移动数据库缓存模型研究

针对移动数据库系统性能有待提高的问题,提出了一种移动数据库缓存模型。采用基于消息摘要的同步算法,通过比较移动客户端与服务器消息摘要表中的消息摘要值,完成缓存同步,维护移动客户端缓存与服务器数据的一致性;该模型还考虑了数据的时效性与事务的优先级,设计了一种基于价值函数的缓存替换算法。实验结果表明,随着缓存数据个数的增加,所提算法的缓存命中率高于最近最少使用(LRU)和LA2U算法,同时随着访问频率的增加,事务的重启率低于LRU和LA2U,有效提高了移动数据库缓存的性能。     

基于协同过滤的Web缓存替换算法研究

针对GDSF替换算法中对访问频率缺少预测的不足,提出了一种基于协同过滤的GDSF缓存替换算法(GDSF-CF)。该算法考虑了Web对象之间相似性与用户访问时间间隔,运用协同过滤算法生成Web对象的预测访问频率,并采用齐普夫定律参数对GDSF算法的目标函数进行了改进。当需要进行缓存替换时,利用目标函数价值计算缓存空间中的每个Web对象缓存价值,将最小缓存价值的Web对象进行替换。仿真实验结果表明,该算法的命中率HR和字节命中率BHR都有较大提升。     

最小驻留价值缓存替换算法

为提高搜索应用的缓存性能,提出一种新的缓存替换算法--最小驻留价值(LCV)算法。该算法通过计算对象访问频率,结合对象大小,优先选取对字节命中率贡献最小的对象集进行缓存替换。同时,将最优替换对象集的选取转化为经典0-1背包问题进行了求解,并给出一种快速近似解法及其算法数据结构。在与最近最少使用(LRU)、先进先出(FIFO)和考虑多重因子(GD-Size)算法的对比实验中,LCV算法在提高字节命中率(BHR)和降低平均延时时间(ALT)方面具有更好的性能。     

输入缓存Clos网络中分布式正交分路路由算法

提出了一种采用输入缓存MSM结构的Clos网络,该结构适用于高速交换网络。提出了这一结构中的路由算法,该算法采用正交分路的方法来减小网络内部的冲突,引入路由优先级来提高网络内部的链路利用率,使用优先级轮转来均衡网络内部负载。针对这一路由算法,还给出了与之对应的信元调度算法。仿真表明,尽管采用共享缓存的MSM结构内部使用了很高的加速比,但是采用了正交分路的路由算法后,输入缓存MSM结构,可以获得比共享缓存MSM结构更好的时延及吞吐性能,更适合在高速大容量多端口的路由器或交换机中采用。     

一种基于流量预测的多优先级队列缓存管理算法

在引入优先级因素的流量预测模型基础上，提出了一种多优先级队列的缓存管理算法，该算法通过固定时刻的缓存预分配，结合时划间隔中的缓存竞争，使缓存的分配更加合理而有预见性，仿真实输表明它能够明最降低高优先级数据包的丢包率，同时兼顾低优先级数据包的转发，并且不产生额外的延迟，表明该算法有很好的应用价值。     

流媒体缓存的自适应统计窗缓存算法

网络带宽和磁盘I/O带宽是视频点播系统的两大瓶颈.为了缓解磁盘I/O带宽问题,本文引入统计窗概念.提出自适应统计窗缓存算法,采用周期性缓存决策方法管理缓存的流媒体数据,同时采用区分优先级缓存策略和冗余释放策略提高算法性能.采用实际点播数据进行的仿真研究表明:自适应统计窗缓存算法的性能优于定长分段、指数分段和自适应分段算法,特别是在VCR操作时,本算法的性能更佳.     

基于多优先级的动态阈值RED算法

提出一种基于多优先级的动态阈值RED算法(PDT-RED)。根据分组优先级和未使用的缓存空间动态调节平均队列阈值,实现对不同类型的分组采取不同的标记/丢弃概率的区分服务。为了提高缓存利用率,在该算法基础上提出优化算法(OPDT-RED)。模拟实验证明,与同类算法相比,OPDT-RED使高优先级的分组丢失率降低了56%~62%,平均分组丢失率降低了33%~41%,并且简单、易于实现,能提高缓存的利用率。     

对象存储中基于高斯分布的分层缓存淘汰算法

在分布式存储系统中,引入分层缓存技术是优化系统读写的重要方法。目前分层缓存技术多使用LRU及其改进算法管理缓存空间,该方式虽然一定程度上改善了缓存性能,但有限的缓存命中率也使其成为性能瓶颈。针对此问题,本文研究聚焦于分布式对象存储在云计算场景下的应用特点,设计了一种基于高斯分布的淘汰算法。算法基于高斯分布的形态特征设计淘汰规则,规避了LRU及其改进算法采用频率估计概率的误差影响。通过仿真实验结果表明,在用户访问符合高斯分布时,本文提出的基于高斯分布的分层缓存淘汰算法能有效提高缓存命中率。     

一种自适应的数据库缓存模型

缓存可以提高应用系统的性能.但应用系统使用数据的情形是动态变化的,特别当数据更改数量大时,固定缓存会使应用系统的性能急剧下降.为了取得更好的性能,缓存应该根据应用系统的动态变化相应动态改变其数据和大小.缓存中的各类数据的查询、更改的频率是不同的,根据这一特点,提出了一种调整缓存的算法.当应用系统繁忙或负载情况发生重大变化时,则进行缓存调整.算法相对比较简单,容易实现.对各种负载情况进行模拟实验证明,这一自适应的缓存算法比固定缓存具有更好的性能.     

一种IA-64架构下的大规模流媒体服务器缓存调度算法

在大规模流媒体服务中,缓存管理是非常关键的问题.特别是随着IA64架构的出现,物理内存的大小可大大得到增加,缓存管理策略正变得越来越重要.目前已经有很多缓存管理算法,其中间隔缓存策略通常被认为是比较有效的一个.但是以往的各种基于间隔的算法大多没有考虑媒体对象的流行程度,致使缓存的利用率受到了影响.通过对媒体对象的流行程度的特点进行研究,并考虑到利用IA64系统中的大内存的思想,提出了一种基于流行程度的间隔缓存策略.同时,为了分析该算法的性能,引入了一个算法的性能分析模型.分析结果显示该算法比传统的间隔缓存策略具有更好的性能.     

Efficient replacement of nonuniform objects in Web caches

Cache performance depends heavily on replacement algorithms, which dynamically select a suitable subset of objects for caching in a finite space. Developing such algorithms for wide-area distributed environments is challenging because, unlike traditional paging systems, retrieval costs and object sizes are not necessarily uniform. In a uniform caching environment, a replacement algorithm generally seeks to reduce cache misses, usually by replacing an object with the least likelihood of re-reference. In contrast, reducing total cost incurred due to cache misses is more important in nonuniform caching environments. The authors present the least-unified value algorithm, which performs better than existing algorithms for replacing nonuniform data objects in wide-area distributed environments     

Caching on the World Wide Web

With the recent explosion in usage of the World Wide Web, the problem of caching Web objects has gained considerable importance. Caching on the Web differs from traditional caching in several ways. The nonhomogeneity of the object sizes is probably the most important such difference. In this paper, we give an overview of caching policies designed specifically for Web objects and provide a new algorithm of our own. This new algorithm can be regarded as a generalization of the standard LRU algorithm. We examine the performance of this and other Web caching algorithms via event- and trace-driven simulation     

A novel cooperative caching algorithm for massive P2P caches

For an ISP (Internet Service Provider) that has deployed P2P caches in more than one ASs (autonomous systems), cooperative caching which makes their caches cooperate with each other can save more cost of carrying P2P traffic than independent caching. However, existing cooperative caching algorithms only use objects’ popularity as the measurement to decide which objects should be cached, and cost on intra-ISP links that has great impact on the benefits of cooperative caching is not considered. In this paper, we first model the cooperative caching problem as a NP-Complete problem, which is based on our analysis about the cost of serving requests with consideration of both the objects’ popularity and the cost on intra-ISP links. Then we propose a novel cooperative caching algorithm named cLGV (Cooperative, Lowest Global Value). The cLGV algorithm uses a new concept global value to estimate the benefits of caching or replacing an object in the cooperative caching system, and the global value of each object is evaluated according to not only objects’ popularity in each AS but also cost on intra-ISP links among ASs. Results of both synthetic and real traces driven simulations indicate that our cLGV algorithm can save the cost of carrying P2P traffic at least 23 % higher than that of existing cooperative caching algorithms.     

Web cache management based on the expected cost of web objects

With the recent explosion in usage of the World Wide Web, Web caching has become increasingly important. However, due to the non-uniform cost/size property of data objects in this environment, design of an efficient caching algorithm becomes an even more difficult problem compared to the traditional caching problems. In this paper, we propose the Least Expected Cost (LEC) replacement algorithm for Web caches that provides a simple and robust framework for the estimation of reference probability and fair evaluation of non-uniform Web objects. LEC evaluates a Web object based on its cost per unit size multiplied by the estimated reference probability of the object. This results in a normalized assessment of the contribution to the cost-savings ratio, leading to a fair replacement algorithm. We show that this normalization method finds optimal solution under some assumptions. Trace-driven simulations with actual Web cache logs show that LEC offers the performance of caches more than twice its size compared with other algorithms we considered. Nevertheless, it is simple, having no parameters to tune. We also show how the algorithm can be effectively implemented as a Web cache replacement module.     

Tulip: A New Hash Based Cooperative Web Caching Architecture

With the exponential growth of WWW traffic, web proxy caching becomes a critical technique for Internet web services. Well-organized proxy caching systems with multiple servers can greatly reduce the user perceived latency and decrease the network bandwidth consumption. Thus, many research papers focused on improving web caching performance with the efficient coordination algorithms among multiple servers. Hash based algorithm is the most widely used server coordination mechanism, however, there's still a lot of technical issues need to be addressed. In this paper, we propose a new hash based web caching architecture, Tulip. Tulip aggregates web objects that are likely to be accessed together into object clusters and uses object clusters as the primary access units. Tulip extends the locality-based algorithm in UCFS to hash based web proxy systems and proposes a simple algorithm to reduce the data grouping overhead. It takes into consideration the access speed dispatch between memory and disk and replaces expensive small disk I/O with less large ones. In case a client request cannot be fulfilled by the server in the memory, the system fetches the whole cluster which contains the required object into memory, the future requests for other objects in the same cluster can be satisfied directly from memory and slow disk I/Os are avoided. It also introduces a simple and efficient data dupllication algorithm, few maintenance work need to be done in case of server join/leave or server failure. Along with the local caching strategy, Tulip achieves better fault tolerance and load balance capability with the minimal cost. Our simulation results show Tulip has better performance than previous approaches.     

一种有效的混合式P2P Web缓存系统HCache

针对当前P2P Web 缓存系统中副本过多的问题,提出了一种有效的混合式P2P Web缓存系统：HCache。HCache根据用户对网页的访问特点及网页的优先级,对网页进行有选择的缓存,进而减少P2P Web缓存系统中的副本个数。根据Web对象当前的流行度,对LRU替换策略进行了改进(ELRU),提高了P2P Web缓存的命中率。在日志驱动的模拟实验表明,HCache缓存系统提高了Web缓存的命中率和性能。     

Efficient segment-based video transcoding proxy for mobile multimedia services

To support various bandwidth requirements for mobile multimedia services for future heterogeneous mobile environments, such as portable notebooks, personal digital assistants (PDAs), and 3G cellular phones, a transcoding video proxy is usually necessary to provide mobile clients with adapted video streams by not only transcoding videos to meet different needs on demand, but also caching them for later use. Traditional proxy technology is not applicable to a video proxy because it is less cost-effective to cache the complete videos to fit all kinds of clients in the proxy. Since transcoded video objects have inheritance dependency between different bit-rate versions, we can use this property to amortize the retransmission overhead from transcoding other objects cached in the proxy. In this paper, we propose the object relation graph (ORG) to manage the static relationships between video versions and an efficient replacement algorithm to dynamically manage video segments cached in the proxy. Specifically, we formulate a transcoding time constrained profit function to evaluate the profit from caching each version of an object. The profit function considers not only the sum of the costs of caching individual versions of an object, but also the transcoding relationship among these versions. In addition, an effective data structure, cached object relation tree (CORT), is designed to facilitate the management of multiple versions of different objects cached in the transcoding proxy. Experimental results show that the proposed algorithm outperforms companion schemes in terms of the byte-hit ratios and the startup latency.     

En-Route Transcoding缓存的优化放置和替换

对en-route transcoding缓存中的缓存路由和协同放置及替换问题进行了研究.提出了CCRA(cost-aware cache routing algorithm)缓存路由算法,能以可控的探测开销来发现潜在的、具有最小访问开销的缓存对象.在此基础上,建立了en-route transcoding缓存的分析模型,将缓存放置和替换问题形式化为一个最优化问题,并利用一种基于动态规划的方法来求解最佳缓存放置策略.仿真结果表明,与已有的元算法放置策略相比,该协同放置和替换策略可以获得更好的CSR性能.     

Integrating Web caching and Web prefetching in client-side proxies

Web caching and Web prefetching are two important techniques used to reduce the noticeable response time perceived by users. Note that by integrating Web caching and Web prefetching, these two techniques can complement each other since the Web caching technique exploits the temporal locality, whereas Web prefetching technique utilizes the spatial locality of Web objects. However, without circumspect design, the integration of these two techniques might cause significant performance degradation to each other. In view of this, we propose in this paper an innovative cache replacement algorithm, which not only considers the caching effect in the Web environment, but also evaluates the prefetching rules provided by various prefetching schemes. Specifically, we formulate a normalized profit function to evaluate the profit from caching an object (i.e., either a nonimplied object or an implied object according to some prefetching rule). Based on the normalized profit function devised, we devise an innovative Web cache replacement algorithm, referred to as Algorithm IWCP (standing for the Integration of Web Caching and Prefetching). Using an event-driven simulation, we evaluate the performance of Algorithm IWCP under several circumstances. The experimental results show that Algorithm IWCP consistently outperforms the companion schemes in various performance metrics.     

Segmentation of multimedia streams for proxy caching

Proxy caching of large multimedia objects on the edge of the Internet has become increasingly important for reducing network latency. For a large media object, such as a two-hour video, treating the whole media as a single object for caching is not appropriate. In this paper, we study three media segmentation approaches to proxy caching: fixed, pyramid, and skyscraper. Blocks of a media stream are grouped into various segments for cache management. The cache admission and replacement policies attach different caching priorities to individual segments, taking into account the access frequency of the media object and the segment distance from the start of the media. These caching policies give preferential treatment to the beginning segments. As such, most user requests can be quickly played back from the proxy servers without delay. Event-driven simulations are conducted to evaluate the segmentation approaches and compare them with whole media caching. The results show that: 1) compared with whole media caching, segmentation-based caching is more effective not only in increased byte-hit ratio but also in lowered fraction of requests that requires delayed start; 2) pyramid segmentation, where segment size increases exponentially, is the best segmentation approach; and 3) segmentation-based caching is especially advantageous when the cache size is limited, when the set of hot media objects changes over time, when the media file size is large, and when there are a large number of distinct media objects.