[petsc-users] MatSetNullSpace results in nan with MUMPS

[Bhargav Subramanya]

Dear Matt,

Thank you for the reply.

On Sun, Apr 3, 2022 at 4:00 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Apr 2, 2022 at 8:59 PM Bhargav Subramanya <
> bhargav.subramanya at kaust.edu.sa> wrote:
>
>> Dear All,
>>
>> I am trying to solve Ax = b in parallel using MUMPS, where x is composed
>> of velocity, pressure, and temperature. There is a null space due to the
>> homogeneous Neumann pressure boundary conditions.
>>
>> 1. Without explicitly removing the null space, the solution is bounded
>> and robust, and I am able to iterate in time, although I need to check if
>> the solution is physically correct. I am not sure if MUMPS is able to
>> detect the null space in this case.
>>
>
> MUMPS should fail with a 0 pivot. I do not understand this.
>
>
>> 2. However, when I explicitly use MatSetNullSpace as shown below, the
>> solution blows up in 2-3 temporal iterations, resulting in nan. Could you
>> please share your thoughts regarding this problem?
>>
>
> Did you check that this is a nullspace vector using
> https://petsc.org/main/docs/manualpages/Mat/MatNullSpaceTest.html
> <https://urldefense.com/v3/__https://petsc.org/main/docs/manualpages/Mat/MatNullSpaceTest.html__;!!Nmw4Hv0!xGtCbXmv5pFV2xcOa8sYXVsvJ5ZBq-DjqjcYkhdAhaqrNUkfz8gR5oPpDoHp0UmKNtnpRIvkN2M3iq_wt9Ep9wMmVJA$>
> ?
>

--My apologies. I had a bug with the null vector assembly. I fixed it and
confirmed it with MatNullSpaceTest. My solution doesn't blow up now. Thanks
a lot.

>
>   Thanks,
>
>      Matt
>
>
>> Thanks,
>> Bhargav
>>
>> ---------------
>> Output for -pc_type svd -pc_svd_monitor
>> SVD: condition number 3.159257564649e+16, 1 of 1080 singular values are
>> (nearly) zero
>>     SVD: smallest singular values: 3.693890048955e-17 2.902795363876e-10
>> 2.902795486255e-10 2.902796682780e-10 7.614221123356e-10
>>     SVD: largest singular values : 1.166981958972e+00 1.166981958972e+00
>> 1.166995008014e+00 1.166995008014e+00 1.166995008014e+00
>> -----------------
>>
>> PetscErrorCode *SetNullVector*(Mat MatGlobalLinear,Mesh mesh,Equations
>> eq){
>>
>> Vec NullVector;
>>
>> PetscInt numNullVectors,disp,i,j;
>>
>> PetscScalar constantValue = 1.0,nrm;
>>
>> MatNullSpace NullSp = NULL;
>>
>> PetscErrorCode ierr;
>>
>>
>> PetscFunctionBeginUser;
>>
>> numNullVectors = 1;
>>
>> disp = mesh.numLayers*mesh.numEdges + (eq.pressure-1)*mesh.numCells;
>>
>> ierr = *MatCreateVecs*(MatGlobalLinear,&NullVector,NULL);CHKERRQ(ierr);
>>
>> *for*(i=0; i<mesh.numLayers; i++){
>>
>> *for*(j=0; j<mesh.numCells; j++){
>>
>> ierr = VecSetValue(NullVector,j + i*mesh.numCells + disp,constantValue,
>> *INSERT_VALUES*);CHKERRQ(ierr);
>>
>> }
>>
>> }
>>
>> ierr = *VecAssemblyBegin*(NullVector);CHKERRQ(ierr);
>>
>> ierr = *VecAssemblyEnd*(NullVector);CHKERRQ(ierr);
>>
>> ierr = *VecNorm*(NullVector,*NORM_2*,&nrm);CHKERRQ(ierr);
>>
>> ierr = *VecScale*(NullVector,1.0/nrm);CHKERRQ(ierr);
>>
>> /* set null space */
>>
>> ierr = *MatNullSpaceCreate*(MPI_COMM_WORLD,*PETSC_FALSE*
>> ,numNullVectors,&NullVector,&NullSp);CHKERRQ(ierr);
>>
>> ierr = *MatSetNullSpace*(MatGlobalLinear,NullSp);CHKERRQ(ierr);
>>
>> /* clean up */
>>
>> ierr = *MatNullSpaceDestroy*(&NullSp);CHKERRQ(ierr);
>>
>> ierr = *VecDestroy*(&NullVector);CHKERRQ(ierr);
>>
>> PetscFunctionReturn(0);
>>
>> }
>> -----------------
>>
>>
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>
>
>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <https://urldefense.com/v3/__http://www.cse.buffalo.edu/*knepley/__;fg!!Nmw4Hv0!xGtCbXmv5pFV2xcOa8sYXVsvJ5ZBq-DjqjcYkhdAhaqrNUkfz8gR5oPpDoHp0UmKNtnpRIvkN2M3iq_wt9EpP92ZqUc$>
>

-- 

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