On the use of virtual channels in networks of workstations withirregular topology

Networks of workstations are becoming increasingly popular as a cost-effective alternative to parallel computers. Typically, these networks connect workstations using irregular topologies, providing the wiring flexibility, scalability, and incremental expansion capability required in this environment. Recently, we proposed two methodologies for the design of adaptive routing algorithms for networks with irregular topology, as well as fully adaptive routing algorithms for these networks. These algorithms increase throughput considerably with respect to previously existing ones, but require the use of at least two virtual channels. In this paper, we propose a very efficient flow control protocol to support virtual channels when link wires are very long and/or have different lengths. This flow control protocol relies on the use of channel pipelining and control flits. Control traffic is minimized by assigning physical bandwidth to virtual channels until the corresponding message blocks or it is completely transmitted. Simulation results show that this flow control protocol performs as efficiently as an ideal network with short wires and flit-by-flit multiplexing. The effect of additional virtual channels per physical channel has also been studied, revealing that the optimal number of virtual channels varies with network size. The use of virtual channel priorities is also analyzed. The proposed flow control protocol may increase short message latency, due to long messages monopolizing channels and hindering the progress of short messages. Therefore, we have analyzed the impact of limiting the number of flits (block size) that a virtual channel may forward once it gets the link. Simulation results show that limiting the maximum block size causes the overall network performance to decrease     

An efficient joint channel assignment and QoS routing protocol for IEEE 802.11 multi-radio multi-channel wireless mesh networks

With the emerging of video, voice over IP (VoIP) and other real-time multimedia services, more and more people pay attention to quality of service (QoS) issues in terms of the bandwidth, delay and jitter, etc. As one effective way of broadband wireless access, it has become imperative for wireless mesh networks (WMNs) to provide QoS guarantee. Existing works mostly modify QoS architecture dedicated for ad hoc or sensor networks, and focus on single radio and single channel case. Meanwhile, they study the QoS routing or MAC protocol from view of isolated layer. In this paper, we propose a novel cross-layer QoS-aware routing protocol on OLSR (CLQ-OLSR) to support real-time multimedia communication by efficiently exploiting multi-radio and multi-channel method. By constructing multi-layer virtual logical mapping over physical topology, we implement two sets of routing mechanisms, physical modified OLSR protocol (M-OLSR) and logical routing, to accommodate network traffic. The proposed CLQ-OLSR is based on a distributed bandwidth estimation scheme, implemented at each node for estimating the available bandwidth on each associated channel. By piggybacking the bandwidth information in HELLO and topology control (TC) messages, each node disseminates information of topology and available bandwidth to other nodes in the whole network in an efficient way. From topology and bandwidth information, the optimized path can be identified. Finally, we conduct extensive simulation to verify the performance of CLQ-OLSR in different scenarios on QualNet platform. The results demonstrate that our proposed CLQ-OLSR outperforms single radio OLSR, multi-radio OLSR and OLSR with differentiated services (DiffServ) in terms of network aggregate throughput, end-to-end packet delivery ratio, delay and delay jitter with reasonable message overheads and hardware costs. In particular, the network aggregate throughput for CLQ-OLSR can almost be improved by 300% compared with the single radio case.     

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     

多信道无线Mesh网络信道分配算法

针对无线Mesh网络的带宽容量问题,文章通过使用无线网络干扰协议模型对无线链路的干扰进行量化,利用整数线性规划公式对信道分配问题进行描述,在信道分配的时候,应用目标函数对无线链路的信道分配进行优化,使网络总的干扰权重最小化,在此基础上提出一个信道分配的启发式算法。仿真结果表明,文章提出的算法能提高网络的吞吐量。     

一种动态分配虚拟输出队列结构的片上路由器

传统虚通道流控技术的片上路由器通过增加虚通道缓解排头阻塞引起的链路吞吐率下降以及网络拥塞的同时,面临缓冲区低利用率、仲裁开销较大等问题.而动态虚通道流控的片上路由器虽可通过动态管理缓冲单元,提高缓冲区利用率与链路吞吐率,但却不可避免流控与仲裁逻辑复杂度与开销的快速增长.为了提高链路吞吐率与缓冲区利用率,获得较好的性能与开销折中,提出一种动态分配虚拟输出队列结构的片上路由器DAVOQ,该结构通过快速链表动态组织虚拟输出队列,同时使用超前路由机制以简化仲裁逻辑,优化流水线.模拟与综合的结果表明,相比传统虚通道路由器,DAVOQ路由器改善报文传输延迟与吞吐率的同时,在0.13μm CMOS工艺下,节省了15.1%的标准单元面积与12.9%的漏电流功耗;而相比动态虚通道路由器,DAVOQ路由器能够以较小的吞吐率损失获得可观的延迟改善,同时节约15.6%的标准单元面积与20.5%的漏电流功耗.     

Asynchronous spatial division multiplexing router

Asynchronous quasi-delay-insensitive (QDI) NoCs have several advantages over their clocked counterparts. Virtual channel (VC) is the most utilized flow control method in asynchronous routers but spatial division multiplexing (SDM) achieves better throughput performance for best-effort traffic than VC. A novel asynchronous SDM router architecture is presented. Area and latency models are provided to analyse the network performance of all router architectures including wormhole, virtual channel and SDM. Performance comparisons have been made with different configurations of payload size, communication distance, buffer size, port bandwidth, network size and number of VCs/virtual circuits. Compared with VC, SDM achieves higher throughput with lower area overhead.     

Performance-based path determination for interprocessorcommunication in distributed computing systems

The different types of messages used by a parallel application program executing in a distributed computing system can each have unique characteristics so that no single communication network can produce the lowest latency for all messages. For instance, short control messages may be sent with the lowest overhead on one type of network, such as Ethernet, while bulk data transfers may be better suited to a different type of network, such as Fibre Channel or HIPPI. This work investigates how to exploit multiple heterogeneous communication networks that interconnect the same set of processing nodes using a set of techniques we call performance-based path determination (PBPD). The performance-based path selection (PBPS) technique selects the best (lowest latency) network among several for each individual message to reduce the communication overhead of parallel programs. The performance-based path aggregation (PBPA) technique, on the other hand, aggregates multiple networks into a single virtual network to increase the available bandwidth. We test the PBPD techniques on a cluster of SGI multiprocessors interconnected with Ethernet, Fibre Channel, and HiPPI networks using a custom communication library built on top of the TCP/IP protocol layers. We find that PBPS can reduce communication overhead in applications compared to using either network alone, while aggregating networks into a single virtual network can reduce communication latency for bandwidth-limited applications. The performance of the PBPD techniques depends on the mix of message sizes in the application program and the relative overheads of the networks, as demonstrated in our analytical models     

The impact of virtual channel allocation on the performance of deterministic wormhole-routed k-ary n-cubes

Virtual channels yield significant improvement in the performance of wormhole-routed networks as they can greatly reduce message blocking over network resources. K-ary n-cubes with deterministic routing have been widely analysed using analytical modelling tools. Most existing models, however, have either entirely ignored the effects of virtual channel multiplexing or have not considered the impact of virtual channels allocation on message latency. This paper discusses two different organisations of virtual channels in k-ary n-cubes, resulting in two deterministic routing algorithms. It then proposes an analytical model to compute message latency for the two routing algorithms. The proposed model is used in a case study to demonstrate the sensitivity of network latency to the way virtual channels are allocated to messages.     

How to Effectively Use Multiple Channels in Wireless Mesh Networks

Operating on a frequency band occupying several nonoverlapping channels, IEEE 802.11 is now widely used in wireless mesh networks (WMNs). Many multichannel MAC protocols are proposed to improve the spatial reuse in the network under the assumption that the transmissions on nonoverlapping channels do not interfere with each other. Some joint routing and channel assignment algorithms are also designed to increase the network throughput based on the premise that we can switch between different channels freely. Although simulations show that great improvements on network throughput can be observed in both cases, two fundamental questions remain: 1) Can we really use multiple nonoverlapping channels freely in WMNs? 2) If we can, what will be the cost when we switch channels dynamically and frequently? In this paper, by conducting extensive experiments on our testbed, we attempt to answer these questions. We find that in spite of interference between both overlapping and nonoverlapping channels, we can still use multiple channels in mesh networks under certain conditions but with care. We also show that the channel switching cost is actually very significant in WMNs. We recommend not to switch the channels too frequently when designing the channel assignment algorithms, and those channel assignment algorithms selecting one channel for each packet are not really beneficial.     

具有拥塞缓解策略的动态虚拟通道研究及其VLSI实现

虚拟通道技术改善了片上网络性能,却带来了巨大的面积与功耗开销.通过分析静态虚拟通道的不足,提出了基于拥塞缓解策略的动态虚拟通道结构.它采用链表方式组织缓冲,可以自动调整通道结构来适应各种流量负载:在较低流量下,该结构扩展通道队列深度,减小了报文传输延迟;在较高流量下,它增加虚拟通道数量,消除队列头阻塞与通道不足阻塞,并缓解拥塞现象发生,减少流反馈次数,提高了网络吞吐率.在90nm CMOS工艺下完成了DVC路由器的VLSI设计,与传统路由器相比,不仅改善了报文传输延迟与吞吐率,而且有效降低了面积与功耗开销.     

Dynamic game with perfect and complete information based dynamic channel assignment

The nodes in Ad Hoc networks compete for channels when communicating, with the features of no center and self-organization. In traditional channel assignment strategy of MAC layer, nodes do not consider the demands to channel resources of other nodes, which hinders improving the network performance. Practically, in the network based on competitive MAC protocol, each node tries to maximize its payoff, while this interferes with the behavior of other nodes at the same time. Game theory is an effective tool to solve problems of distributed resources, which can be used effectively in channel assignment. In this paper, we propose a new protocol, namely, DGPCI-DCA (Dynamic Game with Perfect and Complete Information based Dynamic Channel Assignment). When all the nodes are rational and greedy, each node selects channels dynamically by backward induction according to strategies of other nodes, thus Nash equilibrium can finally be achieved. Experiments show that the network performance is effectively improved, i.e., the throughput and saturation throughput can be increased, and the packet loss rate and network delay can be reduced.     

Alleviating consumption channel bottleneck in wormhole routed k-aryn-cube systems

This paper identifies performance degradation in wormhole routed k-ary n-cube networks due to limited number of router-to-processor consumption channels at each node. Many recent research in wormhole routing have advocated the advantages of adaptive routing and virtual channel flow control schemes to deliver better network performance. This paper indicates that the advantages associated with these schemes cannot be realized with limited consumption capacity. To alleviate such performance bottlenecks, a new network interface design using multiple consumption channels is proposed. To match virtual multiplexing on network channels, we also propose each consumption channel to support multiple virtual consumption channels. The impact of message arrival rate at a node on the required number of consumption channels is studied analytically. It is shown that wormhole networks with higher routing adaptivity, dimensionality, degree of hot-spot traffic, and number of virtual lanes have to take advantage of multiple consumption channels to deliver better performance. The interplay between system topology, routing algorithm, number of virtual lanes, messaging overheads, and communication traffic is studied through simulation to derive the effective number of consumption channels required in a system. Using the ongoing technological trend, it is shown that wormhole-routed systems can use up to two-four consumption channels per node to deliver better system performance     

Anchored opportunity queueing: a low-latency scheduler for fair arbitration among virtual channels

Many wormhole interconnection networks for parallel systems, and more recently system area networks, implement virtual channels to provide a number of services including improved link utilization and lower latencies. The forwarding of flits from the virtual channels on to the physical channel is typically accomplished using flit-based round-robin (FBRR) scheduling. This paper presents a novel scheduling strategy, anchored opportunity queueing (AOQ), which preserves the throughput and fairness characteristics of FBRR while significantly reducing the average delay experienced by packets. The AOQ scheduler achieves lower average latencies by trying, as far as possible, to complete the transmission of a complete packet before beginning the transmission of flits from another packet. The AOQ scheduler achieves provable fairness in the number of opportunities it offers to each of the virtual channels for transmissions of flits over the physical channel. We prove this by showing that the relative fairness bound, a popular measure of fairness, is a small finite constant in the case of the AOQ scheduler. Finally, we present simulation results comparing the delay characteristics of AOQ with other schedulers for virtual channels. The AOQ scheduler is simple to implement in hardware, and also offers a practical solution in other contexts such as in scheduling ATM cells in Internet backbone switches.     

ORA——一种负载平衡的虚通道分配算法

MPP互联网中通常使用虚通病来防止死锁和提高网络吞吐率。但通常的虚通道分配算法会导致通道的负载不平衡,从而降低网络的性能。针对采用虫孔路由技术和维序路由算法下的Torus互联网,提出了ORA虚通道负载平衡分配算法。与Naive分配算法和Scott分配算法的比较表明,ORA能够较好地实现负载平衡,能够较好地提高网络的性能。     

Deep reinforcement learning-based cooperative interactions among heterogeneous vehicular networks

Most real-world vehicle nodes can be structured into an interconnected network of vehicles. Through structuring these services and vehicle device interactions into multiple types, such internet of vehicles becomes multidimensional heterogeneous overlay networks. The heterogeneousness of the overlays makes it difficult for the overlay networks to coordinate with each other to improve their performance. Therefore, it poses an interesting but critical challenge to the effective analysis of heterogeneous virtual vehicular networks. A variety of virtual vehicular networks can be easily deployed onto the native network by applying the concept of SDN (Software Defined Networking). These virtual networks reflect their heterogeneousness due to their different performance goals, and they compete for the same physical resources of the underlying network, so that a sub-optimal performance of the virtual networks may be achieved. Therefore, we propose a Deep Reinforcement Learning (DRL) approach to make the virtual networks cooperate with each other through the SDN controller. A cooperative solution based on the asymmetric Nash bargaining is proposed for co-existing virtual networks to improve their performance. Moreover, the Markov Chain model and DRL resolution are introduced to leverage the heterogeneous performance goals of virtual networks. The implementation of the approach is introduced, and simulation results confirm the performance improvement of the latency sensitive, loss-rate sensitive and throughput sensitive heterogeneous vehicular networks using our cooperative solution.     

A Comparison of Router Architectures for Virtual Cut-Through and Wormhole Switching in a NOW Environment

Most multicomputer interconnection networks use wormhole switching, leading to fast and compact routers. Current routers incorporate virtual channels and even fully adaptive routing. Networks of workstations (NOWs) inherited multicomputer technology. Most commercial routers designed for NOWs implement wormhole switching. However, wormhole switching is not well suited for NOWs. The long wires required in this environment lead to large buffers to prevent buffer overflow during flow control signaling. Moreover, wire length is limited by buffer size. Virtual cut-through (VCT) achieves a higher throughput than wormhole switching. However, buffer requirements and packetizing overhead prevented its widespread use in multicomputers. Nevertheless, wormhole and VCT switching require similar buffer capacity in NOWs. Moreover, some messaging layers such as Illinois Fast Messages (FM) and BIP split messages into packets for increased performance. Therefore, the traditional disadvantages of VCT switching disappear in NOWs. In this paper, we show that VCT routers can be simpler than wormhole routers, while still achieving the advantages of using virtual channels and adaptive routing. We also propose a fully adaptive routing algorithm for VCT switching in a NOW environment. Moreover, we show that VCT routers outperform wormhole routers in a NOW environment at a lower cost. Also, VCT routers require buffer capacity independent of wire length, making them suitable for networks of workstations.     

Mesh网中高效无死锁自适应路由算法

提出了一种新的应用于三维Mesh网中的无死锁路由算法.在当今的商用多计算机系统中,二维和三维的Mesh网是多处理器网络最为常用的拓扑结构之一.在应用于Mesh网的平面自适应路由(Planar Adaptive Routing)算法中,每条物理通道只需三条虚拟通道就可以有效地在三维以及更高维的Mesh网中避免死锁的产生.然而,采用该算法,网络拓扑一维和三维分别有两条和一条虚拟通道始终处于空闲状态.该文所提出的算法针对三维Mesh网,每条物理通道只需两条虚拟通道就可以有效地避免死锁.文中通过充分的模拟数据验证了此算法的有效性.     

Exploring virtual network selection algorithms in DSM cache coherence protocols

Distributed shared memory (DSM) multiprocessors typically require disjoint networks for deadlock-free execution of cache coherence protocols. This is normally achieved by implementing virtual networks with the help of virtual channels or virtual lanes multiplexed on a single physical network. To keep the coherence protocol simple, messages are usually assigned to virtual lanes in a predefined static manner based on a cycle-free lane assignment dependence graph. However, this static split of virtual networks (such as request and reply networks) may lead to underutilization of certain virtual networks while saturating the other networks. In this paper, we explore different static and dynamic schemes to select the virtual lanes for outgoing messages and mix the load among them without restricting any particular type of message to be carried only by a particular virtual network. We achieve this by exposing the selection algorithms to the coherence protocol itself, so that it can inject messages into selected virtual lanes based on some local information, and still enjoy deadlock-freedom. Our execution-driven simulation on five applications from the SPLASH-2 suite shows that as the system scales, the virtual network selection algorithms play an important role. For 128-node systems, our dynamic selection algorithm speeds up parallel execution by as much as 22 percent over an optimized baseline system running a modified SGI Origin 2000 protocol. We also explore how network latency, the number of message buffers per virtual lane, and the depth of network interface output queues affect the relative performance of various virtual lane selection algorithms.     

Real-Time Virtual Channel Flow Control

Real-time communication system support for large scale parallel multicomputers becomes an important issue as the number of real-time applications developed on these systems increases. Flow control is a key component that affects the performance of the communication subsystem. We develop a range of new real-time virtual channel flow control schemes for wormhole networks. The flow control schemes differ in their priority mapping strategies, priority adjustment methods, and arbitration functions. The priority mapping strategy and priority adjustment method of a flow control scheme determine the priority of a message. The priority of a message is used for the virtual channel assignment and the physical channel arbitration. We discuss the trade-off between the performance and the hardware cost of each flow control scheme. A simulator is implemented for studying the performance of the schemes, and simulation experiments are designed to compare the importance of priority mapping, priority adjustment and arbitration toward the system performance. As wormhole networks scale to larger sizes, the average distance between source and destination nodes increases. The flits of messages in wormhole networks, which are buffered in nodes along the path from the source to the destination, consume network resources in these nodes. Therefore, increased scaling may lead to increased resource consumption, congestion, and late messages. In real-time systems, messages lose their value when they miss their deadlines. In order to reduce congestion, we provide a scheme for dropping messages that miss their deadlines.     

Synchronous bandwidth allocation in FDDI networks

It is well known that an FDDI token ring network provides a guaranteed throughput for synchronous messages and a bounded medium access delay for each node/station. However, this fact alone cannot effectively support many real-time applications that require the timely delivery of each critical message. The reason for this is that the FDDI guarantees a medium access delay bound to nodes, but not to messages themselves. The message-delivery delays may exceed the medium-access delay bound even if a node transmits synchronous messages at a rate not greater than the guaranteed throughput. We solve this problem by developing a synchronous bandwidth allocation (SEA) scheme which calculates the synchronous bandwidth necessary for each application to satisfy its message-delivery delay requirement. The result obtained in this paper is essential for effective use of the FDDI token ring networks in supporting such real-time communication as digital video/audio transmissions, and distributed control/monitoring