[petsc-users] Speed of MatGetRowSum

[Derek Gaston]

Thanks for your response!  I suppose I should have just checked the code
out myself :-)

Makes sense - I won't worry about it then unless it shows up in profiling
(which I doubt).  I just wanted to make sure it wasn't an N^2 operation
(which it's not since my matrices are very sparse).

Thanks again,
Derek

On Fri, Apr 27, 2018 at 12:00 PM Smith, Barry F. <bsmith at mcs.anl.gov> wrote:

>
>   Here is the current code: ;-)
>
> PetscErrorCode MatGetRowSum(Mat mat, Vec v)
> {
>   Vec            ones;
>   PetscErrorCode ierr;
>
>   PetscFunctionBegin;
>   PetscValidHeaderSpecific(mat,MAT_CLASSID,1);
>   PetscValidType(mat,1);
>   PetscValidHeaderSpecific(v,VEC_CLASSID,2);
>   if (!mat->assembled)
> SETERRQ(PetscObjectComm((PetscObject)mat),PETSC_ERR_ARG_WRONGSTATE,"Not for
> unassembled matrix");
>   MatCheckPreallocated(mat,1);
>   ierr = MatCreateVecs(mat,&ones,NULL);CHKERRQ(ierr);
>   ierr = VecSet(ones,1.);CHKERRQ(ierr);
>   ierr = MatMult(mat,ones,v);CHKERRQ(ierr);
>   ierr = VecDestroy(&ones);CHKERRQ(ierr);
>   PetscFunctionReturn(0);
> }
>
> An optimized for AIJ one would avoid creating the matrix and avoid the
> multiples by 1 in the summation and avoid the communication.
>
> I wouldn't worry about its performance; it won't be the bottle neck
> generally. Writing a custom routine for AIJ (seq and MPI) would be easy,
> just structure it like the MatMult_xxx() routine.
>
>    Barry
>
>
> > On Apr 27, 2018, at 11:58 AM, Derek Gaston <friedmud at gmail.com> wrote:
> >
> > I was wondering about the comment on MatGetRowSum that says:
> >
> > "This code is slow since it is not currently specialized for different
> formats"
> >
> > How slow are we talking about here?  Is it actually going to loop over
> i,j from 0 to N on a sparse matrix?
> >
> > I need to get a vector that represents the "lumped" mass matrix: but I
> would like to get it fairly quickly.  I would just compute the lumped
> matrix directly and then use MatGetDiag - but I actually want both
> (unlumped/consistent and lumped) and MatGetRowSum seemed like the best
> direction.  Willing to take advice though!
> >
> > Derek
>
>
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