Evaluating Parallel Algorithms Theoretical and Practical Aspects

The motivation for the work reported in this thesis has been to lessen the gap between theory and practice within the eld of parallel computing When looking for new and faster parallel algorithms for use in massively parallel systems it is tempting to investigate promising alternatives from the large body of research done on parallel algorithms within the eld of theoretical computer science These algorithms are mainly described for the PRAM Parallel Random Access Machine model of computation This thesis proposes a method for evaluating the practical value of PRAM algorithms The approach is based on implementing PRAM algorithms for execution on a CREW Concurrent Read Exclusive Write PRAM simula tor Measuring and analysis of implemented algorithms on nite problems provide new and more practically oriented results than those traditionally obtained by asymptotical analysis O notation The evaluation method is demonstrated by investigating the practical value of a new and important parallel sorting algorithm from theoretical computer science known as Cole s Parallel Merge Sort algorithm Cole s algorithm is compared with the well known Batcher s bitonic sorting algo rithm Cole s algorithm is asymptotically probably the fastest among all known sorting algorithms and also cost optimal Its O logn time con sumption implies that it is faster than bitonic sorting which is O log n time provided that the number of items to be sorted n is large enough However it is found that bitonic sorting is faster as long as n is less than i e about Giga Tera items Consequently Cole s logarithmic time algorithm is not fast in practice The thesis also gives an overview of complexity theory for sequential and parallel computations and describes a promising alternative for parallel programming called synchronous MIMD programming