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This paper considers the use of feedback to improve the performance of computing systems that offer differentiated services. The motivation of the work is based on the increasing demand on application servers. It is not always sufficient to buy high-performance software for the server. Multiple servers may be needed. To guarantee that QoS requirements are satisfied, it is possible to statically assign resources for a specific class. This often results in underutilization of resources. This paper describes a novel technique that is based on control theory principles applied to a server cluster that provides differentiated service. The paper shows that feedback can be used to adjust the number of client requests concurrently being processed based on dynamic information such as CPU utilization. The paper also compares the use of the proposed technique with a dynamic non control-theoretic approach that is not based on control theory principles. Results show a dramatic increase in the number of served users using the control theory principles compared with a non control-theoretic approach during the same experiment duration. The improvement provided by the proposed technique exceeded 20%.