We consider single-pass, lossless, queueing systems at steady-state subject to Poisson job arrivals at an unknown rate. Service rates are allowed to depend on the number of jobs in the system, up to a fixed maximum, and power consumption is an increasing function of speed. The goal is to control the state dependent service rates such that both energy consumption and delay are kept low. We consider a linear combination of the mean job delay and energy consumption as the performance measure. We examine both the 'architecture' of the system, which we define as a specification of the number of speeds that the system can choose from, and the 'design' of the system, which we define as the actual speeds available. Previous work has illustrated that when the arrival rate is precisely known, there is little benefit in introducing complex (multi-speed) architectures, yet in view of parameter uncertainty, allowing a variable number of speeds improves robustness. We quantify the tradeoffs of architecture specification with respect to robustness, analysing both global robustness and a newly defined measure which we call local robustness.Keywords: parameter uncertainty; robust design; controlled single server queue; speed scaling
IntroductionPerformance analysis, design and control by means of stochastic queueing models (cf. Wolff 1989) has affected a variety of fields, including not only telecommunications and computing systems but also service engineering, manufacturing, logistics, health-care, road traffic and biological modelling. A typical queueing model abstracts unknown job arrival and service requirements by means of stochastic processes and distributions. The resulting dynamics of queue-length, workload or other performance processes are analysed yielding performance measures that ultimately allow for better design and control of the system at hand. Design of the system often refers to an off-line specification of parameters whereas control of the system typically refers to an on-line decision making based on state measurements (e.g. setting service speeds). In this paper we shall use a third term, architecture selection, referring to the action of deciding what are the design and control parameters that are available to process.Almost all of the queueing theoretic, performance analysis, design, control and architecture selection literature is based on the underlying assumption that the probability laws of arrival and service processes are precisely known. A few exceptions to this rule are mentioned later in this section. In practice, this assumption is often too strong, especially due to the fact that obtaining precise a priori parameter estimates is not possible in many settings. Our