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- Fahl, Gustav
(author)
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Object views of relational data in multidatabase systems
- 1994
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Licentiate thesis (other academic/artistic)abstract
- In a multidatabase system it is possible to access and update data residing in multiple databases. The databases may be distributed, heterogeneous, and autonomous. The first part of the thesis provides an overview of different kinds of multidatabase system architectures and discusses their relative merits. In particular, it presents the AMOS multidatabase system architecture which we have designed with the purpose of combining the advantages and minimizing the disadvantages of the different kinds of proposed architectures.A central problem in multidatabase systems is that of data model heterogeneity: the fact that the participating databases may use different conceptual data models. A common way of dealing with this is to use a canonical data model (CDM). Object-oriented data models, such as the AMOS data model, have all the essential properties which make a data model suitable as the CDM. When a CDM is used, the schemas of the participating databases are mapped to equivalent schemas in the CDM. This means that the data model heterogeneity problem in AMOS is equivalent to the problem of defining an object-oriented view (or object view for short) over each participating database.We have developed such a view mechanism for relational databases. This is the topic of the second part of the thesis. We discuss the relationship between the relational data model and the AMOS data model and show, in detail, how queries to the object view are processed.We discuss the key issues when an object view of a relational database is created, namely: how to provide the concept of object identity in the view; how to represent relational database access in query plans; how to handle the fact that the extension of types in the view depends on the state of the relational database; and how to map relational structures to subtype/supertype hierarchies in the view.A special focus is on query optimization.
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- Fjellborg, Björn
(author)
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An approach to extraction of pipeline structures for VLSI high-level synthesis
- 1990
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Licentiate thesis (other academic/artistic)abstract
- One of the concerns in high-level synthesis is how to efficiently exploit the potential concurrency in a design. Pipelining achieves a high degree of concurrency, and a certain structural regularity through exploitation of locality in communication. However, pipelining cannot be applied to all designs. Pipeline extraction localizes parts of the design that can benefit form pipelining. Such extraction is a first step in pipeline synthesis. While current pipeline synthesis systems are restricted to exploitation of loops, this thesis addresses the problem of extracting pipeline structures from arbitrary designs without apparent pipelining properties. Therefore, an approach that is based on pipelining of individual computations is explored. Still, loops constitute an important special case, and can be encompassed within the approach in an efficient way. The general formulation of the approach cannot be applied directly for extraction purposes, because of a combinatorial explosion of the design space. An iterative search strategy to handle this problem i presented. A specific polynomial-time algorithm based on this strategy, using several additional heuristics to reduce complexity, has been implemented in the PiX system, which operates as a preprocessor to the CAMAD VLSI design system. The input to PiX is an algorithmic description in a Pascal-like language, which is translated into the Extended Timed Petri Net (ETPN) representation. The extraction is realized as analysis of and transformations on the ETPN. Preliminary results from PiX show that the approach is feasible and useful for realistic designs.
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- Ohlsson, Mattias
(author)
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Extensions and explorations of the elastic arms algorithm
- 1993
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In: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 77:1, s. 19-32
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Journal article (peer-reviewed)abstract
- The deformable templates method for track finding in high energy physics is reviewed and extended to handle multiple and secondary vertex positions. An automatized minimization method that handles different types of parametrizations is derived. It is based on the gradient descent method but modified with an explicit calculation of the natural metric. Also a simplified and more intuitive derivation of the algorithm using Potts mean field theory equations is given.
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- Ohlsson, Mattias, et al.
(author)
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Neural Networks for Optimization Problems with Inequality Constraints: The Knapsack Problem
- 1993
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In: Neural Computation. - : MIT Press - Journals. - 0899-7667 .- 1530-888X. ; 5:2, s. 331-339
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Journal article (peer-reviewed)abstract
- A strategy for finding approximate solutions to discrete optimization problems with inequality constraints using mean field neural networks is presented. The constraints x ≤ 0 are encoded by x⊖(x) terms in the energy function. A careful treatment of the mean field approximation for the self-coupling parts of the energy is crucial, and results in an essentially parameter-free algorithm. This methodology is extensively tested on the knapsack problem of size up to 103 items. The algorithm scales like NM for problems with N items and M constraints. Comparisons are made with an exact branch and bound algorithm when this is computationally possible (N ≤ 30). The quality of the neural network solutions consistently lies above 95% of the optimal ones at a significantly lower CPU expense. For the larger problem sizes the algorithm is compared with simulated annealing and a modified linear programming approach. For "nonhomogeneous" problems these produce good solutions, whereas for the more difficult "homogeneous" problems the neural approach is a winner with respect to solution quality and/or CPU time consumption. The approach is of course also applicable to other problems of similar structure, like set covering.
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- Ohlsson, Mattias, et al.
(author)
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Track finding with deformable templates - the elastic arms approach
- 1992
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In: Computer Physics Communications. - : Elsevier BV. - 0010-4655. ; 71:1-2, s. 77-98
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Journal article (peer-reviewed)abstract
- A novel algorithm for particle tracking is presented and evaluated. It is based on deformable templates that converge using a deterministic annealing algorithm. These deformable templates are initialized by Hough transforms. The algorithm, which effectively represents a merger between neuronic decision making and parameter fitting, naturally lends itself to parallel execution. Very good performance is obtained for both non-magnetic and magnetic tracks. For the latter simulated TPC tracks from the CERN DELPHI detector are used.
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