Incorporating Cost of Control into the Design of a Load Balancing Controller

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  • Venue:
  • RTAS '04 Proceedings of the 10th IEEE Real-Time and Embedded Technology and Applications Symposium
  • Year:
  • 2004

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Abstract

Load balancing is widely used in computing systems as away to optimize performance by reducing bottleneck utilizations,such as adjusting the size of buffer pools to balanceresource demands in a database management system. Loadbalancing is generally approached as a constrained optimizationproblem in which only the benefits of load balancingare considered. However, the costs of control are importantas well. Herein, we study the value of including in controllerdesign the trade-off between the cost of transient imbalancesin resource utilizations and the cost of changingresource allocations. An example of the latter are actionssuch as resizing buffer pools that can reduce throughputs.This is because requests for data in pools whose memory isreduced immediately have longer access times whereas requestsfor data in pools whose memory is increased must fillthis memory with data from disk before accessed times arereduced. We frame our study of control costs in terms of thewidely used linear quadratic regulator (LQR). We develop acost model that allows us to specify the LQR Q and R matricesbased on the impact on system performance of changingresource allocations and transient load imbalances. Ourstudies of a DB2 Universal Database Server using benchmarksfor online transaction processing and decision supportworkloads show that incorporating our cost model intothe MIMO LQR controller results in a 14% improvement inperformance beyond that achieved by dynamically allocatingthe size of buffers without properly considering the costof control.