Department of Computer Science Series of publications

 

Recent Submissions

  • Rissanen, Jorma; Myllymäki, Petri; Roos, Teemu; Santhanam, Narayana Prasad (University of Helsinki, Department of Computer Science, 2014)
  • Fagerholm, Fabian; Paasivaara, Maria; Jedlitschka, Andreas; Kuvaja, Pasi; Kuhrmann, Marco; Männistö, Tomi; Münch, Jürgen; Raatikainen, Mikko (University of Helsinki, Department of Computer Science, 2014)
  • Suomalainen, Lauri; Nikkhouy, Emad; Ding, Aaron Yi; Tarkoma, Sasu (University of Helsinki, Department of Computer Science, 2014)
    Software-Defined Networking (SDN) is a novel solution to network configuration and management. Its openness and programmability features have greatly motivated the open source communities where numerous applications and tools are developed for various R&D purposes. For the strength of SDN, the upcoming 5th Generation mobile networks (5G) can also benefit from the modular and open design to innovate the network architecture and services. In this report, we present a survey of existing open source platforms, applications and tools for SDN and 5G research. We discuss the potential directions and share our perspectives in this domain.
  • Schaeffer, Satu Elisa (University of Helsinki, Department of Computer Science, 2014)
    In Spring 2014, a small group of students at University of Helsinki took on the task of adapting and applying usability-evaluation techniques for evaluating four different types of augmented-reality applications. This report combines their final reports so that other students, researchers, and IT professionals around the world facing similar situations can draw from their experiences and findings. The course was instructed by the editor of this work. ACM Computing Classification System (CCS): H.5 [Human-centered computing] I.3.2 [Computing methodologies]
  • Järvinen, Ilpo; Chemmagate, Binoy; Ding, Aaron Yi; Daniel, Laila; Kojo, Markku (University of Helsinki, Department of Computer Science, 2012)
    This experimental study analyzes the effects of larger TCP initial window on competing interactive media and Web traffic in a larger number of cellular access configurations. In addition, we analyze the effect of shorter initial RTO on TCP performance in cellular access configurations. Both simulation and real network experiments were conducted. The initial window of ten segments reduces TCP elapsed times when the number of flows is small enough, however, with large number of flows it introduces losses that require TCP timeout. The initial RTO change from three to one second improves elapsed time in limited number of configurations, but in other cellular configurations spurious timeouts trigger almost alway during TCP three-way handshake due to the lower timeout.
  • Norta, Alex (2011)
    The management and coordination of business-process collaboration experiences changes because of globalization, specialization, and innovation. Service-oriented computing (SOC) is a means towards businessprocess automation and recently, many industry standards emerged to become part of the service-oriented architecture (SOA) stack. In a globalized world, organizations face new challenges for setting up and carrying out collaborations in semi-automating ecosystems for business services. For being efficient and effective, many companies express their services electronically in what we term business-process as a service (BPaaS). Companies then source BPaaS on the fly from third parties if they are not able to create all service-value inhouse because of reasons such as lack of reasoures, lack of know-how, cost- and time-reduction needs. Thus, a need emerges for BPaaS-HUBs that not only store service offers and requests together with information about their issuing organizations and assigned owners, but that also allow an evaluation of trust and reputation in an anonymized electronic service marketplace. In this paper, we analyze the requirements, design architecture and system behavior of such a BPaaS-HUB to enable a fast setup and enactment of business-process collaboration. Moving into a cloud-computing setting, the results of this paper allow system designers to quickly evaluate which services they need for instantiationg the BPaaS-HUB architecture. Furthermore, the results also show what the protocol of a backbone service bus is that allows a communication between services that implement the BPaaS-HUB. Finally, the paper analyzes where an instantiation must assign additional computing resources vor the avoidance of performance bottlenecks.
  • Bertsekas, Dimitri P.; Huizhen, Yu (2010)
    We consider the classical finite-state discounted Markovian decision problem, and we introduce a new policy iteration-like algorithm for finding the optimal Q-factors. Instead of policy evaluation by solving a linear system of equations, our algorithm requires (possibly inexact) solution of a nonlinear system of equations, involving estimates of state costs as well as Q-factors. This is Bellman's equation for an optimal stopping problem that can be solved with simple Q-learning iterations, in the case where a lookup table representation is used; it can also be solved with the Q-learning algorithm of Tsitsiklis and Van Roy [TsV99], in the case where feature-based Q-factor approximations are used. In exact/lookup table representation form, our algorithm admits asynchronous and stochastic iterative implementations, in the spirit of asynchronous/modified policy iteration, with lower overhead and/or more reliable convergence advantages over existing Q-learning schemes. Furthermore, for large-scale problems, where linear basis function approximations and simulation-based temporal difference implementations are used, our algorithm resolves effectively the inherent difficulties of existing schemes due to inadequate exploration.
  • Huizhen, Yu (2010)
    We consider approximate policy evaluation for finite state and action Markov decision processes (MDP) with the least squares temporal difference algorithm, LSTD(λ), in an explorationenhanced off-policy learning context. We establish for the discounted cost criterion that the off-policy LSTD(λ) converges almost surely under mild, minimal conditions. We also analyze other convergence and boundedness properties of the iterates involved in the algorithm. Our analysis draws on theories of both finite space Markov chains and weak Feller Markov chains on topological spaces. Our results can be applied to other temporal difference algorithms and MDP models. As examples, we give a convergence analysis of an off-policy TD(λ) algorithm and extensions to MDP with compact action and state spaces.
  • Bertsekas, Dimitri P.; Yu, Huizhen (2010)
    We consider the classical nite-state discounted Markovian decision problem, and we introduce a new policy iteration-like algorithm for fi nding the optimal Q-factors. Instead of policy evaluation by solving a linear system of equations, our algorithm requires (possibly inexact) solution of a nonlinear system of equations, involving estimates of state costs as well as Q-factors. This is Bellman's equation for an optimal stopping problem that can be solved with simple Q-learning iterations, in the case where a lookup table representation is used; it can also be solved with the Q-learning algorithm of Tsitsiklis and Van Roy [TsV99], in the case where feature-based Q-factor approximations are used. In exact/lookup table representation form, our algorithm admits asynchronous and stochastic iterative implementations, in the spirit of asynchronous/modi ed policy iteration, with lower overhead and more reliable convergence advantages over existing Q-learning schemes. Furthermore, for large-scale problems, where linear basis function approximations and simulation-based temporal di erence implementations are used, our algorithm resolves e ffectively the inherent difficulties of existing schemes due to inadequate exploration.
  • Yu, Huizhen (2010)
    We consider approximate policy evaluation for finite state and action Markov decision processes (MDP) in the off-policy learning context and with the simulation-based least squares temporal difference algorithm, LSTD(λ). We establish for the discounted cost criterion that the off-policy LSTD(λ) converges almost surely under mild, minimal conditions. We also analyze other convergence and boundedness properties of the iterates involved in the algorithm, and based on them, we suggest a modification in its practical implementation. Our analysis uses theories of both finite space Markov chains and Markov chains on topological spaces, in particular, the e-chains.
  • Bingham, Ella; Koivisto, Mikko; Leino, Yrjö; Mannila, Heikki (2010)
    Linkage disequilibrium (LD) refers to the statistical dependency of the DNA content at nearby locations of the chromosome. Numerous approaches to analyze genome data rely on the well documented fact that LD decays monotonously with the distance of the studied loci. This decay, though noisy and modi ed by a number of factors, can be attributed to the recombination process, a major source of genetic variation in diploid organisms. In this work we take first steps toward analyzing the extent of LD between very distant loci, even loci from different chromosomes. This is in contrast to traditional "genome-wide" analyses which merely study the LD within each chromosome separately. We design several measures of LD, and use them for analyzing the HapMap data. We also consider LD between supermarkers determined by haplotype clusters in windows of a few SNPs. We report on suggestive pairs of loci where unusually large correlations are observed within all ethnic groups. We describe how the computations can be arranged in a way that enables an all-pairs analysis of the data, that is, all pairs of loci across all the 22 autosomal chromosomes. This kind of "genome times genome" analysis is computationally very burdensome due to the sheer number of possible pairs. We show ways to make it feasible.
  • Galbrun, Esther (2010)
    Phrase-Based Statistical Machine Translation systems model the translation process using pairs of corresponding sequences of words extracted from parallel corpora. These biphrases are stored in phrase tables that typically contain several millions such entries, making it di cult to assess their quality without going to the end of the translation process. Our work is based on the examplifying study of phrase tables generated from the Europarl data, from French to English. We give some statistical information about the biphrases contained in the phrase table, evaluate the coverage of previously unseen sentences and analyse the e ects of pruning on the translation.
  • Ikonen, Marko (2009)
    Software projects are born and die but their problems remain. While equipment and tools are more powerful than ever before, the success rates of the projects are still poor. This report addresses project failures and their reasons based on the literature. In addition, key subjects for improving one’s own projects are presented. Awareness of underlying weaknesses in these projects makes it possible to find targets for improvements. The report mainly focuses on software engineering projects. However, the viewpoint can be applied education (e.g. capstone software project) as well as to improve situational awareness in student projects and to recognize problems in early stages of those projects. As a result, this report eases the work of project managers and supervisors by addressing key points of failures and success. With this knowledge, managers and supervisors can tailor a specific questionnaire to find targets of improvements from their projects.