Spectral Versus Pseudospectral Solutions of the Wave Equation by Waveform Relaxation Methods

We examine spectral and pseudospectral methods as well as waveform relaxation methods for the wave equation in one space dimension. Our goal is to study block Gauss–Jacobi waveform relaxation schemes which can be efficiently implemented in a parallel computing environment. These schemes are applied to semidiscrete systems written in terms of sparse or dense matrices. It is demonstrated that the spectral formulations lead to the implicit system of ordinary differential equations Wã = Sã + g(t)w, with sparse matrices W and S which can be effectively solved by direct application of any Runge–Kutta method. We also examine waveform relaxation iterations based on splittings W = W ₁–W ₂ and S = S ₁ + S ₂ and demonstrate that these iterations are only linearly convergent on finite time windows. Waveform relaxation methods applied to the explicit system ã = W ^–1 Sã + g(t)W ^–1 w are somewhat faster but less convenient to implement since the matrix W ^–1 S is no longer sparse. The pseudospectral methods lead to the system = D + g(t)w with a differentiation matrix D of order one and the corresponding waveform relaxation iterations are much faster than the iterations corresponding to the spectral cases (both implicit and explicit).     

分布计算环境的实现技术

本文论述了分布式计算环境以及远过程调用、线程、目录服务等支持机制的概念；并以ＯＳＦ（ＯｐｅｎＳｏｆｔｗａｒｅＦｏｕｎｄａｔｉｏｎ）ＤＣＥ（ＤｉｓｔｒｉｂｕｔｅｄＣｏｍｐｕｔｉｎｇＥｎｖｉｒｏｒｍｅｎｔ）１．０为实例介绍ＤＣＥ的实现技术。ＯＳＦＤＣＥ提供了建立一个分布式应用的基础结构，随着它的逐步完善，建立在其上的分布式应用的增多，它趋向于成为分布式应用的标准平台。     

分布式环境下的访问控制综述

论述了分布式环境下的访问控制问题。介绍了多域间安全互操作的机制、信任管理和证书的分布查找等相关研究的进展。通过分析和总结,指出了分布式访问控制研究的发展方向。     

A useful technique for the point Successive Over Relaxation method

By gradually decreasing the relaxation parameter in the SOR method, it is shown that the number of iterations required for the unknown variables to converge to reasonable values can be reduced. This practical SOR method can also be applied for composite substances with arbitrary geometry. When applying the method to a simple example of a composite substance, the iteration number was reduced by about 20% as compared with the ordinary SOR method using an optimal relaxation parameter.     

Distributed Context Monitoring for the Adaptation of Continuous Services

This paper describes a middleware designed for distributed context acquisition and reconciliation intended to support the adaptation of continuous Internet services, like e.g., multimedia streaming. These services persist in time, and are characterized by multiple transmissions of data by the service provider, as a result of a single request from the user. Adapting these services to the current context requires the continuous monitoring of context data, and a real-time adjustment of the adaptation parameters upon the detection of a relevant context change. The proposed solution is based on asynchronous context change notifications, and specific techniques have been designed to minimize the number of unnecessary updates and the re-evaluation of policies. The paper also provides experimental results obtained by developing an adaptive video streaming system and running it on top of the proposed middleware.     

Scalability Analysis of the Hierarchical Architecture for Distributed Virtual Environments

A distributed virtual environment (DVE) is a shared virtual environment where multiple users at their workstations interact with each other over a network. Some of these systems may support a large number of users, for example, multiplayer online games. An important issue is how well the system scales as the number of users increases. In terms of scalability, a promising system architecture is a two-level hierarchical architecture. At the lower level, multiple servers are deployed; each server interacts with its assigned users. At the higher level, the servers ensure that their copies of the virtual environment are as consistent as possible. Although the two-level architecture is believed to have good properties with respect to scalability, not much is known about its performance characteristics. In this paper, we develop a performance model for the two-level architecture and obtain analytic results on the workload experienced by each server. Our results provide valuable insights into the scalability of the architecture. We also investigate the issue of consistency and develop a novel technique to achieve weak consistency among copies of the virtual environment at the various servers. Simulation results on the consistency/scalability trade-off are presented.     

开放分布式环境中信任管理综述

信任是以网络为媒介的交互主体之间重要决策的依据,为主体处理交互过程中的不确定因素、不可控因素、模糊性和不完备信息等提供新思路。信任管理是描述、建立、验证和维护主体之间信任关系的方法,正日益成为研究热点。首先在分析信任及信任管理相关概念的基础上,给出了信任管理的描述性定义和形式化定义,并讨论了二者的关系;其次,分类对典型的信任管理系统进行分析、点评和对比;最后,总结了当前研究中存在的不足并讨论了信任管理未来的发展趋势。     

Evaluating the Impact of the Communication System on Distributed Virtual Environments

This paper analyzes the impact of the communication system on distributed virtual environments. We present a platform model that express the communication requirements of this kind of applications and evaluate it via simulation. Performance measurements varied the number of participants, the partition of the virtual environment, a participant's speed, and the size of the virtual world. Results show that the division of the environment improves the delay both to enter and to exchange groups, and reduces the number of messages exchanged. This partition allows both a participant's speed and the number of participants to increase, thereby ensuring a good interactivity.     

Reliable and Efficient Distributed Checkpointing System for Grid Environments

In Fine-Grained Cycle Sharing (FGCS) systems, machine owners voluntarily share their unused CPU cycles with guest jobs, as long as their performance degradation is tolerable. However, unpredictable evictions of guest jobs lead to fluctuating completion times. Checkpoint-recovery is an attractive mechanism for recovering from such “failures”. Today’s FGCS systems often use expensive, high-performance dedicated checkpoint servers. However, in geographically distributed clusters, this may incur high checkpoint transfer latencies. In this paper we present a distributed checkpointing system called Falcon that uses available disk resources of the FGCS machines as shared checkpoint repositories. However, an unavailable storage host may lead to loss of checkpoint data. Therefore, we model the failures of a storage host and develop a prediction algorithm for choosing reliable checkpoint repositories. We experiment with Falcon in the university-wide Condor testbed at Purdue and show improved and consistent performance for guest jobs in the presence of irregular resource availability.     

Optimized Overlapping Schwarz Waveform Relaxation for a Class of Time-Fractional Diffusion Problems

For parabolic PDEs with integer-order temporal derivative, if we use the Schwarz waveform relaxation (SWR) algorithm with Robin transmission conditions as the solver, the so-called equioscillation principle is an important concept to get a good Robin parameter, which has a significant effect on the convergence rate of the algorithm. Surprisingly, as we show in this paper such a principle may result in rather disappointing Robin parameter for the SWR algorithm when we use it to solve time-fractional PDEs. For a class of time-fractional diffusion equations, by analyzing a new min–max problem we get much better Robin parameter, which is found very close to the best one that we can make through numerical optimizations and numerical experiments. To use the SWR algorithm in practice, we apply the kernel reduction technique proposed recently by Baffet and Hesthaven to treat the convolutions with kernel function of the form \(\mathscr {K}_{\gamma }(t)=t^{-\gamma }/\varGamma (1-\gamma )\), where \(\gamma \in (0, 1)\). For time-fractional PDEs with this kind of kernel function, the kernel reduction technique results in efficient one-step numerical schemes. Numerical results obtained by using this technique confirm our theoretical conclusions very well.     

通用组件的改装方法

在用组件组装应用系统的过程中 ,可重用组件一般都需要通过改装才能符合新系统的要求。组件改装方法主要有包容、聚集和继承等     

分布式虚拟仿真场景生成方法研究及性能评析

在分布式虚拟仿真系统中,有三种方法生成虚拟场景：基于几何式的、基于图像的和混合式的,几何式的方法和基于图像的方法各有优缺点,混合式的方法试图结合前两种方法的优点来创建虚拟场景。分别介绍了这三种生成分布式虚拟场景的方法及步骤,并重点介绍了一种混合式方法——IBRAC技术,分析了它的两个关键技术：图像绘制加速技术、压缩技术。接着对三种虚拟场景生成方法的性能进行了评析,并给出厂性能比较的实验结果,最后对它们的应用背景进行了总结。     

Component Based Simulation Environments of Distributed Discrete Event Simulation

Component based design is a software design method developing from object-oriented design. It uses hierarchical, rnodular ideas to analyze and design systerns. It improves the reusability of software, decreases the cost of system developments. This paper applies the ideas of component based software design and distributed discrete event simulation （DDES）, and puts forward simulation component model standards and distributed simulation method, and develops the simulation environment. This method allows the users to reuse existing models and to build simulation by assembling basic models. It is fit for modeling and simulating large and complex systems in especial domain such as communication network, and supports the reuse of models effectively. The simulation environment has good flexibility and expansibility.     

分布式环境下用证书实现dRBAC

对跨越多个管理域的系统而言,dRBAC是一个可升级的、分散的信任管理和访问控制机制。属性证书本质上是面向授权使用PKI的一种扩展方法,以支持与授权相关的任务。本文提出了用证书来实现dRBAC的思想,详细阐述了如何使用属性证书在动态结盟环境下实现dRBAC模型。     

XML路由技术在分布式虚拟环境中的应用

在对XML路由技术进行研究和分析的基础上,提出一种基于XML路由的分布式虚拟环境系统构建方法.该方法将XML路由技术与主动兴趣管理相结合,采用双向共享组播树作为通信拓扑结构,根据发布-订购模式实现了数据包的主动过滤和转发.不仅减少系统的网络通信量,而且可以快速、动态地改变通信关系,有效地提高了分布式虚拟环境的可扩展性.最后通过原型系统的实现验证了该方法的可行性和有效性.     

分布式环境中的入侵检测与安全策略研究

提出在分布式系统中的一种新的入侵检测方法和安全策略架构.该解决方案能够保证安全策略的一致性,有效阻止对本地策略的偶然或恶意更改.方案采用两层入侵检测机制保证分布式策略的完整性和一致性,在容错性和应对DoS攻击方面具有良好的表现.该方法能够提高大规模系统的安全性.     

分布式多视点软件开发环境中的不一致性检测与管理

针对基于分布式多视点环境的软件开发中存在的不一致性问题,提出了一个不一致性管理模型ＩＭＭ,建立了水平不一致性和垂直不一致性的新概念。讨论了不一致性的检测方法和处理策略。     

分布式数字视频监控系统的设计与实现

介绍了数字视频监控系统在分布式网络环境中的应用、该系统的设计原理和实现方法,对系统中的关键技术,如多线程调度、视频编解码技术和网络拥塞控制等进行了研究。     

分布式环境下大对象数据发布的设计及实现

大对象数据的发布方式直接决定了系统能否构建成分布式体系结构.通过分析HTTP协议的请求、响应状态及WEB服务器的内容协商机制,本文提出三种实现大对象数据WEB环境发布的方法,分别针对不同数据类型.首先将所有数据存入数据库中,对于图像数据可以使用HTML标签方式发布,对于格式化文档使用HTML表单方式发布,对于静态HTML文件使用Apache重定向模块实现发布,这三种发布方式服务器端均使用Java Servlet程序访问数据库,以流模式实现大对象数据的发布.