并行计算模型

1.引言为提高系统性能,并行机系统设计者在体系结构上采用了多种新技术,然而目前并行软件(包括系统软件和应用软件)的研究和开发远远落后于体系结构的进步,即体系结构上的进步并未充分反映到并行软件的设计中。相比之下,串行计算和串行软件的研究是成功的,原因之一在于有一个简单而又代表了串行计算主要性能特征的计算模型(Random Ac-cess Machines,RAM)。而并行计算缺少这样一个简单、被广泛接受并在并行计算中起同等重要作用的模型。“确定少量的并行计算模型以作为程序设计语言的自然基础,并有助于高性能硬件的实现”,这一课题仍是国际公认的并行处理中具挑战性课题之一。     

Computation and Dynamical Models of Mind

Van Gelder (1995) has recently spearheaded a movement to challenge the dominance of connectionist and classicist models in cognitive science. The dynamical conception of cognition is van Gelder's replacement for the computation bound paradigms provided by connectionism and classicism. He relies on the Watt governor to fulfill the role of a dynamicist Turing machine and claims that the Motivational Oscillatory Theory (MOT) provides a sound empirical basis for dynamicism. In other words, the Watt governor is to be the theoretical exemplar of the class of systems necessary for cognition and MOT is an empirical instantiation of that class. However, I shall argue that neither the Watt governor nor MOT successfully fulfill these prescribed roles. This failure, along with van Gelder's peculiar use of the concept of computation and his struggle with representationalism, prevent him from providing a convincing alternative to current cognitive theories.     

A Comparison of Multicast Pull Models

We consider the setting of a web server that receives requests for documents from clients, and returns the requested documents over a multicast/broadcast channel. We compare the quality of service (QoS) obtainable by optimal schedules under various models of the capabilities of the server and the clients to send and receive segments of a document out of order. We show that allowing the server to send segments out of order does not improve any reasonable QoS measure. However, the ability of the clients to receive data out of order can drastically improve the achievable QoS under some, but not all, reasonable/common QoS measures.     

Significance of Models of Computation,from Turing Model to Natural Computation

The increased interactivity and connectivity of computational devices along with the spreading of computational tools and computational thinking across the fields, has changed our understanding of the nature of computing. In the course of this development computing models have been extended from the initial abstract symbol manipulating mechanisms of stand-alone, discrete sequential machines, to the models of natural computing in the physical world, generally concurrent asynchronous processes capable of modelling living systems, their informational structures and dynamics on both symbolic and sub-symbolic information processing levels. Present account of models of computation highlights several topics of importance for the development of new understanding of computing and its role: natural computation and the relationship between the model and physical implementation, interactivity as fundamental for computational modelling of concurrent information processing systems such as living organisms and their networks, and the new developments in logic needed to support this generalized framework. Computing understood as information processing is closely related to natural sciences; it helps us recognize connections between sciences, and provides a unified approach for modeling and simulating of both living and non-living systems.     

中国农业发展仿真模型海外版之比较分析（续）

该文对海外出现的有关未来中国农业发展问题的七个模型进行了比较和分析，找出了模型中预测结果产生差异的原因，同时，为了便于读者查阅和比较，本文还开发了一张对话式的计算机框架式软件，最后根据讨论结果，提出了一个适合中国国情的通用农业发展预测模型。     

网格计算中的任务调度模型研究

任务调度对于获取高性能具有十分重要的作用，各种任务调度算法都是基于不同的假设模型。从任务模型、网络平台模型和性能目标模型3个方面讨论了各种模型，结论为网格任务调度模型的主要发展方向是异构、非专用和多目标。     

Parallel Biomolecular Computation: Models and Simulations

This paper is concerned with the development of techniques for massively parallel computation at the molecular scale, which we refer to as molecular parallelism. While this may at first appear to be purely science fiction, Adleman [Ad1] has already employed molecular parallelism in the solution of the Hamiltonian path problem, and successfully tested his techniques in a lab experiment on DNA for a small graph. Lipton [L] showed that finding the satisfying inputs to a Boolean expression of size n can be done in O(n) lab steps using DNA of length O(n log n) base pairs. This recent work by Adleman and Lipton in molecular parallelism considered only the solution of NP search problems, and provided no way of quickly executing lengthy computations by purely molecular means; the number of lab steps depended linearly on the size of the simulated expression. See [Re3] for further recent work on molecular parallelism and see [Re4] for an extensive survey of molecular parallelism. Our goal is to execute lengthy computations quickly by the use of molecular parallelism. We wish to execute these biomolecular computations using short DNA strands by more or less conventional biotechnology engineering techniques within a small number of lab steps. This paper describes techniques for achieving this goal, in the context of well defined abstract models of biomolecular computation. Although our results are of theoretical consequence only, due to the large amount of molecular parallelism (i.e., large test tube volume) required , we believe that our theoretical models and results may be a basis for more practical later work, just as was done in the area of parallel computing. We propose two abstract models of biomolecular computation. The first, the Parallel Associative Memory (PAM) model, is a very high-level model which includes a Parallel Associative Matching (PA-Match) operation, that appears to improve the power of molecular parallelism beyond the operations previously considered by Lipton [L]. We give some simulations of conventional sequential and parallel computational models by our PAM model. Each of the simulations use strings of length O(s) over an alphabet of size O(s) (which correspond to DNA of length O(s log s) base pairs). Using O(s log s) PAM operations that are not PA-Match (or O(s) operations assuming a ligation operation) and t PA-Match operations, we can: 1. simulate a nondeterministic Turing Machine computation with space bound s and time bound 2 ^O(s) , with t = O(s) , 2. simulate a CREW PRAM with time bound D, with M memory cells, and processor bound P, where here s = O( log (PM)) and t = O(D+s), 3. find the satisfying inputs to a Boolean circuit constructible in s space with n inputs, unbounded fan-out, and depth D, where here t = O(D+s). We also propose a Recombinant DNA (RDNA) model which is a low-level model that allows operations that are abstractions of very well understood recombinant DNA operations and provides a representation, which we call the complex , for the relevant structural properties of DNA. The PA-Match operation for lengthy strings of length s cannot be feasibly implemented by recombinant DNA techniques directly by a single step of complementary pairing in DNA; nevertheless we show this Matching operation can be simulated in the RDNA model with O(s) slowdown by multiple steps of complementary pairing of substrings of length 2 (corresponding to logarithmic length DNA subsequences). Each of the other operations of the PAM model can be executed in our RDNA model, without slowdown. We further show that, with a further O(s)/ log (1/ε) slowdown, the simulations can be done correctly with probability 1/2 even if certain recombinant DNA operations (e.g., Separation) can error with a probability ε. We also observe efficient simulations can be done by PRAMs and thus Turing Machines of our molecular models. Received December 30, 1995; revised December 30, 1996, and January 22, 1998.     

Computation of Random Errors in Digital Terrain Models

Geostatistical methods make it possible to estimate the random errors in digital terrain models (DTMs) from one single set of measurements. After average interpolating trend removal, the error is derived from the variogram of the residual data set. This method is compared with a procedure that uses triangulation and resampling of two overlapping DTMs for extracting the error component. Eighteen small areas measured with a multibeam echo sounder are selected for the comparison. In the experiment the two methods come out with similar values for the random error. A by-product of the experiment is the development of a model for the average noise reduction caused by resampling of triangle models (TINs). The method we apply for the trend computation has time complexity O(n). Since the noise computation only requires the part of the variogram close to the origin, it is possible to formulate the computation of the random error in DTMs in O(n) time.

Using Algorithms of Decomposition for Computation of Linear Stochastic Models

The mathematical apparatus of decomposition is used to solve the problem of analysis and computation of stiff stochastic systems of differential equations. A theorem substantiating the adequacy of a solution obtained is formulated and an algorithm of computation of stiff stochastic systems by the method of depression of equations is given.     

Computation of Equilibria in OLG Models with Many Heterogeneous Households

This paper develops a decomposition algorithm by which a market economy with many households may be solved through the computation of equilibria for a sequence of representative agent economies. The paper examines local and global convergence properties of the sequential recalibration (SR) algorithm. The SR algorithm is then demonstrated to efficiently solve Auerbach–Kotlikoff OLG models with a large number of heterogeneous households. We approximate equilibria in OLG models by solving a sequence of related Ramsey optimal growth problems. This approach can provide improvements in both efficiency and robustness as compared with integrated complementarity-based solution methods.     

Molecular Verification of Rule-Based Systems Based on DNA Computation

Various graphic techniques have been developed to analyze structural errors in rule-based systems that utilize inference (propositional) logic rules. Four typical errors in rule-based systems are: redundancy (numerous rule sets resulting in the same conclusion); circularity (a rule leading back to itself); incompleteness (deadends or a rule set conclusion leading to unreachable goals); and inconsistency (rules conflicting with each other). This study presents a new DNA-based computing algorithm mainly based upon Adleman's DNA operations. It can be used to detect such errors. There are three phases to this molecular solution: rule-to-DNA transformation design, solution space generation, and rule verification. We first encode individual rules using relatively short DNA strands, and then generate all possible rule paths by the directed joining of such short strands to form longer strands. We then conduct the verification algorithm to detect errors. The potential of applying this proposed DNA computation algorithm to rule verification is promising given the operational time complexity of O(n*q), in which n denotes the number of fact clauses in the rule base and q is the number of rules with longest inference chain.     

随机Petri网性能计算软件关键技术的研究

性能计算对随机Petri网模型的性能分析具有重要作用。以研究随机Petri网性能计算软件关键技术为主要目的,提出了一种独立于计算（CIM）的性能计算软件体系结构元模型,并以此元模型为依据,遵循面向对象的系统分析与设计方法给出该软件独立于平台（PIM）的软件体系结构模型,对核心算法设计方案采用先计算基础性能数据,再计算扩展性能数据的策略分步骤完成,并给出了基于MVC模式实现的PSM模型实例和一个电子农务应用案例。最后,给出了同类性能计算软件的设计可适用此开发方法的初步结论。     

On Dice and Coins: Models of Computation for Random Generation

To examine the concept of random generation in bounded, as opposed to expected, polynomial time, a model of a probabilistic Turing machine (PTM) with the ability to make random choices with any (small) rational bias is necessary. This ability is equivalent to that of being able to simulate rolling any k-sided die (where [k] is polynomial in the length of the input). We would like to minimize the amount of hardware required for a machine with this capability. This leads to the problem of efficiently simulating a family of dice with a few different types of biased coins as possible. In the special case of simulating one n-sided die, we prove that only two types of biased coins are necessary, which can be reduced to one if we allow irrationally biased coins. This simulation is efficient, taking O(log n) coin flips. For the general case we get a tight time vs number of biases tradeoff; for example, with O(log n) different biases, we can simulate, for any i < n, an i-sided die in O(log n) coin flips.     

A Comparison of the Axiomatic and Functional Models of Structured Programming

This paper discusses axiomatic and functional models of the semantics of structured programming. The models are presented together with their respective methodologies for proving program correctness and for deriving correct programs. Examples using these methodologies are given. Finally, the models are compared and contrasted.     

Display of Molecular Models with Interactive Graphics

This paper details a scheme for producing display images of atomic models. The scheme relies on a combination of a restricted atomic model, specialized sorting for visibility, and dedicated hardware to produce icons. Building such an imaging device became possible due to sharply decreasing costs of off-the-shelf memory chips and related support hardware. The device is a test bed for understanding the display procedure necessary for this task;insight gained from operation of the device is leading to guides for a reimplementation in dedicated bit-slice technology and specialized VLSI devices.     

中文指代消解模型的对比研究

指代消解的基本任务是把指向现实世界中相同实体的所有实体表达关联起来。与英文指代消解的研究相比,中文指代消解的研究相对较少,至今没有对现存的中文指代消解模型进行公平的测评和比较,主要原因是现存的中文指代消解模型在训练和测评时采用了不同的语料,且所选用的特征属性也不相同。基于上述原因,实现了5类基本的中文指代消解模型,并在ACE2005中文语料上进行同平台、同语料、同特征的测评。通过测评比较了各类模型的性能,同时分析和探讨了影响中文指代消解模型精度的各种因素。