I am trying to solve a sparse system of linear equations using GMRES (7600 x 7600). I have not done any preconditioning yet. THe procedure of calling the subroutines follows the example for unsymmetric matrices given in the help files.
The problem I am facing is that after a certain number of iterations, the residual between two consecutive steps does not change. Well, it does change but very very slowly. I am talking about 1d-12 or smaller per iteration.
The system of equations solved nicely with PARDISO. As I need to solve a large system later on, I really need to get GMRES to work. Does anyone know any remedy to this problem? Another thing I should mention is that the iterative solver does not stop even when the maximum number of iterations specified has been exceeded. It just kept on iterating. Why is this so? The condition number of the matrix is about 5e6.
One final thing, does the arrangement of the system of equations affect the convergence of the solver? I have tried arranging it two ways and both of them gave the same outcome. Just to point out, I failed also to obtain a solution for the same matrix using the GMRES solver in Matlab.