An Effective Cache Replacement Algorithm in Transcoding-Enabled Proxies

In this paper, we address the problem of cache replacement for transcoding proxy caching. Transcoding proxy is a proxy that has the functionality of transcoding a multimedia object into an appropriate format or resolution for each client. We first propose an effective cache replacement algorithm for transcoding proxy. In general, when a new object is to be cached, cache replacement algorithms evict some of the cached objects with the least profit to accommodate the new object. Our algorithm takes into account of the inter-relationships among different versions of the same multimedia object, and selects the versions to replace according to their aggregate profit which usually differs from simple summation of their individual profits as assumed in the existing algorithms. It also considers cache consistency, which is not considered in the existing algorithms. We then present a complexity analysis to show the efficiency of our algorithm. Finally, we give extensive simulation results to compare the performance of our algorithm with some existing algorithms. The results show that our algorithm outperforms others in terms of various performance metrics.     

On exploring aggregate effect for efficient cache replacement in transcoding proxies

Recent technology advances in mobile networking have ushered in a new era of personal communication. Users can ubiquitously access the Internet via many emerging mobile appliances, such as portable notebooks, personal digital assistants (PDAs), and WAP-enabled cellular phones. While the transcoding proxy is attracting an increasing amount of attention in this environment, it is noted that new caching strategies are required for these transcoding proxies. We propose an efficient cache replacement algorithm for transcoding proxies. Specifically, we formulate a generalized profit function to evaluate the profit from caching each version of an object. This generalized profit function explicitly considers several new emerging factors in the transcoding proxy and the aggregate effect of caching multiple versions of the same object. It is noted that the aggregate effect is not simply the sum of the costs of caching individual versions of an object, but rather, depends on the transcoding relationship among these versions. The notion of a weighted transcoding graph is devised to evaluate the corresponding aggregate effect efficiently. Utilizing the generalized profit function and the weighted transcoding graph, we propose, in this paper, an innovative cache replacement algorithm for transcoding proxies. In addition, an effective data structure is designed to facilitate the management of the multiple versions of different objects cached in the transcoding proxy. Using an event-driven simulation, it is shown that the algorithm proposed consistently outperforms companion schemes in terms of the delay saving ratios and cache hit ratios.     

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.     

Coordinated multimedia object replacement in transcoding proxies

Finding replacement candidates for accommodating a new object is an important research issue in web caching. Due to the new emerging factors in the transcoding proxy and the aggregate effect of caching multiple versions of the same multimedia object, this problem becomes more important and complex as audio and video applications have proliferated over the Internet, especially in the environment of mobile computing systems. This paper addresses coordinated cache replacement in transcoding proxies. First, we propose an original model which determines cache replacement candidates on all candidate nodes in a coordinated fashion with the objective of minimizing the total cost loss for linear topology. We formulate this problem as an optimization problem and present a low-cost optimal solution for deciding cache replacement candidates. Second, we extend this problem to solve the same problem for tree networks. Finally, we conduct extensive simulations to evaluate the performance of our solutions by comparing with existing models.     

Cost-based cache replacement and server selection for multimedia proxy across wireless Internet

Multimedia proxy plays an important role in multimedia streaming over wireless Internet. Since wireless network exhibits different characteristics from the Internet, multimedia proxy caching over wireless Internet faces additional challenges. In this paper, we present a study of cache replacement for a single server and server selection for multiple servers across wireless Internet. By considering multiple objectives of multimedia proxy, we design a unified cost metric to measure proxy performance in wireless Internet. Based on the defined unified cost metric, we propose a novel replacement algorithm for single-server and a new server-selection policy for multiple servers to improve the end-to-end performance such as throughput, media quality, and start-up latency. To effectively handle errors occurred on wireless link, channel-adaptive unequal error protection is deployed according to distinct quality of service requirements of layered or scalable media. Simulation results demonstrate that our approaches achieve significantly better performance than the known cache-replacement algorithms and sever selection schemes, respectively.     

A weighted segment-based caching algorithm for video streaming objects over heterogeneous networking environments

Traditional caching technology is not applicable to cache video streaming objects over heterogeneous networking environments. The popularity of mobile devices in the heterogeneous networking environments make the access of Internet content become a common phenomenon. To support different mobile devices in the heterogeneity networking environments, a transcoding proxy is used to transcode different versions of the streaming videos according to clients’ requests. In this paper, we propose a weighted caching replace strategy for video streaming objects over heterogeneous networking environments. A new caching algorithm with static weight transcoding graph and dynamic caching relation tree is introduced. The proposed algorithm is compared with LRU, LFU, CP and PF cache algorithms in three parts: hit ratio, byte hit ratio, and average transmission delay. Experimental results show that the proposed algorithm outperforms than traditional LRU, LFU, CP and PF cache algorithms.     

一种基于SQUID的流媒体代理CACHE的设计与实现

提出了一种基于SQUID的流媒体CACHE的设计和实现机制，它对层次编码的流能提供自适应的服务质量。首先给出了一个CACHE的体系结构，然后分别阐述了CACHE中的存储管理、预取策略和替换策略，最后给出了实验结果。     

An effectiveness-based adaptive cache replacement policy

Belady’s optimal cache replacement policy is an algorithm to work out the theoretical minimum number of cache misses, but the rationale behind it was too simple. In this work, we revisit the essential function of caches to develop an underlying analytical model. We argue that frequency and recency are the only two affordable attributes of cache history that can be leveraged to predict a good replacement. Based on those two properties, we propose a novel replacement policy, the Effectiveness-Based Replacement policy (EBR) and a refinement, Dynamic EBR (D-EBR), which combines measures of recency and frequency to form a rank sequence inside each set and evict blocks with lowest rank. To evaluate our design, we simulated all 30 applications from SPEC CPU2006 for uni-core system and a set of combinations for 4-core systems, for different cache sizes. The results show that EBR achieves an average miss rate reduction of 12.4%. With the help of D-EBR, we can tune the weight ratio between ‘frequency’ and ‘recency’ dynamically. D-EBR can nearly double the miss reduction achieved by EBR alone. In terms of hardware overhead, EBR requires half the hardware overhead of real LRU and even compared with Pseudo LRU the overhead is modest.     

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.     

流媒体对象的缓存管理策略

基于流媒体服务的代理技术是流媒体研究领域中的重要课题.随着流媒体技术在Internet和无线网络环境中的高速发展,对流媒体代理服务器的研究也正在逐步深入.本文主要讨论通过代理技术改善媒体的服务质量,降低媒体的传输延迟以及减轻网络负载.在Internet环境下,对流媒体代理服务器的研究集中于流媒体的访问特性、缓存替换算法,构建和实现一个流媒体代理服务器是对流媒体代理技术研究的基础.     

自适应一致性替换算法的设计与实现

针对代理缓存的一致性策略和替换策略还没有很好地结合起来的技术现状,基于最优化模型,提出、设计和实现了一种新的优化代理缓存的自适应一致性--替换算法(即 ACR算法).这种算法包括一致性策略和替换策略两部分,一致性策略采用自适应TTL机制,替换策略是基于成本/价值模型的算法.通过Trace-Driven模拟实验,结果表明ACR算法在陈旧命中比上均优于传统的几个替换算法.     

基于期望预测价值的时移代理缓存替换算法

分析移动多媒体广播上时移业务的特点,描述了可预测用户行为的时移代理服务器资源调度策略,并提出一种基于期望预测价值的时移代理服务器缓存替换算法。仿真实验结果表明,该算法与传统的FIFO算法相比较,提高了缓存的预测命中率,减少了用户平均等待延迟。     

A novel server-side proxy caching strategy for large-scale multimedia applications

Nowadays, server-side Web caching becomes an important technique used to reduce the User Perceived Latency (UPL). In large-scale multimedia systems, there are many Web proxies, connected with a multimedia server, that can cache some most popular multimedia objects and respond to the requests for them. Multimedia objects have some particular characteristic, e.g., strict QoS requirements. Hence, even some efficient conventional caching strategies based on cache hit ratio, meant for non-multimedia objects, will confront some problems in dealing with the multimedia objects. If we consider additional resources of proxy besides cache space, say bandwidth, we can readily observe that high hit ratios may deteriorate the entire system performance. In this paper, we propose a novel placement model for networked multimedia systems, referred to as the H^k/T model, which considers the combined influence of arrival rate, size, and playback time to select the objects to be cached. Based on this model, we propose an innovative Web caching algorithm, named as the ART-Greedy algorithm, which can balance the load among the proxies and achieve a minimum Average Response Time (ART) of the requests. Our experimental results conclusively demonstrate that the ART-Greedy algorithm outperforms the most popular and commonly used LFU (Least Frequently Used) algorithm significantly, and can achieve a better performance than the byte-hit algorithm when the system utilization is medium and high.     

一种新的基于分区的多媒体代理协作管理策略

多媒体代理缓存是提高媒体流化质量和减轻网络负载的重要手段，将基于协作的多媒体代理缓存技术引入无线网络，提出一种新的分区缓存管理模型（Partition-based caching manager,PBCM)，通过对系统中各个代理的部分缓存空间（称为静态分区）采取集中式管理，可以合理配置缓存资源，降低缓存的冗余度，从全局角度优化系统的整体性能，具体地，PBCM中制定了一种全局对象放置策略来管理静态分区缓存的内容，该策略被模型化为一个优化问题，并采用改进遗传算法来求解，仿真实验表明该模型及算法是有效可行的。     

Performance evaluation of Web proxy cache replacement policies

The continued growth of the World-Wide Web and the emergence of new end-user technologies such as cable modems necessitate the use of proxy caches to reduce latency, network traffic and Web server loads. In this paper we analyze the importance of different Web proxy workload characteristics in making good cache replacement decisions. We evaluate workload characteristics such as object size, recency of reference, frequency of reference, and turnover in the active set of objects. Trace-driven simulation is used to evaluate the effectiveness of various replacement policies for Web proxy caches. The extended duration of the trace (117 million requests collected over 5 months) allows long term side effects of replacement policies to be identified and quantified.

Our results indicate that higher cache hit rates are achieved using size-based replacement policies. These policies store a large number of small objects in the cache, thus increasing the probability of an object being in the cache when requested. To achieve higher byte hit rates a few larger files must be retained in the cache. We found frequency-based policies to work best for this metric, as they keep the most popular files, regardless of size, in the cache. With either approach it is important that inactive objects be removed from the cache to prevent performance degradation due to pollution.     

一种端到端网络的不相交多路径QoS路由算法

不相交多路径路由算法旨在一个端到端的网络中为应用流选择多个路径,且这些路径在瓶颈链路上是彼此不相交的。本文提出的不相交多路径QoS路由（DMQR）算法在Dijkstra最短路径和最短最宽路径（SWP）算法的基础上,能够动态地计算时延最短、带宽最宽、在瓶颈链路上互不相交的路径,且保证每个路径都是满足一定服务质量的。在视频会议、远程医疗和远程教育等重要的视频通信场合,要求应用层和网络层必须协同工作以保证一些必要的QoS,例如端到端的带宽、时延和包丢失率等。本文针对端到端的网络,重点讨论不相交多路径QoS路由算法在应用层的设计和实现。性能分析和模拟结果显示,所提出的不相交多路径QoS路由算法总是收敛的,且当网络流量增加时,该算法具有较低的包丢失率和较高的吞吐量。     

基于高斯混合模型的Web代理服务器缓存替换策略

Web代理服务器缓存能够在一定程度上解决用户访问延迟和网络拥塞问题,Web代理缓存的缓存替换策略直接影响缓存的命中率,从而影响网络请求响应的效果;为此,使用一种通过固定大小的循环滑动窗口提取Web日志数据的多项特征,并使用高斯混合模型对Web日志数据进行聚类分析,预测在窗口时间内可能再次访问到Web对象,结合最近最少使用(LRU)算法,提出一种新的基于高斯混合模型的Web代理服务器缓存替换策略;实验结果表明,与传统的缓存替换策略LRU、LFU、FIFO、GDSF相比,该策略有效提高了Web代理缓存的请求命中率和字节命中率。     

Modeling and Optimization of Meta-Caching Assisted Transcoding

The increase of aggregate Internet bandwidth and the rapid development of 3G wireless networks demand efficient delivery of multimedia objects to all types of wireless devices. To handle requests from wireless devices at runtime, the transcoding-enabled caching proxy has been proposed to save transcoded versions to reduce the intensive computing demanded by online transcoding. Constrained by available CPU and storage, existing transcoding-enabled caching schemes always selectively cache certain transcoded versions, expecting that many future requests can be served from the cache. But such schemes treat the transcoder as a black box, leaving no room for flexible control of joint resource management between CPU and storage. In this paper, we first introduce the idea of meta-caching by looking into a transcoding procedure. Instead of caching certain selected transcoded versions in full, meta-caching identifies intermediate transcoding steps from which certain intermediate results (called metadata) can be cached so that a fully transcoded version can be easily produced from the metadata with a small amount of CPU cycles. Achieving big saving in caching space with possibly small sacrifice on CPU load, the proposed meta-caching scheme provides a unique method to balance the utilization of CPU and storage resources at the proxy. We further construct a model to analyze the meta-caching scheme. Based on the analysis, we propose AMTrac, Adaptive Meta-caching for Transcoding, which adaptively applies meta-caching based on the client request patterns and available resources. Experimental results show that AMTrac can significantly improve the system throughput over existing approaches.      

基于P2P的3G流媒体代理缓存策略

3G时代已经到来,伴随着3G的快速发展,移动流媒体服务的需求迅速增长。由于移动网络带宽远达不到互联网的带宽,流媒体业务的增长将增加网络延迟,影响流媒体的观看质量,因此提出了一种新的基于P2P的3G流媒体缓存代理结构PSPA,配合PSPA设计了代理缓存置换策略SCOP,该设计能有效降低对移动核心网带宽的占用。最后,仿真实验和实验结果分析表明,PSPA体系与代理缓存置换策略SCOP的结合,能有效提高移动流媒体的服务质量,对3G流媒体的应用具有一定的参考价值。