[petsc-users] Handling singular matrix in KSP and ASM

[Matthew Knepley]

On Wed, May 27, 2015 at 10:52 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   The built in PETSc LU factorizations have special options to attempt to
> "shift" a singular matrix (by adding something to a zero pivot). These
> methods are ad hoc but sometimes perform satisfactory inside a
> preconditioner. See the manual page for PCLU in particular
> -pc_factor_shift_type <shifttype> - Sets shift type or PETSC_DECIDE for the
> default; use '-help' for a list of available types  See also
> http://www.mcs.anl.gov/petsc/documentation/faq.html#zeropivot


The other option is to use -pc_type svd for the coarse grid.

  Matt


>
>   Barry
>
>
> > On May 27, 2015, at 10:40 PM, Amneet Bhalla <mail2amneet at gmail.com>
> wrote:
> >
> > Hi Folks,
> >
> > We are dealing with a singular matrix that arises from Stokes' equation
> with Dirichlet velocity BCs. The way we are handling it is to pass
> nullspace info to KSP, i.e, vector of {u,p} = {0,1}, and letting the matrix
> to be singular. However, we are using ASM as the preconditioner and when
> the subpc covers the entire domain (which can be the case in the
> preconditioning step of our MG algorithm), LU decomposition fails. We are
> thinking of pinning one of the pressure DOFs to zero explicitly, so that we
> can avoid singular matrix for the LU decomposition. Is there another method
> of doing it, which avoids explicit pinning down of an arbitrary pressure
> DOF?
> >
> > Thanks,
> > --Amneet
> >
> > --
> > --Amneet
> >
> >
> >
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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