[petsc-users] external solvers

[Umut Tabak]

Dear all,

I was trying to use some external solvers interfaced to PETSc however I 
am experiencing some problems in the solution process. First of all, my 
matrices are ill-conditioned and if I do not use 
-pc_factor_shift_nonzero or -pc_factor_shift_positive_definite, I can 
not solve the systems, with these command line options, solution phase 
goes fine(However I could not understand what these options are really 
doing, adding some entries to the diagonal, the operator matrix is 
changed and some kind of other operations are used to back I suppose, 
keeping this in mind, could not the same shifting applied for iterative 
methods as well to make the system better conditioned, excuse me if this 
question is too naive and lacking theoretical background).

If I try to use the same options with some external solvers by the 
command line option, for example mumps, spooles, superlu with 
-pc_factor_mat_solver_package mumps, I can not get the right solution. 
What might be going wrong or ,highly probably, what could be the mistake 
I am doing?

Best regards,

Umut