Design of Abstract Domains using First-order Logic In this paper we propose a simple framework based on rst-order logic, for the design and decomposition of abstract domains for static analysis. An assertion language is chosen that speci es the properties of interest, and abstract domains are de ned to be suitably chosen sets of assertions. Composition and decomposition of abstract domains is facilitated by their logical speci cation in rst-order logic. In particular, the operations of reduced product and disjunctive completion are formalized in this framework. Moreover, the notion of (conjunctive) factorization of sets of assertions is introduced, that allows one to decompose domains in`disjoint' parts. We illustrate the use of this framework by studying typical abstract domains for ground-dependency and aliasing analysis in logic programming. In the theory of abstract interpretation 3], abstract domains are (computer) representations of properties. The semantics of an abstract domain is given by a function called concretization, that maps elements of the abstract domain into elements of a`concrete domain'. Two fundamental aspects of the study of abstract domains are the investigation of representations supporting e cient implementations, and the comparative study of the properties represented by abstract domains. This paper is concerned with the latter aspect.. In the standard approach the design phase is not clearly distinguished from the representation one. In general, once the concrete domain is chosen, a representation of an abstract domain is directly de ned by means of a suitable lattice structure equipped with a concretization function. Then the image of the abstract domain under its concretization function is used to study its properties, as well as for comparing the domain with other ones (w.r.t. the same concrete domain). There are two equivalent techniques for the study and comparison of the properties represented by abstract domains (cf. 3]): Galois connections, where an abstract domain is a complete lattice together with a Galois connection (or insertion) that relates the abstract domain with the`concrete' one; and closure operators, where an abstract domain is The paper is organized as follows. The next section introduces a methodology for the design and decomposition of abstract domains using rst-order logic. Section 3 presents an assertion language for the design of typical abstract domains for logic programming. Section 4 contains a comparative study of various abstract domains for logic programming. Section 5 discusses some related work. Finally, in Section 6 we conclude with a discussion on other applications and on future work. A preliminary version of this paper appeared in 20]. In this paper a simple framework based on rst-order logic has been proposed for the design of abstract domains for static analysis. The correspondent of the operations of reduced product and disjoint completion of abstract domains have been de ned in the logical framework. Moreover, the notion of conjunctive factorization has been introduced, for decomposing abstract domains in`disjoint' parts. The usefulness of this framework has been illustrated by analyzing typical abstract domains used in abstract interpretation of logic programs.. The framework can also be used for de ning operators on abstract domains. For instance, an important operator in abstract interpretation of logic programs is the`projection' away from the variables that are not in V . The projection operation corresponds to existential quanti cation ( 23] The logical representation of these domains can be used to prove that this de nition is correct and optimal, (i.e. it provides the minimal representation). In fact, correctness amounts to prove that the following is an equivalence (in M):. ASub (G; R)^ Sharing ( ) , ASub (G 0 ; R 0 )^ Sharing ( 0 ): Optimality amounts to prove the following two conditions:. 1. for every x in V :. ASub (G 0 ; R 0 ) ) ground(x) i Sharing ( 0 ) ) ground(x);. 2. for every distinct variables x; y of V :. ASub (G 0 ; R 0 ) ) :share(x; y) i Sharing ( 0 ) ) :share(x; y).. The proof of the above statements is not di cult, using the de nitions of Sharing and ASub . We conclude by mentioning some interesting topics for future work. The speci c framework for logic programming could be applied for proving the correctness of abstract uni cation algorithms. This could be done by describing the uni cation by means of a suitable predicate transformer on L, in the style of 25], and by de ning a transformation which reduces the considered abstract uni cation algorithm to an instance of the predicate transformer. However, this is not an easy task, for it is already di cult to design a speci c correct abstract uni cation algorithm (see e.g. 4]).. Another interesting topic that seems worth of investigation, is the study of the relationship between abstract interpretations and proof methods. This topic has been tackled in the functional programming setting, where a domain-theoretic approach is used in 17] for proving that strictness analysis by abstract interpretation and non-standard type inference are equivalent. For logic programming, our framework could be used for de ning a program logic for the comparison of data-driveness analysis using type inference (cf. e.g. 1]) and abstract interpretation (cf. 23]). The standard techniques used for the comparative study of the properties represented by abstract domains are based on two equivalent approaches: Galois connections and closure operators.. In the original approach ( 3] ), comparison of abstract domains is performed by means of the notion of abstraction, where an abstract domain is more abstract than another one if there is a Galois insertion from the rst into the latter. This notion is weakened in 9], where the comparison is de ned w.r.t. a given property, by means of the notion of quotient of one abstract domain w.r.t. another one, describing the part of the former abstract domain that is useful for computing the information described by the latter one. A similar analysis is possible by means of our framework, where the domain and the property are rst speci ed in the logic, and next factorized. Then the reduced product of the common factors of the domains corresponds to the quotient of the domain w.r.t. the property.. The approach based on closure operators has been employed in two recent works 6, 14] , that investigate the`inverse' of the two important operators on abstract domains, namely the reduced product and the disjunctive completion, respectively. In 6], this approach is used for investigating domain complementation in abstract interpretation, a kind of inverse of the reduced product. The authors formalize the concept of decomposition of an abstract domain, as a set of abstract domains whose reduced product yields the initial abstract domain and use the notion of pseudo-complement for decomposing abstract domains. The distinguishing feature of our approach from this work is the use of an assertion language for describing the properties of interest, and the explicit role of the set of considered program variables (or more in general of syntactic objects). As a consequence, domains are decomposed by inspecting the form of their assertions. The relative de nition of conjunctive factorization is rather intuitive, since it resembles the notion of factorization of integers into pairwise prime factors: it is always applicable, and the resulting factors are`disjoint'. This is not the case for the method based on the notion of psuedo-complement: the notion of domain decomposition is introduced, that amounts to condition (c) of De nition 2.5, and the pseudo-complement D C of a domain D w.r.t. another domain C is used to provide the (binary) decomposition (D C ; C ) of D, where the factors are not necessarily`disjoints'.. In 14], the approach based on closure operators is used for investigating the inverse of the operation of disjunctive completion. They introduce the notion of disjunctive basis for an abstract domain as the most abstract domain inducing the same disjunctive completion, and study the disjunctive basis of typical abstract domains used in abstract interpretation of functional and logic programming. In this paper we have introduced a similar notion, called _-reduced domain (on L). The main di erence is that here L determines the granularity of the _-reduced domains, while in 14] all the closure operators on the concrete domain are considered. Moreover, the fact that L is a rst-order assertion language guarantees that the disjunctive completion of a domain is equal to the disjunctive completion of a _-reduced domain.. The abstract domains analyzed in Section 4 have been extensively studied in previous work. In 8] it is proven that the part of Sharing describing groundness dependencies is contained in Pos. In 6] this result is strengthen by showing that this part coincides with Def, and that Sharing + is the pseudo-complement of Def in Sharing. In this paper these results are directly derived from the logical characterization of Sharing. Moreover, we have obtained the nest (in L) decomposition of Sharing. Finally, the factors of this decomposition have been used for other purposes, e.g. for comparing the expressiveness of the abstract domains.. The classes of Boolean functions used to represent Def and Pos have been analyzed in 7, 2]. The di erence from these works is that they focus on the representation, while we focus on the design and reasoning, by considering a syntactic characterization of Def (Def ) in rst-order logic.

O O B I O B I O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O B I O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O B I O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


A Big-Stepped Probability Approach for Discovering Default Rules This paper deals with the extraction of default rules from a database of examples. The proposed approach is based on a special kind of probability distributions, called "big-stepped probabilities", which are known to provide a semantics for non-monotonic reasoning. The rules which are learnt are genuine default rules, which could be used (under some conditions) in a non-monotonic reasoning system and can be encoded in possibilistic logic.

O O O O O O B I O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O B I O O O O O O O O O B I O O O O O O O O


On control laws for discrete linear repetitive processes with dynamic boundary conditions Abstract Repetitive processes are characterized by a series of sweeps, termed passes, through a set of dynamics defined over a finite duration known as the pass length. On each pass an output, termed the pass profile, is produced which acts as a forcing function on, and hence contributes to, the dynamics of the next pass profile. This can lead to oscillations in the sequence of pass profiles produced which increase in amplitude in the pass-to-pass direction and cannot be controlled by application of standard control laws. Here we give new results on the design of physically based control laws for so-called discrete linear repetitive processes which arise in applications areas such as iterative learning control. This paper has considered the influence of the boundary conditions on the stability and control properties of discrete linear repetitive processes. In particular, the case when the pass state initial vector sequence is an explicit function of the previous pass profile has been studied. The presence of such boundary conditions can cause the process to be asymptotically unstable (the weakest form of stability property) and the major new results in this paper show how to design a control law to ensure this property. This control law is feed forward in the sense that it is fully activated by the previous pass profile. A second feature examined here relates to the fact that control law design for asymptotic stability may conflict with other essential requirements, e.g. stability along the pass and tracking performance. In such cases it may be necessary to consider a multi-loop control strategy.. Here we have developed a solution where control is introduced through the pass state initial vector sequence, i.e. a form of boundary control as in partial differential equation (or distributed parameter) control. One area to be investigated in further work is that of minimizing the number previous pass points required for this task, i.e. can we use a control law matrix which has some (but not all) zero sub-blocks and hence a more efficient implementation?. A natural question to ask is if it is possible to undertake a robustness analysis? The answer with the approach used here is negative due to the fact the block matrices M 0 and N 0 contain different products of matrices which prevent effective progress. Hence the development of alternative settings for a robustness analysis of the processes considered here is an obvious subject for further research. This again highlights the critical role of the boundary conditions and, in particular, the pass state initial vector sequence in the analysis and control of discrete linear repetitive processes since if they are of the form (2) a well developed robust control theory is available-see Rogers et al. (2007) for the details. One possible approach here would be to try to exploit the structure of so-called wave repetitive processes (see Rogers et al. (2007) for the definition and currently available results) as a setting for analysis. Wroclaw, Poland in 1972 , 1977 and 1994 , respectively. In October 1996

O O O O O B I I O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Using Data Accessibility for Resource Selection in Large-Scale Distributed Systems Abstract-Large-scale distributed systems provide an attractive scalable infrastructure for network applications. However, the loosely coupled nature of this environment can make data access unpredictable, and in the limit, unavailable. We introduce the notion of accessibility to capture both availability and performance. An increasing number of data-intensive applications require not only considerations of node computation power but also accessibility for adequate job allocations. For instance, selecting a node with intolerably slow connections can offset any benefit to running on a fast node. In this paper, we present accessibility-aware resource selection techniques by which it is possible to choose nodes that will have efficient data access to remote data sources. We show that the local data access observations collected from a node's neighbors are sufficient to characterize accessibility for that node. By conducting trace-based, synthetic experiments on PlanetLab, we show that the resource selection heuristics guided by this principle significantly outperform conventional techniques such as latency-based or random allocations. The suggested techniques are also shown to be stable even under churn despite the loss of prior observations. L ARGE-SCALE distributed systems provide an attractive scalable infrastructure for network applications. This virtue has led to the deployment of several distributed systems in large-scale, loosely coupled environments such as peer-to-peer computing [1] , distributed storage systems [2] , [3] , [4] , and Grids [5] , [6] , [7] . In particular, the ability of large-scale systems to harvest idle cycles of geographically distributed nodes has led to a growing interest in cyclesharing systems [8] and @home projects [9] , [10] . However, a major challenge in such systems is the network unpredictability and limited bandwidth available for data dissemination. For instance, the BOINC project [11] reports an average throughput of only about 289 Kbps, and a significant proportion of BOINC hosts shows an average throughput of less than 100 Kbps [1] . In such an environment, even a few megabytes of data transfer between poorly connected nodes can have a large impact on the overall application performance. This has severely restricted the amount of data used in such computation platforms, with most computations taking place on small data objects.. Emerging scientific applications, however, are dataintensive and require access to a significant amount of dispersed data. Such data-intensive applications encompass a variety of domains such as high-energy physics [12] , climate prediction [13] , astronomy [14] , and bioinformatics [15] . For example, in high-energy physics applications such as the Large Hadron Collider (LHC), thousands of physicists worldwide will require access to shared, immutable data at the scale of petabytes [16] . Similarly, in the area of bioinformatics, a set of gene sequences could be transferred from a remote database to enable comparison with input sequences [17] . In these examples, performance depends critically on efficient data delivery to the computational nodes. Moreover, the efficiency of data delivery for such applications would critically depend on the location of data and the point of access. Hence, in order to accommodate data-intensive applications in loosely coupled distributed systems, it is essential to consider not only the computational capability, but also the data accessibility of computational nodes to the required data objects. The focus of this paper is on developing resource selection techniques suitable for such data-intensive applications in large-scale computational platforms.. Data availability has been widely studied over the past few years as a key metric for storage systems [2] , [3] , [4] . However, availability is primarily used as a server-side metric that ignores client-side accessibility of data. While availability implies that at least one instance of the data is present in the system at any given time, it does not imply that the data are always accessible from any part of the system. For example, while a file may be available with 5 nines (i.e., 99.999 percent availability) in the system, real access from different parts of the system can fail due to reasons such as misconfiguration, intolerably slow connections, and other networking problems. Similarly, the availability metric is silent about the efficiency of access from different parts of the network. For example, even if a file is available to two different clients, one may have a much worse connection to the file server, resulting in much greater download time compared to the other. Therefore, in the context of data-intensive applications, it is important to consider the metric of data accessibility: how efficiently a node can access a given data object in the system.. The challenge we address is the characterization of accessibility from individual client nodes in large distributed systems. This is complicated by the dynamics of wide-area networks, which rule out static a priori measurement, and the cost of on-demand information gathering, which rules out active probing. Additionally, relying on global knowledge obstructs scalability, so any practical approach must rely on local information. To achieve accessibility-aware resource selection, we exploit local historical data access observations. This has several benefits. First, it is fully scalable as it does not require global knowledge of the system. Second, it is inexpensive as we employ observations of the node itself and its directly connected neighbors (i.e., one-hop away). Third, past observations are helpful to characterize the access behavior of the node. For example, a node with a thin access link is likely to show slow access most of the time. Last, by exploiting relevant access information from the neighbors, it is possible to obviate the need for explicit probing (e.g., to determine network performance to the server), thus minimizing system and network overhead.. Our key research contributions are as follows:. . We present accessibility estimation heuristics which employ local data access observations, and demonstrate that the estimated data download times are fairly close to real measurements, with 90 percent of the estimates lying within 0.5 relative error in live experimentation on PlanetLab. . We infer the latency to the server based on the prior neighbor measurement without explicitly probing the server. For this, we extend existing estimation heuristics [18] , [19] , [20] to more accurately work with a limited number of neighbors. Our enhancement gives accurate results even with only a few neighbors. . We present accessibility-aware resource selection techniques based on our estimation functions and compare to the optimal and conventional techniques such as latency-based and random selection. Our results indicate that our approach not only outperforms the conventional techniques, but does so over a wide range of operating conditions. . We investigate the impact of churn prevalent in loosely coupled distributed systems. In fact, churn is critical to our resource selection techniques because we assume that nodes lose all past observations when they join the system again. Despite this stringent memory-loss property, the results show that our techniques perform well even under a certain degree of churn. Accessibility is a crucial concern for an increasing number of data-intensive applications in loosely coupled distributed systems. Such applications require more sophisticated resource selection due to bandwidth and connectivity unpredictability. In this paper, we presented decentralized, scalable, and efficient resource selection techniques based on accessibility. Our techniques rely only on local, historic observations, so it is possible to keep network overhead tolerable. We showed that our estimation techniques are sufficiently accurate to provide a meaningful rank order of nodes based on their accessibility. Our techniques outperform conventional approaches and are reasonably close to the optimal selection. In particular, the self-estimationbased selection approached 1.4 of optimal over time, the neighbor estimation-based selection was within 2.6 of optimal with 16 neighbors, compared to a proximity-based selection that was over three times the optimal. With respect to the mean elapsed time, the self-and neighbor-estimationbased selections were, respectively, 52 and 70 percent more efficient than proximity-based selection. We also investigated how our techniques work under node churn and showed that they work well under churn circumstances in which nodes suffer from loss of observations. Finally, we showed that our techniques consistently outperform conventional techniques in replicated environments.. In this work, we focused on performance for the accessibility metric. The next step is to capture availability as well as performance to take dynamism into account. In addition to this, we plan to extend our work by providing system-wide dissemination of observations so that the node has more chances to see relevant observations in estimation. This is reasonable since neighbor estimation has no constraints on topological or geographical similarities to utilize observations coming from other nodes. Jon B. Weissman received the BS degree from Carnegie-Mellon University in 1984, and the MS and PhD degrees from the University of Virginia in 1989 and 1995, respectively, all in computer science. He is a leading researcher in the area of high-performance distributed computing. His involvement dates back to the influential Legion project at the University of Virginia during his PhD. He is currently an associate professor of computer science at the University of Minnesota, where he leads the Distributed Computing Systems Group. His current research interests are in Grid computing, distributed systems, high-performance computing, resource management, reliability, and e-science applications. He works primarily at the boundary between applications and systems. He is a senior member of the IEEE and the IEEE Computer Society and an awardee of the US National Science Foundation CAREER Award (1995).. . For more information on this or any other computing topic, please visit our Digital Library at www.computer.org/publications/dlib. We conducted over 100,000 actual downloading experiments for a span of five months with 241 PlanetLab nodes geographically distributed across the globe. For this, we deployed a Pastry [25] , [40] network, a structured overlay based on a DHT ring. We distributed data objects of four sizes: 1, 2, 4, and 8 MB, over the network, each object with a unique key. We then generated a series of random queries so that the selected nodes perform downloading the relevant objects. Table 1 provides the details of the download traces. In the simulations, we use a mixture of all traces rather than individual traces, unless otherwise mentioned.. To evaluate resource selection techniques, we design and implement a simulator which inputs the ping maps and the collective downloading traces and outputs performance results according to the selection algorithms. Initially, the simulator constructs a network in which nodes are randomly connected to each other with a predefined neighbor size without any locality or topological considerations. To minimize error due to the construction, we repeated simulations 50 times and report the results with 95 percent confidence intervals as needed. After constructing the network, the simulator runs each resource selection algorithm. Initially, it constructs a virtual trace in which the list of candidates and the download time from each candidate are recorded. The candidate nodes are randomly chosen for each allocation. As the candidate may have more than one actual download record for a server, the download time is also randomly selected from them. The simulator then selects a worker based on each selection algorithm. Based on the selected worker, the download time is returned from the virtual trace.. For our evaluation, we compare the resource selection techniques based on our estimation techniques with two conventional techniques: random and latency-based selections. The following describes the resource selection techniques:. . OMNI: Optimal selection; . RANDOM: Random selection; . PROXIM: Latency-based selection; . SELF: SELF basically performs the selection by selfestimation. One exception is that it allows the node to make an estimation by partial observations if it has any observations to the object server. 4 This can improve accuracy. If no estimate is available, it performs random selection; . NEIGHBOR: NEIGHBOR performs the selection based on neighbor estimation. If no estimate is available, it performs random selection.

O B I O B I O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Mind change efficient learning This paper studies efficient learning with respect to mind changes. Our starting point is the idea that a learner that is efficient with respect to mind changes minimizes mind changes not only globally in the entire learning problem, but also locally in subproblems after receiving some evidence. Formalizing this idea leads to the notion of strong mind change optimality. We characterize the structure of language classes that can be identified with at most mind changes by some learner (not necessarily effective): a language class L is identifiable with mind changes iff the accumulation order of L is at most . Accumulation order is a classic concept from point-set topology. We show that accumulation order is related to other established notions of structural complexity, such as thickness and intrinsic complexity. To aid the construction of learning algorithms, we show that the characteristic property of strongly mind change optimal learners is that they output conjectures (languages) with maximal accumulation order. We illustrate the theory by describing strongly mind change optimal learners for various problems such as identifying linear subspaces, one-variable patterns, and fixed-length patterns. One of the goals of computational learning theory is to design learning algorithms for which we can provide performance guarantees. Identification in the limit is a central performance goal in Gold's language learning paradigm [11] . A well-studied refinement of this notion is identification with bounded mind changes [9, 1] . In this paper, we investigate a further refinement that we term strong mind change optimality (SMC-optimality). Briefly, a learner is SMC-optimal if the learner achieves the best possible mind change bound not only for the entire problem, but also relative to any data sequences that the learner may observe.. The general theory in this paper has two main goals: (1) To provide necessary and sufficient conditions for a language collection to be identifiable with a given (ordinal) mind-change bound by some learner (not necessarily effective). (2) To provide necessary and sufficient conditions for a learner to be SMC-optimal. The results addressing (1) help us determine when an SMC-optimal learning algorithm exists, and the results addressing (2) help us to construct optimal learning algorithms when they do exist.. We situate our study in the framework of point-set topology. Previous work has shown the usefulness of topology for learning theory [34,chapter 10,28,17,4,29] We show how to view a language collection as a topological space; this allows us to apply Cantor's classic concept of accumulation order which assigns an ordinal acc(L) to a language collection, if L has bounded accumulation order. We show that a language collection L is identifiable with mind change bound by a learner if and only if acc(L) = . This result establishes a purely information-theoretic and structural necessary condition for identification with bounded mind changes. Based on the concept of accumulation order, we provide necessary and sufficient conditions for a learner to be SMC-optimal. These results show that SMC-optimality strongly constrains the conjectures of learners. We illustrate these results by analyzing various learning problems, such as identifying a linear subspace and a one-variable pattern.. The paper is organized as follows. Section 2 reviews standard concepts for language identification and presents our definition of mind change optimality. Then we establish the correspondence between mind change complexity and accumulation order. Section 4 gives necessary and sufficient conditions for a learner to be strongly mind change optimal. Next, we describe some general principles for constructing SMC-optimal effective learners and illustrate them with one-variable and fixed-length pattern languages. In Section 6, we show strong relationships between the concept of accumulation order and other structural notions studied in learning theory, such as thickness [41] , elasticity [44, 30] , and intrinsic complexity [10, 13] .

O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O B I O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O


Online matching on a line We prove a lower bound ρ ≥ 9.001 for the competitive ratio of the so-called online matching problem on a line. As a consequence, the online matching problem is revealed to be strictly more difficult than the "cow problem". We consider a special class of online server problems, where a number of servers, located on the real line, is to serve a sequence of requests r 1 , r 2 , . . . , r k ∈ R. In contrast to classical server problems (cf, e.g. [3] ), however, each server can serve at most one request. So the optimal offline solution is the min cost matching of the requests into the set of server positions s i . The problem is therefore also known as the online matching problem on a line ( [5] ). As an application, consider a ski rental with different ski lengths s 1 , s 2 , . . . at its disposal to meet requested lengths r 1 , r 2 , . . . of entering clients.. For notational convenience, we consider a "universal" instance with infinitely many servers, one at each integer s ∈ Z. One may equally well consider finite versions with servers at positions s 1 , . . . , s n ∈ R, requests r 1 , . . . r k ∈ R and k ≤ n (or even k = n). These are, however, easily seen to be of approximately the same difficulty: Any ρ-competitive algorithm for one version with ρ <ρ implies aρ-competitive version for the other withρ <ρ.. An online matching algorithm is ρ-competitive if, after serving r 1 , . . . , r t (t ∈ N), the current length L of the online matching constructed so far is at most ρ times the current optimal matching cost. It is a challenging open question to prove or disprove the existence of ρ-competitive online algorithms with finite competitive ratio ρ. The basic difficulty for an online algorithm is to decide which server to use for matching a new request r. There are essentially two choices: Either the server s − that is closest to r from left or the server s + that is closest to r from right (among those servers that are currently still unmatched). Indeed, serving r from a server at s < s − can be interpreted as moving s to s − and serving r from s − .. Assume, e.g., the first 2m 0 requests are at r = 0, ±1, ±2, . . . , ±(m 0 − 1), 0. The first 2m 0 − 1 requests will then be served from the servers at these positions, whereas the last request r = 0 will be served, say, from s = −m 0 . Assume the following requests r 2m 0 +1 , r 2m 0 +2 , . . . are then exactly at the positions where a server has just been moved off to serve the previous request. So r 2m 0 +1 = −m 0 etc. In order to stay ρ-competitive, the online algorithm may first serve a number of requests from left, but must eventually switch to serving some request r = i ≤ −m 0 from right, i.e., from s = m 0 . (Indeed, |i| ≤ ρ/2m 0 ). It may then continue to serve a number of requests from right, but eventually it will have to switch again, serving some request r = j ≥ m 0 from left etc. Thus the online algorithm basically must behave like the famous cow searching for a bridge to cross the river ( [1] , [4] ). We therefore refer to the request sequence constructed as above as a cow sequence with parameter m 0 , started at r = 0.. This analogy yields a lower bound of ρ ≥ 9 for the competitive ratio of any online algorithm for matching on a line (even for cow sequences), cf. [1] or section 2. The main purpose of our paper is to slightly improve this bound to ρ ≥ 9.001. Since a 9-competitive algorithm for the "cow problem" is known, our result proves the online matching problem to be strictly more difficult than the cow problem. In Section 4 we analyze online algorithms based on so-called work functions and show that they have infinite competitive ratio.

O B O O O O O O O O O O O O O O O O O O O B O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O B O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O


Complementary sequences for ISI channel estimation Abstract-A merit factor based on the sequence autocorrelation function, whose minimization leads to the reduction in the Crámer-Rao lower bound (CRLB) for the variance of "two-sided" intersymbol interference (ISI) channel estimation is introduced. Pairs of binary pilot symbol sequences (a preamble and a postamble) for channel estimation are jointly designed to minimize this merit factor. Given that the number of channel taps is and the length of a pilot symbol sequence is ( + 1), where , we distinguish between the case when is even and the case when it is odd. For even , we show that complementary sequences not only minimize the merit factor, but also the CRLB. For a subset of odd we construct almost-complementary periodic sequence pairs that minimize the merit factor. The optimal pilot symbol block signaling requires alternating between two (in most cases) different binary sequences that form the merit-minimizing pair. Snapshots of periodic complementary sequence pairs are optimal for two-sided ISI channel estimation for even , where is the length of a pilot-symbol block and is the number of channel taps. These sequence pairs allow for the minimum possible estimation variance for channel estimation that attains the CRLB. The minimum possible variance, , of the "one-sided" channel estimator is attainable when either and or (see [8] ).. No complementary sequences exist for odd and, therefore, the minimum ML estimation variance is larger than for Fig. 6 . ML sequence detection performance based on optimal two-sided and optimal one-sided pilot symbol sequence pairs: fixed channel. odd . An upper bound on the estimation CRLB is based on the Gerschgorin discs of the matrix formed by summing the autocorrelation matrices of the two training sequences that frame a data block. This upper bound can be minimized by minimizing over all sequence pairs. For even , periodic complementary sequences achieve the minimum possible merit factor . Periodic sequence pairs that minimize the merit factor have been constructed for a subset of odd . We have shown that these pairs achieve the minimum possible for odd . The pairs with odd that achieve have been termed almost-complementary sequences.. For two constructed almost-complementary sequences we have shown that they allow for channel estimation variance that is slightly larger than and equal to the minimum possible variance for given pilot-symbol block lengths.. Simulations have demonstrated that a significant detection performance improvement can be achieved when optimal pilot symbol sequence pairs are used for given parameters , , and . Furthermore, an optimization over which neglects the effect of bandwidth expansion can provide an additional performance gain.

B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O


Experimental identification of turbulent fluid forces applied to fuel assemblies using an uncertain model and fretting-wear estimation Anas Batou, Christian Soize. Experimental identification of turbulent fluid forces applied to fuel assemblies using an uncertain model and fretting-wear estimation. Mechanical Systems and Signal Processing, Elsevier, 2009, 23 (7) Systems and Signal Processing April 7, 2009 identification process of the stochastic loads. Then, the stochastic nonlinear dynamical computational model of fuel assemblies on which the identified stochastic loads are applied yield interesting results concerning the robustness of the estimation of the fretting wear of the fuel rods. A fuel assembly is made up of thousands of fuel rods and tubes which are held in position by grids. This dynamical system bathes in a flow of a liquid (water)which induces turbulent forces that are likely to induce frettingwear of the fuel rods. A fuel assembly is a very complex nonlinear dynamical system for which an accurate computational model (called the reference computational model) would be time consuming and generally, would induce many numerical problems due to the high modal density of such a structure.. Therefore, the computational model must be simplified from an engineering design point of view. The model uncertainties are thus due to the simplification introduced by the mathematical-mechanical modeling process. The measurements are realized with an experimental setup which is constituted of a half fuel assembly which bathes in a turbulent fluid. This experimental setup has been designed in order to improve the understanding of the dynamical behavior of fuel assemblies. The objectives of this paper are to identify the parameters of the mathematical model of the stochastic forces induced by the turbulent fluid which are applied to the experimental setup, using an uncertain stochastic simplified computational model and experimen-2 tal responses. The general methodology used to solve this problem has been presented in [2] and validated on a very simple academic example. In the present paper, we applied this methodology to a complex industrial structure and the identification of the stochastic model is carried out using measure- (1) The model uncertainties induced by the introduction of simplifications in the model. This type model uncertainties are taken into account using the nonparametric probabilistic approach (see [9, 10] ) which consists in modeling the reduced mass and stiffness matrices by full random matrices defined on a probability space (Θ, T , P). The dispersion parameters introduced by this approach will be identified using the maximum likelihood method and a reference computational model made up of an accurate finite element model. ( 2) The mean model of the stochastic loads (induced by the statistical fluctuations of the turbulent pressure applied to the structure) is a vectorvalued Gaussian centered second-order stationary stochastic process defined on a probability space (Θ , T , P ). Consequently, the mean model of the stochastic loads is completely defined by the nominal value of the matrix- (3) The uncertainties concerning the stochastic loads are taken into account by replacing the nominal value of the matrix-valued spectral density function (defined above) by a random matrix-valued spectral density function defined on a probability space (Θ , T , P ). By construction, the mean value of this random matrix-valued spectral density function is equal to the nominal value of the matrix-valued spectral density function. The probability distribution of this random matrix-valued spectral density function de-pends on a dispersion parameter which controls the level of uncertainties and which is identified using the maximum likelihood method and experimental responses.. (4) The uncertainties induced by measurement errors are taken into account in introducing an additive random variable for the experimental observation. This random noise (modeling the effects of the measurement errors) is defined as a Gaussian centered random variable on a probability space. Section 2 deals with the experimental modal analysis carried out on the experimental setup. Section 3 presents the reference computational model which will be used as an observation for the stochastic simplified computational model constructed in Section 4. The uncertain stochastic loads applied on the stochastic simplified computational model are identified in Section 5.. In Section 6, the identified stochastic loads and the stochastic simplified computational model are used to construct statistics on quantities of interest, in particular the fretting-wear on the rods. We have presented a complete methodology for the identification of turbulent fluid forces applied to fuel assemblies using an uncertain simplified

O B O O O O O O B I O O O O O O O O O O O O B O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O B I O O O O


Semantic Web Support for the Business-to-Business E-Commerce Pre-Contractual Lifecycle semantic web, e-commerce, matchmaking, automated negotiation, DAML+OIL, service descriptionIf an e-services approach to electronic commerce is to become widespread, standardisation of ontologies, message content and message protocols will be necessary. In this paper, we present a lifecycle of a business-to-business e -commerce interaction, and show how the Semantic Web can support a service description language that can be used throughout this lifecycle. DAML+OIL is a sufficiently expressive and flexible service description language to be used not only in advertisements, but also in matchmaking queries, negotiation proposals and agreements. We also identify which operations must be carried out on this description language if the B2B lifecycle is to be fully supported. We do not propose specific standard protocols, but instead argue that our operators are able to support a wide variety of interaction protocols, and so will be fundamental irrespective of which protocols are finally adopted. If an e-services approach to electronic commerce is to become widespread, standardisation of ontologies, message content and message protocols will be necessary. In this paper, we present a lifecycle of a business-to-business e-commerce interaction, and show how the Semantic Web can support a service description language that can be used throughout this lifecycle. DAML+OIL is a sufficiently expressive and flexible service description language to be used not only in advertisements, but also in matchmaking queries, negotiation proposals and agreements. We also identify which operations must be carried out on this description language if the B2B lifecycle is to be fully supported. We do not propose specific standard protocols, but instead argue that our operators are able to support a wide variety of interaction protocols, and so will be fundamental irrespective of which protocols are finally adopted. Electronic commerce is having a revolutionary effect on business. It is changing the way businesses interact with consumers, as well as the way they interact with each other. Electronic interactions are increasing the efficiency of purchasing, and are allowing increased reach across a global market.. With the increasing availability of the web, a more open e-commerce environment is developing, allowing businesses to trade more flexibly with each other. Some of this openness is achieved by competition between web portals, while some competition occurs within a single web portal, acting as a marketplace for buyers and sellers to meet.. The e-commerce community is creating new infrastructures to support highlevel business-to-business (B2B) and business-to-consumer interactions on the web. The effort is concentrated on defining a new generation of electronic data interchange protocols, mostly based on XML (like OASIS, BizTalk and RosettaNet) and on creating new kinds of e-business services such as agent-mediated B2B e-commerce, and knowledge-driven customer relationship management.. On the other hand, with the semantic web initiative, the World Wide Web Consortium (W3C) is developing a range of proposals aimed at supporting intelligent information intensive operations over the web. The emphasis is on enriching the web's data markup languages with knowledge representation features, to permit inference over the content of web pages (prominent initiatives include DAML+OIL, and RDF). Its goals include the production of internet-scale inference mechanisms, knowledge markup languages, and active information-seeking services.. The goal of this paper is to explore how semantic web technology can provide support to the business-to-business e-commerce lifecycle. In particular, we study the early (pre-contractual) phases of the lifecycle. We present a conceptual framework for modelling business-to-business interactions. Within it, we experiment with semantic web technology (in particular DAML+OIL) as a mean to express semantically rich descriptions of services and goods. It is important to note that the environment here presented can be used both for automated interactions, but also to provide structured interactions between humans This paper is structured as follows. In section 2 we introduce our conceptual framework. In section 3 we explore the phases of matchmaking and negotiation in detail, with particular attention to the operations that are carried out on the messages that participants exchange. In section 4, we identify the need for a declarative language for service descriptions, derive requirements for it and show that DAML+OIL satisfies them. We also present a set of example service descriptions used at various stages in the B2B e-commerce interaction lifecycle. In section 5, we specify the operations that are required during the B2B lifecycle, and demonstrate that they can be straightforwardly be implemented on a description logics reasoner (section 6). We then discuss related work (section 7) and we conclude presenting our future work intentions (section 8). In this paper, we have presented an analysis of the B2B e-commerce interaction lifecycle in terms of roles, information constructs and operations necessary to carry out the interactions. We have argued that a variety of protocols can be used for matchmaking and negotiation, but that the same information constructs and operations can be used to support them all. For this reason, we advocate standardization of these constructs and operations, as opposed to standardization on a single protocol. We have assessed the requirements on an appropriate service description language for this, and have argued that DAML+OIL meets these requirements. We have shown how DAML+OIL can be used to represent advertisements, queries, negotiation templates, proposals and agreements. Furthermore, and more importantly, we have shown that the key operations necessary to support B2B interactions can be expressed in terms of satisfiability and subsumption -two operations which existing Description Logics reasoners are capable of executing. Hence, the Semantic Web provides an ideal framework for the standardization of the B2B e-commerce interaction lifecycle.. Research on automation of negotiation requires the ability to assess the likely utility of a given advertisement or negotiation proposal. In our service description language, such proposals and advertisements can be complex structures. Up to now, most work on negotiation has assumed that only one parameter (usually price) is being negotiated. Some work has been carried out on multiattribute negotiation (e.g. [32] ) but this assumes a relatively simple utility model. If we are to be able to assign utilities to complex proposals, then research on tools to help people assess the value of different proposals (preference extraction) will be necessary. It will also be necessary to represent the relative utilities of a space of possible proposals. The application of Multi-Attribute Utility Theory to negotiation [33] is a promising approach to do this. We are currently working on ways of extending this work to assign utilities to complex service descriptions.. In this paper, we have not addressed the operations involved in moving from the matchmaking phase to the negotiation phase. If only one matchmaking query is made, and only one advertisement selected, then this process is straightforward: the negotiation template is taken to be the intersection between the query and the advertisement. However, if many queries are made and many advertisements are matched, then the problem becomes more complex. Clusters of potentially compatible participants must be formed, together with appropriate negotiation templates. We hope to explore this issue in the future. Work on service description for use in matchmaking is an important part of developing open agent-based systems. However, work on developing such description languages [27] [28] [29] [30] has focussed on their application in matchmaking and brokering, ignoring the potential role of negotiation. Matchmaking is not used to locate potential trade partners, but rather to determine the functionality of another agent prior to execution. As a result of this, agents advertise exact specifications of their service (with some small amount of flexibility left to the discretion of the service user). This works for a cooperative community of agents, but will not work for a competitive environment such as in e-commerce. Instead, we treat the advertisement as an invitation to trade, and so it will be less constrained. Additionally, we define appropriate operations to support the negotiation phase, to refine an advertisement to a final agreement, and use the same service description language throughout this process. DAML-S [24] and the Web Service Modeling Framework (WSMF) [31] are general approaches to modelling electronic services. Their scope is not limited to B2B e-commerce, as they define general mechanisms to enrich Web Services with declarative semantic information. It is important to note that DAML-S and WSMF are not incompatible with our approach. In fact, both DAML-S and WSMF are good candidate technologies for implementing our framework. In particular, WSMF provides a mediation architecture that would allow trading partners to interact even when they use heterogeneous ontologies or product catalogs. As mentioned in the previous section, we have used some DAML-S concepts in our implementation. However, these standards are still at the early stages and support tools are not mature or non-existent. We have a prototype system based on the Jade [25] agent platform for the matchmaking and negotiation phases. There are four types of agents in this system: Seeker, Provider, Negotiation Host and Negotiation Participant.. The negotiation rules are processed by the Jess rule-engine [26] . To see some examples of rules in Jess format for the English Auction or the Continuous Double Auction, see [17] .. To implement the operations presented in section 5 for the validation rule, the improvement rule and the agreement formation rule, we are using the Racer [21] reasoner system. Racer supports almost all of the DAML+OIL description logic and is one of the few reasoners to provide some support for concrete domains. Racer currently supports integers and rationals, which covers most of our needs.. We have chosen to implement the improvement rule using the description logic reasoner rather than the rule engine. To this end, we have added a construct which is internal to the negotiation host, the Validation Template. For a negotiation in a given state, the Validation Template is the intersection of the negotiation template with generated class expressions representing improvement rule constraints. Because the state of the negotiation evolves with time (for instance the value of the current highest price), the negotiation host must generate a validation template each time a new proposal is received. By checking that the Validation Template subsumes the incoming proposal, the negotiation host checks both rules at the same time: the validation rule and the improvement rule.. In this implementation, we have chosen to use DAML-S Service Profiles as the basis to represent the e-commerce constructs presented above. DAML-S [24] is a DAML+OIL service description ontology. It aims at facilitating discovery, execution, interoperation, composition and execution monitoring of web services. DAML-S defines the notion of Service Profile (what the service does), Service Model (how the service works) and Service Grounding (how to use the service). In this work, we are only concerned by the fact that a service is represented by input and output properties of the Service Profile. Since DAML-S is in DAML+OIL it is possible to apply property restrictions on the service parameters in a similar way than in the examples presented above. We model the participants with DAML-S Actors and use the more general providedBy and requestedBy properties (instead of seller and buyer).. In terms of performance requirements, the matchmaking and negotiation phases are very different. To find a match for a particular advertisement, the reasoner needs to check the satisfiability of the intersection of the advertisement with each advertisement that has been previously submitted. With the dataset we have (around 100 advertisements and queries), the time spent by the reasoner is barely noticeable. We need to design a way to automatically generate large amounts of meaningful data to put the system under a bigger load and study its behaviour. For negotiation, the number of descriptions to manipulate is function of the number of participants in the negotiation. Compared to matchmaking, the negotiation phase uses few but more complex descriptions (which current reasoners can handle).

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Ensuring Latency Targets in Multiclass Web Servers AbstractÐTwo recent advances have resulted in significant improvements in web server quality-of-service. First, both centralized and distributed web servers can provide isolation among service classes by fairly distributing system resources. Second, session admission control can protect classes from performance degradation due to overload. The goal of this work is to design a general ªfront-endº algorithm that uses these two building blocks to support a new web service model, namely, multiclass services which control response latencies to within prespecified targets. Our key technique is to devise a general service abstraction to adaptively control not only the latency of a particular class, but also to bound the interclass relationships. In this way, we capture the extent to which classes are isolated or share system resources (as determined by the server architecture and system internals) and hence their effects on each other's QoS. For example, if the server provides class isolation (i.e., a minimum fraction of system resources independent of other classes), yet also allows a class to utilize unused resources from other classes, the algorithm infers and exploits this behavior, without an explicit low level model of the server. Thus, as new functionalities are incorporated into web servers, the approach naturally exploits their properties to efficiently satisfy the classes' performance targets. We validate the scheme with trace driven simulations. T HE explosive growth in web traffic [17] has led to excessive latencies due to overloaded web servers. Consequently, reducing and controlling server latencies is a key challenge for delivering end-to-end quality-of-service.. Towards this end, two key mechanisms have been introduced to improve web QoS. First, admission control has been proposed as a mechanism to prevent web servers from entering overload situations [4] , [10] , [16] . Specifically, by admitting new sessions only if the measured load is below a prespecified threshold, admission control can prevent the server from entering a regime in which latencies are excessive, or session throughput collapses due to dropped requests and aborted sessions.. Second, web servers can now provide performance isolation and differentiation among the different service classes hosted by the site. In particular, a server may support a number of service classes which may represent different classes of users or different applications (news, email, static documents, dynamic content, etc.). Whether such classes are supported in a single-node server or a distributed cluster, mechanisms devised in [3] , [6] , [7] and [2] , respectively, can ensure that each service class receives a certain share of system resources (disk, CPU, memory, etc.). Moreover, by appropriately weighting the share of system resources, differentiation among service classes is achieved. Similarly, as delays are also incurred in the system's request queues, prioritization of incoming requests can further differentiate the performance among classes [4] , [12] .. Thus, differentiation and isolation can be achieved by prioritized scheduling of system resources, and protection from overload can be achieved by admission control. However, even if taken together, these two mechanisms cannot ensure that a request's targeted delay will be satisfied. Consequently, because end-to-end latency is a key component of user-perceived quality-of-service, new mechanisms are needed to ensure that the service class' request delays are limited to within the targeted value.. In this paper, we introduce a new framework for multiclass web server control which can satisfy per class latency constraints and devise an algorithm termed Latency-Targeted Multiclass Admission Control (LMAC). Our key technique is to design a scheme within a general framework of request and service envelopes. Such envelopes statistically describe the server's request load and service capacity as a function of interval length, resulting in a high-level service abstraction which circumvents the need to model or measure the components of a request's delay. For example, a request incurs delays in the request queue, CPU processing, memory, disk in the case of cache misses, and so on; individually controlling the latency in each subsystem is a difficult task in a modern server. Instead, we utilize the envelopes as a simple tool for controlling class quality-of-service while maximizing utilization of system resources.. Our approach has three key distinctions. First, it enables web servers to support a strong service model with class latencies bounded to a prespecified target, i.e., a minimum fraction of accepted requests will be serviced within the class delay target.. Second, it provides a mechanism to characterize and control the interclass relationships. For example, suppose server resources are allocated to classes in a weighted-fair manner so that classes have performance isolation, yet a class is able to utilize unused resources of other classes. In general, the extent to which an increased load in one class affects the performance of another class is a complex function of the total system load, the particular resource scheduling algorithm, and the low-level interactions among the server's resources. Building on the results of [19] , we use the envelopes as a way to characterize the high-level isolation/sharing relationships among classes and design a general multiclass algorithm to exploit these effects.. Finally, by decoupling access control and resource allocation from the internals of the server, we obtain a general solution that applies to a broad class of servers, including single-node and distributed servers, and servers with varying levels of quality-of-service support. Consequently, as the server is enhanced with functionalities such as weighted-fair resource allocation, the algorithm naturally exploits these features to better utilize the available resources and support an increased number of sessions per service class.. To evaluate our scheme, we perform a broad set of tracedriven-simulation experiments. We first compare our scheme with an uncontrolled system and illustrate that the algorithm is able to prevent performance degradation due to overload. Next, comparing the delays obtained in simulations with the class QoS objectives, we find that, in many cases, latencies can be controlled to within several percent of the targeted value. Moreover, in the single class case, we compare with a simple queuing theoretic approach, and find that envelopes control the system to a significantly higher degree of accuracy. Finally, in the multiclass case, simulations indicate that substantial interclass resource sharing gains are available. Here, we find that the approach is able to extract these gains and efficiently utilize system resources while satisfying each class' delay targets.. The remainder of this paper is organized as follows: In Section 2, we describe the server architecture and the system abstraction used for QoS management. Next, in Section 3, we describe a simple single-class queuing theoretic approach to serve as a benchmark for performance analysis, and illustration of the key problems in meeting delay targets. In Section 4, we introduce the request and service envelopes and show how they can be computed. Next, we develop an access control algorithm based on the properties of these envelopes in Section 5. We describe the simulation scenario and present experimental results in Sections 6 and 7, respectively. Finally, in Section 8, we conclude. In this paper, we developed a scheme termed Latencytargeted Multiclass Admission Control (LMAC). The algorithm uses measurements of requests and service latencies to control each class' quality-of-service. By abstracting system resources into a high-level virtual server rather than modeling the intricate interactions of low-level system resources, our approach can be applied to off-theshelf servers enhanced with monitoring and admission control. Moreover, as QoS and performance functionalities are added to servers, e.g., class-based request scheduling, operating systems enhanced with quality-of-service mechanisms, or locality aware load balancing, we have shown that the LMAC algorithm exploits these features and realizes a corresponding increase in utilization to various service classes.. In future work, we plan to experiment with additional traces. Further, since dynamic content is becoming a major component in Web server workloads, we plan to extend LMAC to take into account dynamic content. We also plan to investigate the performance of LMAC in presence of persistent connections. We are also investigating the performance of LMAC and similar measurement-based admission control approaches in large-scale Internet Data Centers (IDC) [20] . . For more information on this or any computing topic, please visit our Digital Library at http://computer.org/publications/dlib.

O O O O B B I O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O B O B I O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O B B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O


Combinatorics of geometrically distributed random variables: run statistics Abstract. For words of length n, generated by independent geometric random variables, we consider the mean and variance, and thereafter the distribution of the number of runs of equal letters in the words. In addition, we consider the mean length of a run as well as the length of the longest run over all words of length n. Let X denote a geometrically distributed random variable, i. e. PfX = kg = pq k?1 for k 2 N and q = 1 ? p. The combinatorics of n geometrically distributed independent random variables X 1 ; : : : ; X n has attracted recent interest, especially because of applications in computer science. We mention just two areas, the skip list 1, 15, 17, 11] and probabilistic counting 5, 9, 10, 12].. In 16] the number of left-to-right maxima was investigated for words a 1 : : : a n , where the letters a i are independently generated according to the geometric distribution. In 13] the study of left-to-right maxima was continued, but now the parameters studied were the mean value and mean position of the r-th maximum. In this article we study runs of consecutive equal letters in a string of n geometrically distributed independent random letters. For example in w = 22211114431 we have 5 runs of equal letters of respective lengths 3; 4; 2; 1; 1. In the sequel we denote by R n (w) the number of runs in the word w, where w is of length n. In section 2 we study the mean and variance of R n . Thereafter, in section 3 we study the distribution of the number of runs, which turns out to be Gaussian. Subsequently, in section 4 we study the average length of the runs per word. Finally, in section 5 we determine the mean and variance of the length of the longest run in a word of length n. The This leads to Proposition 1. The mean value of the number of runs for n 1 is given by n = E R n = z n ]G(z) = 2q 1 + q n + p 1 + q :. Now that we see such a simple result, we are tempted to look for a simple proof as well. Indeed, since the expectation is additive, we have n ? 1 times the expectation of having unequal digits at two adjacent random variables, plus 1. This expectation is given by X i6 =j pq i?1 pq j?1 = 2q 1 + q :. Then the expected number of runs is given by (n ? 1) 2q. 1 + q + 1 = 2q 1 + q n + p 1 + q :. If we want to compute the variance, such a simple argument seems to be out of reach.. Henceforth, we di erentiate (2.2) twice, and use the notation V (z) = @ 2 @u 2 F(z; 1).. We see Adding the expectation and subtracting the square of the expectation, we obtain the variance.. Proposition 2. The variance of the number of runs is given for n 2 by 2 n = VR n = 2q(1 ? q) 2 (2 + q 2 ) (1 + q) 2 (1 ? q 3 ) n ? 2q(1 ? q) 2 (3 ? q + q 2 ) (1 + q) 2 (1 ? q 3 ) : It is a folklore result that the limit q ! 1 maps the model of words, generated by geometrically distributed random variables to the model of random permutations. For permutations, there are obviously n runs, and the expected value con rms that, with the variance being exactly zero, as it should.

O O O O O O O O O O O O O O O O O B I I O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B O O O O O O O O O O O O O O O O


Continuous Range Query Processing over Moving Objects In this paper, we propose a continuous range query processing method over moving objects. To efficiently process continuous range queries, we design a main-memory-based query index that uses smaller storage and significantly reduces the query processing time. We show through performance evaluation that the proposed method outperforms the existing methods. To provide high-quality location-based services, one of the most important technical problems is the efficient query processing methods of moving objects [1] , [2] . When objects frequently move, the results of many continuous queries must be changed rapidly. Therefore, such continuous queries are needed for location-based services. Continuous range queries are used to monitor moving objects inside the query ranges [6] . For an example, a taxi cab service company can dispatch a taxi to a customer at a specific location using the result of a continuous range query such as "Find all the taxi cabs currently located within 5 blocks away from a specific location." As another example, for a security service, a continuous range query such as "Find all people who enter a security range without permission." can be used.. A method of continuous range query processing using a query index was first proposed in [6] and many methods have been proposed until recently [3] - [5] . These methods are classified by disk-based methods [5] , [6] and mainmemory-based methods [3] , [4] . We only mention the mainmemory-based method because our method is based on main-memory and the disk-based method is not as efficient as the main-memory-based method even if the disk-based methods are modified for main-memory access [4] . The CES method shows better performance than other existing methods [4] .. The existing methods have performance limitations caused by two characteristics of query indices. First, query identifiers are used to find which queries are affected by the movement of an object. In other words, for the affected queries, it modifies the query results when an object enters into query ranges newly or goes out from query ranges in which the object lay previously. To efficiently process continuous range queries, it is important to quickly and efficiently find which queries are affected by the movement of objects. If the process is conducted with query identifiers, it needs to compare between the set of query identifiers which contain the previous position of an object and the set of query identifiers which contain the new position of an object. The process involves a number of operations to compare objects of one set and objects of another set. In addition, the process is conducted although there are no queries affected by the movement of an object. Second, a query identifier is redundantly stored in each cell contained in the query range to manage the relationship between cells and queries and leads to a waste of storage space. In the memory based method [3] , [4] , the query index uses the grid structure composed of the same-sized cells and the number of redundantly saving query identifiers is increased according to the growth of query range.. To overcome the limitations of the existing methods, we propose a novel query index. Each query has a bit identifier and each cell in a grid has a bit pattern which represents the relationship between cells and queries. Using the bit patterns, we can compute quickly which queries overlap a cell in a grid and reduce the number of unnecessary operations by comparing the bit patterns without comparing the query identifiers for query reevaluation when objects move. In addition, the management of cells in the grid by groups prevents from wasting the storage space through the increase of the length of the bit pattern and increasing the comparison costs of bit patterns. We also propose an efficient continuous range query processing method with the index.. The rest of this paper is organized as follows. Section 2 proposes the novel query index and Sect. 3 shows the performance evaluation. Finally, Sect. 4 presents the conclusions of this paper. We have proposed an efficient continuous range query processing and query indexing method. We used bit identifiers to represent queries and bit patterns to let cells represent the relationship between cells and queries. In addition, the management of cells in the grid by group prevents the waste of storage space through the increase of the length of the bit pattern and increasing the comparison costs of bit patterns.

O O O O O B I O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B I O O O O O O O O O O O O B I O O O O O O B I O O O O O O O O O O O O B I O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O