Analysis of task assignment policies in scalable distributedweb-server systems

A distributed multiserver Web site can provide the scalability necessary to keep up with growing client demand at popular sites. Load balancing of these distributed Web-server systems, consisting of multiple, homogeneous Web servers for document retrieval and a Domain Name Server (DNS) for address resolution, opens interesting new problems. In this paper, we investigate the effects of using a more active DNS which, as an atypical centralized scheduler, applies some scheduling strategy in routing the requests to the most suitable Web server. Unlike traditional parallel/distributed systems in which a centralized scheduler has full control of the system, the DNS controls only a very small fraction of the requests reaching the multiserver Web site. This peculiarity, especially in the presence of highly skewed load, makes it very difficult to achieve acceptable load balancing and avoid overloading some Web servers. This paper adapts traditional scheduling algorithms to the DNS, proposes new policies, and examines their impact under different scenarios. Extensive simulation results show the advantage of strategies that make scheduling decisions on the basis of the domain that originates the client requests and limited server state information (e.g., whether a server is overloaded or not). An initially unexpected result is that using detailed server information, especially based on history, does not seem useful in predicting the future load and can often lead to degraded performance     

Web应用服务器自适应负载平衡服务

Web应用服务器是为事务性Web应用提供一系列运行时服务的分布式系统.基于中间件的自适应负载平衡服务是为Web应用服务器提供高可信赖性和高伸缩性的一种有效方法,但目前还存在许多不足,如缺乏服务端透明性、负载策略不可替换等,不能满足Web应用服务器特有的需求.分析了Web应用服务器负载平衡服务的关键需要,设计了一种自适应负载平衡服务,阐述了在J2EE应用服务器WebFrame2.0上实现该服务的若干关键技术及其解决办法,包括可热插拔、负载策略可替换、负载反馈与自适应控制、状态迁移以及容错技术等,最后是相关工作介绍及其比较.该负载平衡服务已在Web应用服务器WebFrame2.0中得以实现.     

Adaptive Server Load Balancing in SDN Using PID Neural Network Controller

Web service applications are increasing tremendously in support of high-level businesses. There must be a need of better server load balancing mechanism for improving the performance of web services in business. Though many load balancing methods exist, there is still a need for sophisticated load balancing mechanism for not letting the clients to get frustrated. In this work, the server with minimum response time and the server having less traffic volume were selected for the aimed server to process the forthcoming requests. The Servers are probed with adaptive control of time with two thresholds L and U to indicate the status of server load in terms of response time difference as low, medium and high load by the load balancing application. Fetching the real time responses of entire servers in the server farm is a key component of this intelligent Load balancing system. Many Load Balancing schemes are based on the graded thresholds, because the exact information about the network flux is difficult to obtain. Using two thresholds L and U, it is possible to indicate the load on particular server as low, medium or high depending on the Maximum response time difference of the servers present in the server farm which is below L, between L and U or above U respectively. However, the existing works of load balancing in the server farm incorporate fixed time to measure real time response time, which in general are not optimal for all traffic conditions. Therefore, an algorithm based on Proportional Integration and Derivative neural network controller was designed with two thresholds for tuning the timing to probe the server for near optimal performance. The emulation results has shown a significant gain in the performance by tuning the threshold time. In addition to that, tuning algorithm is implemented in conjunction with Load Balancing scheme which does not tune the fixed time slots.     

Performance guarantees for Web server end-systems: acontrol-theoretical approach

The Internet is undergoing substantial changes from a communication and browsing infrastructure to a medium for conducting business and marketing a myriad of services. The World Wide Web provides a uniform and widely-accepted application interface used by these services to reach multitudes of clients. These changes place the Web server at the center of a gradually emerging e-service infrastructure with increasing requirements for service quality and reliability guarantees in an unpredictable and highly-dynamic environment. This paper describes performance control of a Web server using classical feedback control theory. We use feedback control theory to achieve overload protection, performance guarantees, and service differentiation in the presence of load unpredictability. We show that feedback control theory offers a promising analytic foundation for providing service differentiation and performance guarantees. We demonstrate how a general Web server may be modeled for purposes of performance control, present the equivalents of sensors and actuators, formulate a simple feedback loop, describe how it can leverage on real-time scheduling and feedback-control theories to achieve per-class response-time and throughput guarantees, and evaluate the efficacy of the scheme on an experimental testbed using the most popular Web server, Apache. Experimental results indicate that control-theoretic techniques offer a sound way of achieving desired performance in performance-critical Internet applications. Our QoS (Quality-of-Service) management solutions can be implemented either in middleware that is transparent to the server, or as a library called by server code     

Web服务器集群系统的实现方法及负载管理

影响WWW网站应用的一个主要因素是用户请求的响应时间太慢,这严重影响了基于Internet的各项应用的发展。本文主要阐述了如何通过多个分布式的Web服务器结点来构筑Web服务器集群系统。文中详细说明了Web服务器集群系统的实现原理及方法,并讨论提出了实出其中各个服务结点之间负载平衡的一些基本技术及机制。     

基于Web内容的集群服务器请求分配系统的研究与实现

随着基于Internet上的Web应用服务迅速发展，提供高性能、高可靠性和高扩展性的各种Web服务已成为用户的迫切需求。通过对现有的请求分配策略的分析，提出一种综合考虑后端服务器的缓存局域性的基于Web请求内容的集群服务器负载分发策略的设计与实现。     

一种LVS服务器权值的动态调节方案

以LVS为基础，实现了一个负载动态均衡方案。该方案对整个系统的负载均衡情况作出评估，选出要被调整的服务器集合，然后根据服务器的负载情况采用负反馈的方法对这些服务器的权值作出适当调整，从而实现了Web服务器集群系统的动态负载均衡。     

Web服务器负载均衡的研究

目前的WWW负载均衡大多是通过分布式Web服务器的设计来实现的。而分布式Web服务器设计中的传统技术依赖于对中心资源的操纵（如路由器或DNS服务器）。本文是对目前负载均衡技术存在问题解决方案的进一步研究,研究的目的是探索基于应用的负载均衡Web服务器（Application Based Load Balancing Web Server),避免中心资源形成系统瓶颈。     

一种基于OSI应用层的Web群集负载平衡调序策略研究

本文总结了目前基于L7的Web群集负载平衡调度研究，分析了影响性能的主要因素，在估计Web负载时考虑了请求强度以及Web服务器自身的性能，提出了处理能力异构服务器群集的最小负载调度算法。在算法中还同时考虑了服务器在进入临界状态时性能急剧下降的因素，避免群集进入临界状态．新算法能较为准确地跟踪群集系统的负载，更好地
均衡分配负载。     

基于服务器实时性能状态的动态负载均衡算法

为实现Web服务器集群合理的作业任务分配,文章提出了一种新的负载均衡算法,综合考虑了负载均衡调度器后端的业务主机的实时性能,实现了负载均衡调度的动态调整.     

Linux集群服务器的性能测试模型

为创建具有良好的可扩展性和卓越性能价格比的网络服务器,以满足快速增长的访问量和数据流量需求,介绍了Linux集群服务器的工作原理、负载均衡方式和调度算法,阐述了收集和计算Linux服务器性能测试数据的方法,创建了输入和系统模型,比较了不同调度算法对系统性能的影响。经使用Automod软件表明,该模型可以发现虚拟服务器的最大处理能力。     

基于移动Agent的双层负载均衡策略

提出了一种适用于Web集群的负载均衡策略,策略采用两次转发结合动态门限调整的双层均衡机制,将移动Agent应用于各子策略中,提高了系统调度决策的实时性,从分组和服务器节点,两个层次上实现了负载均衡,提高了服务器的cache命中率。同时,该策略还大幅度减少了均衡过程中的节点间交互,较好解决了额外通信的问题。     

基于热点内容的下载服务Web集群动态负载均衡

文中针对提供下载服务的Web集群，提出了一种新的负载均衡方案，通过对热点内容的动态冗余来提高整个集群的性能，使整个集群达到负载均衡，并由仿真实验和具体的实例验证了文中提出的方案，证明了该方案的实际效果。     

Analysis of burstiness monitoring and detection in an adaptive Web system

Due to the heavy tailed pattern of Internet traffic, it is crucial to monitor the incoming arrival rate in a Web system to preserve its performance. In this study, we focus on the arrival rate processing mechanism as part of the design of an adaptive load balancing Web algorithm. The arrival rate is one of the most important metrics to be monitored in a Web site to avoid the possible congestion of Web servers. Six methods are analysed to detect the burstiness of incoming traffic in a Web system. We define six burstiness factors to be individually included in an adaptive load balancing algorithm, which also needs to monitor some Web servers’ parameters continuously, such as the arrival rate at the server or its CPU utilization in order to avoid an unexpected overload situation.We also define adaptive time slot scheduling based on the burstiness factor, which reduces considerably the overhead of the monitoring process by increasing the monitoring frequency when bursty traffic arrives at the system and by decreasing the frequency when no bursts are detected in the arrival rate. Simulation results of the behaviour of the six burstiness factors and adaptive time slot scheduling when sudden changes are detected in the arrival rate are presented and discussed. We have considered a scenario made up of a locally distributed cluster-based Web information system for simulations.     

基于预测机制的自适应负载均衡算法

工作负载特征对Web服务器集群中负载均衡调度算法的性能有重要影响。针对负载特征在调度算法所起作用的分析和讨论,提出基于预测机制的自适应负载均衡算法（RR_MMMCS-A-P）。通过监测工作负载,预测后续请求到达率和请求大小,快速调整相应参数,实现集群中各服务器之间的负载均衡。实验表明,无论是对计算密集型任务还是数据密集型任务,RR_MMMCS-A-P同基于CPU和CPU-MEM的调度算法相比在缩短平均响应时间方面具有较好的性能。     

Distributed Downloads of Bulk,Replicated Grid Data

Data-sharing scientific communities use storage systems as distributed data stores by replicating content. In such highly replicated environments, a particular dataset can reside at multiple locations and can thus be downloaded from any one of them. Since datasets of interest are significantly large in size, improving download speeds either by server selection or by co-allocation can offer substantial benefits. In this paper, we present an architecture for co-allocating Grid data transfers across multiple connections, enabling the parallel download of datasets from multiple servers. We have developed several co-allocation strategies comprising of simple brute-force, predictive and dynamic load balancing techniques as a means both to exploit rate differences among the various client–server links and to address dynamic rate fluctuations. We evaluate our approaches using the GridFTP data movement protocol in a wide-area testbed and present our results.     

一种异构Web服务器集群动态负载均衡算法

针对Web服务器集群系统中负载动态变化特性,提出了一种临界加速递减动态请求负载分配算法．通过负载权值的等效变换更准确地反映集群中单台服务器的当前负载状态;通过临界递减因子来有效抑制服务器可能出现的“拒绝访问”现象;通过随机概率分配方式替代固定转发分配方式,使访问负载的分布更均匀;通过实际测试获取算法中所需的计算参数,使配置操作更为简单．实验结果表明,该算法对较大负载的文件集的大密度访问情况效果明显。     

Flexible and extensible load balancing

This paper presents the design philosophy and implementation of the BALANCE system. BALANCE is a flexible, network independent and computer architecture independent load balancing system which allows the building of reusable parallel and distributed applications. By implementing related services as bi generic servers with their connection endpoints registered in BALANCE, the clients can easily access the servers by server system calls. To demonstrate the flexibility of BALANCE, several widely different applications have been implemented and evaluated, including system servers, parallel and distributed applications and a scheduling testbed. The use of generic servers to improve system modularity and code reuse is also discussed. © 1997 John Wiley & Sons, Ltd.     

一种自适应文件系统元数据服务负载均衡策略

随着大数据时代的到来,全球信息存储量呈现爆发式的增长,传统的存储系统在存储性能、存储容量、数据可靠性和成本等方面存在诸多不足。近年来,以云计算平台为依托的存储技术得到了飞速的发展,成为了处理海量数据的重要工具。本文针对分布式文件系统元数据管理的问题,提出了一种自适应元数据服务负载均衡策略。该策略主要包括以下三点内容：第一,介绍了一种实时的元数据服务器的性能评价模型;第二,提出了一种基于服务器负载变化的检测周期自适应调整机制;第三,提出了一种基于元数据服务器性能指标的自适应负载均衡算法。实验证明了该方法的可行性,有效性和稳定性。     

B／S模式服务器集群系统的研究与应用

传统三层B／S模式Web应用系统在Web服务器和数据库服务器连接时,存在瓶颈问题。本文基于此问题给出了一种基于Web集群和数据库集群的负载均衡系统。试验结果表明该系统能较好的满足要求。