The emerging interests within the IT industry on utility computing and virtualization technologies have created a need for more effective workload management tools, one that dynamically controls resource allocation to a hosted application to achieve quality of service (QoS) goals. These goals can in turn be driven by the utility of the service provided, typically based on the application's service level agreement (SLA) as well as the cost of resources allocated to the application. In this paper, we build on our earlier work on dynamic CPU allocation to applications on shared servers, and present a feedback control system consisting of two nested integral control loops for managing the QoS metric of the application along with the utilization of the allocated CPU resource. The control system was implemented on a lab testbed using the Apache Web server as the application and the 90th percentile of the response times as the QoS metric. By testing the system using a synthetic workload based on an industry benchmark, we validate the two important features of the nested control design. First, compared to a single loop for controlling response time only, the nested design is less sensitive to the bimodal behavior of the system resulting in more robust performance. Second, compared to a single loop for controlling CPU utilization only, the new design provides a framework for dealing with the tradeoff between better QoS and lower cost of resources, therefore resulting in better overall utility of the service.
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