No description available.

No description available.

Distributed systems are gradually being accepted as the dominant computing paradigm of the future. However, due to the diversity and multiplicity of resources, and the need for transparency to users, global resource management raises many questions. On the performance level the potential benefits of the load balancing in resolving the occasional congestion experienced by some nodes while others are idle or lightly loaded are commonly accepted. It is also acknowledged that no single load balancing algorithm deals satisfactorily with the changing system characteristics and dynamic workload environment. In modelling distributed systems for load balancing, optimistic assumptions of system characteristics are commonly made, with no evaluation of alternative system design options such as communications protocols. When realistic assumptions are made on system attributes such as communication bandwidth, load balancing overheads, and workload model, doubts are cast on the capability of load balancing to improve the performance of distributed systems significantly. A taxonomy is developed for the components as well as the attributes aspects of load balancing algorithms to provide a common terminology and a comprehensive view to load balancing in distributed systems. For adaptive algorithms the taxonomy is extended to identify the issues involved and the ways of adding adaptability along different dimensions. A design methodology is also outlined. A review of related work is used to identify the most promising load balancing strategies and the modelling assumptions made in previous load balancing studies. Subsequently the research problems addressed in this thesis and the design of new algorithms are detailed. A simulated system developed to allow an experimentation with various load balancing algorithms under different workload models and system attributes is described. Based on the nature of the file system structure and the classes of nodes processing speed involved, different models of loosely-coupled distributed systems can be defined. Four models are developed: disk-based homogeneous nodes, diskless homogeneous nodes, diskless heterogeneous nodes, and disk-based heterogeneous nodes. The nodes are connected through a broadcast transfer device. A set of representative load balancing algorithms covering a range of strategies are evaluated and compared for the four models of distributed systems. The algorithms developed include a new algorithm called Diffuse based on explicit adaptability for the homogeneous systems. In the case of heterogeneous systems, novel modifications are made to a number of algorithms to take into account the heterogeneity of nodes speed. The evaluation on homogeneous systems is two-fold: an assessment of the effect of system attributes on the performance of the distributed system subject to these algorithms, and a comparison of the relative merits of the algorithms using different performance metrics, and in particular a classification of the performance of the Diffuse algorithm with regard to others in the literature. For the heterogeneous systems the performance of the adapted algorithms is compared to that of the standard versions and to the no load balancing case. As a result of this evaluation, for a set of combinations of performance objectives, distributed system attributes, and workload environment, we identify the most . appropriate load balancing algorithm and optimal values for adjustable parameters of the algorithm.

Computer technology, particularly at the circuit level, is fast approaching its physical limitations. As future needs for greater power from computing systems grows, increases in circuit switching speed (and thus instruction speed) will be unable to match these requirements. Greater power can also be obtained by incorporating several processing units into a single system. This ability to increase the performance of a system by the addition of processing units is one of the major advantages of multiprocessor systems. Four major characteristics of multiprocessor systems have been identified (28) which demonstrate their advantage. These are:- Throughput Flexibility Availability Reliability The additional throughput obtained from a multiprocessor has been mentioned above. This increase in the power of the system can be obtained in a modular fashion with extra processors being added as greater processing needs arise. The addition of extra processors also has (in general) the desirable advantage of giving a smoother cost-performance curve ( 63). Flexibility is obtained from the increased ability to construct a system matching the user 'requirements at a given time without placing restrictions upon future expansion. With multiprocessor systems; the potential also exists of making greater use of the resources within the system. Availability and reliability are inter-related. Increased availability is achieved, in a well designed system, by ensuring that processing capabilities can be provided to the user even if one (or more) of the processing units has failed. The service provided, however, will probably be degraded due to the reduction in processing capacity. Increased reliability is obtained by the ability of the processing units to compensate for the failure of one of their number. This recovery may involve complex software checks and a consequent decrease in available power even when all the units are functioning.

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The thesis is concerned with the transmission and reception of digital data at 19,200 bit/s over voice-frequency telephone channels. Following a feasibility study based on both practical and theoretical constraints, the telephone network itself is investigated to determine methods of circuit characterisation and the causes and effects of distortion and other signal impairments.

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This thesis reports on two graphene-based structures that have been proposed and fabricated as possible prototypes for high-spatial-resolution Hall sensors with potential application in research on high-density magnetic recording technology such as bit patterned magnetic recording (BPMR) and other areas where the measurement of highly inhomogeneous fields is required. There is a direct graphene-metal contact in the first structure, which is named as TYPE I in this thesis, so that the anomalous Hall effect (AHE) in the ferromagnetic islands deposited on the graphene could be detected. Meanwhile, the graphene and the metal are isolated by an h-BN layer in the second structure which is named as TYPE II, so that only the stray field from the islands can be detected using the ordinary Hall effect (OHE).The transport measurements performed on TYPE I devices revealed there is no AHE or stray field signal detectable, and their Hall resistance relations are non-linear and do not pass through the origin point. A finite element simulation comparing the resistance of the empty graphene cross and the island-occupied cross indicates that the current in the graphene may not redistribute through the metallic islands due to interface current blocking, resulting in the non-appearance of the expected AHE signal. Moreover, an analysis on the data of the longitudinal magnetoresistance (MR) reveals that a two-fluid model and effective medium theory (EMT) model might be the major graphene MR mechanisms in the regime away from and near to the charge neutrality point (CNP) respectively. As a combined result of the above findings, a joint MR-Hall effect model under the condition of the presence of a pre-existing transverse offset current, is proposed to explain the unusual behaviour of the Hall measurement data of the TYPE I devices. The model gives qualitatively correct fitting for all longitudinal and transverse transport data of TYPE I devices. In addition, the nature of the graphene/metal contact is considered as the reason responsible for the non-appearance of the expected AHE and stray field signal, although further experimental work is needed, and suggested in the thesis, to clarify this issue. On the other hand, the TYPE II devices have shown their potential to be developed as a Hall sensor being able to detect a sub-micron magnetic island in the future, but there is still a large space for the performance of the devices to be improved. At the end of the thesis, future experimental work, which could lead to the eventual development of a high-sensitivity high-spatial-resolution Hall sensor on the basis of TYPE I and TYPE II structures, are suggested and described.