> There is a way to specify a DMlibMesh field decomposition in terms of
> libMesh variables per split. It's a PETSc runtime option and a bit clunky
> (there is a similar hookup to define (G)ASM splits).
>
> I am testing a patch that improves the option syntax and adds a DMlibMesh
> API for setting the splits programmatically. This would require casting the
> solver to a PetscDMNonlinearSolver and pulling the DM out, though.
>
> It would be easy to add a "native" libMesh API at the NonlinearSolver
> level: since the splits are consumed by the solver implementation (PETSc)
> as needed, there would be no change to the Trilinos impl needed. This would
> allow one, among other things, to define splits in a higher-level package
> (e.g., Moose input file), which is the intended use.
>
> Serving up the (near)nullspaces is even easier and, again, can be consumed
> by the solver impl as needed.
> I want to test it on basic elasticity problems first.
> Both patches should be in by the end of the week or sooner, once I'm back
> from travel.
>
> Dmitry.
>
> On Monday, September 17, 2012, Jed Brown wrote:
>
>> On Mon, Sep 17, 2012 at 5:54 PM, Roy Stogner <[email protected]
>> >wrote:
>>
>> > Maybe having a
>> > non-positive-definite matrix is the trouble, irrespective of whether
>> > the matrix is actually indefinite or not?
>> >
>>
>> Stokes *is* indefinite: it has lots of negative eigenvalues due to
>> pressure
>> being a Lagrange multiplier.
>>
>> The one-dimensional null space is not the problem. (We recommend setting
>> it
>> using MatSetNullSpace(), then just ensure that your right hand sides are
>> consistent and everything should work fine.) The indefiniteness/saddle
>> point is the problem.
>>
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