USENIX 2001 Abstract
Kernel Mechanisms for Service Differentiation in Overloaded Web Servers
Thiemo Voigt, Swedish Institute of Computer Science; Renu Tewari and Douglas Freimuth, IBM T.J. Watson Research Center; and Ashish Mehra, iScale Networks
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 kernel-based 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 different 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 effectiveness 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.
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