Performance evaluation for VBR Continuous Media File Server admission control

A file server for continuous media must provide resource guarantees and only admit requests that do not violate the resource availability. This paper addresses the admission performance of a server that explicitly considers the variable bit rate nature of the continuous media streams. A prototype version of the server has been implemented and evaluated in several heterogeneous environments. The two system resources for which admission control is evaluated are the disk bandwidth and the network bandwidth. Performance results from both measurement and simulation are shown with respect to different admission methods and varying scenarios of stream delivery patterns. We show that the vbrSim algorithm developed specifically for the server outperforms the other options for disk admission especially with request patterns that have staggered arrivals, while the network admission control algorithm is able to utilize a large percentage of the network bandwidth available. We also show the interactions between the limits of these two resources and how a system can be configured without wasted capacity on either one of the resources. Copyright © 2004 John Wiley & Sons, Ltd.     

关于高端集群式流媒体服务器仿真的研究

流媒体应用在近年来获得了飞速的发展,流媒体服务器在用户并发度和存储容量两个方面的需求日益增高,在这种背景下,集群式流媒体服务器逐渐成为高端流媒体服务器的主流.集群式流媒体服务器的研究工作非常需要科学的仿真工具来协助,一方面能够测试算法的性能和普适性,另一方面可以通过仿真分析对服务器的硬件资源进行优化配置.本文在介绍集群式流媒体服务器的体系结构和性能分析的基础上,设计并实现了一套分布式流媒体仿真工具和集群式流媒体服务器实验床.最后基于仿真实验的数据,得到一些提高集群式流媒体服务器性能和资源优化配置的有益结论.     

Constructing a video server with tertiary storage: Practice and experience

Handling a tertiary storage device, such as an optical disk library, in the framework of a disk-based stream service model, requires a sophisticated streaming model for the server, and it should consider the device-specific performance characteristics of tertiary storage. This paper discusses the design and implementation of a video server which uses tertiary storage as a source of media archiving. We have carefully designed the streaming mechanism for a server whose key functionalities include stream scheduling, disk caching and admission control. The stream scheduling model incorporates the tertiary media staging into a disk-based scheduling process, and also enhances the utilization of tertiary device bandwidth. The disk caching mechanism manages the limited capacity of the hard disk efficiently to guarantee the availability of media segments on the hard disk. The admission controller provides an adequate mechanism which decides upon the admission of a new request based on the current resource availability of the server. The proposed system has been implemented on a general-purpose operating system and it is fully operational. The design principles of the server are validated with real experiments, and the performance characteristics are analyzed. The results guide us on how servers with tertiary storage should be deployed effectively in a real environment. RID="*" ID="*" e-mail: hjcha@cs.yonsei.ac.kr     

Modeling and generating realistic streaming media server workloads

Currently, Internet hosting centers and content distribution networks leverage statistical multiplexing to meet the performance requirements of a number of competing hosted network services. Developing efficient resource allocation mechanisms for such services requires an understanding of both the short-term and long-term behavior of client access patterns to these competing services. At the same time, streaming media services are becoming increasingly popular, presenting new challenges for designers of shared hosting services. These new challenges result from fundamentally new characteristics of streaming media relative to traditional web objects, principally different client access patterns and significantly larger computational and bandwidth overhead associated with a streaming request. To understand the characteristics of these new workloads we use two long-term traces of streaming media services to develop MediSyn, a publicly available streaming media workload generator. In summary, this paper makes the following contributions: (i) we propose a framework for modeling long-term behavior of network services by capturing the process of file introduction, non-stationary popularity of media accesses, file duration, encoding bit rate, and session duration. (ii) We propose a variety of practical models based on the study of the two workloads. (iii) We develop an open-source synthetic streaming service workload generator to demonstrate the capability of our framework to capture the models.     

Using eager strategies to improve NFS I/O performance

Typical network file system (NFS) clients write lazily: they leave dirty pages in the page cache and defer writing to the server. This reduces network traffic when applications repeatedly modify the same set of pages. However, this approach can lead to memory pressure, when the number of available pages on the client system is so low that the system must work harder to reclaim dirty pages. We show that NFS performance is poor under memory pressure and present two mechanisms to solve it: eager writeback and eager page laundering. These mechanisms change the client's data management policy from lazy to eager, in which dirty pages are written back proactively, resulting in higher throughput for sequential writes. In addition, we show that NFS servers suffer from out-of-order file operations, which further reduce performance. We introduce request ordering, a server mechanism to process operations, as much as possible, in the order they were sent by the client, which improves read performance substantially. We have implemented these techniques in the Linux operating system. I/O performance is improved, with the most pronounced improvement visible for sequential access to large files. We see 33% improvement in the performance of streaming write workloads and more than triple the performance of streaming read workloads. We evaluate several non-sequential workloads and show that these techniques do not degrade performance, and can sometimes improve performance.     

Dynamic Scheme Transition Adaptable to Variable Video Popularity in a Digital Broadcast Network

To provide on-demand video streaming services through the network, video objects with both high and low client request rates must be served efficiently. In this paper, we propose a dynamic scheme transition to provide on-demand streaming services efficiently regardless of video popularity. This approach can maintain quality-of-service (QoS) by transitioning the service scheme according to the request rate. The server provides services by heuristically broadcasting video segments when the video popularity is low and by a Periodic Broadcast when the video popularity is high. The server identifies the variations in client request rates from the number of service channels and determines transitions to more efficient service schemes autonomously. We evaluated this scheme transition for various parameters and traces from a company providing streaming services. The results show that the performance of scheme transition is very efficient.      

流媒体技术及其文件格式

为了全面、深刻地理解流媒体技术,以达到进一步提高流服务器工作效率的目的,该文就流媒体文件格式展开深入研究。在剖析常用流媒体系统和文件格式的基础上,笔者结合在研发新的流媒体系统过程中积累的经验,论证了流媒体文件格式在流媒体系统中占有重要地位,设计合理的文件格式是提高流媒体服务器工作效率最直接和最有效的办法。最后该文对已有流媒体文件格式对流服务器性能的影响进行了分析,提出了一种新的流媒体文件格式框架。实践表明,使用该文建议的流媒体文件格式可以大幅度提高流媒体服务器的工作效率。     

基于流行度预测的流媒体代理缓存替换算法

针对流行度随时间变化的特性，利用回归分析技术给出了一种流媒体文件的流行度预测算法，并在增加少量存储空间及计算时间消耗的情况下，将该预测算法应用于流媒体代理缓存服务器的缓存替换算法之中，模拟实验表明，该方法能减少缓存的替换次数，提高缓存命中率，性能较优。     

基于可变窗口的流媒体缓存算法研究与实现

缓存管理是影响流媒体服务器性能的关键因素之一。通过对流媒体访问特性的分析,提出并实现了基于可变窗口的流媒体缓存替换算法CABVW。该算法通过动态调整拆分代价和合并代价的阈值,实现了磁盘带宽和缓存的负载平衡。实验结果表明,该算法与传统主流算法相比明显地增加了流媒体服务器的并发用户数量;算法计算复杂度低,也较易于实现。     

基于RTMP的流媒体服务器的研究

多媒体数据要在网络上传输,必须先对多媒体数据进行流化处理。要想流化多媒体数据,就需要流媒体服务器。所以流媒体服务器对多媒体数据的传输有着至关重要作用。其中流化处理就是对多媒体数据进行封装,把音视频数据打包成能进行流传输的数据包。而且提高流媒体服务器的性能一个有效途径是提高缓存的利用率,使系统为更多的媒体流服务。文章以RTMP为基础,首先分析了该传输协议,并提出了一种改进的缓存策略,使流媒体服务器性能有了提高。     

Throughput optimization for video streaming proxy servers based on video staging

A video streaming proxy server needs to handle hundreds of simultaneous connections between media servers and clients. Inside, every video arrived at the server and delivered from it follows a specific arrival and delivery schedule. While arrival schedules compete for incoming network bandwidth, delivery schedules compete for outgoing network bandwidth. As a result, a proxy server has to provide sufficient buffer and disk cache for storage, together with memory space, disk space and disk bandwidth. In order to optimize the throughput, a proxy server has to govern the usage of these resources. In this paper, we first analyze the property of a traditional smoothing algorithm and a video staging algorithm. Then we develop, based on the smoothing algorithm, a video staging algorithm for video streaming proxy servers. This algorithm allows us to devise an arrival schedule based on the delivery schedule. Under this arrival and delivery schedule pair, we can achieve a better resource utilization rate gracefully between different parameter sets. It is also interesting to note that the usage of the resources such as network bandwidth, disk bandwidth and memory space becomes interchangeable. It provides the basis for inter-resource scheduling to further improve the throughput of a video streaming proxy server system.

A Statistical Admission Control Scheme for Continuous Media Servers Using Caching

In continuous media servers, disk load can be reduced by using buffer cache. In order to utilize the saved disk bandwidth by caching, a continuous media server must employ an admission control scheme to decide whether a new client can be admitted for service without violating the requirements of clients already being serviced. A scheme providing deterministic QoS guarantees in servers using caching has already been proposed. Since, however, deterministic admission control is based on the worst case assumption, it causes the wastage of the system resources. If we can exactly predict the future available disk bandwidth, both high disk utilization and hiccup-free service are achievable. However, as the caching effect is not analytically determined, it is difficult to predict the disk load without substantial computation overhead. In this paper, we propose a statistical admission control scheme for continuous media servers where caching is used to reduce disk load. This scheme improves disk utilization and allows more streams to be serviced while maintaining near-deterministic service. The scheme, called Shortsighted Prediction Admission Control (SPAC), combines exact prediction through on-line simulation and statistical estimation using a probabilistic model of future disk load in order to reduce computation overhead. It thereby exploits the variation in disk load induced by VBR-encoded objects and the decrease in client load by caching. Through trace-driven simulations, it is demonstrated that the scheme provides near-deterministic QoS and keeps disk utilization high.     

A distributed admission control model for QoS assurance in large-scale media delivery systems

Conventional admission control models incur some performance penalty. First, admission control computation can overload a server that is already heavily loaded. Also, in large-scale media systems with geographically distributed server clusters, performing admission control on each cluster can result in long response latency, if the client request is denied at one site and has to be forwarded to another site. Furthermore, in prefix caching, initial frames cached at the proxy are delivered to the client before the admission decisions are made. If the media server is heavily loaded and, finally, has to deny the client request, forwarding a large number of initial frames is a waste of critical network resources. In this paper, a novel distributed admission control model is presented. We make use of proxy servers to perform the admission control tasks. Each proxy hosts an agent to coordinate the effort. Agents reserve media server's disk bandwidth and make admission decisions autonomously based on the allocated disk bandwidth. We develop an effective game theoretic framework to achieve fairness in the bandwidth allocation among the agents. To improve the overall bandwidth utilization, we also consider an aggressive admission control policy where each agent may admit more requests than its allocated bandwidth allows. The distributed admission control approach provides the solution to the stated problems incurred in conventional admission control models. Experimental studies show that our algorithms significantly reduce the response latency and the media server load.     

面向直播HTTP Streaming系统的HTTP缓存服务器行为优化

HTTP缓存服务器是提高HTTP Streaming系统客户并发量的关键环节。但当前主流HTTP缓存服务器,如Nginx、Squid、Varnish等,在缓存资源更新期间的行为都存在不足,当被应用在面向直播的HTTP Streaming系统中时,会周期性地把大量客户端请求转发至源服务器,从而制约了HTTP Streaming系统的可伸缩性。提出一种优化的HTTP缓存服务器在缓存更新期间的行为,即缓存服务器仅向源服务器转发一路客户端请求,缓存更新期间,拒绝其他关于该资源的请求。优化策略在使用最为广泛的Nginx服务器的基础上进行了实现。实验证明,优化后系统的伸缩性得到了显著提高。     

Peer Assisted Video Streaming With Supply-Demand-Based Cache Optimization

In this paper, we consider a hybrid P2P video on-demand architecture that utilizes both the server and the peer resources for efficient transmission of popular videos. In our system architecture, each peer dedicates some cache space to store a particular segment of a video file as well as some of its upload bandwidth to serve the cached segment to other peers. Peers join the system and issue a streaming request to a control server. Control server directs the peers to streaming servers or to other peers who have the desired video segments. Control server also decides which peer should cache which video segment. Our main contribution in this paper is to determine the proper caching strategies at peers such that we minimize the average load on the streaming servers.      

一种直播服务器的设计与实现

提出一种直播服务器设计方案,媒体接收端根据节目链接访问媒体源,获取媒体数据流;媒体数据缓冲区是一个双循环缓冲区,循环缓存媒体接收端获取的媒体数据流;媒体服务器发送端根据媒体数据缓冲区建立子会话,生成访问该节目的直播服务器链接,终端通过链接访问直播服务器,直播服务器从媒体数据缓冲区中循环读取媒体数据发送给终端。设计方案具有适合多种网络、架设过程简单、可多台服务器级联等特点,并且成本低廉,可扩展性强。     

Storage technique for real-time streaming of layered video

Scalable streaming technology has been proposed to effectively support heterogeneous devices with dynamically varying bandwidth. From the file system’s point of view, scalable streaming introduces another dimension of complexity in disk scheduling. Most of the existing efforts on multimedia file systems are dedicated to I/O scheduling algorithm and data placement scheme that efficiently guarantee I/O bandwidth. The important underlying assumption in these efforts is that most of the multimedia file accesses are simple playback operations and therefore are sequential. However, this workload characteristic is not valid in scalable streaming environment. In a scalable streaming environment, i.e., when only a subset of imagery is retrieved, the playback does not necessarily coincide with the sequential access on the file. The current file structure and the file system organization leaves much to be desired for supporting scalable streaming service. In this work, we propose a file system scheme, Harmonic Placement to efficiently support scalable streaming. The basic idea of Harmonic placement is to cluster the frequently accessed layers together to avoid unnecessary disk seeks. The data blocks are partitioned into two sets with respect to the layers: lower layers and upper layers. In Harmonic placement, the data blocks in the lower layers are placed with respect to their frame sequence and the data blocks in the upper layers are clustered according to the layers they belong to. We develop elaborate performance models for three different file system schemes: Progressive placement, Interleaved Placement and Harmonic Placement. We investigate the performance of the file server with different file system schemes. It was found that file system performance is very sensitive to the file organization scheme. When most of the service requests are for low-quality video (e.g., 128 Kbits/s ISDN), Progressive placement scheme supports twice as many sessions as the Interleaved placement scheme. When most of the service requests are for high-quality video (e.g., 1.5 Mbits/s MPEG-2 DVD quality), Interleaved placement can support twice as many requests as Progressive placement. In both cases, Harmonic placement scheme yields the most promising performance. Primitive version of this work has appeared on Proceedings of NOSSDAV ’06, Providence, Rhode Island, USA. This work is in part funded by KOSEF throught National Research Lab (ROA-2007-000-200114-0) and by HY-SDR center at Hanyang University.     

改进的最小效用流媒体代理缓存替换算法

在SCU-K算法的基础上,提出了基于流行度和将来访问次数的最小效用替换算法（SCU-PFUT）。此外算法还考虑了流媒体文件的字节有效性和文件块大小的因素,使得替换出内存的数据块更加合理。不但避免LRU和LFU算法中出现的媒体文件被连续替换的问题,相对于LRU、LFU和SCU-2,其在缓存命中率、字节命中率和空间利用率都得到了提升。     

移动网格中负载自平衡的集群式边缘流媒体服务器

迅速发展的3G网络和覆盖范围可达整个城域的Wi-Fi网络的逐渐普及,使移动网络环境中对流媒体服务的需求迅速增长.由于移动用户主要依赖Internet上的流媒体服务资源提供服务,所以位于无线网络和Internet交界处的边缘流媒体服务器可以充当桥梁和缓冲区,对于降低网络负载和提高服务质量有显著的效果,因而提出一种新的集群架构的边缘流媒体服务器(CESS)的设计,而且针对集群服务器中最重要的负载均衡问题做了分析和测试,提出了一种新的MCLBS缓存替换算法来使CESS能够达到自适应的负载均衡.最后,实验测试和结果分析证明,相对于传统的缓冲替换算法,MCLBS算法更适合集群式的服务器体系结构,在同样的条件下,缓存命中率明显提高,大大降低远程服务器的带宽.     

影响多媒体服务器性能的关键因素研究

在构建大规模视频服务系统时 ,基于层次型多服务器群的体系结构在吞吐率、可扩展性、经济性等方面都有其突出的优势 ,尤其适合于在因特网上的应用 .但是 ,要充分发挥和提高视频服务系统的性能 ,还要针对一些主要的瓶颈(如服务器磁盘 I/ O带宽与网络带宽 ) ,解决好一系列的问题 .本文分析了影响多媒体视频服务器性能的一些主要因素 ,如视频服务器的体系结构、服务器与客户端之间的数据传送方式、媒体数据在视频服务器存储子系统中的分布与放置方式、对磁盘访问请求的调度、单服务器中的缓存及多服务器间协同缓存的管理、接入控制策略、流调度策略等 ,这些因素对视频服务器的性能与吞吐率有着极大的影响 .本文还介绍了一些适用于大规模视频服务系统的性能优化技术 ,如广播、批处理等流调度策略 .在构建视频服务器系统时 ,只有综合考虑这些因素 ,才能真正提高服务器乃至整个视频服务系统的吞吐率 ,并较好地满足客户的 Qo S要求