Effects of Neglecting Buffers in Feed-Forward Design for Web Servers

In English This paper addresses a problem related to control of web servers. Due to implementation issues, relevant measurements are only taken over the a limit part of the server system, which leaves the controller unaware of what happens in buffers proceeding the actual web-server (for instance in the TCP/IP layers). We model this and reveals that the unmeasured buffering introduces an extra zero in the relationship between the process control-signal and output. Furthermore, it is shown that a measurement of a disturbance-input actually is state dependent. We also investigate how these new properties affects a traditional feed--forward design. The stability analysis suggests that under certain circumstances, the system becomes unstable due to the feedback mechanism introduced by the designed feed-forward. We verify the analysis by both simulations and experiments.