MIMO control of an Apache web server: modeling and controller design

This paper considers the efficacy of feedback control in improving the performance of computing systems. Computing systems typically have many competing performance goals which are affected by several external variables. A feedback control strategy is desirable because well established techniques exist to handle these performance trade-offs and external disturbances. In order to employ such a strategy, decisions need to be made about the inputs, outputs, sample time, model type, and performance measures. This paper describes this process, which is often nebulous for computing systems, in the context of an Apache web server. A linear multi-input multi-output model of the system is identified experimentally and used to design several feedback controllers. Experimental results are presented showing the problems associated with a pure pole placement design and effectiveness of LQ control based techniques. The paper concludes with a discussion of future work.