This thesis concentrates on the derivation of a modularised version of the DMOS distributed garbage collection algorithm and the implementation of this algorithm in a distributed computational environment. DMOS appears to exhibit a unique combination of attractive characteristics for a distributed garbage collector but the original algorithm is known to contain a bug and, previous to this work, lacks a satisfactory, understandable implementation. The relationship between distributed termination detection algorithms and distributed garbage collectors is central to this thesis. A modularised DMOS algorithm is developed using a previously published distributed garbage collector derivation methodology that centres on mapping centralised collection schemes to distributed termination detection algorithms. In examining the utility and suitability of the derivation methodology, a family of six distributed collectors is developed and an extension to the methodology is presented. The research work described in this thesis incorporates the definition and implementation of a distributed computational environment based on the ProcessBase language and a generic definition of a previously unimplemented distributed termination detection algorithm called Task Balancing. The role of distributed termination detection in the DMOS collection mechanisms is defined through a process of step-wise refinement. The implementation of the collector is achieved in two stages; the first stage defines the implementation of two distributed termination mappings with the Task Balancing algorithm; the second stage defines the DMOS collection mechanisms.

An investigation is presented into an interrupt-generating object model which is designed to reduce the effort of programming distributed memory multicomputer networks. The object model is aimed at the natural modelling of problem domains in which a number of concurrent entities interrupt one another as they lay claim to shared resources. The proposed computational model provides for the safe encapsulation of shared data, and incorporates inherent arbitration for simultaneous access to the data. It supplies a predicate triggering mechanism for use in conditional synchronization and as an alternative mechanism to polling. Linguistic support for the proposal requires a novel form of control structure which is able to interface sensibly with interrupt-generating active data objects. The thesis presents the proposal as an elemental language structure, with axiomatic guarantees which enforce safety properties and aid in program proving. The established theory of CSP is used to reason about the object model and its interface. An overview is presented of a programming language called HUL, whose semantics reflect the proposed computational model. Using the syntax of HUL, the application of the interrupt-generating active data object is illustrated. A range of standard concurrent problems is presented to demonstrate the properties of the interrupt-generating computational model. Furthermore, the thesis discusses implementation considerations which enable the model to be mapped precisely onto multicomputer networks, and which sustain the abstract programming level provided by the interrupt-generating active data object in the wider programming structures of HUL.

Organizations face challenges constantly owing to limited resources. As such, to take advantage of new opportunities and to mitigate possible risks they look for new ways to collaborate, by sharing knowledge and competencies. Coordination among partners is critical in order to achieve success. The segmented South African public sector is no different. Driven by the desire to ensure proper service delivery in this sector, various government bodies and service providers play different roles towards the attainment of common goals. This is easier said than done, given the complexity of the distributed nature of the environment. Heterogeneity, autonomy, and the increasing need to collaborate provoke the need to develop an integrative and dynamic coordination support service system in the SA public sector. Thus, the research looks to theories/concepts and existing coordination practices to ground the process of development. To inform the design of the proposed artefact the research employs an interdisciplinary approach championed by coordination theory to review coordination-related theories and concepts. The effort accounts for coordination constructs that characterize and transform the problem and solution spaces. Thus, requirements are explicit towards identifying coordination breakdowns and their resolution. Furthermore, how coordination in a distributed environment is supported in practice is considered from a socio-technical perspective in an effort to account holistically for coordination support. Examining existing solutions identified shortcomings that, if addressed, can help to improve the solutions for coordination, which are often rigidly and narrowly defined. The research argues that introducing a mediating technological artefact conceived from a virtual community and service lenses can serve as a solution to the problem. By adopting a design-science research paradigm, the research develops a model as a primary artefact to support coordination from a collaboration standpoint. The suggestions from theory and practice and the unique case requirement identified through a novel case analysis framework form the basis of the model design. The proposed model support operation calls for an architecture which employs a design pattern that divides a complex whole into smaller, simpler parts, with the aim of reducing the system complexity. Four fundamental functions of the supporting architecture are introduced and discussed as they would support the operation and activities of the proposed collaboration lifecycle model geared towards streamlining coordination in a distributed environment. As part of the model development knowledge contributions are made in several ways. Firstly, an analytical instrument is presented that can be used by an enterprise architect or business analyst to study the coordination status quo of a collaborative activity in a distributed environment. Secondly, a lifecycle model is presented as meta-process model with activities that are geared towards streamlining the coordination of dynamic collaborative activities or projects. Thirdly, an architecture that will enable the technical virtual community-centric, context-aware environment that hosts the process-based operations is offered. Finally, the validation tool that represents the applied contribution to the research that promises possible adaptation for similar circumstances is presented. The artefacts contribute towards a design theory in IS research for the development and improvement of coordination support services in a distributed environment such as the South African public sector.

Objects are entities which encapsulate data and those operations which manipulate the data. A distributed, object-based programming system (or DOBPS) is a distributed operating system which has been designed to support an object-based programming language and, in particular, an object abstraction. DOBPSs have the benefits of simplifying program construction and improving the performance of programs by providing efficient, system-level support for the abstractions used by the language. Many DOBPSs also permit hardware and software failures to be tolerated. This thesis introduces a definition for the term "distributed, object-based programming system" and identifies the features, that are related to objects, which are required by an operating system of a DOBPS. A classification scheme is presented that categorizes and characterizes these features to permit a number of implementation techniques to be easily examined, compared, and contrasted. / Science, Faculty of / Computer Science, Department of / Graduate

Bibliography: pages 104-110. / Two technologies, distributed operating systems and UNIX are very relevant in computing today. Many distributed systems have been produced and many are under development. To a large extent, distributed systems are considered to be the only way to solve the computing needs of the future. UNIX, on the other hand, is becoming widely recognized as the industry standard for operating systems. The transputer, unlike. UNIX and distributed systems is a relatively new innovation. The transputer is a concurrent processing machine based on mathematical principles. Increasingly, the transputer is being used to solve a wide range of problems of a parallel nature. This thesis combines these three aspects in creating a distributed implementation of UNIX on a network of transputers. The design is based on the satellite model. In this model a central controlling processor is surrounded by worker processors, called satellites, in a master/ slave relationship.

Bibliography: pages 90-94. / The DISTRIX operating system is a multiprocessor distributed operating system based on UNIX. It consists of a number of satellite processors connected to central servers. The system is derived from the MINIX operating system, compatible with UNIX Version 7. A remote procedure call interface is used in conjunction with a system wide, end-to-end communication protocol that connects satellite processors to the central servers. A cached file server provides access to all files and devices at the UNIX system call level. The design of the file server is discussed in depth and the performance evaluated. Additional information is given about the software and hardware used during the development of the project. The MINIX operating system has proved to be a good choice as the software base, but certain features have proved to be poorer. The Inmos transputer emerges as a processor with many useful features that eased the implementation.

This paper presents the G-network, a new topological design which is a suitable architecture for point-to-point communication and interconnection networks, We show that the G-network has the following desirable characteristics: Efficient routing, small number of links, and fault tolerance. The performance of the G-network is compared to that of the Barrel Shifter and Illiac Mesh networks.

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In this thesis a system which extracts 3D surfaces of arbitrary scenes under natural illumination is constructed using low-cost, off-the-shelf components. The system is implemented over a network of workstations using standardized distributed software technology. The architecture of the system is highly influenced by the performance requirements of multimedia applications which require 3D computer vision. Visible scene surfaces are extracted using a passive multi-baseline stereo technique. The implementation efficiently supports any number of cameras in arbitrary positions through an effective rectification strategy. The distributed software components interact through CORBA and work cooperatively in parallel. Experiments are performed to assess the effects of various parameters on the performance of the system and to demonstrate the feasibility of this approach. / Thesis / Master of Engineering (ME)