Skipping and Fetching: Insights on Non-conventional Product-Form Solutions

Complex models of computer systems are often difficult to study with numerical or analytical approaches because of the state space explosion problem. The class of product-form models is one of the most significant tools for overcoming this problem, and in many applications, this tool is the only way to perform a quantitative analysis. In this paper, we study the duality between two different product-form models. The first consists of a queuing network with finite capacity waiting rooms governed by the skip-over policy. The second is a recently presented product-form model in which a job fetching policy is applied. To investigate the relationships between these two models, we first extend the fetching queuing model to allow for finite capacity warehouses and for a Repetitive Service Blocking with Random Destination (RS-RD) discipline. Subsequently, we represent their distinctive features in terms of Generalized Stochastic Petri Nets which precisely specify their semantics in a modular manner and provide clear and intuitive interpretations of these policies. With these two preliminary results, we prove that it is possible to structurally transform a model of one class into one of the other and vice versa, thus choosing the representation that is computationally more convenient to compute the performance measures of interest.