I would not expect this. Are you sure that direct solvers are actually working well? You can run -ksp_type gmres -pc_type lu to "accelerate" the direct solver. Under normal circumstances the direct solver will converge in one iteration but if, for example, the problem is singular or nearly singular several iterations might solve the linear system but one may not.
Barry On Aug 30, 2013, at 9:08 PM, subramanya sadasiva <[email protected]> wrote: > Hi, > Is there a reason that the convergence of the newton methods in SNES is much > better with iterative solvers instead of with direct solvers? This is > particularly in reference with the VI solvers. I get nearly quadratic > convergence with the iterative solvers, but the code just diverges or > converges linearly when i run with -ksp_type preonly . > Thanks, > Subramanya
