Feedback control of a Lotus Notes server: modeling and control design

This paper considers the modeling and feedback control of a mail server running Lotus Notes^TM. Computing systems such as this typically have two competing control goals: maximize throughput and minimize response time. To achieve these goals, a control input (tuning control) is used to limit the number of users allowed to connect to the mail server at any one time. The measured output is the server queue length-the number of requests that are waiting to be processed. Because response time is a client metric and thus difficult to measure at the server, we formulate the control problem as tracking a reference server queue length. A linear input-output model of the system is identified experimentally and used to design an integral controller. Losses in the queue length measurement due to the fact that requests are only logged after they are served are accounted for by another linear model. Experimental results are presented showing the effectiveness of a low-gain controller and the saturation problems experienced by a high-gain controller. The paper concludes with a discussion of future work