Parallelization of the maximum likelihood approach to phylogenetic inference

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Title: Parallelization of the maximum likelihood approach to phylogenetic inference
Author: Garnham, Janine
Abstract: Phylogenetic inference refers to the reconstruction of evolutionary relationships among various species, usually presented in the form of a tree. DNA sequences are most often used to determine these relationships. The results of phylogenetic inference have many important applications, including protein function determination, drug discovery, disease tracking and forensics. There are several popular computational methods used for phylogenetic inference, among them distance-based (i.e. neighbor joining), maximum parsimony, maximum likelihood, and Bayesian methods. This thesis focuses on the maximum likelihood method, which is regarded as one of the most accurate methods, with its computational demand being the main hindrance to its widespread use. Maximum likelihood is generally considered to be a heuristic method providing a statistical evaluation of the results, where potential tree topologies are judged by how well they predict the observed sequences. While there have been several previous efforts to parallelize the maximum likelihood method, sequential implementations are more widely used in the biological research community. This is due to a lack of confidence in the results produced by the more recent, parallel programs. However, because phylogenetic inference can be extremely computationally intensive, with the number of possible tree topologies growing exponentially with the number of species, parallelization is necessary to reduce the computation time to a reasonable amount. A parallel program was developed for phylogenetic inference based on the trusted algorithms of fastDNAml, a sequential program for phylogenetic inference utilizing the maximum likelihood approach. Parallelization is achieved using the popular master/workers scheme, where workers evaluate potential tree topologies in parallel. Three innovative optimizations are employed to alleviate the associated communication bottleneck encountered when using the master/workers technique with large-scale systems and problems. First, message packing reduces the number of messages sent out by the master, along with the associated overheads. Secondly, allowing workers to keep the best trees evaluated reduces the number of messages received by the master, as low-scoring results are discarded by the workers. Finally, multiple masters are utilized to parallelize the responsibilities of what is traditionally a single master process. These last two optimizations led to a dramatic improvement in performance over the unoptimized parallelization under the conditions tested. Message packing, however, demonstrated a slight reduction in performance. Although testing with large-scale systems and problems was not possible, results for all three optimizations suggested likely performance enhancement under such conditions, potentially leading to relief of the bottleneck.
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Date: 2007-08

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