Papers by Winfried Grassmann
Springer International Publishing eBooks, 2022
Springer International Publishing eBooks, 2022
Springer International Publishing eBooks, 2022
Springer International Publishing eBooks, 2022
Springer International Publishing eBooks, 2022
Numerical Methods for Solving Discrete Event Systems
The objective of this project is to simulate a Peer-to-Peer type of environment with the JADE mul... more The objective of this project is to simulate a Peer-to-Peer type of environment with the JADE multi-agent system platform to investigate the use of social networks to optimize the speed of search and to improve quality of service in the Peer-to-Peer environment. Our project uses the Gnutella protocol as a starting point. The Gnutella protocol broadcasts messages for searching files. This message passing generates much traffic in the network. This degrades the quality of service. We propose a model where each peer has a "friends list", for each category of interest. Once peers generate their "friends list", they use these lists for searching files in the network. The model has been implemented and some initial experiments have been performed.
Proceedings - Winter Simulation Conference
The state in which a discrete event simulation is started causes the estimators for equilibrium m... more The state in which a discrete event simulation is started causes the estimators for equilibrium measures obtained from the simulation to be biased, and to reduce this bias, the collection of data is delayed until a so-called warm-up period is completed. In this paper, we determine the optimal warm-up periods for steady-state discrete event simulations. We do this by using deterministic numerical methods, that is, methods not using random numbers. We found that in the systems investigated, transient expectations give no indication regarding the optimal length of the warm-up periods, which is counterintuitive. This requires some re-evaluation of some of commonly held opinions about the factors one should take into account when determining warm-up periods. Such factors will also be discussed.
Behavior Research Methods & Instrumentation, 1974
Often one wishes to find which factors determine selections or ranking orders of certain objects.... more Often one wishes to find which factors determine selections or ranking orders of certain objects. This is what this program does. The objects are assumed to be selected or ranked according to their performance or ability. The performance of the jth element, gj is a linear function of certain known independent variables, Xij, and an error term, Uj. Thus, one has
INFOR: Information Systems and Operational Research, 2010
The objective of this paper is to find equilibrium probabilities of Quasi-Birth-Death (QBD) proce... more The objective of this paper is to find equilibrium probabilities of Quasi-Birth-Death (QBD) processes with a special structure. A QBD process is a Markov chain in which the states are divided into levels, with each level containing several phases. Moreover, only transitions between neighboring levels are allowed, that is, no event can increase or decrease the level by more than 1. Transitions between levels are governed by three matrices: the rates of going up one level are given by the matrix A 0 , the rates of staying at the same level by the matrix A 1 , and the rates of going down one level by A 2. QBD processes are frequently used to model queueing systems, with the level often being the number in some queue, and in many queueing situations, one can go up one level only when in specific phases, and it is easy to see that the only rows of A 0 containing non-zero elements are the rows corresponding to these phases. Hence, we investigate the case where A 0 has only a small number of rows which are not zero, and we show how this can be exploited in algorithm T, an algorithm due to Neuts, and in algorithm U suggested by Latouche. In many cases, A 0 is lower triangular, and this can be exploited by algorithm U, but not by algorithm T. There is also a logarithmic reduction method, but unfortunately, we have not found any way to significantly reduce the computational complexity of this method when A 0 has vanishing rows, or A 0 is lower triangular. We also show how to deal with the case where A 0 does not have the full rank, but few or no rows of zero.
winter simulation conference, Dec 6, 1982
ABSTRACT This paper analyses several stochastic systems and obtains expressions for the initial b... more ABSTRACT This paper analyses several stochastic systems and obtains expressions for the initial bias. The bias is compared with the estimation error of the same system starting in statistical equilibrium. It turns out that the bias and the estimation error are closely related to each other. Furthermore, it is shown that if the estimation error is small, the bias becomes negligible.
International Series in Operations Research & Management Science, 2000
Computational probability is the science of calculating probabilities and expectations. As this b... more Computational probability is the science of calculating probabilities and expectations. As this book demonstrates, there are many mathematical challenges in the area of computational probability. To set the stage, we discuss the objectives of computational probability in more detail, and we point out the difficulties one encounters in this area. We also contrast computational probability with other approaches.
Linear Algebra, Markov Chains, and Queueing Models, 1993
In this paper, we study the total reward connected with a Markov reward process from time zero to... more In this paper, we study the total reward connected with a Markov reward process from time zero to time m. In particular, we determine the average reward within this time period, as well as its variance. Though the emphasis is on discrete time Markov processes, continuous-time reward processes will also be considered. For the discrete time reward process, the determination of the expected reward form 0 to m is of course trivial. It is of interest, however, that the deviation of this expectation from its steady state equivalent can be obtained from equations which are identical to the equations for the equilibrium probabilities, except that a vector of constants is added. We also consider the variance, both for transient systems and for systems in equilibrium. It is shown that the variance per time unit in equilibrium can also be obtained from equations which differ from the equations for the equilibrium probabilities only by a constant vector. Since there are three different sets of variables which satisfy similar equations, the LU factorization suggests itself. Instead of the LU factorization, we will use a U L factorization which reflects the probabilistic interpretation of the problem. This interpretation allows us to extend the factorization to systems with an infinite number of states as will be demonstrated, using the Wiener-Hopf factorization of the GIIGII queue as an example.
The world population is expected to reach an estimated 9.8 billion by 2050, necessitating substan... more The world population is expected to reach an estimated 9.8 billion by 2050, necessitating substantial increases in food production. Achieving such increases will require large-scale application of computer informatics within the agricultural sector. In particular, application of informatics to crop breeding has the potential to greatly enhance our ability to develop new varieties quickly and economically. Achieving this potential, however, will require capabilities for analyzing huge volumes of data acquired from various field-deployed image acquisition technologies. Although numerous frameworks for big data processing have been developed, there are relatively few published case studies that describe user experiences with these frameworks in particular application science domains. In this paper, we describe our efforts to apply Apache Spark to three applications of initial interest within the Plant Phenotyping and Imaging Research Centre (P2IRC) at the University of Saskatchewan. We...
The Gnutella protocol requires peers to broadcast messages to their neighbours when they search f... more The Gnutella protocol requires peers to broadcast messages to their neighbours when they search files. The message passing generates a lot of traffic in the network, which degrades the quality of service. We propose using social networks to optimize the speed of search and to improve the quality of service in a Gnutella based peerto-peer environment. Once peers generate their “friends lists”, they use these lists to semantically route queries in the network. This helps to reduce the search time and to decrease the network traffic by minimizing the number of messages circulating in the system as compared to standard Gnutella. We demonstrate by simulating such an environment with the JADE multi-agent system platform that by learning other peers’ interests, building and exploiting their social networks (friends lists) to route queries semantically, peers can get more relevant resources faster and with less traffic generated, i.e. that the performance of the Gnutella system can be impro...
International Series in Operations Research & Management Science, 2000
This chapter shows how to find the equilibrium probabilities in processes of GI/M/1 type, and M/G... more This chapter shows how to find the equilibrium probabilities in processes of GI/M/1 type, and M/G/1 type, and GI/G/1 type by matrix analytic methods. GI/M/1-type processes are Markov chains with transition matrices having the same structure as the imbedded Markov chain of a GI/M/1 queue, except that the entries are matrices rather than scalars. Similarly, M/G/1 type processes have transition matrices of the same form as the imbedded Markov chain of the M/G/1 queue, except that the entries are matrices. In the imbedded Markov chain of the GI/M/1 queue, all columns but the first have the same entries, except that they are displaced so that the diagonal block entry is common to all. Similarly, in the M/G/1 queue, all rows except the first one are equal after proper centering.
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Papers by Winfried Grassmann