Self-* overload control for distributed web systems

Unexpected increases in demand and most of all flash crowds are considered the bane of every web application as they may cause intolerable delays or even service unavailability. Proper quality of service policies must guarantee rapid reactivity and responsiveness even in such critical situations. Previous solutions fail to meet common performance requirements when the system has to face sudden and unpredictable surges of traffic. Indeed they often rely on a proper setting of key parameters which requires laborious manual tuning, preventing a fast adaptation of the control policies. We contribute an original Self-* Overload Control (SOC) policy. This allows the system to self-configure a dynamic constraint on the rate of admitted sessions in order to respect service level agreements and maximize the resource utilization at the same time. Our policy does not require any prior information on the incoming traffic or manual configuration of key parameters. We ran extensive simulations under a wide range of operating conditions, showing that SOC rapidly adapts to time varying traffic and self-optimizes the resource utilization. It admits as many new sessions as possible in observance of the agreements, even under intense workload variations. We compared our algorithm to previously proposed approaches highlighting a more stable behavior and a better performance.Comment: The full version of this paper, titled "Self-* through self-learning: overload control for distributed web systems", has been published on Computer Networks, Elsevier. The simulator used for the evaluation of the proposed algorithm is available for download at the address: http://www.dsi.uniroma1.it/~novella/qos_web

Control-theoretic Analysis of Admission Control Mechanisms for Web Server Systems

Web sites are exposed to high rates of incoming requests. The servers may become overloaded during temporary traffic peaks when more requests arrive than the server is designed for. An admission control mechanism rejects some requests whenever the arriving traffic is too high and thereby maintains an acceptable load in the system. This paper presents how admission control mechanisms can be designed with a combination of queueing theory and control theory. In this paper we model an Apache web server as a GI/G/1-system and then design a PI-controller, commonly used in automatic control, for the server. The controller has been implemented as a module inside the Apache source code. Measurements from the laboratory setup show how robust the implemented controller is, and how it corresponds to the results from the theoretical analysis

Estudio de mecanismos de acceso dinámico al espectro en radio cognitiva

La radio cognitiva está considerada como una prometedora solución al problema de la ineficiencia en el uso de las bandas frecuenciales en redes de acceso inalámbrico causada por el esquema de licencias tradicional que actualmente se emplea. En el marco de trabajo de la radio cognitiva, el acceso dinámico al espectro (Dynamic Spectrum Access, DSA) se refiere al procedimiento de permitir a usuarios sin licencia acceder de forma temporal a una parte del espectro asociado a un usuario con licencia. El objetivo de un protocolo DSA es, por tanto, permitir a usuarios secundarios aprovechar los spectrum holes u oportunidades espectrales con el menor impacto posible en la calidad del servicio experimentada por los usuarios primarios (por ejemplo, la probabilidad de bloqueo). Un sistema DSA puede ser implementado tanto de forma centralizada como de forma distribuida. Cuando un sistema central gestiona el espectro, obtiene la información necesaria para tomar decisiones óptimas dados unos ciertos objetivos y restricciones. Una implementación centralizada, además, puede incorporar un sistema de subasta que permita establecer el precio de las oportunidades espectrales en tiempo real. Este aspecto sin embargo no será contemplado en este proyecto, dado que estará centrado en optimizar las probabilidades de bloqueo de cada tipo de usuario. Debido a que estas decisiones son tomadas de forma dinámica (por ejemplo, el acceso a una red con usuarios que entran y salen de la misma) la programación dinámica es una herramienta matemática válida para la formulación rigurosa de un problema de decisión DSA. Adicionalmente, el proceso de ocupación del espectro, del mismo modo que cualquier otro sistema de comunicaciones, puede ser caracterizado con un modelo de Markov que, aplicado a la formulación de la programación dinámica, resulta en un proceso de decisión de Markov (MDP).Escuela Técnica Superior de Ingeniería de Telecomunicació

Avoiding Bad Query Mixes to Minimize Unsuccessful Client Requests Under Heavy Loads

In three-tiered web applications, some form of admission control is required to ensure that throughput and response times are not significantly harmed during periods of heavy load. We propose Q-Cop, a prototype system for improving admission control decisions that computes measures of load on the system based on the actual mix of queries being executed. This measure of load is used to estimate execution times for incoming queries, which allows Q-Cop to make control decisions with the goal of minimizing the number of requests that are not serviced before the client, or their browser, times out. Using TPC-W queries, we show that the response times of different types of queries can vary significantly, in excess of 50% in our experiments, depending not just on the number of queries being processed but on the mix of other queries that are running simultaneously. The variation implies that admission control can benefit from taking into account not just the number of queries being processed, but also the mix of queries. We develop a model of expected query execution times that accounts for the mix of queries being executed and integrate that model into a three-tiered system to make admission control decisions. This approach makes more informed decisions about which queries to reject, and our results show that it significantly reduces the number of unsuccessful client requests. Our results show that this approach makes more informed decisions about which queries to reject and as a result significantly reduces the number of unsuccessful client requests. For comparison, we develop several other models which represent related work in the field, including an MPL-based approach and an approach that considers the type of query but not the mix of queries. We show that Q-Cop does not need to re-compute any modelling information in order to perform well, a strong advantage over most other approaches. Across the range of workloads examined, an average of 47% fewer requests are denied than the next best approach

Limites de capacidade e proteçao de servidores em redes gigabit

Orientadora: Cristina Duarte MurtaDissertaçao (mestrado) - Universidade Federal do Paraná, Setor de Ciencias Exatas, Programa de Pós-Graduaçao em Informática. Defesa: Curitiba, 2005Inclui bibliografi

Diseño de sistemas de subasta dinámica de espectro en radio cognitiva

En un sistema de radio cognitiva, el acceso dinámico al espectro (Dynamic Spectrum Access, DSA) hace referencia a un protocolo mediante el cual se permite a usuarios no licenciados o “secundarios” el uso temporal de una parte del espectro asignado a usuarios con licencia o “primarios”. En este trabajo se presenta una triple formulación: MDP, optimización mediante programación lineal y simulación, para modelar un escenario basado en un protocolo DSA de implementación centralizada con acceso basado en subasta, cuyo objetivo principal es la obtención del máximo rendimiento económico posible manteniendo la menor probabilidad de bloqueo para los usuarios primarios.Escuela Técnica Superior de Ingeniería de Telecomunicació

Effective Resource and Workload Management in Data Centers

The increasing demand for storage, computation, and business continuity has driven the growth of data centers. Managing data centers efficiently is a difficult task because of the wide variety of datacenter applications, their ever-changing intensities, and the fact that application performance targets may differ widely. Server virtualization has been a game-changing technology for IT, providing the possibility to support multiple virtual machines (VMs) simultaneously. This dissertation focuses on how virtualization technologies can be utilized to develop new tools for maintaining high resource utilization, for achieving high application performance, and for reducing the cost of data center management.;For multi-tiered applications, bursty workload traffic can significantly deteriorate performance. This dissertation proposes an admission control algorithm AWAIT, for handling overloading conditions in multi-tier web services. AWAIT places on hold requests of accepted sessions and refuses to admit new sessions when the system is in a sudden workload surge. to meet the service-level objective, AWAIT serves the requests in the blocking queue with high priority. The size of the queue is dynamically determined according to the workload burstiness.;Many admission control policies are triggered by instantaneous measurements of system resource usage, e.g., CPU utilization. This dissertation first demonstrates that directly measuring virtual machine resource utilizations with standard tools cannot always lead to accurate estimates. A directed factor graph (DFG) model is defined to model the dependencies among multiple types of resources across physical and virtual layers.;Virtualized data centers always enable sharing of resources among hosted applications for achieving high resource utilization. However, it is difficult to satisfy application SLOs on a shared infrastructure, as application workloads patterns change over time. AppRM, an automated management system not only allocates right amount of resources to applications for their performance target but also adjusts to dynamic workloads using an adaptive model.;Server consolidation is one of the key applications of server virtualization. This dissertation proposes a VM consolidation mechanism, first by extending the fair load balancing scheme for multi-dimensional vector scheduling, and then by using a queueing network model to capture the service contentions for a particular virtual machine placement

Performance modelling and analysis of e-commerce systems using class based priority scheduling. An investigation into the development of new class based priority scheduling mechanisms for e-commerce system combining different techniques.

Recently, technological developments have affected most lifestyles, especially with the growth in Internet usage. Internet applications highlight the E-commerce capabilities and applications which are now available everywhere; they receive a great number of users on a 24-7 basis because online services are easy to use, faster and cheaper to acquire. Thus E-commerce web sites have become crucial for companies to increase their revenues. This importance has identified certain effective requirements needed from the performance of these applications. In particular, if the web server is overloaded, poor performance can result, due to either a huge rate of requests being generated which are beyond the server¿s capacity, or due to saturation of the communication links capacity which connects the web server to the network. Recent researches consider the overload issue and explore different mechanisms for managing the performance of E-commerce applications under overload condition. This thesis proposes a formal approach in order to investigate the effects of the extreme load and the number of dropped requests on the performance of E- III commerce web servers. The proposed approach is based on the class-based priority scheme that classifies E-commerce requests into different classes. Because no single technique can solve all aspects of overload problems, this research combines several techniques including: admission control mechanism, session-based admission control, service differentiation, request scheduling and queuing model-based approach. Request classification is based on the premise that some requests (e.g. buy) are generally considered more important than others (e.g. browse or search). Moreover, this research considers the extended models from Priority Scheduling Mechanism (PSM). These models add a new parameter, such as a review model or modify the basic PSM to low priority fair model, after the discovery of ineffectiveness with low priority customers or to add new features such as portal models. The proposed model is formally specified using the ¿ -calculus in early stage of models design and a multi-actor simulation was developed to reflect the target models as accurately as possible and is implemented as a Java-based prototype system. A formal specification that captures the essential PSM features while keeping the performance model sufficiently simple is presented. Furthermore, the simplicity of the UML bridges the gap between ¿-calculus and Java programming language. IV There are many metrics for measuring the performance of E-commerce web servers. This research focuses on the performance of E-commerce web servers that refer to the throughput, utilisation, average response time, dropped requests and arrival rate. A number of experiments are conducted in order to test the performance management of the proposed approaches

Revenue maximization problems in commercial data centers

As IT systems are becoming more important everyday, one of the main concerns is that users may face major problems and eventually incur major costs if computing systems do not meet the expected performance requirements: customers expect reliability and performance guarantees, while underperforming systems loose revenues. Even with the adoption of data centers as the hub of IT organizations and provider of business efficiencies the problems are not over because it is extremely difficult for service providers to meet the promised performance guarantees in the face of unpredictable demand. One possible approach is the adoption of Service Level Agreements (SLAs), contracts that specify a level of performance that must be met and compensations in case of failure. In this thesis I will address some of the performance problems arising when IT companies sell the service of running ‘jobs’ subject to Quality of Service (QoS) constraints. In particular, the aim is to improve the efficiency of service provisioning systems by allowing them to adapt to changing demand conditions. First, I will define the problem in terms of an utility function to maximize. Two different models are analyzed, one for single jobs and the other useful to deal with session-based traffic. Then, I will introduce an autonomic model for service provision. The architecture consists of a set of hosted applications that share a certain number of servers. The system collects demand and performance statistics and estimates traffic parameters. These estimates are used by management policies which implement dynamic resource allocation and admission algorithms. Results from a number of experiments show that the performance of these heuristics is close to optimal.EThOS - Electronic Theses Online ServiceQoSP (Quality of Service Provisioning) : British TelecomGBUnited Kingdo