In-kernel Mechanisms for Adaptive Control of Overloaded Web Servers

The increasing number of Internet users and innovative new services such as e-commerce are placing new demands on Internet servers, for example web servers. It is becoming essential for Internet servers to be highly available, have fast response times, and provide continuous service during overload at least to preferred customers. It is necessary, therefore, to protect Internet servers from overload since during server overload clients experience increased response times and service failures. In this paper we present a kernel-based architecture that protects Internet servers against overload by controlling the amount and rate of work entering the system. Our basic control algorithm limits the incoming TCP SYN requests based on connection attributes. By dropping non-compliant connection requests, the server can provide continuous service to preferred clients even under overload. We present a second mechanism that reorders the listen queue of a server socket based on the priorities of the incoming connection requests. Our experiments show that both mechanisms enable service differentiation during overload. We describe an adaptive architecture that uses these mechanisms to dynamically protect web servers from overload.