> On 28 Jan 2021, at 17:54, Matthew Knepley <knepley@gmail.com> wrote:
>
> On Thu, Jan 28, 2021 at 12:04 PM Artur Palha <artur.palha@gmail.com> wrote:
> Hello Everyone,
>
> I am trying to solve a (linear) system of equations I am constructing as a result of a specific discretisation of a PDE. As I try to solve with a direct solver (MUMPS) I notice that in some circumstances (mesh size and polynomial degree) I get a solve error:
>
> ConvergenceError: Nonlinear solve failed to converge after 0 nonlinear iterations.
> Reason:
>    DIVERGED_LINEAR_SOLVE
>
> After much digging around I managed to (I think) reproduce the error on the mixed Poisson example. For that I simply changed the solver parameters. I add below the full code with the two options of solver parameters (if broken_parameters is set to True then I get the error, if set to False it works fine). The difference is that the broken one uses a direct solver (MUMPS) and the working one uses an iterative solver (gmres). Note that I changed the domain into a periodic domain and I solve for a purely Neumann problem, hence I add the nullspace in the solve step.
>
> The nullspace you set is only used in the iterative solver, since it projects out the nullspace from the approximation. MUMPS tries to factorize your rank-deficient
> matrix and fails.
>

There are some magic `mat_mumps_icntl` flags you can set to ask mumps to try and find the nullspace. Run with `-help` on the commandline to see what they are.

Lawrence