Browsing by Author "Jahnsson, Niklas"
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Jahnsson, Niklas (2015)Bayesian networks are graphical models used to represent the joint probability distribution for all variables in a data set. A Bayesian network can be constructed by an expert, but learning the network from the data is another option. This thesis is centered around exact scorebased Bayesian network structure learning. The idea is to optimize a scoring function measuring the fit of the network to the data, and the solution represents an optimal network structure. The thesis adds to the earlier literature by extending an external memory frontier breadthfirst branch and bound algorithm by Malone et al. [MYHB11], which searches the space of candidate networks using dynamic programming in a layered fashion. To detect duplicates during the candidate solution generation, the algorithm uses efficiently both semiconductor and magnetic disk memory. Inmemory duplicate detection is performed using a hash table, while a delayed duplicate detection strategy is employed when resorting to the disk. Delayed duplicate detection is designed to work well against long disk latencies, because hash tables are currently still infeasible on disk. Delayed duplicate detection allows the algorithm to scale beyond search spaces of candidate solutions fitting only in the comparatively expensive and limited semiconductor memory at disposal. The sortingbased delayed duplicate detection strategy employed by the original algorithm has been found to be inferior to a hashingbased delayed duplicate strategy in other application domains [Kor08]. This thesis presents an approach to use hashingbased delayed duplicate detection in Bayesian network structure learning and compares it to the sortingbased method. The problem faced in the hashing of candidate solutions to disk is dividing the candidate solutions in a certain stage of the search in an efficient and scalable manner to files on the disk. The division presented in this thesis distributes the candidate solutions into files unevenly, but takes into account the maximum number of candidate solutions that can be held in the semiconductor memory. The method works in theory as long as operating system has free space and inodes to allocate for new files. Although the hashingbased method should in principle be faster than the sortingbased, the benchmarks presented in this thesis for the two methods show mixed results. However, the analysis presented also highlights the fact that by improving the efficiency of, for example, the distribution of hashed candidate solutions to different files, the hashingbased method could be further improved.
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