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1.
Yueh-Min Huang Jen-Wen Ding Shiao-Li Tsao 《The VLDB Journal The International Journal on Very Large Data Bases》1999,8(1):44-54
To provide high accessibility of continuous-media (CM) data, CM servers generally stripe data across multiple disks. Currently,
the most widely used striping scheme for CM data is round-robin permutation (RRP). Unfortunately, when RRP is applied to variable-bit-rate
(VBR) CM data, load imbalance across multiple disks occurs, thereby reducing overall system performance. In this paper, the
performance of a VBR CM server with RRP is analyzed. In addition, we propose an efficient striping scheme called constant
time permutation (CTP), which takes the VBR characteristic into account and obtains a more balanced load than RRP. Analytic
models of both RRP and CTP are presented, and the models are verified via trace-driven simulations. Analysis and simulation
results show that CTP can substantially increase the number of clients supported, though it might introduce a few seconds/minutes
of initial delay.
Received June 9, 1998 / Accepted January 21, 1999 相似文献
2.
We consider the problem of storing a given set of files containing continuous-media data on a compact disc by interleaving
them so that the required total area is minimal. We show that the problem is NP-hard and consider a number of special cases.
The fact that these special cases are related to well-known combinatorial optimization problems has been used in solution
techniques developed to handle such cases. We propose an approximation algorithm based on these techniques for the general
case. 相似文献
3.
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
4.
The multimedia multicasting problem 总被引:4,自引:0,他引:4
This paper explores the problems associated with the multicasting of continuous media to support multimedia group applications.
The interaction between multicasting and the delivery of multiple time-correlated continuous-media streams with real-time
delay requirements poses various new and interesting problems in research on communication protocols and architectures. We
describe these problems, and identify where the opportunities are for effective solutions, all in the context of providing
an overview of the current state of research in multimedia multicasting. The issues we discuss include quality of service,
resource reservations, routing, error and traffic control, heterogeneity, and the use of hierarchical coding and open-loop
control techniques. 相似文献
5.
Kelvin K.W. Law John C.S. Lui Leana Golubchik 《The VLDB Journal The International Journal on Very Large Data Bases》1999,8(2):133-153
Advances in high-speed networks and multimedia technologies have made it feasible to provide video-on-demand (VOD) services
to users. However, it is still a challenging task to design a cost-effective VOD system that can support a large number of
clients (who may have different quality of service (QoS) requirements) and, at the same time, provide different types of VCR
functionalities. Although it has been recognized that VCR operations are important functionalities in providing VOD service,
techniques proposed in the past for providing VCR operations may require additional system resources, such as extra disk I/O,
additional buffer space, as well as network bandwidth. In this paper, we consider the design of a VOD storage server that
has the following features: (1) provision of different levels of display resolutions to users who have different QoS requirements,
(2) provision of different types of VCR functionalities, such as fast forward and rewind, without imposing additional demand
on the system buffer space, I/O bandwidth, and network bandwidth, and (3) guarantees of the load-balancing property across
all disks during normal and VCR display periods. The above-mentioned features are especially important because they simplify
the design of the buffer space, I/O, and network resource allocation policies of the VOD storage system. The load-balancing
property also ensures that no single disk will be the bottleneck of the system. In this paper, we propose data block placement,
admission control, and I/O-scheduling algorithms, as well as determine the corresponding buffer space requirements of the
proposed VOD storage system. We show that the proposed VOD system can provide VCR and multi-resolution services to the viewing
clients and at the same time maintain the load-balancing property.
Received June 9, 1998 / Accepted April 26, 1999 相似文献
6.
In this paper, we study
the problem of how to maximize the throughput of a
continuous-media system, given fixed amounts of
buffer space and disk bandwidth both pre-determined
at design time. Our approach is to
maximize the utilizations of disk and buffers. We
propose doing so in two ways. First, we analyze a
scheme that allows multiple streams to share buffers.
Our analysis and preliminary simulation results
indicate that buffer sharing could lead to as much as
50% reduction in total buffer requirement. Second,
we develop three prefetching strategies: SP, IP1 and
IP2. As demonstrated by SP, straightforward
prefetching is not effective at all. In contrast, IP1 and
IP2, which prefetch more intelligently than does SP,
could be valuable in maximizing the effective use of
buffers and disk. Our preliminary simulation results
show that IP1 and IP2 could lead to a 40%
improvement in throughput. 相似文献
7.
Periodic broadcast and scheduled multicast have been shown to be very effective in reducing the demand on server bandwidth.
While periodic broadcast is better for popular videos, scheduled multicast is more suitable for less popular ones. Work has
also been done to show that a hybrid of these techniques offer the best performance. Existing hybrid schemes, however, assume
that the characteristic of the workload does not change with time. This assumption is not true for many applications, such
as movie on demand, digital video libraries, or electronic commerce. In this paper, we show that existing scheduled multicast
techniques are not suited for hybrid designs. To address this issue, we propose a new approach and use it to design an adaptive
hybrid strategy. Our technique adjusts itself to cope with a changing workload. We provide simulation results to demonstrate
that the proposed technique is significantly better than the best static approach in terms of service latency, throughput,
defection rate, and unfairness. 相似文献
8.
We consider the problem of scheduling a set of pages on a single broadcast channel using time-multiplexing. In a perfectly periodic schedule, time is divided into equal size slots, and each page is transmitted in a time slot precisely every fixed interval of time (the period of the page). We study the case in which each page i has a given demand probability , and the goal is to design a perfectly periodic schedule that minimizes the average time a random client waits until its
page is transmitted. We seek approximate polynomial solutions. Approximation bounds are obtained by comparing the costs of
a solution provided by an algorithm and a solution to a relaxed (non-integral) version of the problem. A key quantity in our
methodology is a fraction we denote by , that depends on the maximum demand probability: . The best known polynomial algorithm to date guarantees an approximation of . In this paper, we develop a tree-based methodology for perfectly periodic scheduling, and using new techniques, we derive
algorithms with better bounds. For small values, our best algorithm guarantees approximation of . On the other hand, we show that the integrality gap between the cost of any perfectly periodic schedule and the cost of
the fractional problem is at least . We also provide algorithms with good performance guarantees for large values of .
Received: December 2001 / Accepted: September 2002 相似文献
9.
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
10.
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
11.
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
12.
Ada Wai-chee Fu Polly Mei-shuen Chan Yin-Ling Cheung Yiu Sang Moon 《The VLDB Journal The International Journal on Very Large Data Bases》2000,9(2):154-173
Abstract. For some multimedia applications, it has been found that domain objects cannot be represented as feature vectors in a multidimensional
space. Instead, pair-wise distances between data objects are the only input. To support content-based retrieval, one approach
maps each object to a k-dimensional (k-d) point and tries to preserve the distances among the points. Then, existing spatial access index methods such as the R-trees
and KD-trees can support fast searching on the resulting k-d points. However, information loss is inevitable with such an approach since the distances between data objects can only
be preserved to a certain extent. Here we investigate the use of a distance-based indexing method. In particular, we apply
the vantage point tree (vp-tree) method. There are two important problems for the vp-tree method that warrant further investigation,
the n-nearest neighbors search and the updating mechanisms. We study an n-nearest neighbors search algorithm for the vp-tree, which is shown by experiments to scale up well with the size of the dataset
and the desired number of nearest neighbors, n. Experiments also show that the searching in the vp-tree is more efficient than that for the -tree and the M-tree. Next, we propose solutions for the update problem for the vp-tree, and show by experiments that the algorithms are
efficient and effective. Finally, we investigate the problem of selecting vantage-point, propose a few alternative methods,
and study their impact on the number of distance computation.
Received June 9, 1998 / Accepted January 31, 2000 相似文献
13.
Asynchronous group mutual exclusion 总被引:1,自引:1,他引:0
Yuh-Jzer Joung 《Distributed Computing》2000,13(4):189-206
Abstract. Mutual exclusion and concurrency are two fundamental and essentially opposite features in distributed systems. However, in
some applications such as Computer Supported Cooperative Work (CSCW) we have found it necessary to impose mutual exclusion
on different groups of processes in accessing a resource, while allowing processes of the same group to share the resource.
To our knowledge, no such design issue has been previously raised in the literature. In this paper we address this issue by
presenting a new problem, called Congenial Talking Philosophers, to model group mutual exclusion. We also propose several criteria to evaluate solutions of the problem and to measure their
performance. Finally, we provide an efficient and highly concurrent distributed algorithm for the problem in a shared-memory
model where processes communicate by reading from and writing to shared variables. The distributed algorithm meets the proposed
criteria, and has performance similar to some naive but centralized solutions to the problem.
Received: November 1998 / Accepted: April 2000 相似文献
14.
Synchronous Byzantine quorum systems 总被引:2,自引:0,他引:2
Rida A. Bazzi 《Distributed Computing》2000,13(1):45-52
Summary. Quorum systems have been used to implement many coordination problems in distributed systems such as mutual exclusion, data
replication, distributed consensus, and commit protocols. Malkhi and Reiter recently proposed quorum systems that can tolerate
Byzantine failures; they called these systems Byzantine quorum systems and gave some examples of such quorum systems. In this
paper, we propose a new definition of Byzantine quorums that is appropriate for synchronous systems. We show how these quorums
can be used for data replication and propose a general construction of synchronous Byzantine quorums using standard quorum
systems. We prove tight lower bounds on the load of synchronous Byzantine quorums for various patterns of failures and we
present synchronous Byzantine quorums that have optimal loads that match the lower bounds for two failure patterns.
Received: June 1998 / Accepted: August 1999 相似文献
15.
《Computer Networks》2003,41(4):451-474
We present a method, called opportunistic scheduling, for exploiting the time-varying nature of the radio environment to increase the overall performance of the system under certain quality of service/fairness requirements of users. We first introduce a general framework for opportunistic scheduling, and then identify three general categories of scheduling problems under this framework. We provide optimal solutions for each of these scheduling problems. All the proposed scheduling policies are implementable online; we provide parameter estimation algorithms and implementation procedures for them. We also show how previous work by us and others directly fits into or is related to this framework. We demonstrate via simulation that opportunistic scheduling schemes result in significant performance improvement compared with non-opportunistic alternatives. 相似文献
16.
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
17.
Update propagation strategies to improve freshness in lazy master replicated databases 总被引:4,自引:0,他引:4
Esther Pacitti Eric Simon 《The VLDB Journal The International Journal on Very Large Data Bases》2000,8(3-4):305-318
Many distributed database applications need to replicate data to improve data availability and query response time. The two-phase
commit protocol guarantees mutual consistency of replicated data but does not provide good performance. Lazy replication has
been used as an alternative solution in several types of applications such as on-line financial transactions and telecommunication
systems. In this case, mutual consistency is relaxed and the concept of freshness is used to measure the deviation between
replica copies. In this paper, we propose two update propagation strategies that improve freshness. Both of them use immediate
propagation: updates to a primary copy are propagated towards a slave node as soon as they are detected at the master node
without waiting for the commitment of the update transaction. Our performance study shows that our strategies can improve
data freshness by up to five times compared with the deferred approach.
Received April 24, 1998 / Revised June 7, 1999 相似文献
18.
Data overload is a generic and tremendously difficult problem that has only grown with each new wave of technological capabilities.
As a generic and persistent problem, three observations are in need of explanation: Why is data overload so difficult to address?
Why has each wave of technology exacerbated, rather than resolved, data overload? How are people, as adaptive responsible
agents in context, able to cope with the challenge of data overload? In this paper, first we examine three different characterisations
that have been offered to capture the nature of the data overload problem and how they lead to different proposed solutions.
As a result, we propose that (a) data overload is difficult because of the context sensitivity problem – meaning lies, not
in data, but in relationships of data to interests and expectations and (b) new waves of technology exacerbate data overload
when they ignore or try to finesse context sensitivity. The paper then summarises the mechanisms of human perception and cognition
that enable people to focus on the relevant subset of the available data despite the fact that what is interesting depends
on context. By focusing attention on the root issues that make data overload a difficult problem and on people’s fundamental
competence, we have identified a set of constraints that all potential solutions must meet. Notable among these constraints
is the idea that organisation precedes selectivity. These constraints point toward regions of the solution space that have
been little explored. In order to place data in context, designers need to display data in a conceptual space that depicts
the relationships, events and contrasts that are informative in a field of practice. 相似文献
19.
In this paper, we investigate a playout scheduling framework for supporting the continuous and synchronized presentations
of multimedia streams in a distributed multimedia presentation system. We assume a situation in which the server and network
transmissions provide sufficient support for the delivery of media objects. In this context, major issues regarding the enforcement
of the smooth presentation of multimedia streams at client sites must be addressed to deal with rate variance of stream presentations
and delay variance of networks. We develop various playout-scheduling algorithms that are adaptable to quality-of-service
parameters. The proposed algorithms permit the local adjustment of unsynchronized presentations by gradually accelerating
or retarding presentation components, rather than abruptly skipping or pausing the presentation materials. A comprehensive
experimental analysis of the proposed algorithms demonstrates that our algorithms can effectively avoid playout gaps (or hiccups)
in the presentations. This scheduling framework can be readily used to support customized multimedia presentations. 相似文献
20.
Wen-Syan Li K.Selçuk Candan Kyoji Hirata Yoshinori Hara 《The VLDB Journal The International Journal on Very Large Data Bases》2001,9(4):312-326
Due to the fuzziness of query specification and media matching, multimedia retrieval is conducted by way of exploration.
It is essential to provide feedback so that users can visualize query reformulation alternatives and database content distribution.
Since media matching is an expensive task, another issue is how to efficiently support exploration so that the system is not
overloaded by perpetual query reformulation. In this paper, we present a uniform framework to represent statistical information
of both semantics and visual metadata for images in the databases. We propose the concept of query verification, which evaluates queries using statistics, and provides users with feedback, including the strictness and reformulation alternatives
of each query condition as well as estimated numbers of matches. With query verification, the system increases the efficiency
of the multimedia database exploration for both users and the system. Such statistical information is also utilized to support
progressive query processing and query relaxation.
Received: 9 June 1998/ Accepted: 21 July 2000 Published online: 4 May 2001 相似文献