[petsc-users] PCFIELDSPLIT question

[Matthew Knepley]

On Tue, Feb 2, 2016 at 2:20 PM, Hom Nath Gharti <hng.email at gmail.com> wrote:

> Hi Matt, hi all,
>
> I am trying to use PCFIELDSPLIT for my solver which consists of 4
> different variables, namely, u (displacement vector), \chi
> (displacement potential), p(pressure), and \phi (gravity potential).
>
> My code segment looks like the following:
> ======================================================
> call KSPCreate(PETSC_COMM_WORLD,ksp,ierr)
> call KSPGetPC(ksp,pc,ierr)
> nsplit=4
> call PCFieldSplitSetBlockSize(pc,nsplit,ierr);
>

You do not need the statement above.


> call PCFieldSplitSetIS(pc,"0",gdofu_is,ierr);
> call ISDestroy(gdofu_is,ierr)
> call PCFieldSplitSetIS(pc,"1",gdofchi_is,ierr);
> call ISDestroy(gdofchi_is,ierr)
> call PCFieldSplitSetIS(pc,"2",gdofp_is,ierr);
> call ISDestroy(gdofp_is,ierr)
> call PCFieldSplitSetIS(pc,"3",gdofphi_is,ierr);
> call ISDestroy(gdofphi_is,ierr)
>
> call PCFieldSplitGetSubKSP(pc,nsplit,subksp,ierr);
>

These SetOperators() calls are wrong. I am not sure why you want them here.
Also, that means you do not need the call above.

  Thanks,

     Matt


> call KSPSetOperators(subksp(1),Amat,Amat,ierr);
> call KSPSetOperators(subksp(2),Amat,Amat,ierr);
> call KSPSetOperators(subksp(3),Amat,Amat,ierr);
> call KSPSetOperators(subksp(4),Amat,Amat,ierr);
>
> call KSPSolve(ksp,bvec,xvec,ierr)
> ======================================================
>
> But I am getting the following error:
> [79]PETSC ERROR: Null argument, when expecting valid pointer
> [79]PETSC ERROR: Null Object: Parameter # 1
> [79]PETSC ERROR: #1 KSPSetOperators() line 536 in
> /home/hgharti/lsoft/petsc-3.6.3-intel16-mpi5/src/ksp/ksp/interf
>
> It looks like I am doing something wrong in "call KSPSetOperators" but
> I could not figure that out.
>
> Could anybody help me to fix this problem? I looked into almost all
> related examples but I could not really figure out the correct steps
> after "call PCFieldSplitSetIS".
>
> Any help will be greatly appreciated.
>
> Best,
> Hom nath
>
> On Sun, Jan 24, 2016 at 7:14 PM, Hom Nath Gharti <hng.email at gmail.com>
> wrote:
> > Thank you so much Matt! I will try.
> >
> > Hom Nath
> >
> > On Sun, Jan 24, 2016 at 6:26 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
> >> On Fri, Jan 22, 2016 at 2:19 PM, Hom Nath Gharti <hng.email at gmail.com>
> >> wrote:
> >>>
> >>> Dear all,
> >>>
> >>> I am new to PcFieldSplit.
> >>>
> >>> I have a matrix formed using MATMPIAIJ. Is it possible to use
> >>> PCFIELDSPLIT operations in this type of matrix? Or does it have to be
> >>> MATMPIBIJ or MATNEST format?
> >>
> >>
> >> Yes, you can split AIJ.
> >>
> >>>
> >>> If possible for MATMPIAIJ, could anybody provide me a simple example
> >>> or few steps? Variables in the equations are displacement vector,
> >>> scalar potential and pressure.
> >>
> >>
> >> If you do not have a collocated discretization, then you have to use
> >>
> >>
> >>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCFieldSplitSetIS.html
> >>
> >>   Thanks,
> >>
> >>      Matt
> >>
> >>>
> >>> Thanks for help.
> >>>
> >>> Hom Nath
> >>
> >>
> >>
> >>
> >> --
> >> 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
>



-- 
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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