[petsc-users] Symmetric matrix: Setting entries below diagonal

[Matthew Knepley]

On Fri, Apr 7, 2017 at 11:23 AM, Barletta, Ivano <ibarletta at inogs.it> wrote:

> So, as far as I understand, the only benefit of PETSc with symmetric
> matrices
> is only when Matrix values are set, by reducing the overhead of
> MatSetValue calls?
>

It halves the storage. There is a slight advantage from not having to load
the lower triangle.

   Matt


> Thanks,
> Ivano
>
> 2017-04-07 17:19 GMT+02:00 Barry Smith <bsmith at mcs.anl.gov>:
>
>>
>> > On Apr 7, 2017, at 6:40 AM, Florian Lindner <mailinglists at xgm.de>
>> wrote:
>> >
>> > Hello,
>> >
>> > two questions about symmetric (MATSBAIJ) matrices.
>> >
>> > + Entries set with MatSetValue below the main diagonal are ignored. Is
>> that by design?
>>
>>    Yes
>>
>> > I rather expected setting A_ij to
>> > have the same effect as setting A_ji.
>>
>>    You need to check the relationship between i and j and swap them if
>> needed before the call.
>>
>> >
>> > + Has MatSetOption to MAT_SYMMETRIC and MAT_SYMMETRIC_ETERNAL any gain
>> on MATSBAIJ matrices?
>>
>>   No
>>
>> >
>> > Thanks,
>> > Florian
>> >
>> > Test programm:
>> >
>> >
>> > #include "petscmat.h"
>> > #include "petscviewer.h"
>> >
>> > int main(int argc, char **argv)
>> > {
>> >  PetscInitialize(&argc, &argv, "", NULL);
>> >  PetscErrorCode ierr = 0;
>> >
>> >  Mat A;
>> >  ierr = MatCreate(PETSC_COMM_WORLD, &A); CHKERRQ(ierr);
>> >  MatSetType(A, MATSBAIJ); CHKERRQ(ierr);
>> >  ierr = MatSetSizes(A, 4, 4, PETSC_DECIDE, PETSC_DECIDE); CHKERRQ(ierr);
>> >  ierr = MatSetUp(A); CHKERRQ(ierr);
>> >  ierr = MatSetOption(A, MAT_SYMMETRIC, PETSC_TRUE); CHKERRQ(ierr);
>> >  ierr = MatSetOption(A, MAT_SYMMETRY_ETERNAL, PETSC_TRUE);
>> CHKERRQ(ierr);
>> >
>> >  // Stored
>> >  ierr = MatSetValue(A, 1, 2, 21, INSERT_VALUES); CHKERRQ(ierr);
>> >  ierr = MatSetValue(A, 1, 1, 11, INSERT_VALUES); CHKERRQ(ierr);
>> >
>> >  // Ignored
>> >  ierr = MatSetValue(A, 2, 1, 22, INSERT_VALUES); CHKERRQ(ierr);
>> >  ierr = MatSetValue(A, 3, 2, 32, INSERT_VALUES); CHKERRQ(ierr);
>> >  ierr = MatSetValue(A, 3, 1, 31, INSERT_VALUES); CHKERRQ(ierr);
>> >
>> >  ierr = MatAssemblyBegin(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
>> >  ierr = MatAssemblyEnd(A, MAT_FINAL_ASSEMBLY); CHKERRQ(ierr);
>> >
>> >  PetscViewer viewer;
>> >  ierr = PetscViewerCreate(PETSC_COMM_WORLD, &viewer); CHKERRQ(ierr);
>> >  ierr = PetscViewerSetType(viewer, PETSCVIEWERASCII); CHKERRQ(ierr);
>> >  ierr = PetscViewerPushFormat(viewer, PETSC_VIEWER_ASCII_DENSE);
>> CHKERRQ(ierr);
>> >  ierr = MatView(A, viewer); CHKERRQ(ierr);
>> >  ierr = PetscViewerPopFormat(viewer); CHKERRQ(ierr);
>> >  ierr = PetscViewerDestroy(&viewer); CHKERRQ(ierr);
>> >
>> >  PetscFinalize();
>> >  return 0;
>> > }
>>
>>
>


-- 
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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20170407/6a2169b2/attachment.html>