Bilgisayar Mühendisliği Bölümü / Department of Computer Engineering
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Publication An Approach to the Design of Bilingual Desktop Publishing Systems(1988) Taylı, Murat; Salamah, A.; Tantawy, A.; 9351Publication Object Oriented Approach to the Design of Real Time Distributed Operating Systems(1990) Bor, M.; Taylı, M.; 9351Publication Object Oriented Approach to the Design of Real Time Distributed Operating Systems(1990) Bor, M.; Taylı, Murat; 9351Publication Proceedings of the 1993 World Transputer Congress(1990) Taylı, Murat; Benmaiza, M.; 9351Publication An efficient circuit switching mechanism for Inter process communication in a transputer network(IEEE, 1990) Taylı, Murat; Benmaiza, M.; 9351Distributed real time system require interprocess communication (IPC) services to be not only reliable and efficient but also predictable. In such dynamic systems, predictability can only be achieved by proper architectural support that embodies adequate mechanisms and policies within the operating system. The authors present the design and implementation of predictable transport level communication services supported by the kernel of a real time distributed operating system (RTDOS). RTDOS IPC services are built on a hybrid platform, involving both message- and circuit-switching. Experimental results showed that RTDOS IPC was able to deliver millisecond-level connection times, guaranteed within the prescribed limits of system configuration.Publication Building Bilingual Microcomputer Systems(1990-05) Taylı, Murat; Salamah, A.; 9351In the Arab world the need for bilingual microcomputer systems is ever increasing. In addition to the ability to process the Arabic and English scripts, an ideal system should support the use of existing applications with Arabic data and the access to the system facilities through Arabic interfaces. The Integrated Arabic System (IAS) was developed to study the feasibility of building such systems using existing microcomputers and software solutions.Publication IPC for Predictable Message Passing in a Multiloop Transputer Network(1993) Benmaiza, M.; Taylı, Murat; 9351Publication Avoiding Deadline Decay Under Transient Overloads(IEEE, 1995) Swim, B.; Taylı, Murat; Benmaiza, M.; Woodward, M.; 9351Deadline scheduling policies based on stochastic computation times perform well in average but fail in presence of transient overloads, causing already scheduled tasks to miss their deadline. This paper presents an approach using medium-term scheduling policies to cope with the problem of transient overload in dynamic real-time systems. The proposed approach has been implemented in a Real-Time Distributed Operating System (RTDOS) running on a Transputer network.Publication An Architecture for IPC Services in Distributed Dynamic Hard Real Time Systems(1996) Taylı, Murat; Benmaiza, M.; 9351Publication Pure Dynamic Task Scheduling in Hard Real Time Distributed Systems(1996) Swim, B.; Benmaiza, M.; Taylı, Murat; Woodward, M.; 9351Publication Prospects for Predictable Dynamic Scheduling in RTDOS(IEE Proceedings Computer and Digital Techniques, 1997) Swim, B. R.; Benmaiza, M.; Taylı, Murat; Woodward, M. C.; 9351Future dynamic distributed hard real-time systems may control unpredictable environments and will need operating systems that can handle unknown and changing task populations. In this extreme case, not only is task scheduling totally dynamic, but the system's topology and architecture must adapt to unforeseen configurations. This paper addresses the difficult problem of dynamic task scheduling in a Real Time Distributed Operating System (RTDOS). RTDOS is unique because it possesses the potential to map, at execution time, a flexible topology of networked nodes onto a network of tasks. Attempts will be made to characterize the predictable adaptability of the scheduler so as to relax the pre-run-time scheduling requirements for an RTDOS application. A scheduler architecture and dynamic deadline guarantee scheme are presented along with some experimental results.Publication Predictable distributed Dynamic Scheduling in RTDOS(IET, 1997) Swim, B. R.; Benmaiza, M.; Taylı, Murat; Woodward, M. C.; 9351Future dynamic distributed hard real-time systems may control unpredictable environments, and will need predictable and flexible runtime systems that can handle unknown and changing task populations. In this extreme case not only is task scheduling dynamic, but the system topology and architecture might be adapted to unforeseen configurations. The paper addresses the difficult problem of dynamic distributed task scheduling. A new predictable dynamic deadline guarantee scheme has been designed and implemented in the authors' real-time distributed operating system (RTDOS). The algorithms have been proven to guarantee task deadlines even during transient overloads. RTDOS is designed to tackle unpredictable and highly dynamic environments; therefore its task model is quite unrestrictive, supporting periodic and aperiodic tasks both with arbitrary release times and deadlines. The authors place no restriction on the inter-arrival times between and aperiodic task instances. Moreover, their resource scheduling considers precedence constraints, device allocation and communication requirements. As a consequence, the results can be generalised and applied to many real-time domains. Furthermore, the complexity of the authors' schedulability test has been proved to be O(n). A domain-wide scheduler is described, which maintains its predicted maximum response time even when local nodes are under heavy load. Experimental results confirm the proven expectations of our scheduling scheme.Publication On Scheduling Project Activities With Variable Expenditure Rates(KLUWER ACADEMIC PUBL, SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS, 1998) Özdamar, LinetCommercial projects are undertaken with the expectation of maximizing financial returns. There are a variety of cash flow models in the literature in which payments, whose amounts depend on the type of contract quoted between the contractor and the client, are tied to the project events or to the completion times of activities. These models are of deterministic nature with respect to cash inflows, that is, the amount and timing of the payments, once determined, are fixed. However, there are situations specifically in the housing industry where the contractor is the owner of the project. In this case, the contractor starts with an initial capital to cover the activity expenditures and then, capital is augmented by the sale of flats which take place randomly over the progress of the project. In this risky environment, the contractor has to decide on the rate of expenditure at each decision time in order to maintain a positive cash balance. Hence, activities are represented by multiple performing modes with different activity durations and the same total cost. A heuristic to construct and re-construct schedules during the progress of the project is proposed here with the aim of maximizing the project Net Present Value while completing the project on time. The heuristic incorporates dynamic mode selection criteria which change adaptively according to the current status of the project. Computational experiments with the heuristic demonstrates that it provides satisfactory results regarding the feasibility of the schedules with respect to the project due date and the nonrenewable resource constraints.Publication Self Synchronizing Clocks for Real Time Systems(1999) Benmaiza, M.; Taylı, Murat; 9351The two functions achieved by clock synchronization algorithms are clock precision and clock accuracy. The former keeps the drift between any two clocks in a given set of clocks within defined limits; the latter maintains the drift between a given clock and a reference clock within defined limits. While distributed non real-time systems need only clock precision for their correct operation, clock accuracy is an absolute must for distributed real-time systems. Many of the existing software-based clock synchronization algorithms implement only clock precision, treating clock accuracy as a secondary problem, often solved at a high cost. As modern distributed real-time systems are emerging more and more, it becomes necessary to devise an approach that deals with the precision and accuracy issues equally and in an integrated way. The work presented in this paper consisted in the design and implementation of a software-based clock synchronization algorithm, which achieves at the same time clock accuracy and clock precision. The proposed algorithm is built upon a new mechanism, referred to as self synchronization, through which a clock can continuously synchronize itself relative to a reference clock, independently of the others and with minimal message exchange. An important aspect of this mechanism is to permit the implementation of a very low cost, fault-tolerant algorithm without resorting to any specific hardware. We finally show that, with proper architectural support, the level of attainable accuracy can match hardware-based solutions.Publication A Hierarchical Planning System for Energy Intensive Production Environments(ELSEVIER SCIENCE BV, PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS, 1999-01-15) Özdamar, Linet; Birbil, SIThis paper describes a hierarchical production planning approach with decision support features for energy intensive industries with particular reference to a tile manufacturing factory. In the tiling industry, the facilities which contribute most to the consumption of energy (and, hence, to the production costs) are usually the kilns where the curing operation is carried out. Frequently, the kilns are also the bottleneck in terms of capacity utilization. Thus, in order to save on energy costs, a planning approach which aims at minimizing the number of active kilns throughout the year is needed besides optimizing the process design in the curing department. To achieve the latter goal, it is necessary to take into account demand fluctuations as well as detailed capacity restrictions while deciding on the lot sizes of the products and the kilns on which the products are loaded. Rather than adopting a monolithic mathematical model for developing a desirable production plan, a hierarchical approach which decomposes the problem into two sub-problems is preferred. In the first level, products and capacity are aggregated over the planning horizon to achieve an overall consideration of demand fluctuations over time. Then, the solution provided by the aggregate solution for the current planning period is disaggregated into a detailed lot sizing and loading solution. The disaggregated problem is difficult to solve and hence, a heuristic is proposed here. This planning approach is sustained by a Decision Support System which enables the elimination of possible inconsistencies in the production plan by providing an effective interaction with the decision maker. (C) 1999 Elsevier Science B.V. All rights reserved.Publication A Genetic Algorithm Approach to a General Category Project Scheduling Problem(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 345 E 47TH ST, NEW YORK, NY 10017-2394 USA, 1999-02) Özdamar, LinetA genetic algorithm (GA) approach is proposed for the general resource constrained project scheduling model, in which activities may be executed in more than one operating mode and renewable as well as nonrenewable resource constraints exist, Each activity operation mode has a different duration and requires different amounts of renewable and nonrenewable resources. The objective is the minimization of the project duration or makespan, The problem under consideration is known to be one of the most difficult scheduling problems, and it is hard to find a feasible solution for such a problem, let alone the optimal one, The GA approach described here incorporates problem-specific scheduling knowledge by an indirect chromosome encoding that consists of selected activity operating modes and an ordered set of scheduling rules, The scheduling rules in the chromosome are used in an iterative scheduling algorithm that constructs the schedule resulting from the chromosome. The proposed GA is denoted as a hybrid GA (HGA) approach since it is integrated with traditional scheduling tools and expertise specifically developed for the general resource constrained project scheduling problem. The results demonstrate that HGA approach produces near-optimal solutions within a reasonable amount of computation time.Publication FRACTOP: A Geometric Partitioning Metaheuristic for Global Optimization(KLUWER ACADEMIC PUBL, SPUIBOULEVARD 50, PO BOX 17, 3300 AA DORDRECHT, NETHERLANDS, 1999-06) Demirhan, M; Özdamar, Linet; Helvacıoğlu, L; Birbil, SIWe propose a new metaheuristic, FRACTOP, for global optimization. FRACTOP is based on the geometric partitioning of the feasible region so that search metaheuristics such as Simulated Annealing (SA), or Genetic Algorithms (GA) which are activated in smaller subregions, have increased reliability in locating the global optimum. FRACTOP is able to incorporate any search heuristic devised for global optimization. The main contribution of FRACTOP is that it provides an intelligent guidance (through fuzzy measures) in locating the subregion containing the global optimum solution for the search heuristics imbedded in it. By executing the search in nonoverlapping subregions, FRACTOP eliminates the repetitive visits of the search heuristics to the same local area and furthermore, it becomes amenable for parallel processing. As FRACTOP conducts the search deeper into smaller subregions, many unpromising subregions are discarded from the feasible region. Thus, the initial feasible region gains a fractal structure with many space gaps which economizes on computation time. Computational experiments with FRACTOP indicate that the metaheuristic improves significantly the results obtained by random search (RS), SA and GA.Publication Hybrid Heuristics for the Multi-stage Capacitated Lot Sizing and Loading Problem(STOCKTON PRESS, HOUNDMILLS, BASINGSTOKE RG21 6XS, HAMPSHIRE, ENGLAND, 1999-08) Linet, ÖzdamarThe multi-stage capacitated lot sizing and loading problem (MCLSLP) deals with the issue of determining the lot sizes of product items in serially-arranged manufacturing stages and loading them on parallel facilities in each stage to satisfy dynamic demand over a given planning horizon. It is assumed that regular time capacity decisions have already been made in the tactical level and allocated to the stages, but it is still an important decision problem whether to augment regular time capacity by overtime capacity. Each item may be processed on a technologically feasible subset of existing facilities with different process and setup times on each facility. Since the solution of the MCLSLP requires the design of a powerful algorithm, simulated annealing (SA) and genetic algorithms (GA) are integrated to enhance their individual performances. Furthermore, these global optimisation methods are incorporated into a Lagrangean relaxation scheme, hence creating a hybrid solution methodology. Numerical results obtained using these methods confirm the mutual benefits of integrating different solution techniques.Publication A Note On the Use of a Fuzzy Approach in Adaptive Partitioning Algorithms for Global Optimization(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 445 HOES LANE, PISCATAWAY, NJ 08855-4141 USA, 1999-08) Demirhan, Melek; Özdamar, LinetIn global optimization, adaptive partitioning algorithms (APA) operate on the basis of partitioning the feasible region into subregions, sampling and evaluating each subregion, and selecting one or more subregions for repartitioning, The purpose of the repartitioning process is to locate a narrow neighborhood around the global optimum. In this correspondence, He propose to use a fuzzy approach in the assessment of subregions using random samples taken from these subregions. We discuss different types of uncertainties involved in APA and ne conclude that the use of a fuzzy approach in the assessment of subregions is in concurrence with APA's convergence property, We provide numerical results for the fuzzy approach on 13 test functions from the literature.Publication A hierarchical planning approach for a production-distribution system(TAYLOR & FRANCIS LTD, ONE GUNPOWDER SQUARE, LONDON EC4A 3DE, ENGLAND, 1999-11-10) Özdamar, Linet; AKTİN, AYŞE TÜLİN; 57877; 109203A production-distribution model involving production and transportation decisions in a central factory and its warehouses is developed. The model is based on the operating system of a multi-national company producing detergents in a central factory from which products are distributed to geographically distant warehouses. The overall system costs are optimized considering factory and warehouse inventory costs and transportation costs. Constraints include production capacity, inventory balance and fleet size integrity. Here, a hierarchical approach is adopted in order to make use of medium range aggregate information, as well as to satisfy weekly fluctuating demand with an optimal fleet size. Thus, a model which involves an aggregation of products, demand, capacity, and time periods is solved. In the next planning phase, the aggregate decisions are disaggregated into refined decisions in terms of time periods, product families, inventory and distribution quantities related to warehouses. Consistency between the aggregate and disaggregation models is obtained by imposing additional constraints on the disaggregation model. Infeasibilities in the disaggregated solution are resolved through an iterative constraint relaxation scheme which is activated in response to infeasible solutions pertaining to different causes. Here, we investigate the robustness of the hierarchical model in terms of infeasibilities occurring due to the highly fluctuating nature of demand in the refined time periods and also due to the aggregation process itself.