Disturbance Rejection and Control in Web Servers

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.

• author: Kjaer, Martin Ansbjerg
• supervisor: Karl-Erik Årzén; Maria Kihl; Anders Robertsson
• opponent: Associate Professor Abdelzaher, Tarek F., University of Illinois at Urbana Champaign, USA
• organization: Department of Automatic Control
• publishing date: 2009
• type: Thesis
• publication status: published
• subject: Control Engineering
• keywords: feed-forward, dynamic prediction, computer systems, queuing theory, Web server
• in: PhD Thesis TFRT-1086
• pages: 178 pages
• defense location: Room 1406, building E, Ole Römers väg 1, Faculty of Engineering, Lund University
• defense date: 2009-11-20 13:15:00
• ISSN: 0280-5316
• ISSN: 0280-5316
• language: English
• LU publication?: yes
• id: e81378aa-35b0-472f-ac91-cbd87234e0ce (old id 1488247)
• date added to LUP: 2016-04-01 12:55:32
• date last changed: 2019-05-23 16:01:48