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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.
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
3.
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
4.
The transfer of prerecorded, compressed variable-bit-rate video requires multimedia services to support large fluctuations in bandwidth requirements on multiple time scales. Bandwidth smoothing techniques can reduce the burstiness of a variable-bit-rate stream by transmitting data at a series of fixed rates, simplifying the allocation of resources in video servers and the communication network. This paper compares the transmission schedules generated by the various smoothing algorithms, based on a collection of metrics that relate directly to the server, network, and client resources necessary for the transmission, transport, and playback of prerecorded video. Using MPEG-1 and MJPEG video data and a range of client buffer sizes, we investigate the interplay between the performance metrics and the smoothing algorithms. The results highlight the unique strengths and weaknesses of each bandwidth smoothing algorithm, as well as the characteristics of a diverse set of video clips 相似文献
5.
We explore a communication paradigm for video on demand, called Range Multicast. This schemeis a shift from the conventional
thinking about multicast where every receiver must obtain the same data packet at any time. A range multicast allows new members
to join at their specified time and still receive the entire video stream without consuming additional server bandwidth. Clients
enjoy better service latency since they can join an existing multicast instead of waiting for the next available server stream.
We also present techniques to support video-cassette-recorder-like interactivity in this environment. Unlike existing methods
which require clients to cache data in a private buffer, the Range Multicast solution utilizes the shared network storage
to make more efficient and cost-effective use of the caching space. Furthermore, since a range multicast can accommodate clients
with different play points in the video, a client has a better chance to join an on-going multicast for normal playback after
finishing a VCR operation. This strategy avoids the need for a new server stream, and thus further alleviates the server load.
Our simulation results confirm the aforementioned benefits. 相似文献
6.
We consider the problem of smoothing real-time streams (such as video streams), where the goal is to reproduce a variable-bandwidth stream remotely, while minimizing bandwidth cost, space requirement, and playback delay. We focus on lossy schedules, where data may be dropped due to limited bandwidth or space. We present the following results. First, we determine the optimal tradeoff between buffer space, smoothing delay, and link bandwidth for lossy smoothing schedules. Specifically, this means that if two of these parameters are given, we can precisely calculate the value for the third which minimizes data loss while avoiding resource wastage. The tradeoff is accomplished by a simple generic algorithm, that allows one some freedom in choosing which data to discard. This algorithm is very easy to implement both at the server and at the client, and it enjoys the nice property that only the server decides which data to discard, and the client needs only to reconstruct the stream.In a second set of results we study the case where different parts of the data have different importance, modeled by assigning a real weight to each packet in the stream. For this setting we use competitive analysis, i.e., we compare the weight delivered by on-line algorithms to the weight of an optimal off-line schedule using the same resources. We prove that a natural greedy algorithm is 4-competitive. We also prove a lower bound of 1.23 on the competitive ratio of any deterministic on-line algorithm. Finally, we give a few experimental results which seem to indicate that smoothing is very effective in practice, and that the greedy algorithm performs very well in the weighted case.Received: 21 November 2001, Accepted: 6 November 2003, Published online: 6 February 2004Research supported in part by Israel Ministry of Science. An extended abstract of this paper appeared in Proc. 19th ACM Symp. on Principles of Distributed Computing, July 2000. 相似文献
7.
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
8.
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. 相似文献
9.
10.
Multimedia streams such as audio and video impose tight temporal constraints for their presentation. Often, related multimedia
streams, such as audio and video, must be presented in a synchronized way. We introduce a novel scheme to ensure the continuous
and synchronous delivery of distributed stored multimedia streams across a communications network. We propose a new protocol for synchronized playback and compute the buffer
required to achieve both, the continuity within a single substream and the synchronization between related substreams. The
scheme is very general and does not require synchronized clocks. Using a resynchronization protocol based on buffer level
control, the scheme is able to cope with server drop-outs and clock drift. The synchronization scheme has been implemented
and the paper concludes with our experimental results. 相似文献
11.
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
12.
The Analysis and Performance of Multi-Rate Service in Distributed Video-on-Demand Systems 总被引:1,自引:0,他引:1
In this paper, we develop an end-to-end analysis of a distributed Video-on-Demand (VoD) system that includes an integrated model of the server and the network subsystems with analysis of its impact on client operations. The VoD system provides service to a heterogeneous client base at multiple playback rates. A class-based service model is developed where an incoming video request can specify a playback rate at which the data is consumed on the client. Using an analytical model, admission control conditions at the server and the network are derived for multi-rate service. We also develop client buffer requirements in presence of network delay bounds and delay jitter bounds using the same integrated framework of server and network subsystems. Results from an extensive simulation show that request handling policies based on limited redirection of blocked requests to other resources perform better than load sharing policies. The results also show that downgrading the service for blocked requests to a lower bitrate improves VoD system performance considerably. Combining the downgrade option with restrictions on access to high bitrate request classes is a powerful tool for manipulating an incoming request mix into a workload that the VoD system can handle. 相似文献
13.
Raymond T. Ng Paul Shum 《The VLDB Journal The International Journal on Very Large Data Bases》1998,7(4):239-252
A multi-clip query requests multiple video clips be returned as the answer of the query. In many applications and situations,
the order in which these clips are to be delivered does not matter that much to the user. This allows the system ample opportunities
to optimize system throughput by using schedules that maximize the effect of piggybacking. In this paper, we study how to
find such optimal schedules. In particular, we consider two optimization criteria: (i) one based on maximizing the number
of piggybacked clips, and (ii) the other based on maximizing the impact on buffer space. We show that the optimal schedule
under the first criterion is equivalent to a maximum matching in a suitably defined bipartite graph, and that under the second
criterion, the optimal schedule is equivalent to a maximum matching in a suitably defined weighted bipartite graph. Our experimental
results, which are based on realistic distributions, indicate that both kinds of optimal schedules can lead to a gain in throughput
of over 300%. And yet the time taken to compute such an optimal schedule is negligible. Finally, we show how to deal with
clips that are variable in length.
Received: June 9, 1998 / Accepted October 13, 1998 相似文献
14.
Control of perceived quality of service in multimedia retrieval services: prediction-based mechanism vs. compensation buffers 总被引:3,自引:0,他引:3
Aurelio La Corte Alfio Lombardo Sergio Palazzo Giovanni Schembra 《Multimedia Systems》1998,6(2):102-112
In multimedia systems end-to-end delay jitter has a great impact on the continuity of information playback. Therefore, it
is necessary to introduce appropriate mechanisms to compensate for delay variations, so that the intramedia and intermedia
temporal relationships can be preserved. In this paper, two methods for compensation of the network delay jitter in a distributed
multimedia retrieval service are compared: the first is based on prediction of the network delay jitter suffered by each information
unit and retrieval time modification at the source site; the second is based on a compensation buffer at the destination site.
Comparison is made by assuming a master/slave relationship between the monomedia streams composing the multimedia data flow. 相似文献
15.
I/O scheduling for digital continuous media 总被引:4,自引:0,他引:4
A growing set of applications require access to digital video and audio. In order to provide playback of such continuous
media (CM), scheduling strategies for CM data servers (CMS) are necessary. In some domains, particularly defense and industrial process control, the timing requirements of these applications
are strict and essential to their correct operation. In this paper we develop a scheduling strategy for multiple access to
a CMS such that the timing guarantees are maintained at all times. First, we develop a scheduling strategy for the steady state,
i.e., when there are no changes in playback rate or operation. We derive an optimal Batched SCAN (BSCAN) algorithm that requires minimum buffer space to schedule concurrent accesses. The scheduling strategy incorporates two key
constraints: (1) data fetches from the storage system are assumed to be in integral multiples of the block size, and (2) playback
guarantees are ensured for frame-oriented streams when each frame can span multiple blocks. We discuss modifications to the
scheduling strategy to handle compressed data like motion-JPEG and MPEG.
Second, we develop techniques to handle dynamic changes brought about by VCR-like operations executed by applications. We define a suite of primitive VCR-like operations that can be executed. We show that an unregulated change in the BSCAN schedule, in response to VCR-like operations, will affect playback guarantees. We develop two general techniques to ensure playback guarantees while responding
to VCR-like operations: passive and active accumulation. Using user response time as a metric we show that active accumulation algorithms
outperform passive accumulation algorithms. An optimal response-time algorithm in a class of active accumulation strategies
is derived. The results presented here are validated by extensive simulation studies. 相似文献
16.
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
17.
Without user interactions, multimedia presentations are just fancy slide shows with sound and video supports. User interactions
by themselves do not change the temporal relationships among multimedia objects, such as texts, graphics, images, audio, and
video, but affect the playback schedules. In this paper, we propose a synchronization mechanism to guarantee the quality of
multimedia presentation with user interactions. In our protocol, each presentation site requests media transmission from the
required media servers at certain time intervals prior to the playback deadlines, where these time intervals are the response
times to cover possible experienced end-to-end delays and packet losses, and waits for an initial setup time to ensure intermedia
synchronization before starting the presentation. Users may interact with the presentation. This synchronization mechanism
solves the problems incurred by user interactions by determining the new presentation scenario produced by the interactive
operation, calculating the corresponding setup time, and then rendering the new playback and retrieval schedules. 相似文献
18.
《Real》2001,7(3):255-273
Video delivery from a server to a client across a network is an important component of many multimedia applications. While delivering a video stream across a resource constrained network, loss of frames may be unavoidable. Under such circumstances, it is desirable to find a server transmission schedule that can efficiently utilize the network resources while maximizing the perceived quality-of-service (QoS) at the client. To address this issue, in this paper we introduce the notion ofselective frame discard at the server and formulate the optimal selective frame discard problem using a QoS-based cost function. Given network bandwidth and client buffer constraints, we develop an O (N log N) algorithm to find the minimum number of frames that must be discarded in order to meet these constraints. The correctness of the algorithm is also formally established. We present a dynamic programming based algorithm for solving the problem of optimal selective frame discard. Since the computational complexity of the optimal algorithm is prohibitively high in general, we also develop several efficient heuristic algorithms for selective frame discard. These algorithms are evaluated using JPEG and MPEG video traces. 相似文献
19.
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
20.
Video can be encoded into multiple-resolution format in nature. A multi-resolution or scalable video stream is a video sequence encoded such that subsets of the full resolution video bit stream can be decoded to recreate lower resolution video streams. Employing scalable video enables a video server to provide multiple resolution services for a variety of clients with different decoding capabilities and network bandwidths connected to the server. The inherent advantages of the multi-resolution video server include: heterogeneous client support, storage efficiency, adaptable service, and interactive operations support.For designing a video server, several issues should be dealt with under a unified framework including data placement/retrieval, buffer management, and admission control schemes for deterministic service guarantee. In this paper, we present a general framework for designing a large-scale multi-resolution video server. First, we propose a general multi-resolution video stream model which can be implemented by various scalable compression techniques. Second, given the proposed stream model, we devise a hybrid data placement scheme to store scalable video data across disks in the server. The scheme exploits both concurrency and parallelism offered by striping data across the disks and achieves the disk load balancing during any resolution video service. Next, the retrieval of multi-resolution video is described. The deterministic access property of the placement scheme permits the retrieval scheduling to be performed on each disk independently and to support interactive operations (e.g. pause, resume, slow playback, fastforward and rewind) simply by reconstructing the input parameters to the scheduler. We also present an efficient admission control algorithm which precisely estimates the actual disk workload for the given resolution services and hence permits the buffer requirement to be much smaller. The proposed schemes are verified through detailed simulation and implementation. 相似文献