Theoretical comparisons of search strategies in branch-and-bound algorithms

Four known search strategies used in branch-and-bound algorithms-heuristic search, depth-first search, best-bound search, and breadth-first search-are theoretically compared from the viewpoint of the performance of the resulting algorithms. Heuristic search includes the other three as special cases. Since heuristic search is determined by a heuristic functionh, we first investigate how the performance of the resulting algorithms depends onh. In particular, we show that heuristic search is stable in the sense that a slight change inh causes only a slight change in its performance. The best and the worst heurstic functions are clarified, and also discussed is how the heuristic functionh should be modified to obtain a branch-and-bound algorithm with an improved performance. Finally, properties and limitations of depth-first search, best-bound search, and breadth-first search viewed as special cases of heuristic search are considered. In particular, it is shown that the stability observed for heuristic search no longer holds for depth-first search.     

基于多任务拍卖的资源调度算法

资源调度是计算网格资源管理系统的主要内容之一。文中借鉴市场模型中的拍卖机制,通过用户代理、资源代理、拍卖师和资源的交互作用,设计并实现了一个基于多任务拍卖的网格资源调度原型系统,系统使用多拍卖师结构,提高了作业的吞吐率并且可以有效地防止拍卖过程中双方的欺骗行为,改进了网格资源的调度性能。同时,与传统算法相比,该算法能够有效地配置资源和满足用户的服务质量需求。     

池式反应堆堆内流场数值模拟

以板型燃料组件池式反应堆为研究对象,采用计算流体动力学程序CFX5对堆内流场进行了数值模拟,结果表明:流过堆芯燃料组件的流速较大,在燃料组件出口位置流速达到最大值;无论是否带有围桶,堆内压降均主要集中在堆芯燃料组件上,入口流量增大,堆芯燃料组件上的压降随之增加;堆芯上部腔室和下部腔室的压力变化很小;在相同的入口流量下,带与不带围桶的堆芯进出口差压非常接近。     

计算物理学在“大学物理”计算机辅助教学中的运用

文章介绍了计算物理学的发展和研究方法，阐述了计算物理学在“大学物理”CAI中的重要性和作用，并通过实例进一步加以说明和展示。     

On the computational power of insertion-deletion systems

Gene insertion and deletion are basic phenomena found in DNA processing or RNA editing in molecular biology. The genetic mechanism and development based on these evolutionary transformations have been formulated as a formal system with two operations of insertion and deletion, called insertion-deletion systems (Kari and Thierrin, 1996; Kari et al., 1997).We investigate the generative power of insertion-deletion systems (InsDel systems), and show that the family INS ¹ ₁ DEL ¹ ₁ is equal to the family of recursively enumerable languages. This gives a positive answer to an open problem posed in Kari et al. (1997) where it was conjectured contrary. This revised version was published online in June 2006 with corrections to the Cover Date.     

Towards Efficient Execution of MPI Applications on the Grid: Porting and Optimization Issues

The message passing interface (MPI) is a standard used by many parallel scientific applications. It offers the advantage of a smoother migration path for porting applications from high performance computing systems to the Grid. In this paper Grid-enabled tools and libraries for developing MPI applications are presented. The first is MARMOT, a tool that checks the adherence of an application to the MPI standard. The second is PACX-MPI, an implementation of the MPI standard optimized for Grid environments. Besides the efficient development of the program, an optimal execution is of paramount importance for most scientific applications. We therefore discuss not only performance on the level of the MPI library, but also several application specific optimizations, e.g., for a sparse, parallel equation solver and an RNA folding code, like latency hiding, prefetching, caching and topology-aware algorithms.     

基于WEB的远程可视化并行计算化学系统

以PC Linux并行计算系统为支撑，研制了基于Web方式的可视远程计算化学系统RVCCS。此系统使用Web技术替代传统Telnet方式的远程计算模式，在Web界面中实现了作业提交、作业后台运行、作业状态实时查看、计算结果图示化、分子结构三维化等功能，提供了一种简单易用的远程可视计算方式。使用此系统建立的Web方式远程计算化学站点GHPCC(http：／／www．ghpcc．gzu．edu．cn)自1998年为国内用户提供计算服务以来，接受了各高校和科研单位化学工作者提交的上万次远程计算，为化学工作者提供了一个分子模拟和化学计算的有力工具。     

Characterizing Grids: Attributes, Definitions, and Formalisms

Grid systems and technologies have evolved over nearly a decade; yet, there is still no widely accepted definition for Grids. In particular, the essential attributes that distinguish Grids from other distributed computing environments have not been articulated. Most approaches to definition adopt a static view and consider only the properties and components of, or the applications supported by, Grids. The definition proposed in this paper is based on the runtime semantics of distributed systems. Rather than attempt to simply compare static characteristics of Grids and other distributed computing environments, this paper analyzes operational differences, from the viewpoint of an application executing in both environments. Our definition is expressed formally as an Abstract State Machine that facilitates the analysis of existing Grid systems or the design of new ones with rigor and precision. This new, semantical approach proposes an alternative to the currently accepted models for determining whether or not a distributed system is a Grid.     

Coordination in networks: An economic equilibrium analysis

Networks utilizing modern communication technologies can offer competitive advantages to those using them wisely. But due to the existence of network effects, planning and operating cooperation networks is difficult. In this paper, the strategic situation of individual agents deciding on network participation is analysed. A systematic equilibrium analysis using computer-based simulations reveals principal solution scenarios for network agents and shows that network cooperation problems might frequently be not as difficult to resolve as often feared. In particular, strategic situations are identified showing that a majority of individuals might be better off getting the optimal solution from a central principal and that in many cases simple information intermediation can solve the start-up problem notorious in network economics.     

High Order Finite Difference Approximations of Electromagnetic Wave Propagation Close to Material Discontinuities

Electromagnetic wave propagation close to a material discontinuity is simulated by using summation by part operators of second, fourth and sixth order accuracy. The interface conditions at the discontinuity are imposed by the simultaneous approximation term procedure. Stability is shown and the order of accuracy is verified numerically.