This paper presents the design, implementation, and evaluation of an adaptive architecture to provide relative delay guarantees for different service classes on web servers under HTTP 1.1. The first contribution of this paper is the architecture based on a feedback control loop that enforces desired relative delays among classes via dynamic connection scheduling and process reallocation. The second contribution is our use of feedback control theory to design the feedback loop with proven

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... ance guarantees. In contrast with ad hoc approaches that often rely on laborious tuning and design iterations, our control theory approach enables us to systematically design an adaptive web server with established analytical methods. The design methodology includes using system identification to establish a dynamic model, and using the Root Locus method to design a feedback controller to satisfy performance specifications of a web server. The adaptive architecture has been implemented by modifying an Apache web server. Experimental results demonstrate that our adaptive server achieves robust relative delay guarantees even when workload varies significantly. Properties of our adaptive web server include guaranteed stability, and satisfactory efficiency and accuracy in achieving the desired relative delay differentiation.