[petsc-users] Problems with preconditioners, which one?

[Filippo Spiga]

  Dear Jed,

Jed Brown wrote:
> Usually people use SNES for nonlinear problems (there's nothing wrong
> with using it for linear problems, and I often recommend doing so, but
> it's usually used for nonlinear problems).  In that case, there will be
> some difference, usually around sqrt(epsilon) (roughly 1e-8), between
> the analytic Jacobian (computed by you) and the finite differenced
> Jacobian.  If you define the residual in terms of the matrix, then the
> finite difference Jacobian will agree with the analytic one.

  In my case that test is not useful, right?


> As Barry concluded, your matrix just needs pivoting, build PETSc with
> support for one of the third-party direct solvers (e.g. umfpack,
> superlu, and/or mumps) and it should work fine with -pc_type lu
> -pc_factor_mat_solver_package umfpack.  If you want to use a domain
> decomposition method, you will probably need to be careful when defining
> the partition, but then you should be able to do direct subdomain solves
> as above.  You might also consider a different formulation, or ordering
> your unknowns so that the fields with zero on the diagonal come last
> (then the diagonal entries fill up during factorization and dynamic
> pivoting is no longer needed).

I didn't try UMFPACK and probably that external library is not compiled. 
I will do that. But can I use "-pc_type lu,-pc_factor_mat_solver_package 
umfpack" in a parallel execution? Anyway, I will try this solution this 
morning. I will keep you updated if I will have success or not.

Many thanks to all of you!

Regards

-- 

Filippo SPIGA

«Nobody will drive us out of Cantor's paradise.»
      -- David Hilbert