ДОСТАТОЧНОЕ УСЛОВИЕ СУЩЕСТВОВАНИЯ СТАЦИОНАРНОГО РЕЖИМА ОЧЕРЕДЕЙ ПЕРВИЧНЫХ ТРЕБОВАНИЙ В ТАНДЕМЕ СИСТЕМ МАССОВОГО ОБСЛУЖИВАНИЯ

Рассматривается тандем из двух систем массового обслуживания с нем- гновенным перемещением требований между системами. В каждую из этих систем поступают первичные требования, формируемые внешней средой. Кроме первичных требований во вторую систему также поступают требования, обслуженные первой системой. Построена математическая модель в виде многомерной марковской цепи, определенной на явно заданном вероятностном пространстве. Проведен анализ марковской цепи, описывающей изменение состояния обслуживающего устройства и флуктуации длин очередей первичных требований. Найдено достаточное условие существования стационарного распределения для этой цеп

Performance analysis of networks on chips

Modules on a chip (such as processors and memories) are traditionally connected through a single link, called a bus. As chips become more complex and the number of modules on a chip increases, this connection method becomes inefficient because the bus can only be used by one module at a time. Networks on chips are an emerging technology for the connection of on-chip modules. In networks on chips, switches are used to transmit data from one module to another, which entails that multiple links can be used simultaneously so that communication is more efficient. Switches consist of a number of input ports to which data arrives and output ports from which data leaves. If data at multiple input ports has to be transmitted to the same output port, only one input port may actually transmit its data, which may lead to congestion. Queueing theory deals with the analysis of congestion phenomena caused by competition for service facilities with scarce resources. Such phenomena occur, for example, in traffic intersections, manufacturing systems, and communication networks like networks on chips. These congestion phenomena are typically analysed using stochastic models, which capture the uncertain and unpredictable nature of processes leading to congestion (such as irregular car arrivals to a traffic intersection). Stochastic models are useful tools for the analysis of networks on chips as well, due to the complexity of data traffic on these networks. In this thesis, we therefore study queueing models aimed at networks on chips. The thesis is centred around two key models: A model of a switch in isolation, the so-called single-switch model, and a model of a network of switches where all traffic has the same destination, the so-called network of polling stations. For both models we are interested in the throughput (the amount of data transmitted per time unit) and the mean delay (the time it takes data to travel across the network). Single-switch models are often studied under the assumption that the number of ports tends to infinity and that traffic is uniform (i.e., on average equally many packets arrive to all buffers, and all possible destinations are equally likely). In networks on chips, however, the number of buffers is typically small. We introduce a new approximation specifically aimed at small switches with (memoryless) Bernoulli arrivals. We show that, for such switches, this approximation is more accurate than currently known approximations. As traffic in networks on chips is usually non-uniform, we also extend our approximation to non-uniform switches. The key difference between uniform and nonuniform switches is that in non-uniform switches, all queues have a different maximum throughput. We obtain a very accurate approximation of this throughput, which allows us to extend the mean delay approximation. The extended approximation is derived for Bernoulli arrivals and correlated arrival processes. Its accuracy is verified through a comparison with simulation results. The second key model is that of concentrating tree networks of polling stations (polling stations are essentially switches where all traffic has the same output port as destination). Single polling stations have been studied extensively in literature, but only few attempts have been made to analyse networks of polling stations. We establish a reduction theorem that states that networks of polling stations can be reduced to single polling stations while preserving some information on mean waiting times. This reduction theorem holds under the assumption that the last node of the network uses a so-called HoL-based service discipline, which means that the choice to transmit data from a certain buffer may only depend on which buffers are empty, but not on the amount of data in the buffers. The reduction theorem is a key tool for the analysis of networks of polling stations. In addition to this, mean waiting times in single polling stations have to be calculated, either exactly or approximately. To this end, known results can be used, but we also devise a new single-station approximation that can be used for a large subclass of HoL-based service disciplines. Finally, networks on chips typically implement flow control, which is a mechanism that limits the amount of data in the network from one source. We analyse the division of throughput over several sources in a network of polling stations with flow control. Our results indicate that the throughput in such a network is determined by an interaction between buffer sizes, flow control limits, and service disciplines. This interaction is studied in more detail by means of a numerical analysis

Manufacturing flow line systems: a review of models and analytical results

The most important models and results of the manufacturing flow line literature are described. These include the major classes of models (asynchronous, synchronous, and continuous); the major features (blocking, processing times, failures and repairs); the major properties (conservation of flow, flow rate-idle time, reversibility, and others); and the relationships among different models. Exact and approximate methods for obtaining quantitative measures of performance are also reviewed. The exact methods are appropriate for small systems. The approximate methods, which are the only means available for large systems, are generally based on decomposition, and make use of the exact methods for small systems. Extensions are briefly discussed. Directions for future research are suggested.National Science Foundation (U.S.) (Grant DDM-8914277

EUROPEAN CONFERENCE ON QUEUEING THEORY 2016

International audienceThis booklet contains the proceedings of the second European Conference in Queueing Theory (ECQT) that was held from the 18th to the 20th of July 2016 at the engineering school ENSEEIHT, Toulouse, France. ECQT is a biannual event where scientists and technicians in queueing theory and related areas get together to promote research, encourage interaction and exchange ideas. The spirit of the conference is to be a queueing event organized from within Europe, but open to participants from all over the world. The technical program of the 2016 edition consisted of 112 presentations organized in 29 sessions covering all trends in queueing theory, including the development of the theory, methodology advances, computational aspects and applications. Another exciting feature of ECQT2016 was the institution of the Takács Award for outstanding PhD thesis on "Queueing Theory and its Applications"

Stability Condition of a Retrial Queueing System with Abandoned and Feedback Customers

This paper deals with the stability of a retrial queueing system with two orbits, abandoned and feedback customers. Two independent Poisson streams of customers arrive to the system, and flow into a single-server service system. An arriving one of type i; i = 1; 2, is handled by the server if it is free; otherwise, it is blocked and routed to a separate type-i retrial (orbit) queue that attempts to re-dispatch its jobs at its specific Poisson rate. The customer in the orbit either attempts service again after a random time or gives up receiving service and leaves the system after a random time. After the customer is served completely, the customer will decide either to join the retrial group again for another service or leave the system forever with some probability

Performance and reliability modelling of computing systems using spectral expansion

PhD ThesisThis thesis is concerned with the analytical modelling of computing and other discrete event systems, for steady state performance and dependability. That is carried out using a novel solution technique, known as the spectral expansion method. The type of problems considered, and the systems analysed, are represented by certain two-dimensional Markov-processes on finite or semi-infinite lattice strips. A sub set of these Markov processes are the Quasi-Birth-and-Death processes. These models are important because they have wide ranging applications in the design and analysis of modern communications, advanced computing systems, flexible manufacturing systems and in dependability modelling. Though the matrixgeometric method is the presently most popular method, in this area, it suffers from certain drawbacks, as illustrated in one of the chapters. Spectral expansion clearly rises above those limitations. This also, is shown with the aid of examples. The contributions of this thesis can be divided into two categories. They are, • The theoretical foundation of the spectral expansion method is laid. Stability analysis of these Markov processes is carried out. Efficient numerical solution algorithms are developed. A comparative study is performed to show that the spectral expansion algorithm has an edge over the matrix-geometric method, in computational efficiency, accuracy and ease of use. • The method is applied to several non-trivial and complicated modelling problems, occuring in computer and communication systems. Performance measures are evaluated and optimisation issues are addressed

Стаціонарний режим для системи масового обслуговування типу M | M | c | c + m із сталою інтенсивністю повторів

Розглядається двовимірний марковський процес {X(t), t≥0}, фазовий простір якого являє собою решітку в напівсмузі S(X) = {0,1,…, c +m}×Z₊. Процес {X(t), t≥0} описує обслуговування вимог у багатоканальній системі з повторними викликами та інтенсивністю повторів, яка не залежить від числа повторних викликів. Спочатку для моделі, що розглядається, знайдена умова ергодичності. Потім отримано матрич новекторне подання стаціонарного розподілу через параметри системи. Метод дослідження базується на апроксимації вихідної моделі за допомогою урізаної з подальшим переходом до границі. Застосування отриманих результатів продемонстровано на числових прикладах, у яких наведена залежність блокуючої ймовірності та середньої величини черги в стаціонарному режимі від параметрів системи.A bivariate Markov process {Х(t)=(X₁(t), X₂(t))^T, t≥0} whose phase space is a lattice semistrip S(X)={0,1,…, c+m, }xZ₊ is considered. The first component X₁(t) ∈{0,1,..., c +m} indicates the summarized number of busy servers and calls in the queue at the instant t ≥0, whereas the second one X₂(t)∈{0,1,...} is the number of retrial sources. Parameter c∈Z₊ is a number of servers and m∈Z is a maximal size of the queue. Local rates of X(t) are defined in such a way that X(t) describes the service policy of a multi-server retrial queue in which the rate of repeated flow does not depend on the number of sources of repeated calls. First, using tools of the theory for the QBD-processes (quasi-birth-and-death processes), we study the ergodicity conditions. Then, under these conditions, we consider a problem of finding the steady state probabilities for X(t). A vector-matrix representation of the probabilities via the model parameters is obtained. The applied technique uses an approximation of the initial model by a truncated one and the direct passage to the limit. The obtained formulae are the adequate method to calculate the steady state probabilities. An application of the main result is demonstrated via numerical examples in which we can see relation graphs of the blocking probability and the average number of retrials versus system parameters

ブロッキング現象を伴うシリーズ構成待ち行列システムの配置戦略と性能評価

学位の種別: 課程博士審査委員会委員 : （主査）東京大学教授 西成 活裕, 東京大学教授 山西 健司, 東京大学教授 合原 一幸, 東京大学教授 森川 博之, 東京大学准教授 柳澤 大地University of Tokyo(東京大学