Web预取技术综述

Web预取是减少用户访问延时、提高网络服务质量的关键技术之一,近年来已成为国内外的研究热点.通过利用WWW访问的空间局部性,Web预取使缓存机制从时间局部性向空间局部性扩展.归纳了Web预取技术的分类,概括和比较了不同类别的优势和局限性,给出了预取模型的基本框架及每部分的主要功能,并对各种评价标准进行了详细介绍.同时,深入分析和探讨了现有的几种典型预取算法,系统地比较了这些算法的优缺点.最后从在线性、协作预取、动态流行度、划分用户会话和基于语义与基于路径相结合等方面指出了Web预取技术的研究方向.     

Web对象可预取性研究

目前的Web预取模型一般是基于预测算法来提前取得Web对象,缺乏对Web对象可预取性的研究.若在预取Web对象之前考虑该对象的可预取性,将减少不必要的预取,不仅可以提高预取算法的命中率,也可以减少网络流量和系统负载.详细讨论了Web对象的可预取性概念及相应的度量方法,并推导出在预取缓存交互的情况下,基于网络资源和系统负载计算预取阈值的公式,可用于Web对象的可预取性判断,指导预取控制,使得预取技术更加有效.     

流媒体代理缓存和预取方法的研究

代理缓存可以降低用户的启动延迟，减轻网络流量和服务器的负载，且在Web中也已广泛使用。但由于流媒体和非流媒体(文本，图像)有着显著的区别，目前，支持流媒体的代理缓存技术还面临着许多挑战。文中针对流媒体的数据量大和高带宽需求等特性，对支持流媒体的代理缓存和预取方法进行了总结。调研、分类、比较了目前已有的一些缓存算法和预取算法的优缺点，为下一步的研究提供启发和借鉴作用，同时指出将来的研究方向和热点问题。     

基于用户访问模式的Web预取算法

缩短Web访问中的用户感知时间,是Web应用中的一个重要问题,服务器需要预测用户未来的HTTP请求和处理当前的网页以提高Web服务器的响应速度,为此提出了一种基于用户访问模式的Web预取算法.该算法根据Web日志信息分析了用户的访问模式,并计算出Web页面间的转移概率,以此作为对用户未来请求预取的依据.实验结果表明,该预取算法能有效提高预测精度和命中率,有效地缩短了用户的感知时间.     

流媒体代理缓存和预取方法的研究

代理缓存可以降低用户的启动延迟,减轻网络流量和服务器的负载,且在Web中也已广泛使用.但由于流媒体和非流媒体(文本,图像)有着显著的区别,目前,支持流媒体的代理缓存技术还面临着许多挑战.文中针对流媒体的数据量大和高带宽需求等特性,对支持流媒体的代理缓存和预取方法进行了总结.调研、分类、比较了目前已有的一些缓存算法和预取算法的优缺点,为下一步的研究提供启发和借鉴作用,同时指出将来的研究方向和热点问题.     

浅析缓存预取技术

针对计算机系统中CPU与I／O性能差距持续扩大的的问题,引出缓存及预取的概念,并分析预取技术需要处理的几个关键问题。根据预取的发展。介绍并比较几类典型的预取算法。针对目前的各种软硬件新技术,探讨预取技术所面临的困难及其发展趋势。     

浅析缓存预取技术

针对计算机系统中CPU与I/O性能差距持续扩大的的问题,引出缓存及预取的概念,并分析预取技术需要处理的几个关键问题。根据预取的发展,介绍并比较几类典型的预取算法。针对目前的各种软硬件新技术,探讨预取技术所面临的困难及其发展趋势。     

基于Web流行度的选择Markov预取模型

Web预取技术是目前WWW中减少用户的访问延迟、提高服务质量的主要解决方案之一。该文利用Zipf第1定律和第2定律对Web对象访问流行度建模,并在此基础上,提出了基于Web流行度的选择Markov预取模型。实验表明,该预取模型不仅具有较高的命中率,而且在一定程度上还减少了对带宽的需求。     

无线局域网的Web 缓存系统研究

由于无线环境中的网络和手持设备的限制,缓存机制在提高移动系统性能方面发挥着重要作用.本文描述了适用于无线LAN的Web缓存系统EasyCache系统的实现,给出了基于数据挖掘理论的移动主机端缓存数据预取算法SEDP.模拟实验数据表明SEDP算法取得了良好的效果.     

一种智能终端数据共享中的预取缓存技术

针对智能终端数据共享中的网络延迟问题,本文提出一种两阶段,主动预取与被动预取相结合的数据预取缓存方法,减少网络延迟,提高用户体验。该方法利用网络空闲时间预取数据,减少用户等待时间;通过两阶段预取策略减少网络带宽消耗;通过主被动配合的预取算法来预取数据,提高预取准确率和预取效率;通过一种权重更新函数来更新客户端的缓存,减少对智能终端存储空间的消耗。实验表明使用此方法能减少用户等待时间58.2%,预取命中率为92%,带来的带宽损耗小于5%。     

Web缓存流量特征模型研究和应用

根据Web缓存流量访问特征建立数学模型,设计实现了Web缓存流量特征模拟生成器（WebSimGen）。利用两层代理缓存结构、基于ADF（Aggregation、Disaggregation和Filtering）模型对Web缓存流量的访问特征和性能进行测试,实验表明模拟日志具有和真实日志类似的访问特性。Web生成器具有较大的灵活性,能够克服真实日志的一些缺点,为进一步提高Web缓存性能和预取技术提供了重要依据。     

A clustering-based prefetching scheme on a Web cache environment

Web prefetching is an attractive solution to reduce the network resources consumed by Web services as well as the access latencies perceived by Web users. Unlike Web caching, which exploits the temporal locality, Web prefetching utilizes the spatial locality of Web objects. Specifically, Web prefetching fetches objects that are likely to be accessed in the near future and stores them in advance. In this context, a sophisticated combination of these two techniques may cause significant improvements on the performance of the Web infrastructure. Considering that there have been several caching policies proposed in the past, the challenge is to extend them by using data mining techniques. In this paper, we present a clustering-based prefetching scheme where a graph-based clustering algorithm identifies clusters of “correlated” Web pages based on the users’ access patterns. This scheme can be integrated easily into a Web proxy server, improving its performance. Through a simulation environment, using a real data set, we show that the proposed integrated framework is robust and effective in improving the performance of the Web caching environment.     

Latency-sensitive hashing for collaborative Web caching

Many geographically distributed proxies are increasingly used for collaborative Web caching to improve performance. In hashing-based collaborative Web caching, the response times can be negatively impacted for those URL requests hashed into geographically distant or overloaded proxies. In this paper, we present and evaluate a latency-sensitive hashing scheme for collaborative Web caching. It takes into account latency delays due to both geographical distances and dynamic load conditions. Each URL request is first hashed into an anchor hash bucket, with each bucket mapping to one of the proxies. Secondly, a number of nearby hash buckets are examined to select the proxy with the smallest latency delay to the browser. Trace-driven simulations are conducted to evaluate the performance of this new latency-sensitive hashing. The results show that (1) with the presence of load imbalance due to skew in request origination or hot-spot references, latency-sensitive hashing effectively balances the load by hashing into geographically distributed proxies for collaborative Web caching, and (2) when the overall system is lightly loaded, latency-sensitive hashing effectively reduces latency delays by directing requests to geographically closer proxies.     

Strongly competitive algorithms for caching with pipelined prefetching

Suppose that a program makes a sequence of m accesses (references) to data blocks; the cache can hold k<m blocks. An access to a block in the cache incurs one time unit, and fetching a missing block incurs d time units. A fetch of a new block can be initiated while a previous fetch is in progress; thus, min{k,d} block fetches can be in progress simultaneously. Any sequence of block references is modeled as a walk on the access graph of the program. The goal is to find a policy for prefetching and caching, which minimizes the overall execution time of a given reference sequence. This study is motivated from the pipelined operation of modern memory controllers, and from program execution on fast processors. In the offline case, we show that an algorithm proposed by Cao et al. [Proc. of SIGMETRICS, 1995, pp. 188-197] is optimal for this problem. In the online case, we give an algorithm that is within factor of 2 from the optimal in the set of online deterministic algorithms, for any access graph, and k,d?1. Better ratios are obtained for several classes of access graphs which arise in applications, including complete graphs and directed acyclic graphs (DAG).     

Web caching: A way to improve web QoS

As the Internet and World Wide Web grow at a fast pace, it is essential that the Web‘s performance should keep up with increased demand and expectations. Web Caching technology has.been widely accepted as one of the effective approaches to alleviating Web traffic and increase the Web Quality of Service (QoS). This paper provides an up-to-date survey of the rapidly expanding Web Caching literature. It discusses the state-of-the-art web caching schemes and techniques, with emphasis on the recent developments in Web Caching technology such as the differentiated Web services, heterogeneous caching network structures, and dynamic content caching.     

Integrating Web Prefetching and Caching Using Prediction Models

Web caching and prefetching have been studied in the past separately. In this paper, we present an integrated architecture for Web object caching and prefetching. Our goal is to design a prefetching system that can work with an existing Web caching system in a seamless manner. In this integrated architecture, a certain amount of caching space is reserved for prefetching. To empower the prefetching engine, a Web-object prediction model is built by mining the frequent paths from past Web log data. We show that the integrated architecture improves the performance over Web caching alone, and present our analysis on the tradeoff between the reduced latency and the potential increase in network load.     

Optimizing Web servers using Page rank prefetching for clustered accesses

This paper presents a Page rank-based prefetching technique for accesses to Web page clusters. The approach uses the link structure of a requested page to determine the “most important” linked pages and to identify the page(s) to be prefetched. The underlying premise of our approach is that in the case of cluster accesses, the next pages requested by users of the Web server are typically based on the current and previous pages requested. Furthermore, if the requested pages have a lot of links to some “important” page, that page has a higher probability of being the next one requested. An experimental evaluation of the prefetching mechanism is presented using real server logs. The results show that the Page rank-based scheme does better than random prefetching for clustered accesses, with hit rates of 90% in some cases.     

Performance improvement of web caching in Web 2.0 via knowledge discovery

Web 2.0 systems are more unpredictable and customizable than traditional web applications. This causes that performance techniques, such as web caching, limit their improvements. Our study was based on the hypotheses that the use of web caching in Web 2.0 applications, particularly in content aggregation systems, can be improved by adapting the content fragment designs. We proposed to base this adaptation on the analysis of the characterization parameters of the content elements and on the creation of a classification algorithm. This algorithm was deployed with decision trees, created in an off-line knowledge discovery process. We also defined a framework to create and adapt fragments of the web documents to reduce the user-perceived latency in web caches. The experiment results showed that our solution had a remarkable reduction in the user-perceived latency even losses in the cache hit ratios and in the overhead generated on the system, in comparison with other web cache schemes.     

Practical prefetching techniques for multiprocessor file systems

Improvements in the processing speed of multiprocessors are outpacing improvements in the speed of disk hardware. Parallel disk I/O subsystems have been proposed as one way to close the gap between processor and disk speeds. In a previous paper we showed that prefetching and caching have thepotential to deliver the performance benefits of parallel file systems to parallel applications. In this paper we describe experiments withpractical prefetching policies that base decisions only on on-line reference history, and that can be implemented efficiently. We also test the ability of those policies across a range of architectural parameters.     

自适应的分段代理缓存传输策略

结合现有的代理缓存策略和传输方案,针对现有的网络条件,提出了一种自适应的分段方法,解决了已有方法对于流媒体对象流行性的变化和用户访问模式的不确定缺乏自身调整能力的缺欠,和一种优化的传输方案,采用了单播和多播相结合, 主动预取和补丁传输相结合的方法,对于缩短启动延时、提高字节命中率以及节省骨干网带宽等方面取得了较明显的效果.