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1.
Dynamic batching policies for an on-demand video server 总被引:18,自引:0,他引:18
In a video-on-demand environment, continuous delivery of video streams to the clients is guaranteed by sufficient reserved
network and server resources. This leads to a hard limit on the number of streams that a video server can deliver. Multiple
client requests for the same video can be served with a single disk I/O stream by sending (multicasting) the same data blocks
to multiple clients (with the multicast facility, if present in the system). This is achieved by batching (grouping) requests
for the same video that arrive within a short time. We explore the role of customer-waiting time and reneging behavior in
selecting the video to be multicast. We show that a first come, first served (FCFS) policy that schedules the video with
the longest outstanding request can perform better than the maximum queue length (MQL) policy that chooses the video with
the maximum number of outstanding requests. Additionally, multicasting is better exploited by scheduling playback of the
most popular videos at predetermined, regular intervals (hence, termed FCFS-). If user reneging can be reduced by guaranteeing that a maximum waiting time will not be exceeded, then performance of FCFS- is further improved by selecting the regular playback intervals as this maximum waiting time. For an empirical workload,
we demonstrate a substantial reduction (of the order of 60%) in the required server capacity by batching. 相似文献
2.
Issues in the design of a storage server for video-on-demand 总被引:2,自引:0,他引:2
Antoine N. Mourad 《Multimedia Systems》1996,4(2):70-86
We examine issues related to the design
of a storage server for video-on-demand (VOD) applications.
The storage medium considered is magnetic disks
or arrays of disks. We investigate disk scheduling policies,
buffer management policies and I/O bus protocol issues.
We derive the number of sessions that can be
supported from a single disk or an array of disks and determine the
amount of buffering required to support a given number of users.
Furthermore,
we propose a scheduling mechanism for disk accesses that significantly
lowers the buffer-size requirements in the case of disk arrays.
The buffer size required under the proposed scheme is independent
of the number of disks in the array. This property allows for striping
video content over a large number of disks to achieve higher
concurrency in access to a particular video object.
This enables the server to satisfy hundreds of independent requests
to the same video object or to hundreds of different objects while
storing only one copy of each video object.
The reliability implications of striping content over a large number of disks
are addressed and two solutions are proposed.
Finally, we examine various policies for dealing with disk thermal calibration
and the placement of videos on disks and disk arrays. 相似文献
3.
A buffer-inventory-based dynamic scheduling algorithm for multimedia-on-demand servers 总被引:2,自引:0,他引:2
We present a producer-consumer model of multimedia-on-demand (MOD) servers. The producer retrieves media data from a disk
and places it into a set of buffers, while the consumer sends out the data in the buffers to the users. We develop for the
producer a buffer-inventory-based dynamic scheduling (BIDS) algorithm that guarantees non-zero inventory and non-overflow
of data in the buffers to meet the continuity requirement and no-loss of data for each media stream. The algorithm can deal
with heterogeneous me dia streams as well as the transient circumstances upon service completions and arrivals of new requests.
To smooth out the impact of bursty data of variable-bit-rate media streams and therefore increase the maximum admissible load
of requests, we also introduce into the scheduling scheme a time-scale-dependent peak consumption rate and a virtual cycle
time. Based on BIDS, an effective admission control mechanism can be easily established by checking two simple conditions
respectively on the overall system load and buffer size. Our algorithm is very easy to implement. Experiments carried out
with an actual disk system and real video stream data verify that it is more robust compared to static scheduling algorithms
previously proposed in the literature, especially when handling variable-bit-rate media streams. 相似文献
4.
Due to recent advances in network, storage and data compression technologies, video-on-demand (VOD) service has become economically
feasible. It is a challenging task to design a video storage server that can efficiently service a large number of concurrent
requests on demand. One approach to accomplishing this task is to reduce the I/O demand to the VOD server through data- and
resource-sharing techniques. One form of data sharing is the stream-merging approach proposed in [5]. In this paper, we formalize a static version of the stream-merging problem, derive an upper bound on the
I/O demand of static stream merging, and propose efficient heuristic algorithms for both static and dynamic versions of the
stream-merging problem. 相似文献
5.
Adaptive piggybacking: a novel technique for data sharing in video-on-demand storage servers 总被引:17,自引:0,他引:17
Recent technology advances have made multimedia on-demand services, such as home entertainment and home-shopping, important
to the consumer market. One of the most challenging aspects of this type of service is providing access either instantaneously
or within a small and reasonable latency upon request. We consider improvements in the performance of multimedia storage servers
through data sharing between requests for popular objects, assuming that the I/O bandwidth is the critical resource in the system. We discuss a novel approach to data sharing,
termed adaptive piggybacking, which can be used to reduce the aggregate I/O demand on the multimedia storage server and thus
reduce latency for servicing new requests. 相似文献
6.
In this paper, we present two novel disk failure recovery methods that utilize the inherent characteristics of video streams
for efficient recovery. Whereas the first method exploits the inherent redundancy in video streams (rather than error-correcting
codes) to approximately reconstruct data stored on failed disks, the second method exploits the sequentiality of video playback
to reduce the overhead of online failure recovery in conventional RAID arrays. For the former approach, we present loss-resilient
versions of JPEG and MPEG compression algorithms. We present an inherently redundant array of disks (IRAD) architecture that combines these loss-resilient compression algorithms with techniques for efficient placement of video streams
on disk arrays to ensure that on-the-fly recovery does not impose any additional load on the array. Together, they enhance
the scalability of multimedia servers by (1) integrating the recovery process with the decompression of video streams, and
thereby distributing the reconstruction process across the clients; and (2) supporting graceful degradation in the quality
of recovered images with increase in the number of disk failures. We present analytical and experimental results to show that
both schemes significantly reduce the failure recovery overhead in a multimedia server. 相似文献
7.
A network that offers deterministic, i.e., worst case, quality-of-service guarantees to variable-bit-rate (VBR) video must
provide a resource reservation mechanism that allocates bandwidth, buffer space, and other resources for each video stream.
Such a resource reservation scheme must be carefully designed, otherwise network resources are wasted. A key component for
the design of a resource reservation scheme is the traffic characterization method that specifies the traffic arrivals on a video stream. The traffic characterization should accurately describe the
actual arrivals, so that a large number of streams can be supported; but it must also map directly into efficient traffic-policing
mechanisms that monitor arrivals on each stream. In this study, we present a fast and accurate traffic characterization method
for stored VBR video in networks with a deterministic service. We use this approximation to obtain a traffic characterization
that can be efficiently policed by a small number of leaky buckets. We present a case study where we apply our characterization
method to networks that employ a dynamic resource reservation scheme with renegotiation. We use traces from a set of 25–30-min
MPEG sequences to evaluate our method against other characterization schemes from the literature. 相似文献
8.
We present efficient schemes for scheduling the delivery of variable-bit-rate MPEG-compressed video with stringent quality-of-service
(QoS) requirements. Video scheduling is being used to improve bandwidth allocation at a video server that uses statistical
multiplexing to aggregate video streams prior to transporting them over a network. A video stream is modeled using a traffic
envelope that provides a deterministic time-varying bound on the bit rate. Because of the periodicity in which frame types
in an MPEG stream are typically generated, a simple traffic envelope can be constructed using only five parameters. Using
the traffic-envelope model, we show that video sources can be statistically multiplexed with an effective bandwidth that is often less than the source peak rate. Bandwidth gain is achieved without sacrificing the stringency of the requested
QoS. The effective bandwidth depends on the arrangement of the multiplexed streams, which is a measure of the lag between the GOP periods of various streams. For homogeneous streams,
we give an optimal scheduling scheme for video sources at a video-on-demand server that results in the minimum effective bandwidth.
For heterogeneous sources, a sub-optimal scheduling scheme is given, which achieves acceptable bandwidth gain. Numerical examples
based on traces of MPEG-coded movies are used to demonstrate the effectiveness of our schemes. 相似文献
9.
Providing QOS guarantees for disk I/O 总被引:1,自引:0,他引:1
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. 相似文献
10.
An optimal bandwidth allocation strategy for the delivery of compressed prerecorded video 总被引:1,自引:0,他引:1
The transportation of prerecorded, compressed video data without loss of picture quality requires the network and video
servers to support large fluctuations in bandwidth requirements. Fully utilizing a client-side buffer for smoothing bandwidth
requirements can limit the fluctuations in bandwidth required from the underlying network and the video-on-demand servers.
This paper shows that, for a fixed-size buffer constraint, the critical bandwidth allocation technique results in plans
for continuous playback of stored video that have (1) the minimum number of bandwidth increases, (2) the smallest peak bandwidth
requirements, and (3) the largest minimum bandwidth requirements. In addition, this paper introduces an optimal bandwidth allocation algorithm which, in addition to the three critical bandwidth allocation properties, minimizes the total number of bandwidth
changes necessary for continuous playback. A comparison between the optimal bandwidth allocation algorithm and other critical
bandwidth-based algorithms using 17 full-length movie videos and 3 seminar videos is also presented. 相似文献
11.
Excessive buffer requirement to handle continuous-media playbacks is an impediment to cost- effective provisioning for on-line
video retrieval. Given the skewed distribution of video popularity, it is expected that often there are concurrent playbacks
of the same video file within a short time interval. This creates an opportunity to batch multiple requests and to service
them with a single stream from the disk without violating the on-demand constraint. However, there is a need to keep data
in memory between successive uses to do this. This leads to a buffer space trade-off between servicing a request in memory mode vs. servicing it in disk-mode. In this work, we develop a novel algorithm to minimize the buffer requirement to support a set of concurrent playbacks.
One of the beauties of the proposed scheme is that it enables the server to dynamically adapt to the changing workload while
minimizing the total buffer space requirement. Our algorithm makes a significant contribution in decreasing the total buffer
requirement, especially when the user access pattern is biased in favor of a small set of files. The idea of the proposed
scheme is modeled in detail using an analytical formulation, and optimality of the algorithm is proved. An analytical framework
is developed so that the proposed scheme can be used in combination with various existing disk-scheduling strategies. Our
simulation results confirm that under certain circumstances, it is much more resource efficient to support some of the playbacks
in memory mode and subsequently the proposed scheme enables the server to minimize the overall buffer space requirement. 相似文献
12.
We present end-to-end performance of digital coded video (JPEG, MPEG-1, and MPEG-2) over a local asynchronuous transfer mode
(ATM) network. We discuss performance in terms of both delay (jitter) and frame loss as a function of load. The experimental
data reveal that the burstiness of the variable bit-rate-coded video streams degrades the performance significantly when the
hosts and the network are stressed. Our results show that traffic smoothing decreases frame loss significantly while maintaining
acceptable jitter and loss bounds. We also discuss requirements for system components, such as the network interface and switch,
which are necessary to support video services efficiently. 相似文献
13.
Nevzat Hurkan Balkir Gultekin Ozsoyoglu 《The VLDB Journal The International Journal on Very Large Data Bases》1998,7(4):294-307
Most multimedia servers reported in the literature are designed to serve multiple and independent video/audio streams. We
think that, in future, multimedia servers will also serve complete presentations. Multimedia presentations provide unique
opportunities to develop algorithms for buffer management and admission control, as execution-time consumption requirements
of presentations are known a priori. In this paper, we examine presentations in three different domains (heavyweight, middleweight,
and lightweight) and provide buffer management and admission control algorithms for the three domains. We propose two improvements
(flattening and dynamic-adjustments) on the schedules created for the heavyweight presentations. Results from a simulation
environment are presented.
Received June 9, 1998 / Accepted October 13, 1998 相似文献
14.
The major drawback of the existing cluster placement scheme is the long response time caused by admission control if the
number of clusters and the number of users are large. A circular skip-cluster placement scheme is proposed to reduce the size
of the data buffer as well as the system response time. Furthermore, the popularity of each video is different in the real
world. We propose a new popularity-based data allocation scheme to allocate data units within a cluster such that the corresponding
data units of these popular videos are stored in those cylinders at one end of each cluster. Due to a higher spatial locality
within these hot cylinders, some data units requested by the users are stored in the same cylinder such that one seek operation,
one rotation, and one transfer operation are required to retrieve these data units. Therefore, the time required to retrieve
data for these requests can be reduced, thus also reducing the system response time. Based on our results, the buffer size
and the system response time can be reduced by half or more. These findings are essential for constructing video-on-demand
systems that provide satisfactory performance. 相似文献
15.
In this paper, we present a placement algorithm that interleaves multi-resolution video streams on a disk array and enables
a video server to efficiently support playback of these streams at different resolution levels. We then combine this placement
algorithm with a scalable compression technique to efficiently support interactive scan operations (i.e., fast-forward and
rewind). We present an analytical model for evaluating the impact of the scan operations on the performance of disk-arr ay-based
servers. Our experiments demonstrate that: (1) employing our placement algorithm substantially reduces seek and rotational
latency overhead during playback, and (2) exploiting the characteristics of video streams and human perceptual tolerances
enables a server to support interactive scan operations without any additional overhead. 相似文献
16.
In this paper, we present an efficient approach for supporting fast-scanning (FS) operations in MPEG-based video-on-demand
(VOD) systems. This approach is based on storing multiple, differently encoded versions of the same movie at the server. A
normal version is used for normal playback, while several scan versions are used for FS. Each scan version supports forward and backward FS at a given speedup. The server responds to an FS request
by switching from the normal version to an appropriate scan version. Scanning versions are produced by encoding a sample of
the raw frames using the same GOP pattern of the normal version. When a scanning version is decoded and played back at the
normal frame rate, it gives a perceptual motion speedup. By being able to control the traffic envelopes of the scan versions,
our approach can be integrated into a previously proposed framework for distributing archived, MPEG-coded video streams. FS
operations are supported using no or little extra network bandwidth beyond what is already allocated for normal playback.
Mechanisms for controlling the traffic envelopes of the scan versions are presented. The actions taken by the server and the
client's decoder in response to various types of interactive requests are described in detail. The latency incurred in implementing
various interactive requests is shown to be within an acceptable range. Striping and disk-scheduling strategies for storing
various versions at the server are presented. Issues related to the implementation of our approach are discussed. 相似文献
17.
A large-scale, distributed video-on-demand (VOD) system allows geographically dispersed residential and business users to
access video services, such as movies and other multimedia programs or documents on demand from video servers on a high-speed
network. In this paper, we first demonstrate through analysis and simulation the need for a hierarchical architecture for
the VOD distribution network.We then assume a hierarchical architecture, which fits the existing tree topology used in today's
cable TV (CATV) hybrid fiber/coaxial (HFC) distribution networks. We develop a model for the video program placement, configuration,
and performance evaluation of such systems. Our approach takes into account the user behavior, the fact that the user requests
are transmitted over a shared channel before reaching the video server containing the requested program, the fact that the
input/output (I/O) capacity of the video servers is the costlier resource, and finally the communication cost. In addition,
our model employs batching of user requests at the video servers. We study the effect of batching on the performance of the
video servers and on the quality of service (QoS) delivered to the user, and we contribute dynamic batching policies which
improve server utilization, user QoS, and lower the servers' cost. The evaluation is based on an extensive analytical and
simulation study. 相似文献
18.
In a video-on-demand (VOD) environment, batching requests for the same video to share a common video stream can lead to significant
improvement in throughput. Using the wait tolerance characteristic that is commonly observed in viewers behavior, we introduce a new paradigm for scheduling in VOD systems.
We propose and analyze two classes of scheduling schemes: the Max_Batch and Min_Idle schemes that provide two alternative
ways for using a given stream capacity for effective batching. In making a video selection, the proposed schemes take into
consideration the next stream completion time, as well as the viewer wait tolerance. We compared the proposed schemes with
the two previously studied schemes: (1) first-come-first-served (FCFS) that schedules the video with the longest waiting request
and (2) the maximum queue length (MQL) scheme that selects the video with the maximum number of waiting requests. We show
through simulations that the proposed schemes substantially outperform FCFS and MQL in reducing the viewer turn-away probability,
while maintaining a small average response time. In terms of system resources, we show that, by exploiting the viewers wait
tolerance, the proposed schemes can significantly reduce the server capacity required for achieving a given level of throughput
and turn-away probability as compared to the FCFS and MQL. Furthermore, our study shows that an aggressive use of the viewer
wait tolerance for batching may not yield the best strategy, and that other factors, such as the resulting response time,
fairness, and loss of viewers, should be taken into account. 相似文献
19.
Design and analysis of a video-on-demand server 总被引:6,自引:0,他引:6
The availability of high-speed networks, fast computers and improved storage technology is stimulating interest in the development
of video on-demand services that provide facilities similar to a video cassette player (VCP). In this paper, we present a
design of a video-on-demand (VOD) server, capable of supporting a large number of video requests with complete functionality
of a remote control (as used in VCPs), for each request. In the proposed design, we have used an interleaved storage method
with constrained allocation of video and audio blocks on the disk to provide continuous retrieval. Our storage scheme interleaves
a movie with itself (while satisfying the constraints on video and audio block allocation. This approach minimizes the starting delay and the
buffer requirement at the user end, while ensuring a jitter-free display for every request. In order to minimize the starting
delay and to support more non-concurrent requests, we have proposed the use of multiple disks for the same movie. Since a
disk needs to hold only one movie, an array of inexpensive disks can be used, which reduces the overall cost of the proposed
system. A scheme supported by our disk storage method to provide all the functions of a remote control such as “fast-forwarding”,
“rewinding” (with play “on” or “off”), “pause” and “play” has also been discussed. This scheme handles a user request independent
of others and satisfies it without degrading the quality of service to other users. The server design presented in this paper
achieves the multiple goals of high disk utilization, global buffer optimization, cost-effectiveness and high-quality service
to the users. 相似文献
20.
A. Savidis N. Maou I. Pachoulakis C. Stephanidis 《Universal Access in the Information Society》2002,1(4):274-287
The concept of ubiquitous computing reflects an infrastructure in which users are engaged in mobile interaction sessions within
environments composed of dynamically varying computational resources. In this paradigm, applications are required to continuously
follow end users and provide high-quality interaction while migrating among different computing devices and dynamically utilizing
the available input/output (I/O) resources of each device. In the context of such interaction scenarios, the principle of
continuity, which emphasizes the uninterrupted sequence of dialogue activities, is put forward as a key design goal. This
paper presents an application experiment demonstrating interface migration, distributed I/O control, and dynamic I/O reconfiguration.
The adopted dialogue design approach is discussed, along with the identified interaction design requirements, from the perspective
of dialogue continuity. Finally, the employed software engineering strategy is presented, elaborating on the way dialogue
mobility, distribution, and dynamic I/O control have been accomplished.
Published online: 11 June 2002 相似文献