KSP different results with the default and direct LU

[Umut Tabak]

Dear all,

I am trying get myself more acquainted with PETSc.

I am trying to solve a linear equation system which is conditioned 
badly. If I use the approach as given in ex1.c of the KSP section(by 
default, I read from the manual that it uses restarted GMRES , 
preconditioned with JACOBI in ex1.c which I am not very aware of these 
numerical procedures that in deep). I get some wrong results for the 
solution.

I checked my results with Matlab, If I use the command line options as 
suggested on page 68 of the manual, to solve it directly, -ksp_type 
preonly -pc_type lu, I can both get the right solution and it is faster 
than the iterative solution which is used by default. So as far as I can 
follow from the mailing list, iterative methods and preconditioners are 
problem dependent, so my question would be, should I find the right 
approach/solver combination by some trial and error by using different 
combinations that are outlined in the manual?(This can also be dependent 
on the problem types that I would like to solve as well I suppose, for 
the moment, quite badly conditioned.)

Any pointers are appreciated.

Best regards,
Umut