Flexible Enlarged Conjugate Gradient Methods

Enlarged Krylov subspace methods and their s-step versions were introduced [7] in the aim of reducing communication when solving systems of linear equations Ax = b. These enlarged CG methods consist of enlarging the Krylov subspace by a maximum of t vectors per iteration based on the domain decomposition of the graph of A. As for the s-step versions, s iterations of the enlarged Conjugate Gradient methods are merged in one iteration. The Enlarged CG methods and their s-step versions converge in less iterations than the classical CG, but at the expense of requiring more memory storage than CG. Thus in this paper we explore different options for reducing the memory requirements of these enlarged CG methods without affecting much their convergence.