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     

Increasing multimedia system throughput with consumption-based buffer management

In a multimedia server, multiple media streams are generally serviced in a cyclic fashion. Due to non-uniform playback rates and asynchronous arrivals of queries, there tends to be spare disk bandwidth in each service cycle. In this paper, we study the issue of dynamically using spare disk bandwidth and buffer to maximize the system throughput of a multimedia server. We introduce the concept of minimizing buffer consumption as the criterion to select an appropriate media stream to utilize the spare system resources. Buffer consumption measures not only the amount of buffer but also the amount of time such buffer space is occupied (i.e., the space-time product). Different alternatives to utilizing spare disk bandwidth are examined, including different rate-adjustable retrievals of an already activated stream and prefetching the next waiting stream. For rate-adjustable retrievals, we study buffer consumption-based and remaining-time-based criteria for selecting an active stream to increase retrievals. Simulations are conducted to evaluate and compare different cases. The results show that (1) minimizing buffer consumption is the right criterion for maximizing the system throughput with spare disk bandwidth; (2) in general, prefetching a waiting stream incurs more buffer consumption, and thus is less effective than rate-adjustable retrieval of active streams in maximizing the system throughput; and (3) the advantage of rate-adjustable retrieval over prefetching is especially significant when service cycle time is small.     

Operating system support for the management of hard real-time disk traffic

Emerging applications like C³I systems, real-time databases, data acquisition systems and multimedia servers require access to secondary storage devices under timing constraints. In this paper, we focus on operating system support needed for managing real-time disk traffic with hard deadlines. We present the design and implementation of a preemptive deadline-driven disk I/O subsystem suitable for real-time disk traffic management. Preemptibility is achieved with a granularity that is automatically controlled by the I/O subsystem according to current workload and filesystem data layout. An admission control test checks the current resource availability for a given workload. We show that contiguous layout is necessary to maintain hard real-time guarantees and a reasonable level of disk throughput. Finally, we show how buffering can be used to obtain utilization factors close to the maximum disk bandwidth possible.     

Prioritized Admission Strategies to Improve User-Perceived Performance in Interactive VOD Servers

In the past, much emphasis has been given to the data throughput of VOD servers. In Interactive Video-on-Demand (IVOD) applications, such as digital libraries, service availability and response times are more visible to the user than the underlying data throughput. Data throughput is a measure of how efficiently resources are utilized. Higher throughput may be achieved at the expense of deteriorated user-perceived performance metrics such as probability of admission and queuing delay prior to admission. In this paper, we propose and evaluate a number of strategies to sequence the admission of pending video requests. Under different request arrival rates and buffer capacities, we measure the probability of admission, queueing delay and data throughput of each strategy. Results of our experiments show that simple hybrid strategies can improve the number of admitted requests and reduce the queuing time, without jeopardizing the data throughput. The techniques we propose are independent of the underlying disk scheduling techniques used. So, they can be employed to improve the user-perceived performance of VOD servers, in general.     

APEX: adaptive disk scheduling framework with QoS support

APEX is an adaptive disk scheduling framework with Quality-of-Service (QoS) support designed for environments with highly varying disk bandwidth usage. APEX is based on a three-layer scheduling architecture: (1) the upper layer realizes different service classes using a set of queues; (2) the mid-layer distributes available disk bandwidth among these queues; and (3) the lower layer is handled by the disk itself, which does the final ordering of disk requests. We demonstrate the use of APEX in an example scenario, a Learning-on-Demand (LoD) application supported by a multimedia system, where students can search for and playback multimedia-based learning material. In this paper, we present the scheduling concepts of APEX which are based on an extended token bucket algorithm. The disk requests scheduled for service are assembled into batches in order to exploit the intelligence of modern disks. Combined with a specialized work-conservation scheme, this enables APEX to apply bandwidth where it is needed, without the loss of efficiency. We demonstrate, through simulations, that APEX provides both higher throughput and lower response times than other mixed-media disk schedulers while still avoiding deadline violations for real-time requests. We also show its robustness with respect to misaligned bandwidth allocation. The work was conducted while Ketil Lund was an employee at UniK – University Graduate Center, Kjeller, Norway.     

An efficient joint channel assignment and QoS routing protocol for IEEE 802.11 multi-radio multi-channel wireless mesh networks

With the emerging of video, voice over IP (VoIP) and other real-time multimedia services, more and more people pay attention to quality of service (QoS) issues in terms of the bandwidth, delay and jitter, etc. As one effective way of broadband wireless access, it has become imperative for wireless mesh networks (WMNs) to provide QoS guarantee. Existing works mostly modify QoS architecture dedicated for ad hoc or sensor networks, and focus on single radio and single channel case. Meanwhile, they study the QoS routing or MAC protocol from view of isolated layer. In this paper, we propose a novel cross-layer QoS-aware routing protocol on OLSR (CLQ-OLSR) to support real-time multimedia communication by efficiently exploiting multi-radio and multi-channel method. By constructing multi-layer virtual logical mapping over physical topology, we implement two sets of routing mechanisms, physical modified OLSR protocol (M-OLSR) and logical routing, to accommodate network traffic. The proposed CLQ-OLSR is based on a distributed bandwidth estimation scheme, implemented at each node for estimating the available bandwidth on each associated channel. By piggybacking the bandwidth information in HELLO and topology control (TC) messages, each node disseminates information of topology and available bandwidth to other nodes in the whole network in an efficient way. From topology and bandwidth information, the optimized path can be identified. Finally, we conduct extensive simulation to verify the performance of CLQ-OLSR in different scenarios on QualNet platform. The results demonstrate that our proposed CLQ-OLSR outperforms single radio OLSR, multi-radio OLSR and OLSR with differentiated services (DiffServ) in terms of network aggregate throughput, end-to-end packet delivery ratio, delay and delay jitter with reasonable message overheads and hardware costs. In particular, the network aggregate throughput for CLQ-OLSR can almost be improved by 300% compared with the single radio case.     

Performance improvement for applying network virtualization in fiber-wireless （FiWi） access networks

Fiber-wireless （FiWi） access networks, which are a combination of fiber networks and wireless networks, have the advantages of both networks, such as high bandwidth, high security, low cost, and flexible access. However, with the increasing need for bandwidth and types of service from users~ FiWi networks are still relatively incapable and ossified. To alleviate bandwidth tension and facilitate new service deployment, we attempt to apply network virtualization in FiWi networks, in which the network＇s control plane and data plane are separated from each other. Based on a previously proposed hierarchical model and service model for FiWi network virtualization, the process of service implementation is described. The performances of the FiWi access networks applying network virtualization are analyzed in detail, including bandwidth for links, throughput for nodes, and multipath flow transmission. Simulation results show that the FiWi network with virtualization is superior to that without.     

Real-time disk scanning for timely retrieval of continuous media objects

In recent years, demands for multimedia applications have rapidly grown with the advances in computing power and network technology. To fulfill the demands, it is crucial to establish the multimedia information systems supporting continuous media (CM) streams without hiccups. Many useful methods have been proposed in the past so that CM steams with VCR-quality can be supported. Although these earlier approaches can be used for the service of VCR-quality streams, they cannot provide a good system performance for the CM streams with various playback rates and rather short durations of playback. In the paper, we propose a new CM playback method that consists of bulk-scanning and an earliest-deadline-first style scheduling algorithm. Our proposed method provides both good response times and high I/O utilization using the efficient disk scheduling. In addition, our mechanism for admission control makes CM streams never experience hiccups until the ends of playback. Through experimental analyses, we show the performance advantages of the proposed method over the earlier methods.     

Design and evaluation of a multimedia storage server for mixed traffic

The relative simplicity of access to digital communications nowadays and the simultaneous increase in the available bandwidth are leading to the definition of new telematic services, mainly oriented towards multimedia applications and interactivity with the user. In the near future, a decisive role will be played in this scenario by the providers of interactive multimedia services of the on-demand type, which will guarantee the end user a high degree of flexibility, speed and efficiency. In this paper, some of the technical aspects regarding these service providers are dealt with, paying particular attention to the problems of storing information and managing service requests. More specifically, the paper presents and evaluates a new storage technique based on the use of disk array technology, which can manage both typical multimedia connections and traditional requests. The proposed architecture is based on the joint use of the partial dynamic declustering and the information dispersal algorithm, which are employed for the allocation and retrieval of the data stored on the disk array. We also define efficient strategies for request management in such a way as to meet the time constraints imposed by multimedia sessions and guarantee good response times for the rest of the traffic. The system proposed is then analyzed using a simulation approach.     

Providing QOS guarantees for disk I/O

In this paper, we address the problem of providing different levels of performance guarantees or quality of service for disk I/O. We classify disk requests into three categories based on the provided level of service. We propose an integrated scheme that provides different levels of performance guarantees in a single system. We propose and evaluate a mechanism for providing deterministic service for variable-bit-rate streams at the disk. We will show that, through proper admission control and bandwidth allocation, requests in different categories can be ensured of performance guarantees without getting impacted by requests in other categories. We evaluate the impact of scheduling policy decisions on the provided service. We also quantify the improvements in stream throughput possible by using statistical guarantees instead of deterministic guarantees in the context of the proposed approach.     

Dynamic real-time scheduling strategies for interactive continuous media servers

In this paper, we propose and study a dynamic approach to schedule real-time requests in a video-on-demand (VOD) server. Providing quality of service in such servers requires uninterrupted and on-time retrieval of motion video data. VOD services and multimedia applications further require access to the storage devices to be shared among multiple concurrent streams. Most of the previous VOD scheduling approaches use limited run-time,0 information and thus cannot exploit the potential capacity of the system fully. Our approach improves throughput by making use of run-time information to relax admission control. It maintains excellent quality of service under varying playout rates by observing deadlines and by reallocating resources to guarantee continuous service. It also reduces start-up latency by beginning service as soon as it is detected that deadlines of all real-time requests will be met. We establish safe conditions for greedy admission, dynamic control of disk read sizes, fast initial service, and sporadic services. We conduct thorough simulations over a wide range of buffer capacities, load settings, and over varying playout rates to demonstrate the significant improvements in quality of service, throughput and start-up latency of our approach relative to a static approach.     

A disk bandwidth allocation mechanism with priority

Virtualization is a popular technology. Services and applications running on each virtual machine have to compete with each other for limited physical computer or network resources. Each virtual machine has different I/O requirement and special priority. Without proper scheduling resource management, a load surge in a virtual machine may inevitably degrade other’s performance. In addition, each virtual machine may run different kinds of application, which have different disk bandwidth demands and service priorities. When assigning I/O resources, we should deal with each case on demand. In this paper, we propose a dynamic virtual machine disk bandwidth control mechanism in virtualization environment. A Disk Credit Algorithm is introduced to support a fine-gained disk bandwidth allocation mechanism among virtual machines. We can assign disk bandwidth according to each virtual machine’s service priority/weight and its requirement. Related experiments show that the mechanism can improve the VMs’ isolation and guarantee the performance of the specific virtual machine well.     

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.     

Improving multimedia systems performance using constant-density recording disks

Multimedia systems store and retrieve large amounts of data which require extremely high disk bandwidth and their performance critically depends on the efficiency of disk storage. However, existing magnetic disks are designed for small amounts of data retrievals geared to traditional operations; with speed improvements mainly focused on how to reduce seek time and rotational latency. When the same mechanism is applied to multimedia systems, overheads in disk I/O can result in dramatic deterioration in system performance. In this paper, we present a mathematical model to evaluate the performance of constant-density recording disks, and use this model to analyze quantitatively the performance of multimedia data request streams. We show that high disk throughput may be achieved by suitably adjusting the relevant parameters. In addition to demonstrating quantitatively that constant-density recording disks perform significantly better than traditional disks for multimedia data storage, a novel disk-partitioning scheme which places data according to their bandwidths is presented.     

Performance Modeling and Evaluation of a Two-Dimensional Disk Array System

This paper presents a performance model of a two-dimensional disk array (TIDA) system, which is composed of several major subsystems including disk cache, intelligent disk array controller, SCSI-like I/O bus, and two-dimensional array of disk devices. Accessing conflict in these subsystems and fork/join synchronization of physical disk requests are considered in the model. The representation for the complex behavior, including the interactions among subsystems, of a whole disk array system distinguishes the model from others that model only individual subsystems. To assist evaluating the architectural alternatives of TIDA, we employ a subsystem access time modeling methodology, in which we model for each subsystem the mean subsystem access time per request (SATPR). Fed with a given set of representative workload parameters, the performance model is used to conduct performance evaluation and the SATPRs of the subsystems are utilized to identify the bottleneck subsystem for performance improvement. The results show that (1) the values of some key design parameters, such as data block size and I/O bus bandwidth that yield the best system throughput are dependent not only on the subsystem performance but also on the interaction among subsystems; (2) an I/O bus bandwidth of 5 Mbytes/s per disk device is large enough for data transfers from/to disk devices equipped with a cache of 1 Mbytes; and (3) the activity of fork/join synchronization of physical disk requests may cause performance degradation, which can be improved by using large I/O bus bandwidth and/or placing a cache in each disk device.     

FDDI-M: a scheme to double FDDI's ability of supporting synchronoustraffic

Synchronous messages are usually generated periodically and each of them is required to be transmitted before the generation of the next message. Due to the inherent deficiency in its medium access control (MAC) protocol, an FDDI token ring can use at most one half of its ring bandwidth to transmit such synchronous traffic. This deficiency greatly reduces the FDDI's capability of supporting multimedia applications like real-time voice/video transmissions. In this paper, we show how a few simple modifications to the FDDI's MAC protocol can remove this deficiency and double a ring's ability of supporting synchronous traffic. The modified protocol, called FDDI-M, can also achieve a higher throughput for asynchronous traffic than the standard FDDI and the FDDI-II, thus making it useful even for those networks without heavy synchronous traffic     

Performance of a Scalable Multimedia Server with Shared-Storage Clusters

The existing SCSI parallel bus has been widely used in various multimedia applications. However, due to the unfair bus accesses the SCSI bus may not be able to fully utilize the potential aggregate throughput of disks. The number of disks that can be attached to the SCSI bus is limited, and link level fault tolerance is not provided. The serial storage interfaces such as Serial Storage Architecture (SSA) provide high data bandwidth, fair accesses, long transmission distance between adjacent devices (disks or hosts) and link level fault tolerance. The fairness algorithm of SSA ensures a fraction of data bandwidth to be allocated to each device. In this paper we would like to know whether SSA is a better alternative in supporting continuous media than SCSI. The scalability of a multimedia server is very important since the storage requirement may grow incrementally as more contents are created and stored. SSA in a shared-storage cluster environment also supports concurrent accesses by different hosts as long as their access paths are not overlapped. This feature is called spatial reuse. Therefore, the effective bandwidth over an SSA can be higher than the raw data bandwidth and the spatial reuse feature is critical to the scalability of a multimedia server. This feature is also included in FC-AL3 with a new mode called Multiple Circuit Mode (MCM). Using MCM, all devices can transfer data simultaneously without collision. In this paper we have investigated the scalability of shared-stroage clusters over an SSA environment.     

An intelligent memory caching architecture for data-intensive multimedia applications

With the rapid developments in cloud computing and mobile networks, multimedia content can be accessed conveniently. Recently, some novel intelligent caching-based approaches have been proposed to improve the memory architectures for multimedia applications. These applications often face bottleneck related challenges which result in performance degradation and service delay issues. Intelligent multimedia network applications access the shared data by using a specific network file system. This results in answering the processing related constraints on hard-drive storage and might result in bringing bottleneck issues. Therefore, to improve the performance of these multimedia network applications, we present an intelligent distributed memory caching system. We integrate the multimedia application message passing interface in a multi-threaded environment and propose an algorithm which can handle concurrent response behavior for different multimedia applications. Results demonstrate that our proposed scheme outperforms traditional approaches in terms of throughput and file read access features.

     

Continuous retrieval of multimedia data using parallelism

Most implementations of workstation-based multimedia information systems cannot support a continuous display of high resolution audio and video data and suffer from frequent disruptions and delays termed hiccups. This is due to the low I/O bandwidth of the current disk technology, the high bandwidth requirement of multimedia objects, and the large size of these objects, which requires them to be almost always disk resident. A parallel multimedia information system and the key technical ideas that enable it to support a real-time display of multimedia objects are described. In this system, a multimedia object across several disk drives is declustered, enabling the system to utilize the aggregate bandwidth of multiple disks to retrieve an object in real-time. Then, the workload of an application is distributed evenly across the disk drives to maximize the processing capability of the system. To support simultaneous display of several multimedia objects for different users, two alternative approaches are described. The first approach multitasks a disk drive among several requests while the second replicates the data and dedicates resources to each individual request. The trade-offs associated with each approach are investigated using a simulation model     

A multi-layer experimental study of multimedia and QoS communication in wireless mesh networks

A significant issue in Mesh networks is to support multimedia transmissions while providing Quality of Service (QoS) guarantees to mobile users. For real-time multimedia streaming, unstable throughput or insufficient bandwidth will incur unexpected delay or jitter, and it remains difficult to provide comprehensive service guarantees for a wireless mesh environment. In this paper, we target the problem of providing multimedia QoS in wireless mesh networks. We design and implement a campus test-bed for supporting multimedia traffic in mobile wireless mesh networks, and investigate in detail some possible improvements on a number of layers to enable multimedia transmission over wireless networks with QoS support. We first study a number of improvements of some existing routing protocols to support multimedia transmissions. Some new admission control and rate control mechanisms are studied and their performance gains are verified in our experiments. In our new cross-layer adaptive rate control (CLARC) mechanism, we adaptively change the video encoder’s output bit rate based on the available network bandwidth to improve the quality of the received video. We also design a mobile gateway protocol to connect the MANET to Internet and a wireless LAN management protocol to automatically manage WLAN to provide some QoS.