WDM光网络动态虚拟拓扑重构算法

针对波分复用光纤网络上业务流量动态改变的问题,为了使光纤网络能支持更多的业务连接,需要对虚拟拓扑进行重构。基于链路最大负载和包平均跳步距离,利用混合线性规划公式对重构问题进行描述,在此基础上提出一个自适应拓扑重构算法,达到提高网络吞吐量的目的。仿真结果表明,该算法可以有效地改善网络性能。     

A new approach to model virtual channels in interconnection networks

Dealing with virtual channels has always been a critical issue in developing analytical performance models for interconnection networks. Almost all previous studies relied on a method proposed by Dally to capture the effect of virtual channels multiplexing in the performance of interconnection networks. This paper presents a new method to model the effect of virtual channel multiplexing in high-speed wormhole-switched interconnection networks. Dally's method loses its accuracy as the traffic load increases due to blocking nature of wormhole-switched networks. Our new method is based on a finite capacity queue, M/G/1/V and comparing to Dally's method achieves a higher degree of accuracy under low, moderate and high traffic loads. Furthermore, its simplicity eases its employment under different network conditions and setup. The presented model is validated by means of an event driven simulator and a detailed comparison with Dally's method is presented.     

On covert channels between virtual machines

Virtualization technology has become very popular because of better hardware utilization and easy maintenance. However, there are chances for information leakage and possibilities of several covert channels for information flow between the virtual machines. Our work focuses on the experimental study of security threats in virtualization, especially due to covert channels and other forms of information leakage. The existence of data leakage during migration shutdown and destruction of virtual machines, is tested on different hypervisors. For empirically showing the possibility of covert channels between virtual machines, three new network based covert channels are hypothesized and demonstrated through implementation, on different hypervisors. One of the covert channels hypothesized is a TCP/IP steganography based covert channel. Other covert channels are a timing covert channel and a new network covert channel having two pairs of socket programs. We propose a VMM (Virtual Machine Monitor) based network covert channel avoidance mechanism, tackling detection resistant covert channel problems. We also address issue of reducing the possibilities of network based covert channels using VMM-level firewalls. In order to emphasize the importance of addressing the issue of information leakage through virtual machines, we illustrate the simplicity of launching network covert channel based attacks, by demonstrating an attack on a virtual machine using covert channels through implementation.     

Exploiting fine-grained idle periods in networks of workstations

Studies have shown that for a significant fraction of the time, workstations are idle. In this paper, we present a new scheduling policy called Linger-Longer that exploits the fine-grained availability of workstations to run sequential and parallel jobs. We present a two-level workload characterization study and use it to simulate a cluster of workstations running our new policy. We compare two variations of our policy to two previous policies: Immediate-Eviction and Pause-and-Migrate. Our study shows that the Linger-Longer policy can improve the throughput of foreign jobs on a cluster by 60 percent with only a 0.5 percent slowdown of local jobs. For parallel computing, we show that the Linger-Longer policy outperforms reconfiguration strategies when the processor utilization by the local process is 20 percent or less in both synthetic bulk synchronous and real data-parallel applications     

High-performance routing in networks of workstations with irregulartopology

Networks of workstations are rapidly emerging as a cost-effective alternative to parallel computers. Switch-based interconnects with irregular topology allow the wiring flexibility, scalability, and incremental expansion capability required in this environment. However, the irregularity also makes routing and deadlock avoidance on such systems quite complicated. In current proposals, many messages are routed following nonminimal paths, increasing latency and wasting resources. In this paper, we propose two general methodologies for the design of adaptive routing algorithms for networks with irregular topology. Routing algorithms designed according to these methodologies allow messages to follow minimal paths in most cases, reducing message latency and increasing network throughput. As an example of application, we propose two adaptive routing algorithms for ANI (previously known as Autonet). They can be implemented either by duplicating physical channels or by splitting each physical channel into two virtual channels. In the former case, the implementation does not require a new switch design. It only requires changing the routing tables and adding links in parallel with existing ones, taking advantage of spare switch ports. In the latter case, a new switch design is required, but the network topology is not changed. Evaluation results for several different tapologies and message distributions show that the new routing algorithms are able to increase throughput for random traffic by a factor of up to 4 with respect to the original up*/down* algorithm, also reducing latency significantly. For other message distributions, throughput is increased more than seven times. We also show that most of the improvement comes from the use of minimal routing     

Parallel computing in networks of workstations with Paralex

Modern distributed systems consisting of powerful workstations and high-speed interconnection networks are an economical alternative to special-purpose supercomputers. The technical issues that need to be addressed in exploiting the parallelism inherent in a distributed system include heterogeneity, high-latency communication, fault tolerance and dynamic load balancing. Current software systems for parallel programming provide little or no automatic support towards these issues and require users to be experts in fault-tolerant distributed computing. The Paralex system is aimed at exploring the extent to which the parallel application programmer can be liberated from the complexities of distributed systems. Paralex is a complete programming environment and makes extensive use of graphics to define, edit, execute, and debug parallel scientific applications. All of the necessary code for distributing the computation across a network and replicating it to achieve fault tolerance and dynamic load balancing is automatically generated by the system. In this paper we give an overview of Paralex and present our experiences with a prototype implementation     

A topology control approach for utilizing multiple channels in multi-radio wireless mesh networks

We consider the channel assignment problem in a multi-radio wireless mesh network that involves assigning channels to radio interfaces for achieving efficient channel utilization. We present a graph–theoretic formulation of the channel assignment guided by a novel topology control perspective, and show that the resulting optimization problem is NP-complete. We also present an ILP formulation that is used for obtaining a lower bound for the optimum. We then develop a new greedy heuristic channel assignment algorithm (termed CLICA) for finding connected, low interference topologies by utilizing multiple channels. Our evaluations show that the proposed CLICA algorithm exhibits similar behavior and comparable performance relative to the optimum bound with respect to interference and capacity measures. Moreover, our extensive simulation studies show that it can provide a large reduction in interference even with a small number of radios per node, which in turn leads to significant gains in both link layer and multihop performance in 802.11-based multi-radio mesh networks.     

Parallel implementation of back-propagation algorithm in networks of workstations

This work presents an efficient mapping scheme for the multilayer perceptron (MLP) network trained using back-propagation (BP) algorithm on network of workstations (NOWs). Hybrid partitioning (HP) scheme is used to partition the network and each partition is mapped on to processors in NOWs. We derive the processing time and memory space required to implement the parallel BP algorithm in NOWs. The performance parameters like speed-up and space reduction factor are evaluated for the HP scheme and it is compared with earlier work involving vertical partitioning (VP) scheme for mapping the MLP on NOWs. The performance of the HP scheme is evaluated by solving optical character recognition (OCR) problem in a network of ALPHA machines. The analytical and experimental performance shows that the proposed parallel algorithm has better speed-up, less communication time, and better space reduction factor than the earlier algorithm. This work also presents a simple and efficient static mapping scheme on heterogeneous system. Using divisible load scheduling theory, a closed-form expression for number of neurons assigned to each processor in the NOW is obtained. Analytical and experimental results for static mapping problem on NOWs are also presented.     

OpenMP on Networks of Workstations for Software DSMs

This paper describes the implementation of a sizable subset of OpenMP on networks of workstations(NOWs) and the source-to-source OpenMP complier(AutoPar） is used for the JIAJIA home-based shared virtual memory system (SVM).The paper suggests some simple modifications and extensions to the OpenMP standard for the difference between SVM and SMP(symmetric multi processor),at which the OpenMP specification is aimed.The OpenMP translator is based on an automatic paralleization compiler,so it is possible to check the correctness of the semantics of OpenMP programs which is not required in an OpenMP-compliant implementation AutoPar is measured for five applications including both programs from NAS Parallel Benchmarks and real applications on a cluster of eight Pentium Ⅱ PCs connected by a 100 Mbps switched Eternet.The evaluation shows that the parallelization by annotaing OpenMPdirectives is simple and the performance of generatd JIAJIA code is still acceptable on NOWs.     

On virtual private networks security design issues

The concept of virtual private networks (VPNs) provides an economical and efficient solution on communicating private information securely over public network infrastructure.In this paper, we discuss two issues on the design of VPN. We first propose the VPN services, the mandatory VPN operations for each VPN service and the design on VPN protocol stack.Afterwards, we propose a list of protocol modules to be used to support the VPN operations and co-relate the mandatory VPN operations to the appropriate VPN protocols. We then propose the design of VPN software that provides guarantees on security, connectivity and quality of service. We also discuss the message processing sequence by the VPN software.     

A new general method to compute virtual channels occupancy probabilities in wormhole networks

Virtual channels have been proposed to develop deadlock free routing algorithms and to overcome the performance degradation due to chains of blocked messages in wormhole switched networks. Hence, capturing the effect of virtual channels has always been an important issue in developing analytical performance models for these interconnection networks. Almost all previous models relayed on a method proposed by Dally to compute the probability of the number of busy virtual channels per physical channel. Dally's method is based on a Markov chain and after extensive investigation our results reveal that its accuracy degrades as traffic increases. In this study we propose and validate a new general method to compute this probability. The new general method is based on an M/G/1 queue and it exhibits a good degree of accuracy at different traffic conditions. We further showed that Dally's method can be deduced as a special case of the general method. Predictions from both, Dally's method and the new general method are validated against results obtained from an event-driven simulator that mimics the behaviour of wormhole-switch networks.     

A new algorithm based on auxiliary virtual topology for sub-path protection in WDM optical networks

This paper proposes a new survivable algorithm named sub-path protection based on auxiliary virtual topology (SPAVT) to tolerate the single-link failure in WDM optical networks. First, according to the protection-switching time constraint, SPAVT searches multiple pairs of primary and backup paths for each node pair in the network by the off-line manner, and then map these paths to the virtual topology. When a connection request arrives, SPAVT only needs to run one time of the Dijkstra’s algorithm to search a virtual route in virtual topology, where the route may consist of multiple pairs of sub-paths, to meet the protection-switching time constraint. Then, according to the shared resources policy, SPAVT chooses an optimal pair of sub-paths. Simulation results show that SPAVT has smaller blocking probability and lower time complexity than conventional algorithms.     

On the existence of virtual exosystems for synchronized linear networks

When dealing with heterogeneous networks, where the agents are governed by non-identical models, interesting questions arise regarding the ability of the network to synchronize to a common non-trivial output trajectory, as well as the nature of such a trajectory. On this topic, Wieland, Allgöwer, and Sepulchre have recently derived results showing that for a class of heterogeneous networks of dynamically controlled linear agents, non-trivial output synchronization implies the existence of an observable virtual exosystem for which the regulator equations are solvable for each agent. Moreover, this virtual exosystem defines the output trajectories on the agreement manifold and is contained within each agent as an internal model. In this paper, we shed further light on this topic by showing that, under a more general set of assumptions, non-trivial output synchronization can occur in the absence of such a virtual exosystem. We propose a modified result for this case that specifies the existence of a possibly unobservable virtual exosystem for which the regulator equations are solvable, and for which the observable part defines the output trajectories on the agreement manifold. We also show that a variation of the virtual exosystem is contained within each agent as an internal model.     

Resource policing to support fine-grain cycle stealing in networks of workstations

We present the design, implementation, and performance evaluation of a suite of resource policing mechanisms that allow guest processes to efficiently and unobtrusively exploit otherwise idle workstation resources. Unlike traditional policies that harvest cycles only from unused machines, we employ fine-grained cycle stealing to exploit resources even from machines that have active users. We developed a suite of kernel extensions that enable these policies to operate without significantly impacting host processes: 1) a new starvation-level CPU priority for guest jobs, 2) a new page replacement policy that imposes hard bounds on physical memory usage by guest processes, and 3) a new I/O scheduling mechanism called rate windows that throttle guest processes' usage of I/O and network bandwidth. We evaluate both the individual impacts of each mechanism, and their utility for our fine-grain cycle stealing.     

TreadMarks: shared memory computing on networks of workstations

Shared memory facilitates the transition from sequential to parallel processing. Since most data structures can be retained, simply adding synchronization achieves correct, efficient programs for many applications. We discuss our experience with parallel computing on networks of workstations using the TreadMarks distributed shared memory system. DSM allows processes to assume a globally shared virtual memory even though they execute on nodes that do not physically share memory. We illustrate a DSM system consisting of N networked workstations, each with its own memory. The DSM software provides the abstraction of a globally shared memory, in which each processor can access any data item without the programmer having to worry about where the data is or how to obtain its value     

Efficient scheduling of MPI applications on networks of workstations

The availability of a large number of workstations connected through a network can represent an attractive option for high-performance computing for many applications. The message-passing interface (MPI) software environment is an effort from many organisations to define a de facto message-passing standard. In other words, the original specification was not designed as a comprehensive parallel programming environment and some researchers agree that the standard should be preserved as simple and clean as possible. Nevertheless, a software environment such as MPI should have somehow a scheduling mechanism for the effective submission of parallel applications on network of workstations. This paper presents an alternative lightweight approach called Selective-MPI (S-MPI), which was designed to enhance the efficiency of the scheduling of applications on an MPI implementation environment.     

On the search of workstations arrangement in pull production systems

This research presents a model to determine the workstation arrangement and kanban number for pull production systems. Several practical production line characters are considered, and evolutionary algorithms are utilized to obtain the optimal/near-optimal result. The proposed system is a flow-shop, pull production line with N unreliable workstations and N ? 1 inter-stage buffers. Workstations may be allocated to any stage in the production line and have a different processing rate at each stage. Workstations will breakdown/recover at fixed rates, and the operating-breakdown-recovery process is formulated as a Markov process in this model. The size of the inter-stage buffers varies according to the arrangements of workstations. A model is established on the objective of minimizing the total production cost per item of the finished product. The costs considered in the system contain the allocation of workstations, operations, inventory, and production shortage costs. An evolutionary algorithm, genetic algorithm, is used to obtain the allocation of workstations in the production line.     

On the use of variable stride in convolutional neural networks

This paper explores the idea of changing the stride value in convolutional neural networks depending on the position of the pixel within the image: a smaller stride value is used when processing the center of the image, while a larger one is used for pixels close to the edges. We show several examples of image classification tasks where the proposed approach outperforms a baseline solution of same computational cost using fixed stride and several counterexamples where it does not – and explain why this is so. The proposed method has been successfully tested using several contemporary datasets and can be easily implemented and extended to other image classification tasks.

     

Performance bottlenecks and potentials of parallel computing on networks of workstations

The network of workstations (NOW) we consider for parallel computing is heterogeneous and nondedicated (time-sharing), where computing power varies among the workstations, and multiple jobs may interact with each other in execution. We address three performance issues in this paper. First, we examine the effects of heterogeneity on co-scheduling and local scheduling policies for parallel computing. Through experimentation and quantitative comparisons, we discuss features and requirements of scheduling policies on heterogeneous NOW. Second, the heterogeneity and non-dedication of NOW introduce new performance factors into parallel computing, which make traditional performance metrics for parallel computing under homogeneous platforms not suitable. We conducted a collection of experimental measurements to show the performance impact to parallel computing. Finally, using network latencies we experimentally evaluate the parallel computing scalability on NOW. Our objective of this study is to provide insights into unique performance bottlenecks and potentials of networks of workstations.     

A survivable routing algorithm with differentiated domain protection based on a virtual topology graph in multi-domain optical networks

In this paper, we propose a novel differentiated domain protection (DDP) scheme to address survivability in multi-domain optical networks. Survivable routing in multi-domain networks is a challenging algorithmic problem because it requires choosing links that provide adequate protection both between domains and within domains that may offer different protection services. To satisfy the survivability requirements of different domains, DDP considers differentiated protection schemes, i.e., dedicated protection, shared protection, or no protection in different domains. In order to respect the principle of transparent management in multiple domains, DDP abstracts a multi-domain optical network to a virtual topology graph (VTG) for solving routing between domains. Simulation results show that DDP, unlike traditional schemes, can provide trade-off between protection methods and also allows better resource utilization.