Construction of a short path in high dimensional First Passage Percolation

For First Passage Percolation in Z^d with large d, we construct a path connecting the origin to {x_1 =1}, whose passage time has optimal order \log d/d. Besides, an improved lower bound for the "diagonal" speed of the cluster combined with a result by Dhar (1988) shows that the limiting shape in FPP with exponential passage times (and thus that of Eden model) is not the euclidian ball in dimension larger than 35

On the Convergence of Population Protocols When Population Goes to Infinity

Population protocols have been introduced as a model of sensor networks consisting of very limited mobile agents with no control over their own movement. A population protocol corresponds to a collection of anonymous agents, modeled by finite automata, that interact with one another to carry out computations, by updating their states, using some rules. Their computational power has been investigated under several hypotheses but always when restricted to finite size populations. In particular, predicates stably computable in the original model have been characterized as those definable in Presburger arithmetic. We study mathematically the convergence of population protocols when the size of the population goes to infinity. We do so by giving general results, that we illustrate through the example of a particular population protocol for which we even obtain an asymptotic development. This example shows in particular that these protocols seem to have a rather different computational power when a huge population hypothesis is considered.Comment: Submitted to Applied Mathematics and Computation. 200

Realis'2014: Reproductibilité expérimentale pour l'informatique en parallélisme, architecture et système

National audienceRealis 2014 : Reproductibilité expérimentale pour l'informatique en parallélisme, architecture et système - 2éme édition La capacité à reproduire les expériences est une condition nécessaire à une démarche scientifique solide. Dans les communautés de physique ou de bio-informatique qui utilisent intensivement l'outil informatique pour des simulations ou de la fouille de données, un mouvement vers une augmentation de la qualité expérimentale est en cours, via la diffusion des processus expérimentaux et des codes sources utilisés. Nos communautés de systèmes d'exploitations, de systèmes parallèles et distribués et d'architecture doivent également travailler dans cette direction, notamment pour adapter les solutions utilisées ailleurs à nos domaines : Comment devons-nous décrire une expérience pour permettre sa reproductibilité ? Quels sont les contenus (environnements, binaires, scripts, configurations) à conserver et diffuser ? Comment les résultats doivent-ils être fournis afin de permettre de s'y comparer ? Nous encourageons les auteurs soumettant un article à l'une des conférences de ComPAS (CFSE, RenPar ou SympA) à soumettre également un article à Realis. Ce deuxième article peut être une extension de l'article soumis à ComPAS, sans limitation de pages, afin de décrire précisément les expériences menées, et pourra s'appuyer sur des ressources externes : environnements, scripts, données nécessaires. Chaque article sera évalué (et les expériences seront, si possible, reproduites complètement ou partiellement) par un autre auteur sous la supervision d'au moins un membre du comité, afin de favoriser les partages d'expériences sur les bonnes pratiques -- les articles seront attribués en adéquation avec les compétences des auteurs dans la mesure du possible. Le comité décernera le label de reproductibilité expérimentale aux articles dont les expériences auront pû être reproduites de manière satisfaisante. Le prix ComPAS de la reproductibilité expérimentale sera décerné au meilleur article

FullSWOF: A free software package for the simulation of shallow water flows

Numerical simulations of flows are required for numerous applications, and are usually carried out using shallow water equations. We describe the FullSWOF software which is based on up-to-date finite volume methods and well-balanced schemes to solve this kind of equations. It consists of a set of open source C++ codes, freely available to the community, easy to use, and open for further development. Several features make FullSWOF particularly suitable for applications in hydrology: small water heights and wet-dry transitions are robustly handled, rainfall and infiltration are incorporated, and data from grid-based digital topographies can be used directly. A detailed mathematical description is given here, and the capabilities of FullSWOF are illustrated based on analytic solutions and datasets of real cases. The codes, available in 1D and 2D versions, have been validated on a large set of benchmark cases, which are available together with the download information and documentation at http://www.univ-orleans.fr/mapmo/soft/FullSWOF/.Comment: 38 page

A Branching-selection process related to censored Galton-Walton processes

We obtain the asymptotics for the speed of a particular case of a particle system with branching and selection introduced by B\'erard and Gou\'er\'e (2010). The proof is based on a connection with a supercritical Galton-Watson process censored at a certain level.Comment: 16 pages, 2 figure

A survey of general-purpose experiment management tools for distributed systems

International audienceIn the field of large-scale distributed systems, experimentation is particularly difficult. The studied systems are complex, often nondeterministic and unreliable, software is plagued with bugs, whereas the experiment workflows are unclear and hard to reproduce. These obstacles led many independent researchers to design tools to control their experiments, boost productivity and improve quality of scientific results. Despite much research in the domain of distributed systems experiment management, the current fragmentation of efforts asks for a general analysis. We therefore propose to build a framework to uncover missing functionality of these tools, enable meaningful comparisons be-tween them and find recommendations for future improvements and research. The contribution in this paper is twofold. First, we provide an extensive list of features offered by general-purpose experiment management tools dedicated to distributed systems research on real platforms. We then use it to assess existing solutions and compare them, outlining possible future paths for improvements

Linux-based virtualization for HPC clusters

International audienceThere has been an increasing interest in virtualization in the HPC community, as it would allow to easily and efficiently share computing resources between users, and provide a simple solution for checkpointing. However, virtualization raises a number of interesting questions, on performance and overhead, of course, but also on the fairness of the sharing. In this work, we evaluate the suitability of KVM virtual machines in this context, by comparing them with solutions based on Xen. We also outline areas where improvements are needed, to provide directions for future works

Moderate hypercapnia exerts beneficial effects on splanchnic energy metabolism during endotoxemia

Purpose: Low tidal volume ventilation and permissive hypercapnia are required in patients with sepsis complicated by ARDS. The effects of hypercapnia on tissue oxidative metabolism in this setting are unknown. We therefore determined the effects of moderate hypercapnia on markers of systemic and splanchnic oxidative metabolism in an animal model of endotoxemia. Methods: Anesthetized rats maintained at a PaCO2 of 30, 40 or 60mmHg were challenged with endotoxin. A control group (PaCO2 40mmHg) received isotonic saline. Hemodynamic variables, arterial lactate, pyruvate, and ketone bodies were measured at baseline and after 4h. Tissue adenosine triphosphate (ATP) and lactate were measured in the small intestine and the liver after 4h. Results: Endotoxin resulted in low cardiac output, increased lactate/pyruvate ratio and decreased ketone body ratio. These changes were not influenced by hypercapnia, but were more severe with hypocapnia. In the liver, ATP decreased and lactate increased independently from PaCO2 after endotoxin. In contrast, the drop of ATP and the rise in lactate triggered by endotoxin in the intestine were prevented by hypercapnia. Conclusions: During endotoxemia in rats, moderate hypercapnia prevents the deterioration of tissue energetics in the intestin