Abstract. A widely used approach to achieving service level objectives for a software system (e.g., an email server) is to add a controller that manipulates the target system's tuning parameters. We describe a methodology for designing such controllers for software systems that builds on classical control theory. The classical approach proceeds in two steps: system identi®cation and controller design. In system identi®cation, we construct mathematical models of the target system. Traditionally, this has been based on a ®rst-principles approach, using detailed knowledge of the target system. Such models can be complex and dif®cult to build, validate, use, and maintain. In our methodology, a statistical (ARMA) model is ®t to historical measurements of the target being controlled. These models are easier to obtain and use and allow us to apply control-theoretic design techniques to a larger class of systems. When applied to a Lotus Notes groupware server, we obtain model-®ts with R 2 no lower than 75% and as high as 98%. In controller design, an analysis of the models leads to a controller that will achieve the service level objectives. We report on an analysis of a closed-loop system using an integral control law with Lotus Notes as the target. The objective is to maintain a reference queue length. Using root-locus analysis from control theory, we are able to predict the occurrence (or absence) of controller-induced oscillations in the system's response. Such oscillations are undesirable since they increase variability, thereby resulting in a failure to meet the service level objective. We implement this controller for a real Lotus Notes system, and observe a remarkable correspondence between the behavior of the real system and the predictions of the analysis. This indicates that the control theoretic analysis is suf®cient to select controller parameters that meet the desired goals, and the need for simulations is reduced.
The authors in REF [11] demonstrate the power of a control theoretic analysis on a controller for doing admission control of a Lotus Notes workgroup server.
Using Control Theory to Achieve Service Level Objectives In Performance Management
Real-Time Systems
Real-Time Systems
Computer Science