Performance modelling for system-level design

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Parameter dependencies for reusable performance specifications of software components

To avoid design-related per­for­mance problems, model-driven performance prediction methods analyse the response times, throughputs, and re­source utilizations of software architectures before and during implementation. This thesis proposes new modeling languages and according model transformations, which allow a reusable description of usage profile dependencies to the performance of software components. Predictions based on this new methods can support performance-related design decisions

Coupled model transformations for QoS enabled component-based software design

This thesis presents the Palladio Component Model and its accompanying transformations for component-based software design with predictable performance attributes. The use of transformations results in a deterministic relationship between the model and its implementation. The introduced Coupled Transformations method uses this relationship to include implementation details into predictions to get better predictions. The approach is validated in several case studies showing the increased accuracy

Specification and refinement of software connectors

Tese de doutoramento em Informática (área de conhecimento de Fundamentos da Computação)Modern computer based systems are essentially based on the cooperation of distributed, heterogeneous component organized into open software architectures that, moreover, can survive in loosely-coupled environments and be easily adapted to changing application requirements. Such is the case, for example, of applications designed to take advantage of the increased computational power provided by massively parallel systems or of the whole business of Internet-based software development. In order to develop such systems in a systematic way, the focus in development method has switched, along the last decade, from functional to structural issues: both data and processes are encapsulated into software units which are connected into large systems resorting, to a number of techniques intended to support reusability and modifiability. Actually, the complexity and ubiquity achieved by software in present times makes it imperative, more than ever, the availability of both technologies and sound methods to drive its development. Programming ‘in–the–large’, component–based programming and software architecture become popular expressions which embody this sort of concerns and correspond to driving forces in current software engineering. In such a context this thesis aims at introducing formal models for software connectors as well as the corresponding notions of equivalence and refinement upon which calculation principles for reasoning and transforming connector-based software architectures can be developed. This research adopts an exogenous coordination point of view in order to deal with components’ temporal and spatial decoupling and, therefore, to provide support for looser levels of inter-component dependency. The thesis also characterises a notion of behavioural interface for components and services. Interfaces and connectors are put together to form configurations, an abstraction for representing software architectures. A prototype implementation of a subset of the proposed models is provided, in the form of a HASKELL library, as a proof of concept. Furthermore, the thesis reports on a case study in which exogenous coordination is applied to the specification of interactive systems.Um número crescente de sistemas computacionais é baseado na cooperação de componentes interdependentes e heterogêneas, organizadas em arquiteturas abertas capazes de sobreviverem em ambientes altamente distribuídos e facilmente adaptáveis a alterações nos requisitos das aplicações que os suportam. Tal é o caso, por exemplo, de aplicações que exploram o poder computacional de sistemas massivamente paralelos ou de sistemas desenvolvidos sobre a Internet. Para desenvolver este tipo de sistemas de forma sistemática, o foco nos métodos de desenvolvimento alterou-se, ao longo da última década, dos aspectos funcionais para os aspectos estruturais dos sistemas: ambos, estruturas de dados e processos são encapsulados em unidades computacionais que são conectadas em grandes sistemas utilizando-se de diversas técnicas que se pretendem capazes de suportar a reutilização e a adaptabilidade do software. Na realidade, a complexidade e ubiqüidade atingidas pelo software nos dias correntes tornam imperativo, mais do que nunca, a disponibilidade de tecnologias e sólidos métodos para conduzir este processo de desenvolvimento. Programação ’em-grande-escala’, programação baseada em componentes e arquiteturas de software são expressões populares que englobam esta preocupação e correspondem aos esforços direcionados pela engenharia de software. Em tal contexto, esta tese tem por objetivo introduzir modelos formais para conectores de software bem como as correspondentes noções de equivalência e refinamento que suportem cálculos para raciocinar e transformar arquiteturas de software baseada em conectores. Esta pesquisa adota um ponto de vista de coordenação exógena para lidar com a separação espacial e temporal das componentes e suportar níveis elevados de independência entre componentes. A tese caracteriza, ainda, uma noção de interface comportamental para componentes e serviços. Interfaces e conectores agregam-se para formar configurações, uma abstração introduzida para representar arquiteturas de software. A implementação, em protótipo, de parte dos modelos propostos, sob a forma de uma biblioteca em HASKELL, é fornecida como prova de conceito. Finalmente, a tese percorre um estudo de caso em que coordenação exôgena é utilizada na especificação de sistemas interactivos.Fundação para a Ciência e a Tecnologia (FCT), SFRH/BD/11083/200

Performance analysis of large-scale resource-bound computer systems

We present an analysis framework for performance evaluation of large-scale resource-bound (LSRB) computer systems. LSRB systems are those whose resources are continually in demand to serve resource users, who appear in large populations and cause high contention. In these systems, the delivery of quality service is crucial, even in the event of resource failure. Therefore, various techniques have been developed for evaluating their performance. In this thesis, we focus on the technique of quantitative modelling, where in order to study a system, first its model is constructed and then the system’s behaviour is analysed via the model. A number of high level formalisms have been developed to aid the task of model construction. We focus on PEPA, a stochastic process algebra that supports compositionality and enables us to easily build complex LSRB models. In spite of this advantage, however, the task of analysing LSRB models still poses unresolved challenges. LSRB models give rise to very large state spaces. This issue, known as the state space explosion problem, renders the techniques based on discrete state representation, such as numerical Markovian analysis, computationally expensive. Moreover, simulation techniques, such as Gillespie’s stochastic simulation algorithm, are also computationally demanding, as numerous trajectories need to be collected. Furthermore, as we show in our first contribution, the techniques based on the mean-field theory or fluid flow approximation are not readily applicable to this case. In LSRB models, resources are not assumed to be present in large populations and models exhibit highly noisy and stochastic behaviour. Thus, the mean-field deterministic behaviour might not be faithful in capturing the system’s randomness and is potentially too crude to show important aspects of their behaviours. In this case, the modeller is unable to obtain important performance indicators, such as the reliability measures of the system. Considering these limitations, we contribute the following analytical methods particularly tailored to LSRB models. First, we present an aggregation method. The aggregated model captures the evolution of only the system’s resources and allows us to efficiently derive a probability distribution over the configurations they experience. This distribution provides full faithfulness for studying the stochastic behaviour of resources. The aggregation can be applied to all LSRB models that satisfy a syntactic aggregation condition, which can be quickly checked syntactically. We present an algorithm to generate the aggregated model from the original model when this condition is satisfied. Second, we present a procedure to efficiently detect time-scale near-complete decomposability (TSND). The method of TSND allows us to analyse LSRB models at a reduced cost, by dividing their state spaces into loosely coupled blocks. However, one important input is a partition of the transitions defined in the model, categorising them into slow or fast. Forming the necessary partition by the analysis of the model’s complete state space is costly. Our process derives this partition efficiently, by relying on a theorem stating that our aggregation preserves the original model’s partition and therefore, it can be derived by an efficient reachability analysis on the aggregated state space. We also propose a clustering algorithm to implement this reachability analysis. Third, we present the method of conditional moments (MCM) to be used on LSRB models. Using our aggregation, a probability distribution is formed over the configurations of a model’s resources. The MCM outputs the time evolution of the conditional moments of the marginal distribution over resource users given the configurations of resources. Essentially, for each such configuration, we derive measures such as conditional expectation, conditional variance, etc. related to the dynamics of users. This method has a high degree of faithfulness and allows us to capture the impact of the randomness of the behaviour of resources on the users. Finally, we present the advantage of the methods we proposed in the context of a case study, which concerns the performance evaluation of a two-tier wireless network constructed based on the femto-cell macro-cell architecture