Browsing by Author "Burns, Alan"
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- Adding local priority-based dispatching mechanisms to P-NET networks: a fixed priority approachPublication . Tovar, Eduardo; Vasques, Francisco; Burns, AlanIn this paper we address the real-time capabilities of P-NET, which is a multi-master fieldbus standard based on a virtual token passing scheme. We show how P-NET’s medium access control (MAC) protocol is able to guarantee a bounded access time to message requests. We then propose a model for implementing fixed prioritybased dispatching mechanisms at each master’s application level. In this way, we diminish the impact of the first-come-first-served (FCFS) policy that P-NET uses at the data link layer. The proposed model rises several issues well known within the real-time systems community: message release jitter; pre-run-time schedulability analysis in non pre-emptive contexts; non-independence of tasks at the application level. We identify these issues in the proposed model and show how results available for priority-based task dispatching can be adapted to encompass priority-based message dispatching in P-NET networks.
- Buffer-aware bounds to multi-point progressive blocking in priority-preemptive NoCsPublication . Indrusiak, Leandro; Burns, Alan; Nikolic, BorislavThis paper aims to reduce the pessimism of the analysis of the multi-point progressive blocking (MPB) problem in real-time priority-preemptive wormhole networks-on-chip. It shows that the amount of buffering on each network node can influence the worst-case interference that packets can suffer along their routes, and it proposes a novel analytical model that can quantify such interference as a function of the buffer size. It shows that, perhaps counter-intuitively, smaller buffers can result in lower upper-bounds on interference and thus improved schedulability. Didactic examples and large-scale experiments provide evidence of the strength of the proposed approach.
- Communication response time in P-NET networks: worst-case analysis considering the actual token utilisationPublication . Tovar, Eduardo; Vasques, Francisco; Burns, AlanFieldbus networks aim at the interconnection of field devices such as sensors, actuators and small controllers. Therefore, they are an effective technology upon which Distributed Computer Controlled Systems (DCCS) can be built. DCCS impose strict timeliness requirements to the communication network. In essence, by timeliness requirements we mean that traffic must be sent and received within a bounded interval, otherwise a timing fault is said to occur. P-NET is a multi-master fieldbus standard based on a virtual token passing scheme. In P-NET each master is allowed to transmit only one message per token visit, which means that in the worst-case the communication response time could be derived considering that the token is fully utilised by all stations. However, such analysis can be proved to be quite pessimistic. In this paper we propose a more sophisticated P-NET timing analysis model, which considers the actual token utilisation by different masters. The major contribution of this model is to provide a less pessimistic, and thus more accurate, analysis for the evaluation of the worst-case communication response time in P-NET fieldbus networks.
- Decoupling Criticality and Importance in Mixed-Criticality SchedulingPublication . Bletsas, Konstantinos; Ali Awan, Muhammad; Souto, Pedro; Åkesson, Benny; Burns, Alan; Tovar, EduardoResearch on mixed-criticality scheduling has flourished since Vestal’s seminal 2007 paper, but more efforts are needed in order to make these results more suitable for industrial adoption and robust and versatile enough to influence the evolution of future certification standards in keeping up with the times. With this in mind, we introduce a more refined task model, in line with the fundamental principles of Vestal’s mode-based adaptive mixed-criticality model, which allows a task’s criticality and its importance to be specified independently from each other. A task’s importance is the criterion that determines its presence in different system modes. Meanwhile, the task’s criticality (reflected in its Safety Integrity Level (SIL) and defining the rules for its software development process), prescribes the degree of conservativeness for the task’s estimated WCET during schedulability testing. We indicate how such a task model can help resolve some of the perceived weaknesses of the Vestal model, in terms of how it is interpreted, and demonstrate how the existing scheduling tests for the classic variant’s of Vestal’s model can be mapped to the new task model essentially without changes.
- Evaluating P-NET message's response time with fixed priority queuing at application process levelPublication . Tovar, Eduardo; Vasques, Francisco; Burns, AlanP-NET is a multi-master fieldbus standard based on a virtual token passing scheme. In P-NET each master is allowed to transmit only one message per token visit. In the worst-case, the communication response time can be derived considering that, in each token cycle, all stations use the token to transmit a message. In this paper, we define a more sophisticated P-NET model, which considers the actual token utilisation. We then analyse the possibility of implementing a local priority-based scheduling policy to improve the real-time behaviour of P-NET.
- Global and Partitioned Multiprocessor Fixed Priority Scheduling with Deferred Pre-emptionPublication . Davis, Robert I.; Burns, Alan; Marinho, José; Nelis, Vincent; Petters, Stefan M.; Bertogna, MarkoThis article introduces schedulability analysis for global fixed priority scheduling with deferred preemption (gFPDS) for homogeneous multiprocessor systems. gFPDS is a superset of global fixed priority pre-emptive scheduling (gFPPS) and global fixed priority non-pre-emptive scheduling (gFPNS). We show how schedulability can be improved using gFPDS via appropriate choice of priority assignment and final non-pre-emptive region lengths, and provide algorithms which optimize schedulability in this way. Via an experimental evaluation we compare the performance of multiprocessor scheduling using global approaches: gFPDS, gFPPS, and gFPNS, and also partitioned approaches employing FPDS, FPPS, and FPNS on each processor.
- Supporting real-time distributed computer-controlled systems with multi-hop P-NET networksPublication . Tovar, Eduardo; Vasques, Francisco; Burns, AlanFieldbus 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.