[petsc-users] 2D Poisson on nonuniform meshes

[Filippo Leonardi]

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,
> > > 
> > > 
> > > > > > > > 
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