A Survey of Timing Verification Techniques for Multi-Core Real-Time Systems

This survey provides an overview of the scientific literature on timing verification techniques for multi-core real-time systems. It reviews the key results in the field from its origins around 2006 to the latest research published up to the end of 2018. The survey highlights the key issues involved in providing guarantees of timing correctness for multi-core systems. A detailed review is provided covering four main categories: full integration, temporal isolation, integrating interference effects into schedulability analysis, and mapping and allocation. The survey concludes with a discussion of the advantages and disadvantages of these different approaches, identifying open issues, key challenges, and possible directions for future research

Analyse temporelle des systèmes temps-réels sur architectures pluri-coeurs

Predictability is of paramount importance in real-time and safety-critical systems, where non-functional properties --such as the timing behavior -- have high impact on the system's correctness. As many safety-critical systems have agrowing performance demand, classical architectures, such as single-cores, are not sufficient anymore. One increasinglypopular solution is the use of multi-core systems, even in the real-time domain. Recent many-core architectures, such asthe Kalray MPPA, were designed to take advantage of the performance benefits of a multi-core architecture whileoffering certain predictability. It is still hard, however, to predict the execution time due to interferences on sharedresources (e.g., bus, memory, etc.).To tackle this challenge, Time Division Multiple Access (TDMA) buses are often advocated. In the first part of thisthesis, we are interested in the timing analysis of accesses to shared resources in such environments. Our approach usesSatisfiability Modulo Theory (SMT) to encode the semantics and the execution time of the analyzed program. To estimatethe delays of shared resource accesses, we propose an SMT model of a shared TDMA bus. An SMT-solver is used to find asolution that corresponds to the execution path with the maximal execution time. Using examples, we show how theworst-case execution time estimation is enhanced by combining the semantics and the shared bus analysis in SMT.In the second part, we introduce a response time analysis technique for Synchronous Data Flow programs. These are mappedto multiple parallel dependent tasks running on a compute cluster of the Kalray MPPA-256 many-core processor. Theanalysis we devise computes a set of response times and release dates that respect the constraints in the taskdependency graph. We derive a mathematical model of the multi-level bus arbitration policy used by the MPPA. Further,we refine the analysis to account for (i) release dates and response times of co-runners, (ii)task execution models, (iii) use of memory banks, (iv) memory accesses pipelining. Furtherimprovements to the precision of the analysis were achieved by considering only accesses that block the emitting core inthe interference analysis. Our experimental evaluation focuses on randomly generated benchmarks and an avionics casestudy.La prédictibilité est un aspect important des systèmes temps-réel critiques. Garantir la fonctionnalité de ces systèmespasse par la prise en compte des contraintes temporelles. Les architectures mono-cœurs traditionnelles ne sont plussuffisantes pour répondre aux besoins croissants en performance de ces systèmes. De nouvelles architectures multi-cœurssont conçues pour offrir plus de performance mais introduisent d'autres défis. Dans cette thèse, nous nous intéressonsau problème d’accès aux ressources partagées dans un environnement multi-cœur.La première partie de ce travail propose une approche qui considère la modélisation de programme avec des formules desatisfiabilité modulo des théories (SMT). On utilise un solveur SMT pour trouverun chemin d’exécution qui maximise le temps d’exécution. On considère comme ressource partagée un bus utilisant unepolitique d’accès multiple à répartition dans le temps (TDMA). On explique comment la sémantique du programme analyséet le bus partagé peuvent être modélisés en SMT. Les résultats expérimentaux montrent une meilleure précision encomparaison à des approches simples et pessimistes.Dans la deuxième partie, nous proposons une analyse de temps de réponse de programmes à flot de données synchroness'exécutant sur un processeur pluri-cœur. Notre approche calcule l'ensemble des dates de début d'exécution et des tempsde réponse en respectant la contrainte de dépendance entre les tâches. Ce travail est appliqué au processeur pluri-cœurindustriel Kalray MPPA-256. Nous proposons un modèle mathématique de l'arbitre de bus implémenté sur le processeur. Deplus, l'analyse de l'interférence sur le bus est raffinée en prenant en compte : (i) les temps de réponseet les dates de début des tâches concurrentes, (ii) le modèle d'exécution, (iii) les bancsmémoires, (iv) le pipeline des accès à la mémoire. L'évaluation expérimentale est réalisé sur desexemples générés aléatoirement et sur un cas d'étude d'un contrôleur de vol

Many-Core Timing Analysis of Real-Time Systems

La prédictibilité est un aspect important des systèmes temps-réel critiques. Garantir la fonctionnalité de ces systèmespasse par la prise en compte des contraintes temporelles. Les architectures mono-cœurs traditionnelles ne sont plussuffisantes pour répondre aux besoins croissants en performance de ces systèmes. De nouvelles architectures multi-cœurssont conçues pour offrir plus de performance mais introduisent d'autres défis. Dans cette thèse, nous nous intéressonsau problème d’accès aux ressources partagées dans un environnement multi-cœur.La première partie de ce travail propose une approche qui considère la modélisation de programme avec des formules desatisfiabilité modulo des théories (SMT). On utilise un solveur SMT pour trouverun chemin d’exécution qui maximise le temps d’exécution. On considère comme ressource partagée un bus utilisant unepolitique d’accès multiple à répartition dans le temps (TDMA). On explique comment la sémantique du programme analyséet le bus partagé peuvent être modélisés en SMT. Les résultats expérimentaux montrent une meilleure précision encomparaison à des approches simples et pessimistes.Dans la deuxième partie, nous proposons une analyse de temps de réponse de programmes à flot de données synchroness'exécutant sur un processeur pluri-cœur. Notre approche calcule l'ensemble des dates de début d'exécution et des tempsde réponse en respectant la contrainte de dépendance entre les tâches. Ce travail est appliqué au processeur pluri-cœurindustriel Kalray MPPA-256. Nous proposons un modèle mathématique de l'arbitre de bus implémenté sur le processeur. Deplus, l'analyse de l'interférence sur le bus est raffinée en prenant en compte : (i) les temps de réponseet les dates de début des tâches concurrentes, (ii) le modèle d'exécution, (iii) les bancsmémoires, (iv) le pipeline des accès à la mémoire. L'évaluation expérimentale est réalisé sur desexemples générés aléatoirement et sur un cas d'étude d'un contrôleur de vol.Predictability is of paramount importance in real-time and safety-critical systems, where non-functional properties --such as the timing behavior -- have high impact on the system's correctness. As many safety-critical systems have agrowing performance demand, classical architectures, such as single-cores, are not sufficient anymore. One increasinglypopular solution is the use of multi-core systems, even in the real-time domain. Recent many-core architectures, such asthe Kalray MPPA, were designed to take advantage of the performance benefits of a multi-core architecture whileoffering certain predictability. It is still hard, however, to predict the execution time due to interferences on sharedresources (e.g., bus, memory, etc.).To tackle this challenge, Time Division Multiple Access (TDMA) buses are often advocated. In the first part of thisthesis, we are interested in the timing analysis of accesses to shared resources in such environments. Our approach usesSatisfiability Modulo Theory (SMT) to encode the semantics and the execution time of the analyzed program. To estimatethe delays of shared resource accesses, we propose an SMT model of a shared TDMA bus. An SMT-solver is used to find asolution that corresponds to the execution path with the maximal execution time. Using examples, we show how theworst-case execution time estimation is enhanced by combining the semantics and the shared bus analysis in SMT.In the second part, we introduce a response time analysis technique for Synchronous Data Flow programs. These are mappedto multiple parallel dependent tasks running on a compute cluster of the Kalray MPPA-256 many-core processor. Theanalysis we devise computes a set of response times and release dates that respect the constraints in the taskdependency graph. We derive a mathematical model of the multi-level bus arbitration policy used by the MPPA. Further,we refine the analysis to account for (i) release dates and response times of co-runners, (ii)task execution models, (iii) use of memory banks, (iv) memory accesses pipelining. Furtherimprovements to the precision of the analysis were achieved by considering only accesses that block the emitting core inthe interference analysis. Our experimental evaluation focuses on randomly generated benchmarks and an avionics casestudy

Efficient Execution of Dependent Tasks on Many-Core Processors

International audienceThe increasing performance requirements of safety-critical real-time embedded systems made traditional single-corearchitectures obsolete. Moving to more complex many-core systems requires new techniques and tools for the certificationof the embedded software. Timing and functional behaviours are subject to specific requirements of certification guidelinessuch as D-178B/C for avionics and ISO26262 for automotive systems. Determinism and predictability of such systems are amajor challenge. Running tasks should be known in advance which makes the static and non-preemptive scheduling a suitableapproach to reach an optimal execution with a guarantee of a certain degree of determinism. Recent work on mapping andscheduling problems [1], [2], [3] consider a known value (or a set of possible values) of the Worst-Case Response Time (WCRT)and computes a mapping that optimizes a predefined cost function. When the response time analysis is too pessimistic, thestatic scheduling may introduce an idle time which reduces the core utilization. Scheduling techniques must rely on tight estimations of the WCRT which in turn depends on co-runner tasks. However, inorder to obtain a tight upper-bound on the response time, a mapping and scheduling should be known in advance. Indeed, theresponse time is highly influenced by the co-runner tasks. Concurrent accesses to the same shared resource may introduceinterferences that should be accounted for in the response time analysis. The search for an optimal scheduling with a tightWCRT analysis that includes the shared resource interferences is a challenging open problem

Response time analysis of synchronous data flow programs on a many-core processor

International audienceIn this paper we introduce a response time analysis technique for Synchronous Data Flow programs mapped to multipleparallel dependent tasks running on a compute cluster of the Kalray MPPA-256 many-core processor. The analysis we derivecomputes a set of response times and release dates that respect the constraints in the task dependency graph. We extend the Multicore Response Time Analysis (MRTA) framework by deriving a mathematicalmodel of the multi-level bus arbitration policy used by the MPPA. Further, we refine the analysis to account for therelease dates and response times of co-runners, and the use of memory banks. Further improvements to the precision ofthe analysis were achieved by splitting each task into two sequential phases, with the majority of the memory accessesin the first phase, and a small number of writes in the second phase. Our experimental evaluation focused on anavionics case study. Using measurements from the Kalray MPPA-256 as a basis, we show that the new analysis leads toresponse times that are a factor of 4.15 smaller for this application, than the default approach of assumingworst-case interference on each memory access

WCET analysis in shared resources real-time systems with TDMA buses

International audiencePredictability is an important aspect in real-time and safety-critical systems, where non-functional properties – such as the timing behavior – have high impact on the system cor-rectness. As many safety-critical systems have a growing performance demand, simple, but outdated architectures are not sufficient anymore. Instead, multi-core systems are more and more popular, even in the real-time domain. To combine the performance benefits of a multi-core architecture with the required predictability, Time Division Multiple Access (TDMA) buses are often advocated. In this paper, we are interested in accesses to shared resources in such environments. Our approach uses SMT (Satisfiability Modulo Theory) to encode the semantics and execution time of the analyzed program in an environment with shared resources. We use an SMT-solver to find a solution that corresponds to the execution path with correct semantics and maximal execution time. We propose to model a shared bus with TDMA arbitration policy. Using examples, we show how the WCET estimation is enhanced by combining the semantics and the shared bus analysis in SMT

Using execution graphs to model a prefetch and write buffers and its application to the Bostan MPPA

International audienceVerifying the temporal properties of critical systems embedded in vehicles, like planes or cars, is crucial to avoid catastrophic issues. A key component of this verification is the Worst Case Execution Time (WCET) of the programs composing these systems. A common and sound approach to compute WCET is based on static analysis of the programs that requires, in turn, to precisely model the behavior and the timings of the hardware. Processor-specific features such as pipelines, caches, and buffers influence the hardware performances significantly. Hence taking processor features into account when estimating WCET is essential. Modeling the processor's features formally to ensure safe and accurate estimation is then a must. In this paper, we present the methodology applied to capture the behavior of prefetch and write buffers of the Kalray Bostan MPPA microprocessor, and to incorporate the established models with the Execution Graph (XG) to obtain WCET estimation. These analyses are then applied to the Mälardalen benchmark suite and the experimentation results validate the feasibility of our approach

A Survey of Timing Verification Techniques for Multi-Core Real-Time Systems

This survey provides an overview of the scientific literature on timing verificationtechniques for multi-core real-time systems. It reviews the key results in the fieldfrom its origins around 2006 to the latest research published up to the end of July2018. The survey highlights the key issues involved in providing guarantees oftiming correctness for multi-core systems. A detailed review is provided coveringfour main categories: full integration, temporal isolation, integrating interferenceeffects into schedulability analysis, and mapping and allocation. The survey concludeswith a discussion of the advantages and disadvantages of these different approaches,identifying open issues, key challenges, and possible directions for futureresearch.TR-2018-9info:eu-repo/semantics/nonPublishe