Les environnements numériques d’apprentissage (ENA) : État des lieux et prospective

Rapport d’analyse et de synthès

Transport haute-performance: problématique et solutions proposées

L'univers du calcul haute-performance a découvert des limitations du protocole TCP, l'empêchant d'utiliser pleinement les liens haut-débit et grande distance prévus pour former l'ossature des grilles de calcul. Cet article présente ces problèmes de performance, puis décrit, analyse et compare les solutions actuellement proposées par la communauté réseau.RES-

A influência da diferenciação das relações líder-membro no comprometimento com a unidade de trabalho: o papel mediador do clima de apoio

We investigated whether the influence of LMX differentiation (the extent to which the quality of the leader-member relationships within a work unit varies) on work unit commitment is mediated by support climate. We tested this mediated relationship in a sample composed of 30 health care units. The results obtained showed that LMX differentiation measured at Time 1 was negatively related to work unit support climate measured one year later (Time 2), which in turn was positively related to work unit commitment measured at Time 2. The negative indirect effect of LMX differentiation on work unit commitment through support climate was statistically significant. Our study contributes to having a better understanding of the role of LMX differentiation in work unit functioning.Investigámos se a influência da diferenciação de LMX (o quanto a qualidade das relações de líder-membro dentro de uma unidade de trabalho varia) no comprometimento com a unidade de trabalho é mediada pelo clima de apoio. Testámos essa relação mediada em uma amostra composta por 30 unidades de saúde. Os resultados obtidos mostraram que a diferenciação da LMX medida no Tempo 1 foi negativamente relacionada ao clima de suporte da unidade de trabalho medido um ano depois (Tempo 2), que por sua vez foi positivamente relacionado ao comprometimento com a unidade de trabalho medido no Tempo 2. O efeito indireto negativo da diferenciação da LMX no comprometimento com a unidade de trabalho por meio do clima de apoio foi estatisticamente significativo. O nosso estudo contribui para uma melhor compreensão do papel da diferenciação do LMX no funcionamento da unidade de trabalho

Towards a User-Oriented Benchmark for Transport Protocols Comparison in very High Speed Networks

Standard TCP faces some performance limitations in very high speed wide area networks, mainly due to a long end-to-end feedback loop and a conservative behaviour with respect to congestion. Many TCP variants have been proposed to overcome these limitations. However, TCP is a complex protocol with many user-configurable parameters and a range of different implementations. It is then important to define measurement methods so that the transport services and protocols can evolve guided by scientific principles and compared quantitatively. The goal of this report is to present some steps towards a user-oriented benchmark, called ITB, for high speed transport protocols comparison. We first present and analyse some results reported in the literature. From this study we identify classes of representative applications and useful metrics. We then isolate infrastructure parameters and traffic factors which influence the protocol behaviour. This enable us to define scenario capturing and synthesising comprehensive and useful properties. We finally illustrate this proposal by preliminary results obtained on our experimental environment, Grid'5000, we have built and are using for contributing in this benchmark design

Impact of the Correlation between Flow Rates and Durations on the Large-Scale Properties of Aggregate Network Traffic

Since the discovery of long-range dependence in network traffic in 1993, many models have appeared to reproduce this property, based on heavy-tailed distributions of some flow-scale properties of the traffic. However, none of these models consider the correlation existing between flow rates and flow durations. In this work, we extend previously proposed models to include this correlation. Based on a planar Poisson process setting, which describes the flow-scale traffic structure, we analytically compute the auto-covariance function of the aggregate traffic's bandwidth and show that it exhibits long-range dependence with a different Hurst parameter. In uncorrelated case, the model that we propose is consistent with existing models, and predict the same Hurst parameter. We also prove that pseudo long-range dependence with a different index can arise from highly variable flow rates. The pertinence of our model choices is validated on real web traffic traces

An Efficient Network API for in-Kernel Applications in Clusters

International audienceRunning parallel applications on clusters with high-speed local networks requires fast communication between computing nodes but also low latency and high bandwidth file access. However, the application programming interfaces of high-speed local networks were designed for MPI communication and do not always meet the requirements of other applications like distributed file systems. In this paper, we explore several solutions to improve the use of high-speed network for in-kernel applications. Distributed file systems implemented on top of the GM interface of Myrinet are first examined to demonstrate how hard it is to get an efficient interaction between such applications and the network. Then, we propose solutions to simplify and improve this interaction and integrate them into the kernel interface of the new Myrinet. Performance comparisons between MX and GM, and their usage in both a distributed file system and a zero-copy protocol show nice improvements. Moreover, we are able to improve the performance of the flexible kernel API we designed in MX that allows to remove some intermediate copy

Executing distributed applications on virtualized infrastructures specified with the VXDL language and managed by the HIPerNET framework

International audienceWith the convergence of computing and communication, and the expansion of cloud computing, new models and tools are needed to allow users to define, create, and exploit on-demand virtual infras- tructures within wide area distributed environments. Optimally design- ing customized virtual execution-infrastructure and executing them on a physical substrate remains a complex problem. This paper presents the VXDL language, a language for specifying and describing virtual infras- tructures and the HIPerNET framework to manage them. Based on the example of a specific biomedical application and workflow engine, this paper illustrates how VXDL enables to specify different customized vir- tual infrastructures and the HIPerNET framework to execute them on a distributed substrate. The paper presents experiments of the deploy- ment and execution of this application on different virtual infrastructures managed by our HIPerNet system. All the experiments are performed on the Grid'5000 testbed substrate

Document d'aide au déploiement d'IPv6 sur Grid5000

Ce document présente succinctement les motivations pour l'utilisation d'IPv6 dans les grilles. Une analyse des opérations nécessaires pour son déploiement dans la plateforme Grid5000 est ensuite présentée et détaillée

We Trust You! A Multilevel-Multireferent Model Based on Organizational Trust to Explain Performance

This study tests organizational trust as the psychosocial mechanism that explains how healthy organizational practices and team resources predict multilevel performance in organizations and teams, respectively. In our methodology, we collect data in a sample of 890 employees from 177 teams and their immediate supervisors from 31 Spanish companies. Our results from the multilevel analysis show two independent processes predicting organizational performance (return on assets, ROA) and performance ratings by immediate supervisors, operating at the organizational and team levels, respectively. We have found evidence for a theoretical and functional quasiisomorphism. First, based on social exchange theory, we found evidence for our prediction that when organizations implement healthy practices and teams provide resources, employees trust their top managers (vertical trust) and coworkers (horizontal trust) and try to reciprocate these benefits by improving their performance. Second, (relationships among) constructs are similar at different levels of analysis, which may inform HRM officers and managers about which type of practices and resources can help to enhance trust and improve performance in organizations. The present study contributes to the scarce research on the role of trust at collective (i.e., organizational and team) levels as a psychological mechanism that explains how organizational practices and team resources are linked to organizational performance