways to guarantee that they will operate in a reliable and efficient manner. Quantitative verification is a technique for analysing quantitative aspects of a system's design, such as timeliness, reliability or performance. It applies formal methods, based on a rigorous analysis of a mathematical model of the system, to automatically prove certain precisely specified properties, e.g. ``the airbag will always deploy within 20 milliseconds after a crash'' or ``the probability of both sensors failing simultaneously is less than 0.001''. The ability to formally guarantee quantitative properties of this kind is beneficial across a wide range of application domains. For example, in safety-critical systems, it may be essential to establish credible bounds on the probability with which certain failures or combinations of failures can occur. In embedded control systems, it is often important to comply with strict constraints on timing or resources. More generally, being able to derive guarantees on precisely specified levels of performance or efficiency is a valuable tool in the design of, for example, wireless networking protocols, robotic systems or power management algorithms, to name but a few. This report gives a short introduction to quantitative verification, focusing in particular on a widely used technique called model checking, and its generalisation to the analysis of quantitative aspects of a system such as timing, probabilistic behaviour or resource usage. The intended audience is industrial designers and developers of systems such as those highlighted above who could benefit from the application of quantitative verification,but lack expertise in formal verification or modelling

220904

This letter proposes a multi-gateway designation framework to design real-time wireless sensor networks (WSNs) improving traffic schedulability, i.e., meeting the traffic time constraints. To this end, we resort to Spectral Clustering un-supervised learning that allows defining arbitrary k disjoint clusters without knowledge of the nodes physical position. In each cluster we use a centrality metric from social sciences to designate one gateway. This novel combination is applied to a time-synchronized channel-hopping (TSCH) WSN under earliest-deadline-first (EDF) scheduling and shortest-path routing. Simulation results under varying configurations show that our framework is able to produce WSN designs that greatly reduce the worst-case network demand. In a situation with 5gateways, 99% schedulability can be achieved with 3.5 times more real-time flows than in a random benchmark.This work was co-financed by national funds through FCT/MCTES (Portuguese Foundation for Science and Technology) within the CISTER Research Unit (UIDB/04234/2020); by the Operational Programme for Competitiveness and Internationalization (COMPETE 2020) under the PT2020 Agreement, through the European Regional Development Fund (ERDF); and also by FCT and the ESF (European Social Fund) through the Regional Operational Programme (ROP) Norte 2020, under PhD grant 2020.06685.BD. The authors also thank to the CYTED AgIoT Project (520RT011), CORFO COTH2O “Consorcio de Gestion de Recursos Hídricos para la Macrozona Centro-Sur” and Proyecto Asociativo UDP “Plataformas Digitales como Modelo Organizacional”.info:eu-repo/semantics/publishedVersio

EDF Scheduling and Minimal-Overlap Shortest-Path Routing for Real-Time TSCH Networks

With the scope of Industry 4.0 and the Industrial Internet of Things (IIoT), wireless technologies have gained momentum in the industrial realm. Wireless standards such as WirelessHART, ISA100.11a, IEEE 802.15.4e and 6TiSCH are among the most popular, given their suitability to support real-time data traffic in wireless sensor and actuator networks (WSAN). Theoretical and empirical studies have covered prioritized packet scheduling in extenso, but only little has been done concerning methods that enhance and/or guarantee real-time performance based on routing decisions. In this work, we propose a greedy heuristic to reduce overlap in shortest-path routing for WSANs with packet transmissions scheduled under the earliest-deadline-first (EDF) policy. We evaluated our approach under varying network configurations and observed remarkable dominance in terms of the number of overlaps, transmission conflicts, and schedulability, regardless of the network workload and connectivity. We further observe that well-known graph network parameters, e.g., vertex degree, density, betweenness centrality, etc., have a special influence on the path overlaps, and thus provide useful insights to improve the real-time performance of the network

220603

This research proposes a co-design framework for scheduling, routing and gateway designation to improve the real-time performance of low-power wireless mesh networks. We target time-synchronized channel hopping (TSCH) networks with centralized network management and a single gateway. The end goal is to exploit existing trade-offs between the three dimensions to enhance traffic schedulability at systems' design time. The framework we propose considers a global Earliest-Deadline-First (EDF) scheduler that operates in conjunction with the minimal-overlap (MO) shortest-path routing, after a centrality-driven gateway designation is concluded. Simulation results over varying settings suggest our approach can lead to optimal or near-optimal real-time network performance, with 3~times more schedulable flows than a naive real-time configuration.This work was partially supported by National Funds through FCT/MCTES (Portuguese Foundation for Science and Technology), within the CISTER Research Unit (UIDB/04234/2020); by the Operational Competitiveness Programme and Internationalization (COMPETE 2020) under the PT2020 Agreement, through the European Regional Development Fund (ERDF); by FCT and the ESF (European Social Fund) through the Regional Operational Programme (ROP) Norte 2020, under PhD grant 2020.06685.BD.N/

Limited Preemptive Scheduling for Real-Time Systems: a Survey

The question whether preemptive algorithms are better than nonpreemptive ones for scheduling a set of real-time tasks has been debated for a long time in the research community. In fact, especially under fixed priority systems, each approach has advantages and disadvantages, and no one dominates the other when both predictability and efficiency have to be taken into account in the system design. Recently, limited preemption models have been proposed as a viable alternative between the two extreme cases of fully preemptive and nonpreemptive scheduling. This paper presents a survey of the existing approaches for reducing preemptions and compares them under different metrics, providing both qualitative and quantitative performance evaluations

230702

This article presents a novel centrality-driven gateway designation framework for the improved real-time performance of low-power wireless sensor networks (WSNs) at system design time. We target time-synchronized channel hopping (TSCH) WSNs with centralized network management and multiple gateways with the objective of enhancing traffic schedulability by design. To this aim, we propose a novel network centrality metric termed minimal-overlap centrality that characterizes the overall number of path overlaps between all the active flows in the network when a given node is selected as gateway. The metric is used as a gateway designation criterion to elect as a gateway the node leading to the minimal number of overlaps. The method is then extended to multiple gateways with the aid of the unsupervised learning method of spectral clustering. Concretely, after a given number of clusters are identified, we use the new metric at each cluster to designate as cluster gateway the node with the least overall number of overlaps. Extensive simulations with random topologies under centralized earliest-deadline-first (EDF) scheduling and shortest-path routing suggest our approach is dominant over traditional centrality metrics from social network analysis, namely, eigenvector, closeness, betweenness, and degree. Notably, our approach reduces by up to 40% the worst-case end-to-end deadline misses achieved by classical centrality-driven gateway designation methods.This work was partially supported by National Funds through FCT/MCTES (Portuguese Foundation for Science and Technology), within the CISTER Research Unit (UIDB/04234/2020); by the Operational Competitiveness Programme and Internationalization (COMPETE 2020) under the PT2020 Agreement, through the European Regional Development Fund (ERDF); also by FCT and the ESF (European Social Fund) through the Regional Operational Programme (ROP) Norte 2020, under PhD grant 2020.06685.BD.info:eu-repo/semantics/publishedVersio

EDF Scheduling and Minimal-Overlap Shortest-Path Routing for Real-Time TSCH Networks

With the scope of Industry 4.0 and the Industrial Internet of Things (IIoT), wireless technologies have gained momentum in the industrial realm. Wireless standards such as WirelessHART, ISA100.11a, IEEE 802.15.4e and 6TiSCH are among the most popular, given their suitability to support real-time data traffic in wireless sensor and actuator networks (WSAN). Theoretical and empirical studies have covered prioritized packet scheduling in extenso, but only little has been done concerning methods that enhance and/or guarantee real-time performance based on routing decisions. In this work, we propose a greedy heuristic to reduce overlap in shortest-path routing for WSANs with packet transmissions scheduled under the earliest-deadline-first (EDF) policy. We evaluated our approach under varying network configurations and observed remarkable dominance in terms of the number of overlaps, transmission conflicts, and schedulability, regardless of the network workload and connectivity. We further observe that well-known graph network parameters, e.g., vertex degree, density, betweenness centrality, etc., have a special influence on the path overlaps, and thus provide useful insights to improve the real-time performance of the network.info:eu-repo/semantics/publishedVersio