Predictable composition of memory accesses on many-core processors

International audienceThe use of many-core COTS processors in safety critical embedded systems is a challenging research topic. The predictable execution of several applications on those processors is not possible without a precise analysis and mitigation of the possible sources of interference. In this paper, we identify the external DDR-SDRAM and the Network on Chip to be the main bottlenecks for both average performance and predictability in such platforms. As DDR-SDRAM memories are intrinsically stateful, the naive calculation of the Worst-Case Execution Times (WCETs) of tasks involves a significantly pessimistic upper-bounding of the memory access latencies. Moreover, the worst-case end-to-end delays of wormhole switched networks cannot be bounded without strong assumptions on the system model because of the possibility of deadlock. We provide an analysis of each potential source of interference and we give recommendations in order to build viable execution models enabling efficient composable computation of worst-case end-to-end memory access latencies compared to the naive worst-case-everywhere approach

Predictable execution on many-core processors

Dans cette thèse, nous étudions l’adéquation de l’architecture distribuée des processeurs pluricoeurs avec les besoins des concepteurs de systèmes temps réels avioniques. Nous proposons d’abord une analyse détaillée d’un processeur sur étagère (COTS), le KALRAY MPPA®-256, et nous identifions certaines de ses ressources partagées comme étant les goulots d’étranglement limitant à la fois la performance et la prédictibilité lorsque plusieurs applications s’exécutent. Pour limiter l’impact de ces ressources sur les WCETs, nous définissons formellement un modèle d’exécution isolant temporellement les applications concurrentes. Son implantation est réalisée au sein d’un hyperviseur offrant à chaque application un environnement d’exécution isolé et assurant le respect des comportements attendus en ligne. Sur cette base, nous formalisons la notion de partition comme l’association d’une application avec un budget de ressources matérielles. Dans notre approche, les applications s’exécutant au sein d’une partition sont garanties d’être temporellement isolées des autres applications. Ainsi, étant donné une application et son budget associé, nous proposons d’utiliser la programmation par contraintes pour vérifier automatiquement si les ressources allouées à l’application sont suffisantes pour permettre son exécution de manière satisfaisante. Dans le même temps, dans le cas où un budget est effectivement valide, notre approche fournit un ordonnancement et un placement complet de l’application sur le sous-ensemble des ressources du processeurallouées à sa partition.In this thesis, we study the suitability of the distributed architecture of many-core processors for the design of highly constrained real-time systems as is the case in avionics. We firstly propose a thorough analysis of an existing COTS processor, namely the KALRAY MPPA®-256, and we identify some of its shared resources to be paths of interference when shared among several applications. We provide an execution model to restrict the access to these resources in order to mitigate their impact on WCETs and to temporally isolate co-running applications. We describe in detail how such an execution model can be implemented with a hypervisor which practically provides the expected property of temporal isolation at run-time. Based on this, we formalize a notion of partition which represents the association of an application with a resource budget. In our approach, an application placed in a partition is guaranteed to be temporally isolated from applications placed in other partitions. Then, assuming that applications and resource budgets are given,we propose to use constraint programming in order to verify automatically whether the amount of resources requested by a budget is sufficient to meet all of the application’s constraints. Simultaneously, when a budget is valid, our approach computes a schedule of the application on the subset of the processor’s resources allocated to it

Exécution prédictible sur processeurs pluri-cœurs

During the last 25 years, the need for computational power aboard aircrafts has been constantly growing. To support these evolutions, aircraft manufacturers need to bring massive computational power aboard in order to host more and more applications of increasing size.The emergence of promising technologies such as many-core processors thus appears as a good opportunity to tackle this challenge. Yet they also raise issues regarding predictable execution of software. In this context, we provide an end-to-end integration framework enabling to share and to leverage the parallel computational power of many-core processors safely. By doing so, we pave the way for the design of future embedded avionics computers based on many-core processors. More precisely, we propose a thorough analysis of an existing COTS processor, namely the Kalray MPPA-256, and we identify some of its hared resources to be paths of interference when shared among several applications. We provide an execution model to restrict the access to these resources in order to mitigate their impact on software execution times. We describe in detail how such an execution model can be implemented with a hypervisor which practically provides the expected property of temporal isolation at run-time. Based on this, we formalize a notion of partition which represents the association of an application with a resource budget. Since we aim to execute applications of industrial size, we automatically schedule them on their partitions' resources using Constraint Programming.Les besoins en puissance de calcul à bord des avions augmentent régulièrement depuis 25 ans. Pour accompagner ces évolutions, les avionneurs doivent concevoir des calculateurs toujours plus puissants devant héberger des applications toujours plus massives et nombreuses.L'émergence de technologies prometteuses telles que les processeurs pluri-cœurs semble ainsi être une bonne opportunité pour répondre à ces attentes mais présente également de nouveaux défis pour l'exécution prédictible de logiciel. Dans ce contexte, nous proposons un atelier d'intégration de bout en bout qui permet le partage et l'exploitation de la puissance de calcul parallèle d'une cible pluri-cœurs avec des applications contraintes, réalisant ainsi un premier pas vers la conception de calculateurs avioniques à base de processeurs pluri-cœurs. Plus précisément, nous proposons une analyse détaillée d'un processeur sur étagère, le Kalray MPPA-256, et identifions certaines de ses ressources partagées comme étant lespoints de contention réduisant la prédictibilité lors d'exécutions parallèles. Pour résoudre ce problème, nous définissons formellement un modèle d'exécution isolant temporellement les applications concurrentes. Son implantation est réalisée au sein d’un hyperviseur qui garantit le respect des comportements attendus en-ligne. Nous formalisons une notion de partition comme l'association d'une application avec un budget de ressources matérielles. Considérant des applications industrielles de grande taille, nous calculons automatiquement l'ordonnancement et le placement d'une application sur les ressources de sa partition en utilisant la programmation par contraintes

Assessment of the drag porosity approach to model the atmospheric boundary layer over urban canopies

International audienceThis present work details the assessment of the drag-porosity model implemented in ARPS (Advanced Regional Prediction System) atmospheric LES solver with comparison with literature and previous experimental results obtained from the URBANTURB project. The flow inside an idealized urban canopy consisting of a staggered array of cubes with a plan area density of 25% modelled with the drag-porosity approach immersed into a neutral atmospheric boundary layer at high Reynolds is investigated. Besides one-points statistics, particular interest was given to the assessment of turbulent coherent structures and their characteristic scales with literature. An analysis of spectrograms, one- and two-dimensional spectra revealed the presence of typical structures found in smooth- and rough-wall bounded turbulent flows. Further investigation to identify the interaction mechanisms between large and small scales based on wavelength spectral filtering highlighted a non-linear mechanism as observed in literature. The goal of this paper is to assess the capacity of the drag-porosity approach to compute turbulent flows in the roughness sublayer and surface layer.Le travail présenté ici se place dans un contexte d’étude des écoulements atmosphériques se développant au-dessus de canopées urbaines. Plus précisément, cette étude s’intéresse à l’évaluation du modèle de porosité-traînée implémenté dans le solveur atmosphérique au grandes-échelles ARPS (Advanced Regional Prediction System) avec la littérature et les résultats expérimentaux du projet URBANTURB. L’écoulement au-dessus d’une canopée urbaine idéalisée, se composant d’obstacles cubiques arrangés en quinconce avec une densité au sol de 25% modélisés par l’approche de porosité-traînée est étudié à travers des simulations atmosphériques neutres aux grandes échelles (LES) à haut Reynolds. En plus des statistiques en un point, un intérêt particulier à été dédié à l’évaluation des structures turbulentes cohérentes et de leur échelles caractéristiques, en comparaison avec la littérature. Une étude des spectrogrammes, et des spectres 1D et 2D ont révélé la présence de structures classiques caractéristiques d’écoulements turbulents se développant sur paroi lisse ou rugueuse. Un filtrage spectral des échelles les plus énergétiques a fait apparaître une interaction non-linéaire entre les petites et grosses structures. Le but de ce papier est d’étudier les capacités de l’approche porosité-trainée pour la reproduction d’un l’écoulement instationnaire au-dessus d’une canopée urbain

Benefits of precursor simulations with a drag-porosity model as inlet conditions for LES of atmospheric flow within urban canopy

International audienceThe study of wind flow characteristics within urban environments is of prior interest for climate, meteorology and air pollution issues. Previous works have shown that within the urban areas the turbulent transfers of momentum, heat and mass between the Atmospheric Boundary Layer (ABL) and the urban canopy are governed by the interactions between large- and small-scale turbulent structures. Hence, to accurately compute flow characteristics at the neighbourhood scale, it is required to compute canopy-generated turbulence, as well as the turbulent structures prevailing in the ABL. This wide range of scales presents a major issue when performing unsteady simulations of turbulent flows such as Large-Eddy Simulations (LES), since the flow field specified at the inlet conditions the downstream development of the flow. In ABL numerical models, the most common method is to use a multi-scale approach based on successive grid-nesting from mesoscale to the local scale. When performing multi-scale nesting to study urban canopy flows, the urban obstacles are generally explicitly taken into account in the smallest high-resolution domain only, larger domains being not refined enough for a realistic representation of the canopy elements. However, it has been shown that adding roughness elements in the larger domain had a noticeable impact on the flow dynamics and statistics within the small domain (Wiersema et al., Mon. Wea. Rev., 2020). This study addresses the problem of the generation of realistic unsteady atmospheric inflow conditions on an idealised urban-like canopy consisting of a staggered array of cubes of constant height and packing density. Here we investigate the performance of using drag-porosity based simulations as precursor calculations for obstacle-resolving LES. The objective is to show that by using a precursor simulation with a less-costly canopy modelling method as inlet, LES of ABL flows within and above a canopy of explicitly resolved obstacles is possible with minimum flow adjustment in the inlet region. The LES atmospheric model ARPS (Advanced Regional Prediction System) is used with an Immersed Boundary Method (IBM) to model the flow over building-like obstacles. In order to assess the proposed approach, three inflow conditions are explored and compared : (1) a periodic precursor calculation over a large domain covered by cube array modelled with IBM, (2) a synthetic turbulence inflow obtained from purposely deteriorating flow information of the periodic case (without modifying moments and spectra), and (3) a drag-porosity precursor calculation (in which the urban canopy is modelled as a porous media depending on canopy averaged morphological characteristics). To the authors' knowledge, the use of an urban-designed drag model as precursor calculation for an obstacle-resolving simulation is the first to be performed within a single atmospheric solver

Interactive Segmentation With Incremental Watershed Cuts

International audienceIn this article, we propose an incremental method for computing seeded watershed cuts for interactive image segmentation. We propose an algorithm based on the hierarchical image representation called the binary partition tree to compute a seeded watershed cut. We show that this algorithm fits perfectly in an interactive segmentation process by handling user interactions, seed addition or removal, in time linear with respect to the number of affected pixels. Run time comparisons with several state-of-the-art interactive and noninteractive watershed methods show that the proposed method can handle user interactions much faster than previous methods, thus improving the user experience on large images

Large-eddy simulation of the urban boundary layer using drag-porosity modeling

International audienceThis work details the assessment of the performance of the drag-porosity model implemented in ARPS(Advanced Regional Prediction System) atmospheric Large-Eddy Simulation (LES) solver for simulating theatmospheric boundary layer developing over the urban canopy with comparison with literature. The flowwithin and above an idealized urban canopy consisting of a staggered array of cubes with various packingdensities modeled with the drag-porosity approach immersed into a neutral, Coriolis-free atmospheric boundarylayer at high Reynolds is investigated. Besides one-points statistics, particular interest was given to the ability ofthe model to reproduce the turbulent coherent structures and their characteristic scales. A detailed analysis ofone-point statistics, one- and two-dimensional spectra and two-point correlation functions revealed the presenceof typical structures and features found in wall-bounded turbulent flows (two-scale behavior in the roughnesssublayer, ejections, sweeps, self-similar wall-attached large scale streaky motions, canopy-independent verylarge scale motions). Further investigation to identify the interaction mechanisms between large and smallscales based on spectral filtering highlighted an interaction mechanism that resembles an amplitude modulationprocess, as observed in literature on wall-bounded flows. These findings therefore show that the proposedapproach is able to reproduce all the key features of the flow developing over urban terrain

Examining word writing in handwriting and smartphone-writing: orthographic processing affects movement production in different ways

International audienceNew technological devices are changing the way we communicate. With the popularization of smartphones, some people spend more time writing on a phone than handwriting or typing on a keyboard. Does phone-writing change the way we process orthographic information? Does this affect movement production? In the present study, French participants had to write words in a spelling to dictation task. They wrote orthographically consistent and inconsistent short and long words. First, they had to write the words by hand in upper-case letters on a digitizer. One month later, they had to write the words on a smartphone. The results revealed that orthographic consistency affects the spelling processes in both handwriting and phone-writing. We observe more spelling errors for inconsistent words than consistent ones. When analyzing the movement production of the words that were spelled correctly, the data revealed that the timing of orthographic processing differs between the two ways of writing. Orthographic consistency seems to affect the time before movement initiation (latency data) in handwriting, especially in short words. In addition, once the participant starts to write, it also mediates movement production throughout the whole word, affecting the timing of the initial and final letters of the word. In phone-writing, orthographic consistency tends to modulate movement production at the end of the word. Inconsistent words require more processing time than consistent words, especially when they are long. These timing differences are not surprising, since the whole word writing process is much longer in handwriting than in phone-writing. We are preparing another phone-writing experiment in which we examine the implementation of word suggestions. With word suggestions, the spelling processes are no longer a mere recall of information on the letter components of a word. While writing the first letters, smartphones suggest words on top of the virtual keyboard to complete the target word before we write the last letters. This back and forth mechanism of writing letters, reading word suggestions and selecting one of them, radically changes the way we process orthographic information during word writing