The performance of a system is determined by its characteristics as well as by the composition of the load being processed. Hence, its quantitative description is a fundamental part of all performance evaluation studies. Several methodologies for the construction of workload models, which are functions of the objective o f the study, of the architecture of the system to be analyzed, and of the techniques adopted, are presented. A survey of a few applications of these methodologies to various types of systems (i.e., batch, interactive, database, network-based, parallel, supercomputer) is given.

The availability of reliable models of computer virus propagation would prove useful in a number of ways, in order both to predict future threats, and to develop new containment measures. In this paper, we review the most popular models of virus propagation, analyzing the underlying assumptions of each of them, their strengths and their weaknesses. We also introduce a new model, which extends the Random Constant Spread modeling technique, allowing us to draw some conclusions about the behavior of the Internet infrastructure in presence of a self-replicating worm. A comparison of the results of the model with the actual behavior of the infrastructure during recent worm outbreaks is also presented.

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Several of the principal results in bottleneck andlysis for closed queueing networks are surveyed. Both product-form closed queueing networks, where exact bottleneck analysis is possible, and non-product-form closed queueing networks, where approximations are given for asymptotic bottleneck behavior, are considered. Algorithms for the asymptotic bottleneck analysis are presented and the switching surfaces of bottlenecks are described.

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