On Tuesday 01 September 2015 06:32:38 you wrote: > > > > > > 1) The KSP view does not say it is shifting. Are you using the latest > > > > > > release? > > > > yes, 3.6. Does PETSc warn for that even if I set the nullspace? I can also > > check MUMPS or something else. > > I am not sure what you think PETSc does here. If shifting were enabled, it > would add some > diagonal matrix to the input matrix and continue the factorization. This > would mean that the > factors were not, in fact, a factorization of the input matrix, and you > would not get the exact > solution in one iterate.
I though PETSc would've replace my pivots with small eps, which is actually not a problem in my case > > > > 2) If it shifted, it would not solve in a single iterate. > > > > even with preonly? > > You would have a large residual. Do you? Actually, I get a perfect solution. > > > > 3) Your GAMG results imply that something is wrong with the coarse > > > solve. > > > > > > This is exactly what would happen if > > > > > > that problem was not solved accurately (its off by > 10 orders of > > > > > > magnitude). > > > > yes, but GAMG builds is own coarse solvers so either the problem is > > already in the definition of A and b (likely) or it is a bug in gamg. > > Yes. GAMG uses the constants to build the basis, on the assumption that > they are in the (near) nullspace of the > operator with no boundary conditions. Since this is far off, I think this > must not be true for your A. > > > > It sounds like your operator is not singular, and its not the Laplacian > > > > > > since it does not look like the Neumann version > > > > > > has constants as a null space. > > > > I'm using periodic boundaries, and constants are in kern(A) > > Did you check? Checked with VecSet and MatMult just in case, I get a machine eps constant vector. > > Matt > > > Thanks a lot for your time. > > > > > Thanks, > > > > > > > > > > > > > >
