337 research outputs found
Dependence-driven techniques in system design
Burstiness in workloads is often found in multi-tier architectures, storage systems, and communication networks. This feature is extremely important in system design because it can significantly degrade system performance and availability. This dissertation focuses on how to use knowledge of burstiness to develop new techniques and tools for performance prediction, scheduling, and resource allocation under bursty workload conditions.;For multi-tier enterprise systems, burstiness in the service times is catastrophic for performance. Via detailed experimentation, we identify the cause of performance degradation on the persistent bottleneck switch among various servers. This results in an unstable behavior that cannot be captured by existing capacity planning models. In this dissertation, beyond identifying the cause and effects of bottleneck switch in multi-tier systems, we also propose modifications to the classic TPC-W benchmark to emulate bursty arrivals in multi-tier systems.;This dissertation also demonstrates how burstiness can be used to improve system performance. Two dependence-driven scheduling policies, SWAP and ALoC, are developed. These general scheduling policies counteract burstiness in workloads and maintain high availability by delaying selected requests that contribute to burstiness. Extensive experiments show that both SWAP and ALoC achieve good estimates of service times based on the knowledge of burstiness in the service process. as a result, SWAP successfully approximates the shortest job first (SJF) scheduling without requiring a priori information of job service times. ALoC adaptively controls system load by infinitely delaying only a small fraction of the incoming requests.;The knowledge of burstiness can also be used to forecast the length of idle intervals in storage systems. In practice, background activities are scheduled during system idle times. The scheduling of background jobs is crucial in terms of the performance degradation of foreground jobs and the utilization of idle times. In this dissertation, new background scheduling schemes are designed to determine when and for how long idle times can be used for serving background jobs, without violating predefined performance targets of foreground jobs. Extensive trace-driven simulation results illustrate that the proposed schemes are effective and robust in a wide range of system conditions. Furthermore, if there is burstiness within idle times, then maintenance features like disk scrubbing and intra-disk data redundancy can be successfully scheduled as background activities during idle times
Dynamic bandwidth allocation in ATM networks
Includes bibliographical references.This thesis investigates bandwidth allocation methodologies to transport new emerging bursty traffic types in ATM networks. However, existing ATM traffic management solutions are not readily able to handle the inevitable problem of congestion as result of the bursty traffic from the new emerging services. This research basically addresses bandwidth allocation issues for bursty traffic by proposing and exploring the concept of dynamic bandwidth allocation and comparing it to the traditional static bandwidth allocation schemes
Methods of Congestion Control for Adaptive Continuous Media
Since the first exchange of data between machines in different locations in early 1960s,
computer networks have grown exponentially with millions of people now using the
Internet. With this, there has also been a rapid increase in different kinds of services offered
over the World Wide Web from simple e-mails to streaming video. It is generally accepted
that the commonly used protocol suite TCP/IP alone is not adequate for a number of
modern applications with high bandwidth and minimal delay requirements. Many
technologies are emerging such as IPv6, Diffserv, Intserv etc, which aim to replace the onesize-fits-all approach of the current lPv4. There is a consensus that the networks will have
to be capable of multi-service and will have to isolate different classes of traffic through
bandwidth partitioning such that, for example, low priority best-effort traffic does not cause
delay for high priority video traffic. However, this research identifies that even within a
class there may be delays or losses due to congestion and the problem will require different
solutions in different classes.
The focus of this research is on the requirements of the adaptive continuous media
class. These are traffic flows that require a good Quality of Service but are also able to
adapt to the network conditions by accepting some degradation in quality. It is potentially
the most flexible traffic class and therefore, one of the most useful types for an increasing
number of applications.
This thesis discusses the QoS requirements of adaptive continuous media and
identifies an ideal feedback based control system that would be suitable for this class. A
number of current methods of congestion control have been investigated and two methods
that have been shown to be successful with data traffic have been evaluated to ascertain if
they could be adapted for adaptive continuous media. A novel method of control based on
percentile monitoring of the queue occupancy is then proposed and developed. Simulation
results demonstrate that the percentile monitoring based method is more appropriate to this
type of flow. The problem of congestion control at aggregating nodes of the network
hierarchy, where thousands of adaptive flows may be aggregated to a single flow, is then
considered. A unique method of pricing mean and variance is developed such that each
individual flow is charged fairly for its contribution to the congestion
Statistical multiplexing and connection admission control in ATM networks
Asynchronous Transfer Mode (ATM) technology is widely employed for the transport of network traffic, and has the potential to be the base technology for the next generation of global communications. Connection Admission Control (CAC) is the effective traffic control mechanism which is necessary in ATM networks in order to avoid possible congestion at each network node and to achieve the Quality-of-Service (QoS) requested by each connection. CAC determines whether or not the network should accept a new connection. A new connection will only be accepted if the network has sufficient resources to meet its QoS requirements without affecting the QoS commitments already made by the network for existing connections. The design of a high-performance CAC is based on an in-depth understanding of the statistical characteristics of the traffic sources
Some aspects of traffic control and performance evaluation of ATM networks
The emerging high-speed Asynchronous Transfer Mode (ATM) networks are expected to integrate through statistical multiplexing large numbers of traffic sources having a broad range of statistical characteristics and different Quality of Service (QOS) requirements. To achieve high utilisation of network resources while maintaining the QOS, efficient traffic management strategies have to be developed. This thesis considers the problem of traffic control for ATM networks. The thesis studies the application of neural networks to various ATM traffic control issues such as feedback congestion control, traffic characterization, bandwidth estimation, and Call Admission Control (CAC). A novel adaptive congestion control approach based on a neural network that uses reinforcement learning is developed. It is shown that the neural controller is very effective in providing general QOS control. A Finite Impulse Response (FIR) neural network is proposed to adaptively predict the traffic arrival process by learning the relationship between the past and future traffic variations. On the basis of this prediction, a feedback flow control scheme at input access nodes of the network is presented. Simulation results demonstrate significant performance improvement over conventional control mechanisms. In addition, an accurate yet computationally efficient approach to effective bandwidth estimation for multiplexed connections is investigated. In this method, a feed forward neural network is employed to model the nonlinear relationship between the effective bandwidth and the traffic situations and a QOS measure. Applications of this approach to admission control, bandwidth allocation and dynamic routing are also discussed. A detailed investigation has indicated that CAC schemes based on effective bandwidth approximation can be very conservative and prevent optimal use of network resources. A modified effective bandwidth CAC approach is therefore proposed to overcome the drawback of conventional methods. Considering statistical multiplexing between traffic sources, we directly calculate the effective bandwidth of the aggregate traffic which is modelled by a two-state Markov modulated Poisson process via matching four important statistics. We use the theory of large deviations to provide a unified description of effective bandwidths for various traffic sources and the associated ATM multiplexer queueing performance approximations, illustrating their strengths and limitations. In addition, a more accurate estimation method for ATM QOS parameters based on the Bahadur-Rao theorem is proposed, which is a refinement of the original effective bandwidth approximation and can lead to higher link utilisation
JMT – Performance Engineering Tools for System Modeling
We present the Java Modelling Tools (JMT) suite, an integrated
framework of Java tools for performance evaluation of computer
systems using queueing models. The suite offers a rich user interface that simplifies the definition of performance models by means of wizard dialogs and of a graphical design workspace.
The performance evaluation features of JMT span a wide range
of state-of-the-art methodologies including discrete-event simulation, mean value analysis of product-form networks, analytical identification of bottleneck resources in multiclass environments, and workload characterization with fuzzy clustering. The discrete-event simulator supports several advanced modeling features such as finite capacity regions, load-dependent service times, bursty processes, fork-and-join nodes, and implements spectral estimation for analysis of simulative results. The suite is open-source, released under the GNU general public license (GPL), and it is available for
free download at http://jmt.sourceforge.net
Stochastic Dynamic Programming and Stochastic Fluid-Flow Models in the Design and Analysis of Web-Server Farms
A Web-server farm is a specialized facility designed specifically for housing Web
servers catering to one or more Internet facing Web sites. In this dissertation, stochastic
dynamic programming technique is used to obtain the optimal admission control
policy with different classes of customers, and stochastic
uid-
ow models
are used to compute the performance measures in the network. The two types of
network traffic considered in this research are streaming (guaranteed bandwidth per
connection) and elastic (shares available bandwidth equally among connections).
We first obtain the optimal admission control policy using stochastic dynamic
programming, in which, based on the number of requests of each type being served,
a decision is made whether to allow or deny service to an incoming request. In
this subproblem, we consider a xed bandwidth capacity server, which allocates the
requested bandwidth to the streaming requests and divides all of the remaining bandwidth
equally among all of the elastic requests. The performance metric of interest in
this case will be the blocking probability of streaming traffic, which will be computed
in order to be able to provide Quality of Service (QoS) guarantees.
Next, we obtain bounds on the expected waiting time in the system for elastic
requests that enter the system. This will be done at the server level in such a way
that the total available bandwidth for the requests is constant. Trace data will be
converted to an ON-OFF source and
fluid-
flow models will be used for this analysis. The results are compared with both the mean waiting time obtained by simulating
real data, and the expected waiting time obtained using traditional queueing models.
Finally, we consider the network of servers and routers within the Web farm where
data from servers
flows and merges before getting transmitted to the requesting users
via the Internet. We compute the waiting time of the elastic requests at intermediate
and edge nodes by obtaining the distribution of the out
ow of the upstream node.
This out
ow distribution is obtained by using a methodology based on minimizing the
deviations from the constituent in
flows. This analysis also helps us to compute waiting
times at different bandwidth capacities, and hence obtain a suitable bandwidth to
promise or satisfy the QoS guarantees.
This research helps in obtaining performance measures for different traffic classes
at a Web-server farm so as to be able to promise or provide QoS guarantees; while at
the same time helping in utilizing the resources of the server farms efficiently, thereby
reducing the operational costs and increasing energy savings
Quality aspects of Internet telephony
Internet telephony has had a tremendous impact on how people communicate.
Many now maintain contact using some form of Internet telephony.
Therefore the motivation for this work has been to address the quality aspects
of real-world Internet telephony for both fixed and wireless telecommunication.
The focus has been on the quality aspects of voice communication,
since poor quality leads often to user dissatisfaction. The scope of the work
has been broad in order to address the main factors within IP-based voice
communication.
The first four chapters of this dissertation constitute the background
material. The first chapter outlines where Internet telephony is deployed
today. It also motivates the topics and techniques used in this research.
The second chapter provides the background on Internet telephony including
signalling, speech coding and voice Internetworking. The third chapter
focuses solely on quality measures for packetised voice systems and finally
the fourth chapter is devoted to the history of voice research.
The appendix of this dissertation constitutes the research contributions.
It includes an examination of the access network, focusing on how calls are
multiplexed in wired and wireless systems. Subsequently in the wireless
case, we consider how to handover calls from 802.11 networks to the cellular
infrastructure. We then consider the Internet backbone where most of our
work is devoted to measurements specifically for Internet telephony. The
applications of these measurements have been estimating telephony arrival
processes, measuring call quality, and quantifying the trend in Internet telephony
quality over several years. We also consider the end systems, since
they are responsible for reconstructing a voice stream given loss and delay
constraints. Finally we estimate voice quality using the ITU proposal PESQ
and the packet loss process.
The main contribution of this work is a systematic examination of Internet
telephony. We describe several methods to enable adaptable solutions
for maintaining consistent voice quality. We have also found that relatively
small technical changes can lead to substantial user quality improvements.
A second contribution of this work is a suite of software tools designed to
ascertain voice quality in IP networks. Some of these tools are in use within
commercial systems today
- …