Real-Time Fixed and Dynamic Priority Driven Scheduling Algorithms: Theory and Experience

Projet REFLECSThere are two main positions regarding real-time scheduling algorithms. The first is based on fixed priorities and the second makes use of dynamic priorities such as deadlines. These two approaches have never really been compared because the emphasis has always been on the ease of implementation rather than the efficiency of the algorithms and the complexity of the associated feasibility conditions. In addition to traditional real-time applications, we believe that starting to look at these two criteria will be very important in the perspective of providing admission control mechanisms and real-time guarantees on large distributed systems like the Internet network. To that end, our purpose is first to provide a general framework based, on the one hand, a representation of preemptive, real-time scheduling in an algebraic structure that enables us to evaluate the distance of the optimality of any scheduling algorithm ; and on the other hand, a consistent representation of the associated feasibility conditions that enables us to evaluate the number of basic operations. As a second step, considering several kinds of traffics, we initiate the comparison by a straight, but limited, application of our general framework. Our preliminary results will notably highlight, in the cases where deadlines are all greater than periods, that fixed priority schedulers (like deadline monotonic) behave as well as EDF while the worst-case response time analysis is less complex. The same remark is valid when the task sets are almost homogeneous but is in favor of EDF in the general case or when a simple feasibility analysis is needed. Therefore, it might be of interest, given a real-time scheduling context (spanning from small embedded system to large distributed system), to take into account these two extra criteria in order to find a right trade-off among several possible solutions

Ailleurs en Europe, ailleurs de l’Europe. Entretien avec Krzysztof Pomian

L’historien et philosophe revient sur son parcours biographique et intellectuel, et sur la manière dont il a compris la situation et les mutations de l’Europe orientale (et singulièrement de la Pologne) dans la place qu’elle occupe au sein de l’histoire, de la culture et de la géopolitique européennes. Il précise l’importance qu’il accorde à la notion, complexe, de frontière dans la définition (et la redéfinition permanente) de l’Europe. Il compare la pensée et l’écriture de l’histoire dans l’Occident chrétien à celles développées par l’historien musulman Ibn Khaldûn, et il définit sa conception de l’histoire globale. Enfin, Krzysztof Pomian s’interroge sur les « nouvelles frontières » de l’Europe, ses « nouveaux ailleurs », qui lui semblent surtout d’ordre social et anthropologique, davantage encore que strictement géopolitique

Morning peak period travel characteristics of a residential suburb in Cape Town during a school and holiday period : what lessons can we learn?

Thesis (MScEng)--University of Stellenbosch, 2008.ENGLISH ABSTRACT: It is argued that an understanding of variability is central to the modelling of travel behaviour and the assessment of policy impacts and is not the peripheral issue that it has often been considered. There is a growing need to assess multi-day data to assess the distribution of user charges for road pricing, or patterns of public transport usage as well as improve the ability to identify mechanisms behind travel behaviour for modelling purposes. Drawing on studies worldwide, in conjunction with a review of the literature, the thesis first examines the relevance for using multi-day data, then discusses the methodology and results of a five week survey undertaken specifically for this study, makes a comparison of the findings with that observed in other studies, and finally discusses issues relating to the application of the data and future research possibilities. Previous studies have shown that behaviour which makes up the daily travel pattern can be highly repetitious in nature but that observing an individual’s behaviour on a single day might not be representative of their routine travel and that this behaviour varies across demographic segments and driver gender. This paper examines day-to-day travel behaviour variability of a residential area, Summer Greens, located in Cape Town (South Africa) using a travel dataset collected recently in November/December 2006. The survey technique employed was the recording of numberplates of all exiting vehicles from 06:00 to 10:00, weekdays from Monday to Friday over a period of five weeks. A total of 5677 vehicles undergoing 44 743 trips was observed and analysed. This research replicates and extends previous work dealing with day-to-day variability in trip-making behaviour that was conducted with data collected by Del Mistro and Behrens (2006) in Buitengracht Street, Cape Town CBD, in July/August 2005. The present research extends the earlier work by including departure time variations as well as conducting the observations during both a school period (3 weeks) and consecutive holiday period (2 weeks). Further, the thesis presents a method to measure day-to-day variability using the available data surveyed. This thesis finds a considerable difference in school and holiday traffic volumes as expected, but that despite this, certain identical travel behaviour patterns (such as vehicle appearance frequencies, following week repeat proportions etc.) is observed during both these periods. It was found that the peak hour for both school and holiday periods occurred during the same time period and greater traffic volume variability was found to occur on Fridays during the holiday period than in the school period. Traffic volumes across all weeks appeared to decrease from Monday to Wednesday and “bounce back” on Thursday and Friday consistent with the findings of another international study. It was found that motorists exhibited more departure time freedom during the holiday period with average holiday departure times much later than during the school period. Departure times were also observed to gradually become later from Monday to Friday during both the school and holiday periods with Thursday and Friday departure times significantly different to the other weekdays. The proportion of unique vehicles observed was found to increase with time of day and the resulting impact of this on the effectiveness of Variable Message Sign (VMS) applications is also discussed. The research concludes by applying the findings to determine the impact of a hypothetical congestion pricing scheme on traffic volumes.AFRIKAANSE OPSOMMING: Die argument is dat ‘n begrip vir variasie sentraal staan tot die modellering van reisgedrag en die assessering van beleidsimiplikasies, en is dus nie net n nagedagte soos wat gereeld gedink word nie. Daar is ‘n toenemende behoefte om multi-dag data te analiseer om die verspreiding in gebruikerskostes te bestudeer vir die waardasie van paaie, of om verhoudings in publieke vervoergebruik uit te lig, asook, vir die verbetering van die vermoë om die meganismes agter reisgedrag te identifiseer vir modelleringsdoeleindes. Die tesis bestudeer eerstens die relevansie van die gebruik van multi-dag data in samewerking met die literatuurstudie, gebaseer op studies wêreldwyd onderneem. Tweedens bespreek dit die metodologieë en resultate van ‘n vyf-week lange opname wat spesifiek vir hierdie studie onderneem is en tref vergelykings met die resultate verkry deur vorige studies. Dit bespreek dan die probleme wat voortspruit uit die toepassing van die data en ook toekomstige moontlikhede tot verdere navorsing. Vorige studies het getoon dat die gedrag wat tydens daaglikse reispatrone voorkom van nature hoogs herhaaldelik is. Wanneer ‘n individu se gedrag observeer word op ‘n enkele dag is dit egter nie noodwendig verteenwoordigend van sy/haar roetine reisgedrag nie en dat hierdie gedrag afhanklik is van demografiese faktore en die geslag van die bestuurder. Hierdie tesis bespreek die variasie in dag-tot-dag reisgedrag van ‘n residensiële gebied, Summer Greens, in Kaapstad (RSA) deur gebruik te maak van ‘n datastel wat onlangs saamgestel is (November/Desember 2006). Die data is ingesamel deur ‘n opname te doen van die nommerplate van alle uitgaande voertuie tussen 06:00 en 10:00, weeksdae van Maandag tot Vrydag, oor ‘n periode van vyf weke. A totale aantal voertuie van 5677 wat 44 743 ritte onderneem het, is waargeneem en ‘n analise is uitgevoer. Die navorsing herhaal en brei uit op vorige werk wat die dag-tot-dag variasie in ritopwekking bestudeer. Del Mistro en Behrens (2006) het data bestudeer wat ingesamel is in Julie/Augustus 2005 in Buitengrachtstraat, in die Kaapse Middestad (RSA). Die huidige navorsing brei uit op hierdie werk deur die variasie in vertrektyd in te sluit asook om waarnemings te doen tydens die skoolperiode (3 weke) en die daaropvolgende vakansieperiode (2 weke). ‘n Metode word ook aangebied om die dag-tot-dag variasie te meet deur middel van die beskikbare data wat ingesamel is. Hierdie tesis vind ‘n merkwaardige verskil in die skool en vakansie verkeersvolumes, soos verwag kan word, maar ten spyte daarvan bestaan daar sekere identiese reisgedragspatrone (byvoorbeeld die frekwensies waarteen ‘n voertuig voorkom, weeklikse herhaling van proporsies, ens.) gedurende beide hierdie periodes. Die bevinding is dat die spitsuur vir skool- en vakansieperiodes gedurende dieselfde tydsperiode plaasvind en dat verkeersvolumes groter variasie toon op Vrydae gedurende die vakansieperiode, as die skoolperiode. Dit wil blyk of verkeersvolumes gedurende al die weke afneem van Maandag tot Woensdag en dan “terug spring” op Donderdae en Vrydae. Hierdie bevinding is in ooreenstemming met resultate van ‘n ander internasionale studie. Motoriste het ‘n neiging getoon tot groter vryheid in terme van vertrektye tydens die vakansieperiode, met die gemiddelde vakansie vertrektye heelwat later as gedurende die skoolperiode. Vertrektye is ook gevind om geleidelik later te word van Maandag tot Vrydag gedurende beide die skool en vakansieperiodes, met Donderdag en Vrydag se vertrektye wat merkwaardig verskil van ander weeksdae. Die verhouding uniek waargenome voertuie het volgens hierdie navorsing se bevindinge toegeneem met die tyd van die dag en die gevolglike impak hiervan op die effektiwiteit van sogenaamde “Vehicle Message Sign” toepassings word bespreek. Die navorsing sluit dan af deur die bevindinge toe te pas om die impak te bepaal wat ‘n hipotetiese kongestieprysskema op verkeersvolumes het

Video data collection method for pedestrian movement variables & development of a pedestrian spatial parameters simulation model for railway station environments

Thesis (PhD)--Stellenbosch University, 2012.ENGLISH ABSTRACT: The design of railway station environments in South Africa and to a certain extent internationally, is based on rules of thumb. These rules, using general macroscopic principles for determining peak passenger loads are inadequate and misleading for detailed design purposes. The principles advocated in local design guideline documents are erroneous and ignore the highly variable flow nature or “micro-peaking” effects that typically occur within railway station environments. Furthermore, there are no procedures proposed in these guideline documents, which leads to ambiguous assessment techniques used by practitioners in the determination of pedestrian spatial areas. It is evident that the knowledge in the area of pedestrian movement contained within the design guidance is far from comprehensive. Without a reliable method for estimating pedestrian levels-of-service and capacities, design of new facilities does not follow a uniform process, resulting in high levels of uncertainty in determining if the time, money and resources invested in upgrading facilities will actually cater to the demand. The situation is further exacerbated by current industry thinking towards pedestrian modelling in South Africa, where it is perceived by both clients and practitioners to be more cost effective to use macroscopic techniques and designing infrastructure according to a “one-level-up” level-of-service method. Working with architects confirmed that the area of circulation design was lacking in data and guidance and that associated quantified assessments of pedestrian movement was rarely, if ever, carried out. Towards addressing these issues, the development of a Spatial Parameter (SP)-model spreadsheet application became the main objective of the study. The model contributes towards addressing the needs of individual station users based on the trade-off between level-of-service and infrastructure costs. The output of the model allows the designer to avoid the under-provision (detrimental to operations) and oversizing of railway station infrastructure (with obvious financial implications). The author recognised the lack of pedestrian movement data in South Africa and addressed this by conducting extensive video-based pedestrian observations aimed at exploring the macroscopic fundamental relationships and the ways in which these relationships might be influenced by the various personal, situational and environmental factors that characterise the context in which pedestrians move. The movement trajectories of 24,410 pedestrians were investigated over three infrastructure environments at Maitland and Bonteheuwel stations in Cape Town, carefully selected to incorporate the cultural diversity common in South Africa. Tracking of pedestrians was achieved via the use of an in-house developed “video annotator” software tool. Boarding and alighting rates of 7,426 passengers were also observed at these stations incorporating contributory attributes such as age, gender, body size, encumbrance, group size, time of day, and location. The research makes a number of significant advances in the understanding of pedestrian flow behaviour within railway station environments and provides recommendations to industry of what issues to consider. The empirical study has provided comprehensive pedestrian movement characteristics incorporating the relationships between density, speed and flow including the effect of culture and other context factors unique to the local South African environment. New methods for determining spatial requirements are proposed, together with new and unique empirical data for use by the local industry. A calibrated spreadsheet SP-model for assessing the design of concourse type railway stations is developed and presented in the study. The advance in local pedestrian flow knowledge, together with the SP-model, is shown to be practical through application to two real railway station case study projects. The results of this study constitute an important contribution to local pedestrian flow knowledge and is considered a valuable resource for those developing pedestrian models in practice. It is expected that the results will be useful in the planning and design of pedestrian environments in South African railway stations and can be applied to other African metro railway stations with similar pedestrian characteristics. Overall, this research has succeeded in advancing the approach to railway station design, empirical data, knowledge and methods held within the local engineering industry. However, the contribution of this study and associated conference papers is an early step in changing the perceptions in this country towards ensuring fully informed and appropriate performance-based spatial designs.AFRIKAANSE OPSOMMING: Die ontwerp van areas binne Suid-Afrikaanse spoorweg stasies en ook tot ‘n sekere mate internasionaal, is gebaseer op historiese ondervindings asook riglyne wat tans in die praktyk gebruik word. Die riglyne gebruik algemene makroskopiese beginsels om die spits passasiersvrag te bepaal vir gedetaileerde ontwerp doeleindes. Hierdie riglyne is egter ongeskik en misleidend aangesien dit nie die hoogs wispelturige natuur van vloei en mikrospits effekte wat binne die stasies plaasvind, in ag nie. Die riglyne ontbreek ook van prosedures wat gevolg moet word vir die bepaling van ruimtelike areas vir voetgangers wat die gevolg het dat dubbelsinnige beramingstegnieke deur praktisyne gebruik word. Die kennis oor voetganger bewegings in die ontwerp riglyne is nie omvattend genoeg nie. Sonder ‘n betroubare beramings metode vir die bepaling van voetganger diensvlak en kapasiteit kan daar nie bepaal word of die tyd, geld en hulpbronne wat in die fasilitieit geinvesteer word, aan die behoeftes gaan voldoen nie. Die situasie word verder vererger deur die huidige persepsie oor voetganger modellering in Suid-Afrika, waar dit deur beide kliënte en praktisyne, as ‘n meer koste effektiewe oplossing gesien word om makroskopiese tegnieke te gebruik en om infrastruktuur te ontwerp volgens ‘n metode waar ‘n hoër diensvlak as die teiken diensvlak gebruik word. In samewerking met argitekte is dit bevestig dat die area van sirkulasie ontwerp ‘n tekort het aan data en riglyne en dat die kwantitatiewe skattings verbonde aan voetganger beweging selde, indien ooit, uitgevoer word. Die ontwikkeling van ‘n Spatial Parameters (SP)-model om die bogenoemde problem te oorkom, is die hoofdoel van hierdie tesis. Die model poog om die behoeftes van individuele stasie gebruikers aan te spreek gebaseer op die wisselwerking tussen diensvlak en infrastruktuur kostes. Die uitsette van die model stel die ontwerper in staat om ondervoorsiening en oorvoorsiening van spoorweg stasie infrastruktuur te voorkom wat nadelige vir die bedryf is en ook ooglopende finansiële implikasies tot gevolg het. Die skrywer het die tekort aan data aangaande voetganger bewegings in Suid-Afrika geidentifiseer en dit aangespreek deur omvattende video gebaseerde voetganger waarnemings te maak met die doel om die basiese makroskopiese verhoudings te ondersoek asook in hoe ‘n mate hierdie verhoudings beinvloed word deur verskeie persoonlike, liggings- en omgewingsfaktore wat die konteks waarin voetgangers beweeg, karakteriseer. Die bewegingsprofiel van 24,410 voetgangers is ondersoek by drie infrastruktuur omgewings by Maitland en Bonteheuwel stasies in Kaapstad. Die stasies is noukeurig uitgesoek om Suid-Afrika se kulturele diversiteit te verteenwoordig. Die voetgangers is nagevolg deur gebruik te maak van ‘n selfontwikkelde video-annoteerder sagteware. Waarneming van die opklim- en afklimspoed van 7,426 passasiers is gemaak by hierdie stasies en faktore soos ouderdom, geslag, liggaamsgrootte, mobiliteit, grootte van groepe, tyd van die dag en ligging was ingesluit by die waarnemings. Hierdie navorsing maak belangrike bydraes tot die begrip van die vloei van voetgangers binne spoorweg stasies en aanbevellings word aan die industrie gemaak oor die faktore wat in ag geneem moet word by ontwerp van fasilitieite. Die empiriese studie het omvattende voetganger beweging karakteristieke uitgewys wat die verhoudings tussen digtheid, spoed en vloei inkorporeer asook die effek van kultuur en ander faktore wat verband hou met die unieke konteks van die plaaslike Suid-Afrikaanse omgewing. Nuwe metodes om ruimtelike-vereistes te bepaal word voorgestel, saam met nuwe en unieke empiriese data vir gebruik deur die plaaslike industrie. ‘n Gekalibreerde en gevalideerde SP-model is ontwikkel om die ontwerp van spoorweg stasies te assesseer en word in hierdie tesis beskyf en aangebied. Die studie toon dat akkurate data en kennis oor plaaslike voetganger vloei met die SP-model verkry kan word, soos bewys uit twee spoorweg stasie studiegevalle. Die resultate van hierdie tesis dien as ‘n belangrike bydrae tot die kennis van plaaslike voetganger vloei en word geag as ‘n waardevolle hulpbron vir die ontwikkeling van voetganger modelle in die praktyk. Hierdie resultate mag nuttig wees gedurende die beplanning en ontwerp van voetganger-areas in Suid-Afrikaanse spoorweg stasies. Dit kan ook toegepas word vir spoorweg stasies in die res van Afrika wat soortgelyke voetganger karaktereienskappe het. Die navorsing het daarin geslaag om die benadering tot spoorweg stasie ontwerp te verbeter, asook om empiriese data, kennis en die metodes wat binne die plaaslike ingenieurs industrie voorgehou word, te verbeter. Let egter daarop dat die bydrae wat hierdie tesis maak, asook bydraes deur relevante konferensie verhandelinge, ‘n vroeë stap is in die verandering van persepsies in Suid-Afrika om geskikte prestasie-gebaseerde ruimte ontwerpe te verseker

Characterization of the Space of Feasible Worst-Case Execution Times for Earliest-Deadline-First scheduling

International audienc

A Norm Approach for the Partitioned EDF Scheduling of Sporadic Task Systems

International audienceIn this paper, we propose a new approach for the partitioned Earliest Deadline First (EDF) scheduling of sporadic task systems. We consider the case of constrained task deadlines where the deadlines of the tasks are less than or equal to their periods. We introduce the concept of the EDF norm, for defining the space of WCET values that result in schedulable systems given fixed periods and relative deadlines. Based on this concept, it is possible to derive a necessary and sufficient feasibility condition to check whether EDF scheduling is valid for a given partitioning. The EDF norm has interesting convexity properties that permit using a Linear Programming approach to reduce the number of points at which the EDF norm needs to be checked. From the EDF norm, we derive a New Worst Fit Decreasing partitioning heuristic and compare its performance with two existing partitioning heuristics based on density partitioning and demand bound function approximation. We then compare the performance of the heuristic in terms of the resource augmentation paradigm

Real-Time Fixed And Dynamic Priority Driven Scheduling Algorithms: Theory And Experience

There are two main positions regarding real-time scheduling algorithms. The first is based on fixed priorities and the second makes use of dynamic priorities such as deadlines. These two approaches have never really been compared because the emphasis has always been on the ease of implementation rather than the efficiency of the algorithms and the complexity of the associated feasibility conditions. In addition to traditional real-time applications, we believe that starting to look at these two criteria will be very important from the point of view of providing admission control mechanisms and real-time guarantees on large distributed systems like the Internet network. To that end, our purpose is first to provide a general framework based, on the one hand, a representation of preemptive, real-time scheduling in an algebraic structure that enables us to evaluate the distance of the optimality of any scheduling algorithm ; and on the other hand, a consistent representation of the assoc..

Performance des Modèles de Similitude et de Prédiction Appliqués aux Affaiblissements Atmosphériques Mesurés en bande EHF

http://ursi-france.institut-telecom.fr/pages/pages_evenements/journees_scient/docs_journees_2009/data/articles/000046.pdfInternational audienceUne expérience de propagation en bande EHF a été réalisée dans le cadre du programme Syracuse 3, nouvelle génération de système militaire français de SATCOM. L'originalité de l'expérience réside dans les fréquences utilisées (20 GHz /44 GHz) et sur l'angle d'élévation relativement bas de la liaison (17°). Les données recueillies pendant la période 2006-2008 au CELAR à Rennes sont utilisées pour cette étude qui comprend deux parties. La première est dédiée à l'analyse statistique et la modélisation des affaiblissements, du coefficient de similitude et des durées d'évènements. L'utilisation par des services opérationnels de la bande EHF nécessite la mise au point de techniques de lutte contre les affaiblissements (FMT). Parmi celles-ci, cette présentation s'intéresse à l'estimation des conditions de propagation sur la liaison montante (à la fréquence la plus élevée) à partir de l'affaiblissement observé sur la liaison descendante quelques seconde plus tôt (typiquement 3-10s) ABSTRACT An Earth-to-satellite propagation experiment operating at EHF band was set up within the framework of the Syracuse 3 program. Syracuse 3 satellites are the new generation of French military SATCOM system. This experiment is a good opportunity to make attenuation measurements and increase our knowledge of tropospheric propagation effects on EHF links at a low elevation angle (17°). This paper presents yearly statistics concerning attenuation level, frequency scaling ratio and fade durations. A comparison with ITU models is given. Fade Mitigation Techniques on Earth-satellite links require the estimation of channel propagation conditions. This is generally done by using downlink measurements at a lower frequency to derive the channel uplink propagation conditions at higher frequencies. A new short term prediction algorithm, allows an accurate upper-bound of the future attenuation to be estimated in real time, which minimizes the cost of Fade Mitigation Techniques and therefore enables the communication system to reach an appropriate percentage of availability. Its originality is that not only the attenuation level, but also the error conditional distribution are predicted. This new model has been tested on Syracuse 3 attenuation measurements and is shown to outperform significantly other existing models

Statistical analysis of the Syracuse 3 satellite EHF propagation experiment

International audienceAn Earth-to-satellite propagation experiment operating in the EHF band was set up as part of the Syracuse 3 program. The Syracuse 3 satellites are the new generation of French military SATCOM systems. This experiment is a good opportunity to carry out attenuation measurements and increase knowledge about tropospheric propagation effects on EHF links at a low elevation angle (17deg). At these frequencies (20 & 44 GHz), the attenuation due to the troposphere can be very strong. This paper presents yearly statistics of attenuation level, frequency scaling ratio and fade durations. A comparison with ITU models is given