STM多事务竞争冲突下竞争管理策略的研究

事务存储系统是一种高层次抽象并行编程模型,目的为方便开发并行程序。事务存储系统中的竞争管理模块用于解决事务之间的冲突。传统的事务竞争管理策略只负责仲裁两个冲突事务之间的冲突．提出将多个事务及事务冲突关联转换成一张无向图,基于全局事务冲突情景,利用图顶点着色技术求解无向图中最大独立集。最大独立集中事务相互不冲突,CM仲裁处理并发执行,实现系统并发最大化。     

Throughput analysis and bandwidth allocation for IEEE 802.11 WLAN with hidden terminals

Motivated by observations from real world wireless local area network (WLAN) deployments, we develop in this paper a novel analytical model to characterize the saturation throughput of an IEEE 802.11-based access point (AP) and stations under the influence of hidden terminals. Unlike existing models, our model can accommodate different numbers of hidden nodes without increasing the model complexity. Given any number of hidden nodes, only four constraints are needed to describe the interaction between stations and the AP with the consideration of both uplink and downlink traffic. Simulation evaluation shows that our model predicts network performance accurately over a wide range of network sizes and indicates the existence of a throughput starvation problem. To address this problem, based on our model, we formulate a bandwidth allocation problem to optimize the network throughput and fairness under some predefined requirements by systematically tuning the AP and stations contention windows. Simulation results show that the starvation problem is resolved with our approach, and the target throughput is met.     

Good programming in transactional memory : Game theory meets multicore architecture

In a multicore transactional memory (TM) system, concurrent execution threads interact and interfere with each other through shared memory. The less interference a thread provokes the better for the system. However, as a programmer is primarily interested in optimizing her individual code’s performance rather than the system’s overall performance, she does not have a natural incentive to provoke as little interference as possible. Hence, a TM system must be designed compatible with good programming incentives (GPI), i.e., writing efficient code for the overall system should coincide with writing code that optimizes an individual thread’s performance. We show that with most contention managers (CM) proposed in the literature so far, TM systems are not GPI compatible. We provide a generic framework for CMs that base their decisions on priorities and explain how to modify Timestamp-like CMs so as to feature GPI compatibility. In general, however, priority-based conflict resolution policies are prone to be exploited by selfish programmers. In contrast, a simple non-priority-based manager that resolves conflicts at random is GPI compatible.     

HiCOO: Hierarchical cooperation for scalable communication in Global Address Space programming models on Cray XT systems

Global Address Space (GAS) programming models enable a convenient, shared-memory style addressing model. Typically this is realized through one-sided operations that can enable asynchronous communication and data movement. With the size of petascale systems reaching 10,000s of nodes and 100,000s of cores, the underlying runtime systems face critical challenges in (1) scalably managing resources (such as memory for communication buffers), and (2) gracefully handling unpredictable communication patterns and any associated contention. For any solution that addresses these resource scalability challenges, equally important is the need to maintain the performance of GAS programming models. In this paper, we describe a Hierarchical COOperation (HiCOO) architecture for scalable communication in GAS programming models. HiCOO formulates a cooperative communication architecture: with inter-node cooperation amongst multiple nodes (a.k.a multinode) and hierarchical cooperation among multinodes that are arranged in various virtual topologies. We have implemented HiCOO for a popular GAS runtime library, Aggregate Remote Memory Copy Interface (ARMCI). By extensively evaluating different virtual topologies in HiCOO in terms of their impact to memory scalability, network contention, and application performance, we identify MFCG as the most suitable virtual topology. The resulting HiCOO architecture is able to realize scalable resource management and achieve resilience to network contention, while at the same time maintaining or enhancing the performance of scientific applications. In one case, it reduces the total execution time of an NWChem application by 52%.     

一种基于自适应退避值的ZigBee网络CSMA/CA接入机制

目前,IEEE 802.15.4/ZigBee网络CSMA/CA接入机制中退避值的初始值采取了固定设置模式,难以适应不同的网络应用环境,从而对ZigBee网络的时延、能耗、吞吐量等性能造成了一定的负面影响。文章提出一种基于自适应退避值的Zigbee网络CSMA/CA接入机制,旨在通过动态调整退避指数和竞争窗口长度,自适应节点数据传输情况,达到提高网络性能的目的。仿真结果表明,文章所提出的自适应接入机制在提高ZigBee网络吞吐量、降低能耗等方面优势较为明显。     

An efficient contention resolution algorithm for input queuing atm cross-connect switches

This paper describes an efficient contention resolution algorithm and its distributed implementation for large capacity input queuing cross-connect switches, which will establish virtual paths in future broadband ATM networks. The algorithm dynamically allocates sending time to cells held in input queues when no contention is indicated in the designated output ports. An expression for the mean delay and the cell loss probability for random traffic are derived through an approximate analysis. Input cells are served on a first-come, first-served basis as conventional contention resolution algorithms whose throughput saturates at 58 per cent because of head of line blocking in input queues. The proposed algorithm achieves a maximum throughput of 76 per cent.     

基于异构竞争窗口优化的水下传感器网络MAC协议

传统水下传感器网络的MAC协议具有能耗过大、带宽资源不足等缺点,通过研究基于竞争窗口CW(Contention Win-dow)的CW-MAC协议无法完全保障信道利用公平性的缺陷,提出了两种基于异构竞争窗口优化策略的HCW-MAC协议,根据传感器节点的欧式距离差异对各节点所分配CW值的进行优化调整.本文采用NS-3仿真平台对CW-MAC与HCW-MAC协议分别进行了仿真比较,仿真结果验证了HCW-MAC协议的吞吐量与能耗性能均优于传统的CW-MAC协议.     

基于跨层竞争的PRMAC协议的优化研究

基于流水路由的MAC协议PRMAC(Pipelined Routing Medium Access Control)是无线传感网络WSNs(Wireless Sensor Networks)的典型的基于跨层竞争的同步MAC协议. PRMAC协议允许一个周期内进行多跳传输.但是,PRMAC协议的传输请求发送包RTSD(Request-To-Send Data)和确认发送数据包CTSD(Confirmation-To-Send Data)在同一个数据包窗口传输,降低了路由性能.为此,对 PRMAC 协议进行改进,提出面向多跳场景的 PRMAC 协议 MH-PRMAC(Multi-Hop PRMAC). MH-PRMAC协议采用新的周期结构,并在休眠窗口内设置两个独立窗口,再由它们分别传输RTSD和CTSD包.实验数据表明,提出的MH-PRMAC协议有效地降低了数据包传输时延,并提高了数据包传输率.     

基于模数的WiMAX网络高负载自适应冲突解决算法

根据现有的部分研究结果发现,IEEE 802.16标准中使用的截断二进制指数后退算法在解决网络冲突时效率很低;并且当网络的负载增多时,该算法的运行性能更差.在此基础上,提出了一种基于模数的自适应冲突解决算法来优化WiMAX网络的冲突解决过程.该算法将所有用户站按照当前时间帧内的传输机会数量分为多个小组,并且要求用户站只能通过特定的传输机会发送请求.与此同时,为了保证整个网络的高性能,本算法要求基站和用户站根据上一轮请求过程中的平均冲突概率和传输机会使用率来调整当前时间帧内的传输机会数量,以及挑选部分用户站参与带宽请求,保证了WiMAX网络在高负载情况下的运行性能.模拟结果表明,本算法在WiMAX网络运行过程中能够较好地解决请求过程中的冲突,对网络性能提高较大.     

Analysis of multiple-bus synchronous and asynchronous communication systems

We consider a multi-user distributed system in which nodes are connected to multiple-buses using partially duplicated bus interfaces. We study the effects of destination conflicts and bus synchronization on the system performance and show how, for a given system bus duplication can be chosen to provide guaranteed delays while increasing the interconnection system reliability. Alternative system configurations are analyzed and compared using exact and approximate Markovian models. The presented results can be used for design and optimization of high-availability distributed systems, such as loosely coupled cluster systems or backend storage networks.