| Abstract: | Abstract In this paper we study the parallel aspects of PCGLS, a basic iterative method based on the conjugate gradient method with preconditioner applied to normal equations and Incomplete Modified Gram-Schmidt (IMGS) preconditioner, for solving sparse least squares problems on massively parallel distributed memory computers. The performance of these methods on this kind of architecture is usually limited because of the global communication required for the inner products. We will describe the parallelization of PCGLS and IMGS preconditioner by two ways of improvement. One is to accumulate the results of a number of inner products collectively and the other is to create situations where communication can be overlapped with computation. A theoretical model of computation and communication phases is presented which allows us to determine the optimal number of processors that minimizes the runtime. Several numerical experiments on the Parsytec GC/PowerPlus are presented. |
