Formal techniques for performance analysis: blending SAN and PEPA

In this paper we consider two performance modelling techniques from the perspectives of model construction, generation of an underlying continuous time Markov process, and the potential for reduction in the Markov process. Such careful comparison of modelling techniques allows us to appreciate the strengths and weaknesses of different approaches, and facilitates cross-fertilization between them. In the present case we take a characteristic of one formalism, functional rates in Stochastic Automata Networks, and introduce it to the other formalism, Performance Evaluation Process Algebra. We investigate the benefits of this cross-fertilization, particularly from the perspectives of Markov process generation and reduction.     

Bio-PEPA: A framework for the modelling and analysis of biological systems

In this work we present Bio-PEPA, a process algebra for the modelling and the analysis of biochemical networks. It is a modification of PEPA, originally defined for the performance analysis of computer systems, in order to handle some features of biological models, such as stoichiometry and the use of general kinetic laws. Bio-PEPA may be seen as an intermediate, formal, compositional representation of biological systems, on which different kinds of analyses can be carried out. Bio-PEPA is enriched with some notions of equivalence. Specifically, the isomorphism and strong bisimulation for PEPA have been considered and extended to our language. Finally, we show the translation of a biological model into the new language and we report some analysis results.     

Cost-based admission control for Internet Commerce QoS enhancement

In many e-commerce systems, preserving Quality of Service (QoS) is crucial to keep a competitive edge. Poor QoS translates into poor system resource utilisation, customer dissatisfaction and profit loss. In this paper, a cost-based admission control (CBAC) approach is described which is a novel approach to preserve QoS in Internet Commerce systems. CBAC is a dynamic mechanism which uses a congestion control technique to maintain QoS while the system is online. Rather than rejecting customer requests in a high-load situation, a discount-charge model which is sensitive to system current load and navigational structure is used to encourage customers to postpone their requests. A scheduling mechanism with load forecasting is used to schedule user requests in more lightly loaded time periods. Experimental results showed that the use of CBAC at high load achieves higher profit, better utilisation of system resources and service times competitive with those which are achievable during lightly loaded periods. Throughput is sustained at reasonable levels and request failure at high load is dramatically reduced.     

A Structural Approach for Modelling Performance of Systems Using Skeletons

In this paper, we discuss a structural approach to automatic performance modelling of skeleton based applications. This uses a synthesis of performance evaluation process algebra (pepa) and a pattern-oriented hierarchical expression scheme. Such approaches are important in parallel and distributed systems where the performance models must be updated regularly based on the current state of the resources.     

A semantic equivalence for Bio-PEPA based on discretisation of continuous values

Bio-PEPA is a process algebra for modelling biological systems. An important aspect of Bio-PEPA is the ability it provides to discretise concentrations resulting in a smaller, more manageable state space. The discretisation is based on a step size which determines the size of each discrete level and also the number of levels. This paper considers the relationship between two discretisations of the same Bio-PEPA model that differ only in the step size and hence the number of levels, by using the idea of equivalence from concurrency and process algebra. We present a novel behavioural semantic equivalence, compression bisimilarity, and investigate when this equates two discretisations of the same model and the circumstances in which this equivalence is a congruence with respect to the synchronisation operator.     

Bio-PEPA: An Extension of the Process Algebra PEPA for Biochemical Networks

In this work we introduce Bio-PEPA, a process algebra for the modelling and the analysis of biochemical networks. It is a modification of PEPA to deal with some features of biological models, such as stoichiometry and the use of generic kinetic laws. Bio-PEPA may be seen as an intermediate, formal, compositional representation of biological systems, on which different kinds of analysis can be carried out. Finally, we show a representation of a model, concerning a simple genetic network, in the new language.