Thanasis Loukopoulos

Survey on Grid Resource Allocation Mechanisms

Grid is a distributed high performance computing paradigm that offers various types of resources (like computing, storage, communication) to resource-intensive user tasks. These tasks are scheduled to allocate available Grid resources efficiently to achieve high system throughput and to satisfy user requirements. The task scheduling problem has become more complex with the ever increasing size of Grid systems. Even though selecting an efficient resource allocation strategy for a particular task helps in obtaining a desired level of service, researchers still face difficulties in choosing a suitable technique from a plethora of existing methods in literature. In this paper, we explore and discuss existing resource allocation mechanisms for resource allocation problems employed in Grid systems. The work comprehensively surveys Gird resource allocation mechanisms for different architectures (centralized, distributed, static or dynamic). The paper also compares these resource allocation mechanisms based on their common features such as time complexity, searching mechanism, allocation strategy, optimality, operational environment and objective function they adopt for solving computing- and data-intensive applications. The comprehensive analysis of cutting-edge research in the Grid domain presented in this work provides readers with an understanding of essential concepts of resource allocation mechanisms in Grid systems and helps them identify important and outstanding issues for further investigation. It also helps readers to choose the most appropriate mechanism for a given system/application.

Middleware Mechanisms for Agent Mobility in Wireless Sensor and Actuator Networks

This paper describes middleware-level support for agent mobility, targeted at hierarchically structured wireless sensor and actuator network applications. Agent mobility enables a dynamic deployment and adaptation of the application on top of the wireless network at runtime, while allowing the middleware to optimize the placement of agents, e.g., to reduce wireless network traffic, transparently to the application programmer. The paper presents the design of the mechanisms and protocols employed to instantiate agents on nodes and to move agents between nodes. It also gives an evaluation of a middleware prototype running on Imote2 nodes that communicate over ZigBee. The results show that our implementation is reasonably efficient and fast enough to support the envisioned functionality on top of a commodity multi-hop wireless technology. Our work is to a large extent platform-neutral, thus it can inform the design of other systems that adopt a hierarchical structuring of mobile components.

An overview of achieving energy efficiency in on-chip networks

Due to the increasing bandwidth demand for the network-on-chip (NoC), interconnection networks become a dominant source of energy consumption in systems-on-chip (SoCs) and chip multi processors (CMPs). Therefore, energy efficient NoC is key to a successful SoC development. This paper presents an overview of different techniques to achieve energy efficiency at the different levels of NoC design including: a) component level where dynamic voltage scaling (DVS) and dynamic link shutdown (DLS) techniques are reviewed; b) circuit level, e.g., voltage swinging of signals; c) architectural level, where specialised tools, such as Wattch and Orion are discussed. We also summarise research on thermal optimisation issues. To the best of our knowledge, this is the first survey of recent research results on the area.

Implementing Replica Placements: Feasibility and Cost Minimization

Given two replication schemes Xold and Xnew, the replica transfer scheduling problem (RTSP) aims at reaching Xnew, starting from Xold, with minimal implementation cost. In this paper we generalize the problem description to include special cases, where deadlocks can occur while in the process of implementing Xnew. We address this impediment by introducing artificial (dummy) transfers. We then prove that RTSP-decision is NP-complete and propose two kinds of heuristics. The first attempts to replace dummy transfers with valid ones, while the second minimizes the implementation cost. Experimental evaluation of the algorithms illustrates the merits of our approach.

GRAL: A Grouping Algorithm to Optimize Application Placement in Wireless Embedded Systems

Recent embedded middleware initiatives enable the structuring of an application as a set of collaborating agents deployed in the various sensing/actuating entities of the system. Of particular importance is the incurred cost due to agent communication which in terms depends on agent positions in the system. In this paper we present GRAL a grouping algorithm that migrates groups of agents with the aim of minimizing communication. The algorithm works in a distributed fashion based on knowledge available locally at each node and can be used both for one-shot initial application deployment and for the continuous updating of agent placement. Through simulation experiments under various scenarios we evaluate the algorithm, comparing the solution quality reached against the optimal obtained from exhaustive search.

Increasing the performance of CDNs using replication and caching: a hybrid approach

Caching and replication have emerged as the two primary techniques for reducing the delay experienced by end users when downloading Web pages. Even though these techniques may benefit from each other, previous research work tends to focus on either one of them separately. In this paper we investigate the potential performance gains by using a CDN server both as a replicator and as a proxy server. We assume a common storage space for both techniques, and develop an analytical model that characterizes caching performance under various system parameters. Based on the models predictions, we can reason whether it is beneficial to reduce the caching space in order to allocate extra replicas. The resulting problem of finding which object replicas should be created where, given that any free space is used for caching, is NP-complete. Therefore, we propose a hybrid heuristic algorithm (based on the greedy paradigm), in order to solve the combined replica placement and storage allocation problem. Our simulation results indicate that a simple LRU caching scheme can considerably improve the response time of HTTP requests, when utilized over a replication-based infrastructure.

Slice-based parallelization in HEVC encoding: Realizing the potential through efficient load balancing

The new video coding standard HEVC (High Efficiency Video Coding) offers the desired compression performance in the era of HDTV and UHDTV, as it achieves nearly 50% bit rate saving compared to H.264/AVC. To leverage the involved computational overhead, HEVC offers three parallelization potentials namely: wavefront parallelization, tile-based and slice-based. In this paper we study slice-based parallelization of HEVC using OpenMP on the encoding part. In particular we delve on the problem of proper slice sizing to reduce load imbalances among threads. Capitalizing on existing ideas for H.264/AVC we develop a fast dynamic approach to decide on load distribution and compare it against an alternative in the HEVC literature. Through experiments with commonly used video sequences, we highlight the merits and drawbacks of the tested heuristics. We then improve upon them for the case of Low-Delay by exploiting GOP structure. The resulting algorithm is shown to clearly outperform its counterparts achieving less than 10% load imbalance in many cases.

Single and Group Agent Migration: Algorithms, Bounds, and Optimality Issues

Recent embedded middleware platforms enable the structuring of an application as a set of collaborating agents deployed on various nodes of the underlying wireless sensor network (WSN). Of particular importance is the network cost incurred due to agent communication, which in turn depends on how the agents are placed within the WSN system. In this paper, we present two agent migration algorithms with the aim of minimizing the total network overhead. The first one takes independent single agent migration decisions, while the second one considers groups of agents for migration. Both algorithms work in a fully distributed fashion based on the knowledge available locally at each node, and can be used both for one-shot initial application deployment as well as for the continuous updating of agent placement. We also propose two methodologies to tackle the problem when WSN nodes have limited capacity. We show through theoretical analysis that one of our algorithms (called GRAL*) always results in an optimal placement, while for the rest of the algorithms, we derive approximation ratios pertaining to their performance. We evaluate the performance of our algorithms through a series of simulation experiments. Results show that group migration algorithms are superior compared to single agent migration algorithms with the performance difference reaching 34% for some settings.

Improved Genetic Algorithms and List Scheduling Techniques for Independent Task Scheduling in Distributed Systems

Given a set of tasks with certain characteristics, e.g., data size, estimated execution time and a set of processing nodes with their own parameters, the goal of task scheduling is to allocate tasks at nodes so that the total makespan is minimized. The problem has been studied under various assumptions concerning task and node parameters with the resulting problem statements usually being NP-complete. List scheduling (LS) heuristics such as MaxMin and MinMin together with genetic algorithms (GAs) were applied in the past to find solutions. In this paper we investigate new heuristics for both the LS and the GA paradigm with the specific aim of improving the performance of the standard algorithms when task computations involve large data transfers. Experimental results under various environment assumptions illustrate the merits of the new algorithms.

The xBR

Spatial indexes, such as those based on Quadtree, are important in spatial databases for efficient execution of queries involving spatial constraints. In this paper, we present improvements of the xBR-tree ai¾?member of the Quadtree family with modified internal node structure and tree building process, called xBR