Experimental Analysis on Autonomic Strategies for Cloud Elasticity

International audienceIn spite of the indubitable advantages of elasticity in Cloud infrastructures, some technical and conceptual limitations are still to be considered. For instance , resource start up time is generally too long to react to unexpected workload spikes. Also, the billing cycles' granularity of existing pricing models may incur consumers to suffer from partial usage waste. We advocate that the software layer can take part in the elasticity process as the overhead of software reconfigurations can be usually considered negligible if compared to infrastructure one. Thanks to this extra level of elasticity, we are able to define cloud reconfigurations that enact elasticity in both software and infrastructure layers so as to meet demand changes while tackling those limitations. This paper presents an autonomic approach to manage cloud elasticity in a cross-layered manner. First, we enhance cloud elasticity with the software elasticity model. Then, we describe how our au-tonomic cloud elasticity model relies on dynamic selection of elasticity tactics. We present an experimental analysis of a subset of those elasticity tactics under different scenarios in order to provide insights on strategies that could drive the autonomic selection of the proper tactics to be applied

Towards QoS-Oriented SLA Guarantees for Online Cloud Services

International audienceCloud Computing provides a convenient means of remote on-demand and pay-per-use access to computing resources. However, its ad hoc management of quality-of-service and SLA poses significant challenges to the performance, dependability and costs of online cloud services. The paper precisely addresses this issue and makes a threefold contribution. First, it introduces a new cloud model, the SLAaaS (SLA aware Service) model. SLAaaS enables a systematic integration of QoS levels and SLA into the cloud. It is orthogonal to other cloud models such as SaaS or PaaS, and may apply to any of them. Second, the paper introduces CSLA, a novel language to describe QoS-oriented SLA associated with cloud services. Third, the paper presents a control-theoretic approach to provide performance, dependability and cost guarantees for online cloud services, with time-varying workloads. The proposed approach is validated through case studies and extensive experiments with online services hosted in clouds such as Amazon EC2. The case studies illustrate SLA guarantees for various services such as a MapReduce service, a cluster-based multi-tier e-commerce service, and a low-level locking service

Exploring a Mediterranean mesozooplankton 13 year time-series.

Zooplankton plays diverse crucial roles within the marine ecosystem and can also be used as bio- indicator of climate changes since it is very sensitive to environmental changes. Therefore it is essential to consider long-term plankton series. Given the analysis of plankton samples is time- consuming, it requires an effective and rapid analytical method. We have used in this work a supervised learning approach adapted for the semi-automatic classification of digital images of the mesozooplankton of the Bay of Calvi (Corsica, France) by using the Zoo/PhytoImage software. Together with a 11-years long zooplankton time-series, a set of nine environmental variables were monitored in order to identify controlling factors and determine whether the communities were sensitive to global environmental changes. The main components of the mesozooplankton community were characterized by both seasonal and inter-annual variability. Additionally, variation of holoplankton and meroplankton differentiated one from each other. The holoplanktonic community could be split into two subgroups according to its variation in function of the environment: cladocerans and appendicularians, and to a lesser extent, copepods on one hand, and cnidarians, chaetognathes and thaliaceans, on the other hand. Regarding inter-annual variation, one year (2007) showed particularly low production of total zooplankton which was also the case for all the different holoplanktonic taxa. Accounting for that phenomenon were identified some potentially underlying environmental factors. Finally, although water temperature increased significantly over the last years along with the frequency of marine heat wave events, no evident change in the global zooplankton composition was observed yet.STARECAPME

Un algorithme équitable d'exclusion mutuelle distribuée avec priorité

National audienceLes algorithmes distribués d'exclusion mutuelle à priorité permettent de définir un ordre d'accès aux sections critiques protégeant des ressources partagées. Ces algorithmes sont très utiles dans les applications temps-réel ou pour assurer différents niveaux de qualité de service [6]. Cependant, la prise en compte des priorités peut conduire à des famines dans le cas où des demandes de sections critique les plus prioritaires empêchent la satisfaction des moins prioritaires. Pour palier ce problème, certains algorithmes comme celui de Kanrar-Chaki proposent d'incrémenter progressivement les priorités des requêtes pendantes mais ceci peut conduire à une violation de l'ordre des priorités. Ainsi, pour minimiser ces violations sans engendrer de famine et de surplus de messages, nous proposons des modifications de l'algorithme de Kanrar-Chaki pour ralentir la fréquence d'incrémentation des priorités. Nos évaluations des performances confirment l'efficacité de notre approche

Centrality-Based Eventual Leader Election in Dynamic Networks

International audienceThis paper presents CEL, a new distributed eventual leader election algorithm for dynamic networks, which exploits topological information to improve the choice of a central leader and reduce message exchanges. The algorithm has a crosslayer neighbors detection, with a neighbor-aware mechanism, to improve the sharing of topological knowledge and elect a central leader faster. It uses a self-pruning mechanism based on topological knowledge, combined with probabilistic gossip, to improve the performance of broadcast propagation. Evaluations were conducted on the OMNeT++ environment, simulating realistic MANET with interference, collision, and messages loss. Using different parameters values, we have compared CEL to Gómez-Calzado et al. algorithm [1], on the Random Walk and the Truncated Lévy Walk mobility models. The results show better performances than [1], including fewer messages sent, shortest paths to the leader, and a more stable algorithm

Experimental Analysis on Autonomic Strategies for Cloud Elasticity

International audienceIn spite of the indubitable advantages of elasticity in Cloud infrastructures, some technical and conceptual limitations are still to be considered. For instance , resource start up time is generally too long to react to unexpected workload spikes. Also, the billing cycles' granularity of existing pricing models may incur consumers to suffer from partial usage waste. We advocate that the software layer can take part in the elasticity process as the overhead of software reconfigurations can be usually considered negligible if compared to infrastructure one. Thanks to this extra level of elasticity, we are able to define cloud reconfigurations that enact elasticity in both software and infrastructure layers so as to meet demand changes while tackling those limitations. This paper presents an autonomic approach to manage cloud elasticity in a cross-layered manner. First, we enhance cloud elasticity with the software elasticity model. Then, we describe how our au-tonomic cloud elasticity model relies on dynamic selection of elasticity tactics. We present an experimental analysis of a subset of those elasticity tactics under different scenarios in order to provide insights on strategies that could drive the autonomic selection of the proper tactics to be applied

A fair starvation-free prioritized mutual exclusion algorithm for distributed systems

International audienceSeveral distributed mutual exclusion algorithms define the order in which requests are satisfied based on the priorities assigned to requests. These algorithms are very useful for real-time applications ones or those where priority is associated to a quality of service requirement. However, priority based strategies may result in starvation problems where high priority requests forever prevent low priority ones to be satisfied. To overcome this problem, many priority-based algorithms propose to gradually increase the priority of pending requests. The drawback of such an approach is that it can violate priority-based order of requests leading to priority inversion. Therefore, aiming at minimizing the number of priority violations without introducing starvation, we have added some heuristics in Kanrar-Chaki priority-based token-oriented algorithm in order to slow down the frequency with which priority of pending requests is increased. Performance evaluation results confirm the effectiveness of our approach when compared to both the original Kanrar-Chaki and Chang's priority-based algorithms

Est-ce la fin de la crise des scolytes ? Suivi du phénomène par télédétection

L’équipe de Gestion des ressources forestières de Gembloux Agro-Bio Tech a réalisé des cartes annuelles de l'état sanitaire de la pessière en Wallonie et dans le Grand-Est pour les années 2017 à 2022, au départ d’images satellitaires, dans l’objectif de retracer l’évolution temporelle des dégâts de scolytes et de mettre en lumière les facteurs prédisposant l’épicéa aux attaques de ces insectes. Les analyses des cartes d’état sanitaire ont montré que les pertes d'épicéas ne sont pas uniformément reparties. Elles sont particulièrement importantes dans les régions de plaines où les dégâts ont causé la perte de la moitié des peuplements. Considérant les prévisions climatiques des prochaines décennies, la plantation d'épicéas en Wallonie ne semble plus opportune en dehors de l’Ardenne en dessous de 400 m, à l'exception de certaines stations forestières particulièrement favorables à l’épicéa. Par ailleurs, on peut raisonnablement émettre l’hypothèse que l’année 2021 a marqué la fin de la crise avec un retour en 2022 à un niveau d'attaque de scolytes endémique similaire à l'année 2017.Het Team van het Beheer van de bosbouwrijkdommen van Agro-Bio Tech van Gembloers heeft jaarkaarten opgemaakt van de gezondheidstoestand van de sparrenbossen in Wallonië en in de Franse regio Grand-Est voor de jaren 2017-2022, vertrekkend van satellietbeelden, om zo een zicht te krijgen op de evolutie gedurende die periode van de door de schorskever (letterzetter) aangerichte schade, en aan het licht te brengen welke factoren de fijnspar kwetsbaar maken voor de aanvallen door dit insect. Uit een analyse van de kaarten blijkt dat de fijnsparren niet overal gelijkmatig werden aangetast. De aantasting van de fijnsparren is het grootst in de laagvlakten, waar de schade heeft geleid tot het verlies van de helft van de bestanden. Gelet op de klimaatvoorspellingen voor de komende decennia, lijkt het niet langer een goed idee om nog fijnsparren aan te planten in Wallonië buiten de Ardennen beneden de 400 m, met uitzondering van bepaalde bossites die bijzonder geschikt zijn voor de fijnspar. Daarenboven kunnen we redelijkerwijze stellen dat het jaar 2021 het einde inluidde van de crisis met een terugkeer in 2022 naar een niveau van endemische aanvallen van de letterzetter vergelijkbaar met 2017

Structural Changes of Seagrass Seascapes Driven by Natural and Anthropogenic Factors: A Multidisciplinary Approach

Seascape ecology has been widely applied to marine habitats, including seagrass meadows, through various approaches all over the world for the past 30 years. However, these methods mainly study seagrass meadows on a single spatial scale and monitor a single driver of heterogeneity. Additionally, few assess the seascape's structural evolution. This creates gaps between the scientific data provided and those required by environmental managers and stakeholders in charge of seagrass meadow conservation. To meet their expectations, in this paper we developed a new multidisciplinary approach based on the coupling of mapping techniques, particle flux, and biometric investigations in a Mediterranean Bay, the Calvi Bay (Corsica, France), to assess the structural changes of Posidonia oceanica (L.) Delile meadows subject to disturbances. We focused our investigations on the structural characteristics, the spatial dynamics, and the particle fluxes of natural sand areas generated by bottom currents and dead matte patches which ensued from anchoring damages at 10, 15, and 20 m depth. Natural sand patches and anchoring patches differed in size, the first the largest. They also displayed different erosion-colonization dynamics. Natural sand patches were eroded at a mean speed of 12 cm.a−1 and colonized at a rate of 7 cm.a−1. Anchoring patches showed a mean erosion speed of 3.5 cm.a−1 and a colonization rate of 6.5 cm.a−1. Regarding particle fluxes, continuous meadow, and natural patch sedimentation and resuspension rates were 3.7 gDW.m−2.d−1 and 4.1 gDW.m−2.d−1 in average, respectively. In contrast, anchoring patches at 20 m depth acted as sediment traps (112.60 gDW.m−2.d−1 in winter) and showed a higher particle resuspension rate. Our results highlighted the dichotomous dynamics of seagrass seascapes influenced by natural and anthropogenic factors. Thus, the smallest anchoring patch will take about 27 years to be recolonized while the biggest requires 60 years to be covered by the plant. With an upscaling approach, together with the newest mapping tools of marine habitats, we suggest a new method to study the evolution of seagrass meadows at a large spatial scale