基于QoS的磁盘调度策略

随着视频点播、视频会议、视频监控、数字图书馆等流媒体应用的普及，流媒体服务器存储资源管理成为制约服务质量的瓶颈之一。根据多媒体服务器的性能要求，提出了一种支持QoS的磁盘调度策略。它由三个主要部分组成：探测模块、负载监测模块和自适应管理模块。探测模块，负责判断当前的资源情况能否满足服务请求；自适应模块，根据负载监删模块检测到的负载变化情况，动态调整服务周期在实时请求和尽力服务请求之间的分配。实验表明此磁盘调度策略能在保证实时请求无抖动执行的同时，明显减少了非实时请求的响应时间。     

Resource Allocation for Session-Based Two-Dimensional Service Differentiation on e-Commerce Servers

A scalable e-commerce server should be able to provide different levels of quality of service (QoS) to different types of requests based on clients' navigation patterns and the server capacity. E-Commerce workloads are composed of sessions. In this paper, we propose a session-based two-dimensional (2D) service differentiation model for online transactions: intersession and intrasession. The intersession model aims to provide different levels of QoS to sessions from different customer classes, and the intrasession model aims to provide different levels of QoS to requests in different states of a session. A primary performance metric of online transactions is slowdown. It measures the waiting time of a request relative to its service time. We present a processing rate allocation scheme for 2D proportional slowdown differentiation. We then introduce service slowdown as a systemwide QoS metric of an e-commerce server. It is defined as the weighted sum of request slowdown in different sessions and in different session states. We formulate the problem of 2D service differentiation as an optimization of processing rate allocation with the objective of minimizing the service slowdown of the server. We prove that the derived rate allocation scheme based on the optimization guarantees client requests' slowdown to be square-root proportional to their prespecified differentiation weights in both intersession and intrasession dimensions. We evaluate this square-root proportional rate allocation scheme and a proportional rate allocation scheme via extensive simulations. Results validate that both schemes can achieve predictable, controllable, and fair 2D service differentiation on e-commerce servers. The square-root proportional rate allocation scheme provides 2D service differentiation at a minimum cost of service slowdown.     

VoD系统中基于优先级的准入控制和带宽分配策略

本文针对VoD系统中不同客户对视频服务质量的不同要求,提出一种基于优先级的准入控制和带宽动态分配策略。在准入控制时,综合考虑请求的优先级和并发流占用的实际带宽等因素,在保证为高优先级请求预留较多固定带宽的同时提高并发流个数;在服务过程中,根据优先级和网络状况动态调整每个流的带宽,使丢包率低于一定的阈值,并保证在相同的网络状态下为高优先级请求提供较高的视频服务质量。     

严格服务等级约定的区分Web服务技术研究

随着Internet的普及和Web应用的发展，满足客户不同业务需求成为人们关注的问题．该文基于业务类的访问资源需求和服务等级给出了一种资源划分策略，用来确定业务类在访问率波动较小条件下达到服务级别所需的资源；提出一种资源效用控制机制，用来适应访问的突发性和保证业务类服务等级要求．通过按周期进行资源划分，同时利用资源控制机制来适应周期内的访问突发，得到了严格服务等级约定的区分Web服务(简称SLADS)．文章通过建立原型与其它方法进行了实验比较，结果表明SLADS方法能够有效支持业务类的严格服务等级和提高资源利用率．     

A unified benchmarking and model-based framework for building QoS-aware streaming media services

A number of technology and workload trends motivate us to consider the appropriate resource allocation mechanisms and policies for streaming media services in shared cluster environments. We present MediaGuard – a model-based infrastructure for building streaming media services – that can efficiently determine the fraction of server resources required to support a particular client request over its expected lifetime. The proposed solution is based on a unified cost function that uses a single value to reflect overall resource requirements such as the CPU, disk, memory, and bandwidth necessary to support a particular media stream based on its bit rate and whether it is likely to be served from memory or disk. We design a novel, time-segment-based memory model of a media server to efficiently determine in linear time whether a request will incur memory or disk access when given the history of previous accesses and the behavior of the server's main memory file buffer cache. Using the MediaGuard framework, we design two media services: (1) an efficient and accurate admission control service for streaming media servers that accounts for the impact of the server's main memory file buffer cache, and (2) a shared streaming media hosting service that can efficiently allocate the predefined shares of server resources to the hosted media services, while providing performance isolation and QoS guarantees among the hosted services. Our evaluation shows that, relative to a pessimistic admission control policy that assumes that all content must be served from disk, MediaGuard (as well as services that are built using it) deliver a factor of two improvement in server throughput.     

Priority scheduling service for E-commerce web servers

Service scheduling is one of the crucial issues in E-commerce environment. E-commerce web servers often get overloaded as they have to deal with a large number of customers’ requests—for example, browse, search, and pay, in order to make purchases or to get product information from E-commerce web sites. In this paper, we propose a new approach in order to effectively handle high traffic load and to improve web server’s performance. Our solution is to exploit networking techniques and to classify customers’ requests into different classes such that some requests are prioritised over others. We contend that such classification is financially beneficial to E-commerce services as in these services some requests are more valuable than others. For instance, the processing of “browse” request should get less priority than “payment” request as the latter is considered to be more valuable to the service provider. Our approach analyses the arrival process of distinct requests and employs a priority scheduling service at the network nodes that gives preferential treatment to high priority requests. The proposed approach is tested through various experiments which show significant decrease in the response time of high priority requests. This also reduces the probability of dropping high priority requests by a web server and thus enabling service providers to generate more revenue.     

An efficient bandwidth allocation algorithm for real-time VBR stream transmission under IEEE 802.16 wireless networks

This paper investigates variable rate control strategies for real-time multimedia variable bit rate (VBR) services over IEEE 802.16 broadband wireless networks. A data rate control mechanism is derived for the case where the uplink channel provides real-time services and the traffic rate parameter remains constant. This paper shows that the common queuing scheduling algorithms have some bandwidth allocation fairness problems for the real-time polling service (rtPS) in the MAC layer. In other words, the use of a VBR for the rtPS by a WiMAX system results in additional access latency jitter and bandwidth allocation disorder in the transmitted multimedia streams during the regular time interval polling of subscribe stations (SSs) for the contention bandwidth request period. However, the proposed scheduling algorithm solves these SSs contending with bandwidth resource allocation problems based on an extended rtPS (ertPS) of quality-of-service (QoS) pre-programming for a ranging response non-contention polling period. The adopted bandwidth allocation of max–min fairness queue scheduling uses a time constraint condition to transmit real-time multimedia VBR streaming in an IEEE 802.16 broadband wireless environment. In addition, we use the ns-2 simulation tool to compare the capacity of multimedia VBR stream and show that the proposed ertPS scheduling algorithm outperforms other rtPS scheduling algorithms.     

Techniques for increasing the stream capacity of a high-performancemultimedia server

High-performance servers and high-speed networks will form the backbone of the infrastructure required for distributed multimedia information systems. A server for an interactive distributed multimedia system may require thousands of gigabytes of storage space and a high I/O bandwidth. In order to maximize the system utilization, and thus minimize the cost, it is essential that the load be balanced among each of the server's components, viz. the disks, the interconnection network and the scheduler. Many algorithms for maximizing retrieval capacity from the storage system have been proposed in the literature. This paper presents techniques for improving the server capacity by assigning media requests to the nodes of a server so as to balance the load on the interconnection network and the scheduling nodes. Five policies for request assignment-round-robin (RR), minimum link allocation (MLA), minimum contention allocation (MCA), weighted minimum link allocation (WMLA) and weighted minimum contention allocation (WMCA)-are developed. The performance of these policies on a server model developed by the authors (1995) is presented. We also consider the issue of file replication, and develop two schemes for storing the replicas: the parent group-based round-robin placement (PGBRRP) scheme, and the group-wide round-robin placement (GWRRP) scheme. The performance of the request assignment policies in the presence of file replication is presented     

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     

无线Mesh网络中基于业务价值的组播带宽分配方案

高延迟及低效益是无线Mesh网络中开展流媒体业务面临的主要问题。为解决该问题,首先赋予组播业务价值,该值反映了业务的网络优先级及效益优先级,并提出一个基于组播价值优先级的组播带宽分配方案。该方案第一阶段实施基于价值优先级的不区分业务类型的静态分配,通过对高价值业务的优先调度,实现了组播价值最大化及带宽效益最大化。在拥塞时,实施第二阶段的基于业务带宽需求调整及抢占的动态分配,保证了低时延约束业务的服务质量。仿真与常见算法的比较表明,所提出的方案在保证业务网络服务质量的同时,能实现带宽效益最大化。     

Joint bandwidth allocation,data scheduling and incentives for scalable video streaming over peer-to-peer networks

The overall performance of a peer-to-peer (P2P) scalable streaming system largely depends on the strategies employed in bandwidth allocation, data scheduling and incentives. In this paper, we develop a credit-based content-aware bandwidth auction model for scalable streaming in P2P networks. It formulates multi-overlay multi-layer bandwidth request and allocation problems as auction games. Each peer in the games acts as both auctioneer and player. Being a auctioneer, it maximizes the total revenue (credits) by selling upload bandwidth; Being a player, it uses the credits earned in bandwidth sales to sequentially bid for layer bandwidth so as to maximize the received video quality. Also, a content-aware bidding strategy is proposed, under which the required bandwidth quantity from a peer is determined by the informative video chunks and the marginal net utility that peer could provide, as well as the available credits and the maximum layer bit rate. The convergence of the proposed auction algorithm is mathematically proved. Finally, the performance of the proposed scheme is verified by simulation results.     

Comparison of scheduling schemes for on-demand IaaS requests

Infrastructure-as-a-service (IaaS) is one of emerging powerful cloud computing services provided by IT industry at present. This paper considers the interaction aspects between on-demand requests and the allocation of virtual machines in a server farm operated by a specific infrastructure owner. We formulate an analytic performance model of the server farm taking into account the quality of service (QoS) guaranteed to users and the operational energy consumption in the server farm. We compare several scheduling algorithms from the aspect of the average energy consumption and heat emission of servers as well as the blocking probabilities of on-demand requests. Based on numerical results of a comparison of different allocation strategies, a saving on the energy consumption is possible in the operational range (where on-demand requests do not face unpleasant blocking probability) with the allocation of virtual machines to physical servers based on the priority.     

Resource allocation optimization for quantitative service differentiation on server clusters

The goal of service differentiation is to provide different service quality levels to meet changing system configuration and resource availability and to satisfy different requirements and expectations of applications and users. In this paper, we investigate the problem of quantitative service differentiation on cluster-based delay-sensitive servers. The goal is to support a system-wide service quality optimization with respect to resource allocation on a computer system while provisioning proportionality fairness to clients. We first propose and promote a square-root proportional differentiation model. Interestingly, both popular delay factors, queueing delay and slowdown, are reciprocally proportional to the allocated resource usage. We formulate the problem of quantitative service differentiation as a generalized resource allocation optimization towards the minimization of system delay, defined as the sum of weighted delay of client requests. We prove that the optimization-based resource allocation scheme essentially provides square-root proportional service differentiation to clients. We then study the problem of service differentiation provisioning from an important relative performance metric, slowdown. We give a closed-form expression of the expected slowdown of a popular heavy-tailed workload model with respect to resource allocation on a server cluster. We design a two-tier resource management framework, which integrates a dispatcher-based node partitioning scheme and a server-based adaptive process allocation scheme. We evaluate the resource allocation framework with different models via extensive simulations. Results show that the square-root proportional model provides service differentiation at a minimum cost of system delay. The two-tier resource allocation framework can provide fine-grained and predictable service differentiation on cluster-based servers.     

一种基于度量的Web服务质量机制

提出了一种简单、准确和高效的带宽调节算法，该算法对每一个服务请求设置一个带宽标准，同时根据对服务请求的分类和Web服务器系统的有效带宽，来分配及调节每一个服务请求的实际带宽，构成一种基于度量的Web服务质量机制，使Web服务器系统能为不同类型的客户请求提供有区分的和有服务质量的Web服务。     

Block I/O Scheduling on Storage Servers of Distributed File Systems

This paper presents a new scheme of I/O scheduling on storage servers of distributed/parallel file systems, for yielding better I/O performance. To this end, we first analyze read/write requests in the I/O queue of storage server (we name them block I/Os), by using our proposed technique of horizontal partition. Then, all block requests are supposed to be divided into multiple groups, on the basis of their offsets. This is to say, all requests related to the same chunk file will be grouped together, and then be satisfied within the same time slot between opening and closing the target chunk file on the storage server. As a result, the time resulted by completing block I/O requests can be significantly decreased, because of less file operations on the corresponding chunk files at the low-level file systems of server machines. Furthermore, we introduce an algorithm to rate a priority for each group of block I/O requests, and then the storage server dispatches groups of I/Os by following the priority order. Consequently, the applications having higher I/O priorities, e.g. they have less I/O operations and small size of involved data, can finish at a earlier time. We implement a prototype of this server-side scheduling in the PARTE file system, to demonstrate the feasibility and applicability of the proposed scheme. Experimental results show that the newly proposed scheme can achieve better I/O bandwidth and less I/O time, compared with the strategy of First Come First Served, as well as other server-side I/O scheduling approaches.     

A speculative load adaptive streaming scheme for efficient video delivery

Video on demand services require video broadcast schemes to provide efficient and reliable performance under various client request loads. In this paper, we have developed an efficient request load adaptive broadcast scheme, speculative load adaptive streaming scheme (SLAS), that requires lower service bandwidth than previous approaches, regardless of request rate. We have provided both analysis and simulation to show the performance gain over previous schemes. In this paper, we provide the theoretic upper bound of the continuous segment allocations on channels. We found that the number of allocated segments of the SLAS is close to the theoretic upper bound when compared with other schemes over various numbers of stream channels. Our analysis of client waiting time is almost identical to simulation results about all client requests. By simulation, we compared the required service bandwidth and storage requirements of the SLAS scheme and other schemes and found the SLAS scheme is an efficient broadcast scheme as compared to well known seamless channel transition schemes.     

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     

一种新颖的带模糊截止时限的磁盘调度算法

设计了一种新的基于截止时限的磁盘调度算法,该算法支持带多优先级的请求。对于某些实时要求,其截止时限是不确定的或者不精确的,该算法采用模糊集来描述这类不确定性,模糊截止时限的隶属度函数表示对请求完成时间的满意程度。调度的目的是最优的指定优先级,使得截止时限的满意程度最大化。根据请求截止时限的不同,把满意程度划分为若干连续的区间。在每个不同的区间内,每个请求都对应有修正的截止时限,把请求按照其修正的截止时限非减的顺序分配优先级,才能实现请求优先级的最优配置。仿真结果表明该算法能有效的分配请求的优先级,降低请求的丢失率,保证了更多的请求得到满足。     

一种资源优化的双最小均衡Web集群区分服务调度算法

在一种新的Web集群体系结构的基础上,提出了一种资源优化的双最小均衡区分服务调度算法：首先在前端调度器按资源均衡度将Web请求分配到各后台服务器．然后将Web请求的优先级与资源均衡度两个特征参数结合起来,综合设计后台服务器的Web请求调度顺序,为了评估该算法的性能,进行了大量的模拟实验．在与其他著名调度策略如分离式调度的对比结果显示：双最小均衡调度算法使Web请求的效率提高了11％,同时很好地实现了区分服务．证实了资源优化调度策略具有一定的普遍意义．     

Web server QoS models: applying scheduling rules from production planning

Most web servers, in practical use, use a queuing policy based on the Best Effort model, which employs the first-in-first-out (FIFO) scheduling rule to prioritize web requests in a single queue. This model does not provide Quality of Service (QoS). In the Differentiated Services (DiffServ) model, separate queues are introduced to differentiate QoS for separate web requests with different priorities. This paper presents web server QoS models that use a single queue, along with scheduling rules from production planning in the manufacturing domain, to differentiate QoS for classes of web service requests with different priorities. These scheduling rules are Weighted Shortest Processing Time (WSPT), Apparent Tardiness Cost (ATC), and Earliest Due Date. We conduct simulation experiments and compare the QoS performance of these scheduling rules with the FIFO scheme used in the basic Best Effort model with only one queue, and the basic DiffServ model with two separate queues. Simulation results demonstrate better QoS performance using WSPT and ATC, especially when requested services exceed the capacity of a web server.