Performance modeling and control of web servers

Abstract: This thesis deals with the task of modeling a web server and designing a
mechanism that can prevent the web server from being overloaded. Four
papers are presented. The first paper gives an M/G/1/K processor sharing
model of a single web server. The model is validated against measurements
ands imulations on the commonly usedw eb server Apache. A description
is given on how to calculate the necessary parameters in the model. The
second paper introduces an admission control mechanism for the Apache
web server basedon a combination of queuing theory andcon trol theory.
The admission control mechanism is tested in the laboratory, implemented
as a stand-alone application in front of the web server. The third paper
continues the work from the secondp aper by discussing stability. This
time, the admission control mechanism is implemented as a module within
the Apache source code. Experiments show the stability and settling time of
the controller. Finally, the fourth paper investigates the concept of service
level agreements for a web site. The agreements allow a maximum response
time anda minimal throughput to be set. The requests are sorted into
classes, where each class is assigneda weight (representing the income for
the web site owner). Then an optimization algorithm is appliedso that the
total profit for the web site during overload is maximized.