Kernal Mechanisms for Service Differentiation in Overloaded Web Servers

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 a kernel-based
architecture that protects Web servers against overload by controlling the
amount and rate of work entering the system. We present three mechanisms
that provide 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 different service classes by reordering
the listen queue of a server socket based on the priorities of the incoming
connection requests. Third, we present URL-based connection control
that uses application-level information such as URLs and cookies to define
priorities and rate control policies.

We have implemented these mechanisms in AIX 5.0. Through numerous experiments
we demonstrate the effectiveness of
these mechanisms in achieving the desired degree of service differentiation during
overload. We also show that the kernel mechanisms are more
efficient and scalable than application level controls implemented
in the Web server.