Disturbance Rejection and Control in Web Servers

Martin Ansbjerg Kjaer

Research output: ThesisDoctoral Thesis (monograph)

232 Downloads (Pure)

Abstract

An important factor for a user of web sites on the Internet
is the duration of time between the request of a web page until an
answer has been returned. If this response time is too long, the user
is likely to abandon the web site and search for other providers of
the service. To avoid this loss of users, it is important for the web
site operator to assure that users are treated sufficiently fast. On
the other hand, it is also important to minimize the effort to
optimize profit. As these objectives often are contradictory, an
acceptable target response-time that can be formulated. The resources
are allocated in a manner that ensures that long response times
do not occur, while, at the same time, using as little resources as
possible to not overprovision.

The work presented in this doctoral thesis takes a
control-theoretic perspective to solve this problem. The resources
are considered as the control input, and the response time as the main
output. Several disturbances affect the system, such as the arrival
rate of requests to the web site. A testbed was designed to
allow repeatable experiments with different controller
implementations. A server was instrumented with sensors and actuators
to handle requests from 12 client computers with capability for
changing work loads.

On the theoretical side, a model of a web server is presented in this
thesis. It explicitly models a specific sensor implementation where
buffering occurs in the computer prior to the sensor. As a result, the
measurement of the arrival rate becomes state dependent under high
load. This property turns out to have some undesirable effects on the
controlled system. The model was capable of predicting the behavior of
the testbed quite well.

Based on the presented model, analysis shows that
feed-forward controllers suggested in the literature can lead to
instability under certain circumstances at high load. This has not
been reported earlier, but is in this doctoral thesis
demonstrated by both simulations and experiments. The analysis
explains why and when the instability arises.

In the attempt to predict future response-times this thesis also
presents a feedback based prediction scheme. Comparisons between
earlier predictions to the real response-times are used to correct a
model based response time prediction. The prediction scheme is
applied to a controller to compensate for disturbances before the
effect propagates to the response time. The method improves the
transient response in the case of sudden changes in the arrival
rate of requests.

This doctoral thesis also presents work on a control solution
for reserving CPU capacity for a given process or a given group of
processes on a computer system. The method uses only existing
operating-system infrastructure, and achieves the desired
CPU capacity in a soft real-time manner.
Original languageEnglish
QualificationDoctor
Awarding Institution
  • Department of Automatic Control
Supervisors/Advisors
  • Årzén, Karl-Erik, Supervisor
  • Kihl, Maria, Supervisor
  • Robertsson, Anders, Supervisor
Award date2009 Nov 20
Publisher
Publication statusPublished - 2009

Bibliographical note

Defence details

Date: 2009-11-20
Time: 13:15
Place: Room 1406, building E, Ole Römers väg 1, Faculty of Engineering, Lund University

External reviewer(s)

Name: Abdelzaher, Tarek F.
Title: Associate Professor
Affiliation: University of Illinois at Urbana Champaign, USA

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Subject classification (UKÄ)

  • Control Engineering

Keywords

  • feed-forward
  • dynamic prediction
  • computer systems
  • queuing theory
  • Web server

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