Measuring concurrency in CCS

A research report submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in partial fulfilment of the requirements for the degree of Master of ScienceThis research report investigates the application of Charron-Bost's measure of currency m to Milner's Calculus of Communicating Systems (CCS). The aim of this is twofold: first to evaluate the measure m in terms of criteria gathered from the literature: and second to determine the feasiblllty of measuring concurrency in CCS and hence provide a new tool for understanding concurrency using CCS. The approach taken is to identify the differences hetween the message-passing formalism in which the measure m is defined, and CCS and to modify this formalism to-enable the mapping of CCS agents to it. A software tool, the Concurrency Measurement Tool, is developed to permit experimentation with chosen CCS agents. These experiments show that the measure m, although intuitively appealing, is defined by an algebraic expression that is ill-behaved. A new measure is defined and it is shown that it matches the evaluation criteria better than m, although it is still not ideal. This work demonstrates that it is feasible to measure concurrency in CCS and that a methodology has been developed for evaluating concurrency measures.Andrew Chakane 201

A measurement-based study of concurrency in a multiprocessor

A systematic measurement-based methodology for characterizing the amount of concurrency present in a workload, and the effect of concurrency on system performance indices such as cache miss rate and bus activity are developed. Hardware and software instrumentation of an Alliant FX/8 was used to obtain data from a real workload environment. Results show that 35% of the workload is concurrent, with the concurrent periods typically using all available processors. Measurements of periods of change in concurrency show uneven usage of processors during these times. Other system measures, including cache miss rate and processor bus activity, are analyzed with respect to the concurrency measures. Probability of a cache miss is seen to increase with concurrency. The change in cache miss rate is much more sensitive to the fraction of concurrent code in the worklaod than the number of processors active during concurrency. Regression models are developed to quantify the relationships between cache miss rate, bus activity, and the concurrency measures. The model for cache miss rate predicts an increase in the median miss rate value as much as 300% for a 100% increase in concurrency in the workload

Logical clocks for could databases

Dissertação de mestrado em Engenharia de InformáticaCloud computing environments, particularly cloud databases, are rapidly increasing in importance, acceptance and usage in major (web) applications, that need the partition-tolerance and availability for scalability purposes, thus sacrificing the consistency side (CAP theorem). With this approach, use of paradigms such as Eventual Consistency became more widespread. In these environments, a large number of users access data stored in highly available storage systems. To provide good performance to geographically disperse users and allow operation even in the presence of failures or network partitions, these systems often rely on optimistic replication solutions that guarantee only eventual consistency. In this scenario, it is important to be able to accurately and e ciently identify updates executed concurrently. In this dissertation we review, and expose problems with current approaches to causality tracking in optimistic replication: these either lose information about causality or do not scale, as they require replicas to maintain information that grows linearly with the number of clients or updates. Then, we propose Dotted Version Vectors (DVV), a novel mechanism for dealing with data versioning in eventual consistent systems, that allows both accurate causality tracking and scalability both in the number of clients and servers, while limiting vector size to replication degree. We conclude with the challenges faced when implementing DVV in Riak (a distributed key-value store), the evaluation of its behavior and performance, and discuss the advantages and disadvantages of it.Ambientes de computação na nuvem, em especial sistemas de base de dados na nuvem, estão rapidamente a aumentar em importância, aceitação e utilização entre as grandes aplicações (web), que precisam de alta disponibilidade e tolerância a partições por razões de escalabilidade, para isso sacrificando o lado da coerência (teorema de CAP). Com esta abordagem, o uso de paradigmas como a Coerência Inevitável tornou-se generalizado. Nestes sistemas, um grande número de utilizadores têm acesso aos dados presentes em sistemas de dados de alta disponibilidade. Para fornecer bom desempenho para utilizadores geograficamente dispersos e permitir a realização de operações mesmo em presença de partições ou falhas de nós, estes sistemas usam técnicas de replicação optimista que garantem apenas uma coerência inevitável. Nestes cenários, é importante que a identificação de escritas concorrentes de dados, seja o mais exata e eficiente possível. Nesta dissertação, revemos os problemas com as abordagens atuais para o registo da causalidade na replicação optimista: estes ou perdem informação sobre a causalidade ou não escalam, já que obrigam as réplicas a manter informação que cresce linearmente com o número de clientes ou escritas. Propomos então, os Dotted Version Vectors (DVV), um novo mecanismo para lidar com o versionamento de dados em ambientes com coerência inevitável, que permite tanto um registo exato e correto da causalidade, bem como escalabilidade em relação ao número de clientes e número de servidores, limitando o seu tamanho ao factor de replicação. Concluímos com os desafios surgidos na implementação dos DVV no Riak (uma base de dados distribuída de chave/valor), a sua avaliação de comportamento e de desempenho, acabando com uma análise das vantagens e desvantagens da mesma

Gender, poverty and intimate partner violence in southern Africa

Includes bibliographical references.This dissertation critiques the hypothesis that the disadvantages experienced by women in terms of income, political representation etc. renders them more vulnerable to HIV infection. Using literature reviews and quantitative research methods applied to Demographic and Household Survey data from Lesotho, Swaziland and Zimbabwe, we argue that this relationship varies from country to country and contest the proposition that either structural factors or individual factors mainly affect HIV

Moment closure approximations in epidemiology

Moment closure approximation (MCA) is a method of obtaining dynamic deterministic approximations to models where spatiality is important. Such approximations track the time evolution of low-order correlations, for instance the correlation of disease status of nearest-neighbours in a square lattice. Thus they are able to capture aspects of population dynamics which traditional mean-field approximations are unable to. This thesis extends the techniques of moment closure approximation and develops novel applications for MCA in epidemiology. Most existing moment closures were intended as deterministic approximations to static regular lattices. However we develop deterministic approximations for dynamic network models and continuous space models. The purpose of applying MCA to a different set of models is not only to demonstrate their flexibility; we also explore the dynamical properties of such models with the moment closure tools we derive and with simulation data. Comparisons are then made between processes on regular lattices and processes in dynamic networks and in continuous space. Additionally, we answer questions relating to the epidemiology of sexually transmitted diseases and epidemics in populations embedded in two-dimensional continuous space. Some of the new techniques we develop can be applied to other models in ecology and epidemiology. We conclude that moment closure approximations continue to provide fertile ground for research, and that application of MCA to models other than static regular lattices can be worthwhile. Chapter 1 consists of background material and an introduction to moment closure approximations. In chapter 2 we look at the properties of moment closure approximations near critical points and during transient phases and consider their accuracy in such cases. Chapters 3 and 4 cover the application of pair approximations to sexually transmitted disease models, and chapter 5 is a preliminary study of a pair approximation for a continuous space model

A simple run--time concurrency measure

A “concurrency measure ” provides an objective means of comparing the level of parallelism achievable by a distributed computation. To date such measures have only been applicable after a computation has successfully terminated. This paper develops a notion of “observed concurrency ” that can be continuously evaluated as a computation proceeds. It is suitable for nested parallelism and evaluation either in situ or during simulation

Measuring Concurrency of Regular Distributed Computations

In this paper, we present a concurrency measure that is especially adapted to distributed programs that exhibit regular run-time behaviours. Such programs are frequently obtained by automatic parallelization of sequential code. This measure is based on the antichain lattice of the partial order that models the distributed execution under consideration. We show under which condition the measure is computable on an infinite execution which is the repetition of a finite pattern. The measure can then be computed by considering only a bounded number of patterns, this bound being at most the number of processors