Multiserver support for large-scale distributed virtual environments

CyberWalk is a distributed virtual walkthrough system that we have developed. It allows users at different geographical locations to share information and interact within a shared virtual environment (VE) via a local network or through the Internet. In this paper, we illustrate that as the number of users exploring the VE increases, the server will quickly become the bottleneck. To enable good performance, CyberWalk utilizes multiple servers and employs an adaptive region partitioning technique to dynamically partition the whole VE into regions. All objects within each region will be managed by one server. Under normal circumstances, when a viewer is exploring a region, the server of that region will be responsible for serving all requests from the viewer. When a viewer is crossing the boundary of two or more regions, the servers of all the regions involved will be serving requests from the viewer since the viewer might be able to view objects within all these regions. This is analogous to evaluating a database query using a parallel database server, which could improve the performance of serving a viewer's request tremendously. We evaluate the performance of this multiserver architecture of CyberWalk via a detail simulation model.     

A framework for multiuser distributed virtual environments

A framework for multi-user distributed virtual environments (DVEs) has been proposed. The proposed framework, incorporating two models (the functional model and the interconnection model), attempts to represent the common functionality, communication issues and requirements found in multi-user DVEs. The functional model concentrates on the DVE's functionality, while the interconnection model concentrates on how the components are interconnected to realize the required functionality. The models have been specified using the Unified Modeling Language (UML). An experimental case study demonstrates the applicability and generality of the proposed approach.     

Rapid prototyping for distributed virtual environments

The authors combine rapid and exploratory prototyping techniques to capture initially unknown, interdependent requirements in the development of distributed virtual environment applications. They employ containerization to accelerate data exchange     

A scalable interest management scheme for distributed virtual environments

With the expansion of the internet and its bandwidth, distributed virtual environment (DVE) applications have become more prevalent. In DVE applications, users frequently crowd in a specific place, and a key aspect to consider is how to provide interactive performance for users. However, existing approaches using multicast require users to receive uninteresting messages. Even though recent works have addressed fine‐grained filtering, they still incur other drawbacks in terms of assigning lots of multicast addresses or handling overhead of multicast groups. This makes the system less scalable as the number of users increases. In this paper, we propose a new scalable filtering scheme that reduces not only the number of messages during interaction in a region and among neighboring regions, but also the number of multicast addresses without significant computational overhead. Interest management in a region dynamically creates groups of users with the same interests. While members communicate with each other with high fidelity, a representative sends information to non‐members with low frequency. For interaction among neighboring regions, we propose a sub‐region concept to select only a subset of users from the neighboring regions based on proximity, the distribution of the users' locations, and the viewing direction of a user. Copyright © 2007 John Wiley & Sons, Ltd.     

Hand motion prediction for distributed virtual environments

We use our hands to manipulate objects in our daily life. The hand is capable of accomplishing diverse tasks such as pointing, gripping, twisting and tearing. However, there is not much work that considers using the hand as input in distributed virtual environments (DVEs), in particular over the Internet. The main reasons are that the Internet suffers from high network latency, which affects interaction, and the hand has many degrees of freedom, which adds additional challenges to synchronizing the collaboration. In this paper, we propose a prediction method specifically designed for human hand motion to address the network latency problem in DVEs. Through a thorough analysis of finger motion, we have identified various finger motion constraints and we propose a constraint-based motion prediction method for hand motion. To reduce the average prediction error under high network latency, e.g., over the Internet, we further propose a revised dead reckoning scheme here. Our performance results show that the proposed prediction method produces a lower prediction error than some popular methods while the revised dead reckoning scheme produces a lower average prediction error than the traditional dead reckoning scheme, in particular at high network latency.     

A novel approach to enhance distributed virtual memory

Distributed virtual memory (DVM) has been introduced in several researches and models in order to improve system memory performance. In this paper, various practical issues are addressed, examined and analyzed to exploit further areas of performance enhancements. In particular, a novel distributed algorithm for DVM management based on cluster cache in order to enhance previous DVM techniques is discussed. The algorithm employs address translation, page out policies and page replacement. The algorithm performance is evaluated across several test benches under variant system loads to show the efficiency of our technique. Compared with practical DVM and conventional virtual memory (CVM), our algorithm outperforms the previous algorithms and reduces the page faults by a considerable amount of 15% and 25%, respectively.     

大规模虚拟环境中实时阴影生成技术

为了解决大规模场景中阴影生成的真实性和实时性问题,通过平均分割和对数分割相结合的方法,利用平行于视觉投影面的分割面将视截体划分成不同的深度部分;然后对每个分割部分生成对应的阴影图;最后利用多个阴影图代替单个阴影图进行渲染。这种方法既减少了运行时的缓冲空间,又提高了阴影质量,尤其适合于动态大规模环境中实时阴影的生成。用VS2005和OpenSceneGraph实现了算法的绘制,达到满意的视觉效果和生成速度。     

ViWoSG: A distributed scene graph of ultramassive distributed virtual environments

An ultra-massive distributed virtual environment generally consists of ultra-massive terrain data and a large quantity of objects and their attribute data, such as 2D/3D geometric models, audio/video, images, vectors, characteristics, etc. In this paper, we propose a novel method for constructing distributed scene graphs with high extensibility. Thismethod can support high concurrent interaction of clients and implement various tasks such as editing, querying, accessing and motion controlling. Some application experiments are performed to demonstrate its efficiency and soundness. Supported by the National Basic Research Program of China (Grant No. 2004CB719403), the National High-Tech Research & Development Program of China (Grant Nos. 2006AA01Z334, 2007AA01Z318, 2009AA01Z324), the National Natural Science Foundation of China (Grant Nos. 60573151, 60703062, 60833007), and the Marine 908-03-01-10 Project     

A data management scheme for effective walkthrough in large-scale virtual environments

As the complexity of virtual environments increases, it becomes a critical issue to maintain the quality of a walkthrough experience. In this paper, we propose an effective data management scheme to address this issue in client-server architecture. First, we propose using real-time scene management to manage the computing resources on the client side by reducing the amount of transmitted geometry data. Second, we propose a prioritized most likelihood movement model to prefetch potential future objects based on the users current motion intention. Lastly, a hybrid coherence cache model is proposed to take advantages of both the temporal and spatial localities of the walkthrough process. We have done extensive experiments to demonstrate how these techniques can improve the effectiveness of walkthrough in a large virtual environment.     

A compensation-based approach for view maintenance in distributed environments

Data integration over multiple heterogeneous data sources has become increasingly important for modern applications. The integrated data is usually stored as materialized views to allow better access, performance, and high availability. In loosely coupled environments, such as the data grid, the data sources are autonomous. Hence, tie source updates can be concurrent and cause erroneous results during view maintenance. State-of-the-art maintenance strategies apply compensating queries to correct such errors, making the restricting assumption that all source schemata remain static over time. However, in such dynamic environments, the data sources may change not only their data but also their schema. Consequently, either the maintenance queres or the compensating queries may fail. In this paper, we propose a novel framework called DyDa that overcomes these limitations and handles both source data updates and schema changes. We identify three types of maintenance anomalies, caused by either source data updates, data-preserving schema changes, or non-data-preserving schema changes. We propose a compensation algorithm to solve the first two types of anomalies. We show that the third type of anomaly is caused by the violation of dependencies between maintenance processes. Then, we propose dependency detection and correction algorithms to identify and resolve the violations. Put together, DyDa extends prior maintenance solutions to solve all types of view maintenance anomalies. The experimental results show that DyDa imposes a minimal overhead on data update processing while allowing for the extended functionality to handle concurrent schema changes.     

Evaluation of the interactivity of students in virtual learning environments using a multicriteria approach and data mining

This work seeks to provide a new multicriteria approach to evaluate and classify the level of interactivity of students in learning management systems (LMS). We describe, step by step, the complete methodological development process of the evaluation model as well as detailing the results obtained when applying it to a higher education teaching experience. This research demonstrates that the combined use of multicriteria decision methodologies and data mining prove to be particularly suitable for identifying behavioural patterns of the users through the analysis of records generated in LMS. The results reveal that the behavioural patterns in LMS offer certain indicators as to students’ academic performance, although the study does not permit to state that those students who adopt passive attitudes in these spheres may necessarily produce low academic performance.     

Portable virtual cycle accounting for large-scale distributed cycle sharing systems

CPU cycle sharing among distributed heterogeneous computers is the key function in large-scale volunteer computing and desktop grid applications. One important problem in large-scale distributed cycle sharing system is how to account for the amount of computation work performed by a CPU cycle provider, in a uniform and portable fashion across heterogeneous hardware and operating system platforms. Such an accounting mechanism is especially desirable when CPU resources are traded and a lack of uniform workload accounting will hinder the enforcement of market-driven CPU pricing/trading policies in distributed cycle sharing systems. Java Virtual Machine (JVM) has proved to be a good match for distributed cycle sharing because of its abilities to run applications on a wide variety of platforms without modification (portability) and to host untrusted applications (safety). In this paper, we present the design, implementation, and evaluation of an efficient, application-transparent virtual cycle accounting scheme integrated into JVM. Our scheme achieves portable workload accounting across heterogeneous computing platforms by accounting for JVM virtual instructions instead of real processor cycles. Different from the existing JVM CPU accounting mechanisms that involve bytecode rewriting, our scheme is transparent to applications and does not require visible changes to application and library code interfaces which would break applications that use Reflection API. Moreover, our scheme is efficient via the use of processor registers for accounting. Our experimental results demonstrate both high accounting accuracy and low runtime overhead of virtual cycle accounting.     

Direction-aware resource discovery in large-scale distributed computing environments

As a system scales up, the peer-to-peer (P2P) approach is attractive to distributed computing environments, such as Grids and Clouds, due to the amount of resources increased. The major issue in large-scale distributed systems is to prevent the phenomenon of a communication bottleneck or a single point of failure. Conventional approaches may not be able to apply directly to such environments due to restricted queries and varied resource characteristics. Alternatively, a fully decentralized resource discovery service based on an unstructured overlay, which relies only on the information of resource attributes and characteristics, may be a feasible solution. One major challenge of such service is to locate desired and suitable resources without the global knowledge of distributed sharing resources. As a consequence, the more nodes the resource discovery service involves, the higher the network overhead incurs. In this paper, we proposed a direction-aware strategy which can alleviate the network traffic among unstructured information systems for distributed resource discovery service. Experimental results have demonstrated that the proposed approach achieves higher success rate at low cost and higher scalability.     

The WALKABOUT: Using virtual environments to assess large-scale spatial abilities

Computer-simulated virtual environments (VEs) offer promise for assessing people's spatial abilities in large real-world environments. This paper introduces the WALKABOUT, a VE-based test that simulates navigation through large spaces. Participants' ability to form an accurate mental representation of a familiar large-scale environment correlated nearly as highly with their performance on the WALKABOUT as it did with their self-reported sense of direction. Performance on two subscales of the test further indicated that the ability to account for changing egocentric relationships as a result of self-movement and the ability to recognize a novel perspective on an environment are both significantly related to spatial ability at the environmental scale.     

虚拟现实环境下的分布式服务器架构策略研究

虚拟现实环境下,数据实时性和系统稳定性的高要求对服务器架构的设计和优化提出了新的挑战。针对虚拟现实环境下海量数据存储效率的提升和系统性能的优化,提出了一种新的分布式服务器架构,该架构基于分布式协调框架ZooKeeper、分布式缓存架构Redis以及MongoDB分片机制,并改进一致性Hash算法来优化Redis缓存架构,同时优化MongoDB分片的负载均衡机制。经过相应的仿真验证,该架构在虚拟现实环境下具有有效性。     

Scheduling multiple virtual environments in cloud federations for distributed calculations

In the pool of cloud providers that are currently available there is a lack of standardised APIs and brokering tools to effectively distribute high throughput calculations among them. Moreover, the current middleware tools are not able to straightforwardly provision the ephemeral and specific environments that certain codes and simulations require. These facts prevent the massive portability of legacy applications to cloud environments. Such an issue can be overcome by effectively scheduling the distributed calculations using the basic capacities offered by cloud federations. In this work, a framework achieving such a goal is presented: a pilot system (GWpilot) that has been improved with cloud computing capabilities (GWcloud). This framework profits from the expertise acquired in grid federations and provides interesting features that make it more efficient, flexible and useable than other approaches. Thus, decentralisation, middleware independence, dynamic brokering, on-demand provisioning of specific virtual images, compatibility with legacy applications, and the efficient accomplishment of short tasks, among other features, are achieved. Not only this, the new framework is multi-user and multi-application, dynamically instantiating virtual machines depending on the available and demanded resources, i.e. allowing users to consolidate their own resource provisioning. Results presented in this work demonstrate these features by efficiently executing several legacy applications with very different requirements on the FedCloud infrastructure at the same time.     

Exploring activity theory as a tool for evaluating interactivity and learning in virtual environments for children

This paper explores the use of Activity Theory for the evaluation of user behaviour in immersive virtual environments. Specifically, the study of user behaviour focuses on interactivity, which is argued to be one of the most important processes that take place between a user and the system in virtual reality. The ultimate intention is to study the role and the effect of interactivity on learning and conceptual change and to examine how interaction and conceptual learning are related in the context of virtual environments developed primarily for informal educational settings. As a first step to this study, a set of exploratory experiments was carried out with children aged 7–12. The children were asked to complete tasks, such as the assembly of ancient columns from parts, which were designed to promote constructivist learning and explore the methods of carrying out in-depth experiments with children. This paper describes the analysis of these exploratory case studies from an Activity Theory perspective.

An Adaptive Dynamic Grid-Based approach to DDM for large-scale distributed simulation systems

Data Distribution Management (DDM) plays a key role in traffic volume control of large-scale distributed simulations. In recent years, several solutions have been devised to make DDM more efficient and adaptive to different traffic conditions. Examples of such systems include the Region-Based, Fixed Grid-Based, Hybrid, and Dynamic Grid-Based (DGB) schemes. However, less effort has been directed toward improving the processing performance of DDM techniques. This paper presents a novel DDM scheme called the Adaptive Dynamic Grid-Based (ADGB) scheme that optimizes DDM time through analysis of matching performance. ADGB uses an advertising scheme in which information about the target cell involved in the process of matching subscribers to publishers is known in advance. An important concept known as the Distribution Rate (DR) is devised. The distribution rate represents the relative processing load and communication load generated at each federate. The matching performance and the distribution rate are used as part of the ADGB method to select, throughout the simulation, the devised advertisement scheme that achieves the maximum gain with acceptable network traffic overhead. If we assume the same worst case propagation delays, when the matching probability is high, the performance estimation of ADGB has shown that a maximum efficiency gain of 66% can be achieved over the Dynamic Grid-Based scheme. The novelty of the ADGB scheme is its focus on improving performance, an important (and often forgotten) goal of DDM strategies.     

On delay adjustment for dynamic load balancing in distributed virtual environments

Distributed virtual environments (DVEs) are becoming very popular in recent years, due to the rapid growing of applications, such as massive multiplayer online games (MMOGs). As the number of concurrent users increases, scalability becomes one of the major challenges in designing an interactive DVE system. One solution to address this scalability problem is to adopt a multi-server architecture. While some methods focus on the quality of partitioning the load among the servers, others focus on the efficiency of the partitioning process itself. However, all these methods neglect the effect of network delay among the servers on the accuracy of the load balancing solutions. As we show in this paper, the change in the load of the servers due to network delay would affect the performance of the load balancing algorithm. In this work, we conduct a formal analysis of this problem and discuss two efficient delay adjustment schemes to address the problem. Our experimental results show that our proposed schemes can significantly improve the performance of the load balancing algorithm with neglectable computation overhead.     

A PKI approach for deploying modern secure distributed e-learning and m-learning environments

While public key cryptography is continuously evolving and its installed base is growing significantly, recent research works examine its potential use in e-learning or m-learning environments. Public key infrastructure (PKI) and attribute certificates (ACs) can provide the appropriate framework to effectively support authentication and authorization services, offering mutual trust to both learners and service providers. Considering PKI requirements for online distance learning networks, this paper discusses the potential application of ACs in a proposed trust model. Typical e-learning trust interactions between e-learners and providers are presented, demonstrating that robust security mechanisms and effective trust control can be obtained and implemented. The application of ACs to support m-learning is also presented and evaluated through an experimental test-bed setup, using the general packet radio service network. The results showed that AC issuing is attainable in service times while simultaneously can deliver flexible and scalable solutions to both learners and e-learning providers.