Mema Roussopoulos
National and Kapodistrian University of Athens
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Publication
Featured researches published by Mema Roussopoulos.
international conference on data engineering | 2006
Peter R. Pietzuch; Jonathan Ledlie; Jeffrey Shneidman; Mema Roussopoulos; Matt Welsh; Margo I. Seltzer
To use their pool of resources efficiently, distributed stream-processing systems push query operators to nodes within the network. Currently, these operators, ranging from simple filters to custom business logic, are placed manually at intermediate nodes along the transmission path to meet application-specific performance goals. Determining placement locations is challenging because network and node conditions change over time and because streams may interact with each other, opening venues for reuse and repositioning of operators. This paper describes a stream-based overlay network (SBON), a layer between a stream-processing system and the physical network that manages operator placement for stream-processing systems. Our design is based on a cost space, an abstract representation of the network and on-going streams, which permits decentralized, large-scale multi-query optimization decisions. We present an evaluation of the SBON approach through simulation, experiments on PlanetLab, and an integration with Borealis, an existing stream-processing engine. Our results show that an SBON consistently improves network utilization, provides low stream latency, and enables dynamic optimization at low engineering cost.
ACM Transactions on Computer Systems | 2005
Petros Maniatis; Mema Roussopoulos; Thomas J. Giuli; David S. H. Rosenthal; Mary Baker
The LOCKSS project has developed and deployed in a world-wide test a peer-to-peer system for preserving access to journals and other archival information published on the Web. It consists of a large number of independent, low-cost, persistent Web caches that cooperate to detect and repair damage to their content by voting in “opinion polls.” Based on this experience, we present a design for and simulations of a novel protocol for voting in systems of this kind. It incorporates rate limitation and intrusion detection to ensure that even some very powerful adversaries attacking over many years have only a small probability of causing irrecoverable damage before being detected.
Mobile Computing and Communications Review | 1999
Petros Maniatis; Mema Roussopoulos; Edward Swierk; Kevin Lai; Guido Appenzeller; Xinhua Zhao; Mary Baker
People are the outsiders in the current communications revolution. Computer hosts, pagers, and telephones are the addressable entities throughout the Internet and telephony systems. Human beings, however, still need application-specific tricks to be identified, like email addresses, telephone numbers, and ICQ IDs. The key challenge today is to find people and communicate with them personally, as opposed to communicating merely with their possibly inaccessible machines---cell phones that are turned off or PCs on faraway desktops.We introduce the Mobile People Architecture which aims to put the person, rather than the devices that the person uses, at the endpoints of a communication session. We describe a prototype that performs person-level routing; the prototype allows people to receive communication regardless of the network, device, or application they use, while maintaining their privacy.
international conference on management of data | 1997
Nick Roussopoulos; Yannis Kotidis; Mema Roussopoulos
The data cube is an aggregate operator which has been shown to be very powerful for On Line Analytical Processing (OLAP) in the context of data warehousing. It is, however, very expensive to compute, access, and maintain. In this paper we define the “cubetree” as a storage abstraction of the cube and realize in using packed R-trees for most efficient cube queries. We then reduce the problem of creation and maintenance of the cube to sorting and bulk incremental merge-packing of cubetrees. This merge-pack has been implemented to use separate storage for writing the updated cubetrees, therefore allowing cube queries to continue even during maintenance. Finally, we characterize the size of the delta increment for achieving good bulk update schedules for the cube. The paper includes experiments with various data sets measuring query and bulk update performance.
international conference on computer communications | 1999
Guido Appenzeller; Mema Roussopoulos; Mary Baker
We are facing a growing user demand for ubiquitous Internet access. As result, network ports and wireless LANs are becoming common in public spaces inside buildings such as lounges, conference rooms and lecture halls. This introduces the problem of protecting networks accessible through these public ports from unauthorized use. In this paper, we study the problem of access control through public network ports. We view this problem as a special case of the more general problem of access control for a service on a network. We present an access control model on which we base our solution. This model has three components: authentication, authorization, and access verification. We describe the design and implementation of a system that allows secure network access through public network ports and wireless LANs. The design requires no special hardware or custom client software, resulting in minimal deployment cost and maintenance overhead. The system has a user-friendly, Web-based interface, offers good security, and scales to a campus-sized community.
international conference on data engineering | 2011
Konstantinos Tsakalozos; Herald Kllapi; Eva Sitaridi; Mema Roussopoulos; Dimitris Paparas; Alex Delis
Modern frameworks, such as Hadoop, combined with abundance of computing resources from the cloud, offer a significant opportunity to address long standing challenges in distributed processing. Infrastructure-as-a-Service clouds reduce the investment cost of renting a large data center while distributed processing frameworks are capable of efficiently harvesting the rented physical resources. Yet, the performance users get out of these resources varies greatly because the cloud hardware is shared by all users. The value for money cloud consumers achieve renders resource sharing policies a key player in both cloud performance and user satisfaction. In this paper, we employ microeconomics to direct the allotment of cloud resources for consumption in highly scalable master-worker virtual infrastructures. Our approach is developed on two premises: the cloud-consumer always has a budget and cloud physical resources are limited. Using our approach, the cloud administration is able to maximize per-user financial profit. We show that there is an equilibrium point at which our method achieves resource sharing proportional to each users budget. Ultimately, this approach allows us to answer the question of how many resources a consumer should request from the seemingly endless pool provided by the cloud.
international workshop on peer to peer systems | 2004
Mema Roussopoulos; Mary Baker; David S. H. Rosenthal; Thomas J. Giuli; Petros Maniatis; Jeffrey C. Mogul
In the hope of stimulating discussion, we present a heuristic decision tree that designers can use to judge how suitable a P2P solution might be for a particular problem. It is based on characteristics of a wide range of P2P systems from the literature, both proposed and deployed. These include budget, resource relevance, trust, rate of system change, and criticality.
Lecture Notes in Computer Science | 1998
Kevin Lai; Mema Roussopoulos; Diane Tang; Xinhua Zhao; Mary Baker
This paper presents results from an eight-day network packet-trace of MosquitoNet. MosquitoNet allows users of laptop computers to switch seamlessly between a metropolitan-area wireless network and a wired network (10 Mbit/s Ethernet) available in offices and on-campus residences. Results include the amount of user mobility between the wired and wireless networks, the amount of mobility within the wireless network, an examination of application end-to-end delays, and an examination of overall packet loss and reordering in the wireless network. We find that the average mobile host switches between the wired and wireless networks 14 times during the trace and moves within the wireless network five times. Round trip latencies in the wireless network are very high, with a minimum of 0.2 seconds. Even higher end-to-end delays, of up to hundreds of seconds, are due to packet loss and reordering. These delays cause users to change their usage patterns when connected to the wireless network. We conclude that latency is a critical problem in the wireless network.
Distributed Computing | 2006
Mema Roussopoulos; Mary Baker
This paper studies the problem of balancing the demand for content in a peer-to-peer network across heterogeneous peer nodes that hold replicas of the content. Previous decentralized load balancing techniques in distributed systems base their decisions on periodic updates containing information about load or available capacity observed at the serving entities. We show that these techniques do not work well in the peer-to-peer context; either they do not address peer node heterogeneity, or they suffer from significant load oscillations which result in unutilized capacity. We propose a new decentralized algorithm, Max-Cap, based on the maximum inherent capacities of the replica nodes. We show that unlike previous algorithms, it is not tied to the timeliness or frequency of updates, and consequently requires significantly less update overhead. Yet, Max-Cap can handle the heterogeneity of a peer-to-peer environment without suffering from load oscillations.
international conference on service oriented computing | 2011
Konstantinos Tsakalozos; Mema Roussopoulos; Alex Delis
Infrastructure-as-a-Service (IaaS) cloud providers often combine different hardware components in an attempt to form a single infrastructure. This single infrastructure hides any underlying heterogeneity and complexity of the physical layer. Given a non-homogeneous hardware infrastructure, assigning VMs to physical machines (PMs) becomes a particularly challenging task. VM placement decisions have to take into account the operational conditions of the cloud (e.g., current PM load) and load balancing prospects through VM migrations. In this work, we propose a service realizing a two-phase VM-to-PM placement scheme. In the first phase, we identify a promising group of PMs, termed cohort, among the many choices that might be available; such a cohort hosts the virtual infrastructure of the user request. In the second phase, we determine the final VM-to-PM mapping considering all low-level constraints arising from the particular user requests and special characteristics of the selected cohort. Our evaluation shows that in large non-homogeneous physical infrastructures, we significantly reduce the VM placement plan production time and improve plan quality.