On-line speed scaling minimizing expected energy consumption for real-time tasks, by Stephan Plassart (Polaris)
On-line speed scaling minimizing expected energy consumption for real-time tasks, by Stephan Plassart (Polaris)
– September 28, 2017
We present a Markov Decision Process (MDP) approach to compute the optimal on-line speed scaling policy to minimize the energy consumption of a processor executing a finite or infinite set of jobs with real-time constraints. The policy is computed off-line but used on-line. We provide several qualitative properties of the optimal policy: monotonicity with respect to the jobs parameters, comparison with on-line deterministic algorithms. Numerical experiments show that our proposition performs well when compared with off-line optimal solutions and out-performs on-line solutions oblivious to statistical information on the jobs.
The Head and tail random process appears, since the beginning of the art of computation, as a fundamental part of computer science. This talk will explore at a very basic level several ideas about computation of numbers, evaluation of quantities, checking techniques or recommendation evaluation. By small examples, we’ll try to establish links through ages between the probabilistic and algorithmic thinking.
One of the key features of small-worlds is the ability to route messages with few hops only using local knowledge of the topology. In 2000, Kleinberg proposed a model based on an augmented grid that asymptotically exhibits such property.
In this paper, we propose to revisit the original model from a simulation-based perspective. Our approach is fueled by a new algorithm that can draw an augmenting link in Õ(1).
The resulting speed gain enables detailed numerical evaluations. We show for example that in practice, the augmented scheme proposed by Kleinberg is more robust than predicted by the asymptotic behavior, even for very large finite grids. We also propose tighter bounds on the performance of Kleinberg's routing algorithm. At last, we show that, fed with realistic parameters, the model gives results in line with real-life experiments.
On-line speed scaling minimizing expected energy consumption for real-time tasks, by Stephan Plassart (Polaris)
– September 28, 2017
We present a Markov Decision Process (MDP) approach to compute the optimal on-line speed scaling policy to minimize the energy consumption of a processor executing a finite or infinite set of jobs with real-time constraints. The policy is computed off-line but used on-line. We provide several qualitative properties of the optimal policy: monotonicity with respect to the jobs parameters, comparison with on-line deterministic algorithms. Numerical experiments show that our proposition performs well when compared with off-line optimal solutions and out-performs on-line solutions oblivious to statistical information on the jobs.
Seminar Takai Kennouche : LTE/WiFi Coexistence: a WiFi interference-evasion approach
– October 9, 2017
LTE/WiFi Coexistence: a WiFi interference-evasion approach
Abstract
The growing spectrum crunch has motivated exploratory efforts in the use of LTE in the 5GHz ISM bands for downlink traffic. However, this paradigm raises concerns of fair sharing of the spectrum and the adverse impact of scheduled LTE frames on Wi-Fi Packet Success Rates (PSR). To address this issue, we propose an interference-evasion mechanism that allows Wi-Fi devices to survive LTE transmissions without any cooperation between these two different standards. Different from existing approaches, we argue that the simple use of Almost Blank Subframes (ABS) within the LTE standard offering short channel access windows overestimates opportunities for Wi-Fi. The pilots embedded in the ABS not only interfere with Wi-Fi but also adversely impact the carrier sensing function. E-Fi mitigates this problem through a two-fold approach. It uses a combination of (i) Wi-Fi Direct with packet relaying and (ii) classical distributed coordination function to reach distant nodes. Second, it ensures load balancing for both Wi-Fi uplink and downlink traffic with high PSR by creating node-groups with dedicated contention-based medium access intervals. Our approach is validated by comprehensive simulation and experimental results that indicate significantly higher throughput in our approach compared to classical Wi-Fi.
Asymptotic Studies of Large Distributed systems with Failures by Philippe Robert (Inria, Paris)
– October 19, 2017
Asymptotic Studies of Large Distributed systems with Failures
The qualitative behavior of a large scale storage network of non reliable file servers is investigated. In such systems a fraction of the processing capacity can be used to duplicate files on new servers when necessary. Due to random losses, with probability 1 all files will be lost eventually if no new files are added. The main question in this context is of understanding the relation between the durability of the system and the parameters of the network: the duplication capacity, the failure rate, the duplication factor and the average load of a node. Different scalings via mean-field limits or stochastic averaging can be used to study the eventual decay of these networks. They will be reviewed and discussed. An analysis of placement algorithms will be also presented.
Association dans les réseaux WiFi, par Isabelle Guerin (Lyon)
– October 26, 2017
Les déploiements de réseaux WiFi denses permettent à une station d’avoir le choix entre plusieurs points d’accès lors de la connexion au réseau WiFi, améliorant ainsi la couverture radio, la qualité des liens radio et facilitant la mobilité. En contrepartie, la densité des points d’accès peut générer plus d’interférences et de contention, ce qui peut amener à une baisse du débit global puisque ces points d’accès doivent partager un nombre limité de canaux. Le retour à une gestion centralisée des réseaux WiFi offre l’opportunité d’une optimisation globale de l’usage de la ressource radio et donc de réduire les problèmes soulevés lors d’un partage intensif de cette dernière.
Dans cet exposé, je vous montrerai, qu’en modifiant l’étape d’association dans les réseaux WiFi (qui est dans les interfaces WiFi actuelles basée sur le RSSI -Received Signal Strength Indicator-), il est possible d’améliorer le débit global du réseau ainsi que l’équité entre les stations.
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