USENIX Association Proceedings of the 2001 USENIX Annual

The increasing number of Internet users and innovative new services such as e-commerce are placing new demands on Web servers. It is becoming essential for Web servers to provide performance isolation, have fast recovery times, and provide continuous service during overload at least to preferred customers. In this paper, we present the design and implementation of three kernelbased mechanisms that protect Web servers against overload by providing admission control and service differentiation based on connection and application level information. Our basic admission control mechanism, TCP SYN policing, limits the acceptance rate of new requests based on the connection attributes. The second mechanism, prioritized listen queue, supports di erent service classes by reordering the listen queue based on the priorities of the incoming connections. Third, we present HTTP header-based connection control that uses application-level information such as URLs and cookies to set priorities and rate control policies. We have implemented these mechanisms in AIX 5.0. Through numerous experiments we demonstrate their e ectiveness in achieving the desired degree of service di erentiation during overload. We also show that the kernel mechanisms are more eÆcient and scalable than application level controls implemented in the Web server. This work was partially funded by the national Swedish Real-time Systems Research Initiative (ARTES). This work was done when the author was visiting the IBM T.J. Watson Research Center.