further about PCComputeExplicitOperator()

[Yujie]

Dear Barry:

 "Note also:
PCComputeExplicitOperator - Computes the explicit preconditioned operator.
this means it computes B*A or A*B (depending on left or right
preconditioning). This beasty
(unless you use Jacobi preconditioning) is always dense and it makes no
sense to store in
sparse format except for fun."

Regarding the above comments, I have checked the array of the explicit
preconditioner obtained in PCComputeExplicitOperator(). It is sparse. Why
say it is dense?
thanks a lot.

Regards,
Yujie

On Fri, May 2, 2008 at 9:50 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>  Application of a preconditioner is almost always a dense operator. The
> only exception is sparse
> approximate inverse and in fact this is why the sparse approximate inverse
> is a lousy preconditioner.
> So I don't think you would ever want to compute the preconditioner into a
> sparse matrix.
>
>  If you want the SPAI preconditioner explicitly as a PETSc sparse matrix
> you should go into that
> code, figure out how the SPAI stores its computed preconditioner and
> convert it to the PETSc
> format.
>
>  Note also:
> PCComputeExplicitOperator - Computes the explicit preconditioned operator.
> this means it computes B*A or A*B (depending on left or right
> preconditioning). This beasty
> (unless you use Jacobi preconditioning) is always dense and it makes no
> sense to store in
> sparse format except for fun.
>
>   Barry
>
>
>
> On May 1, 2008, at 8:08 PM, Yujie wrote:
>
> when 1 processor is used, the matrix M in PCComputeExplicitOperator(pc,&M)
> > uses MATSEQDENSE type. Now, I want to use MATSEQAIJ, I change the codes as
> > follows:
> > 1563 if (size == 1) {
> > 1564 //05/01/08
> > 1565 //ierr = MatSetType(*mat,MATSEQDENSE);CHKERRQ(ierr);
> > 1566 //ierr =
> > MatSeqDenseSetPreallocation(*mat,PETSC_NULL);CHKERRQ(ierr);
> > 1567 ierr = MatSetType(*mat,MATSEQAIJ);CHKERRQ(ierr);
> > 1568 ierr = MatSeqAIJSetPreallocation(*mat,0,PETSC_NULL);CHKERRQ(ierr);
> > 1569
> > 1570 } else {
> > 1571 ierr = MatSetType(*mat,MATMPIAIJ);CHKERRQ(ierr);
> > 1572 ierr =
> > MatMPIAIJSetPreallocation(*mat,0,PETSC_NULL,0,PETSC_NULL);CHKERRQ(ierr);
> > 1573 }
> >
> > PCApply is fast when running. However,  MatSetValues() is very very slow
> > when some arraies need to set. I find that the problem likely lies in
> > MatSeqXAIJReallocateAIJ(A,A->rmap.n,1,nrow,row,col,rmax,aa,ai,aj,rp,ap,imax,nonew,MatScalar)
> > in MatSetValues_SeqAIJ() after debugging the codes.
> > I can't further figure out where is the problem. Because it is difficult
> > to debug. Could you give me some advice? thanks a lot.
> >
> > the version of PETSc is 2.3.3-p8.
> >
> > thanks a lot.
> >
> > Regards,
> > Yujie
> >
> >
> >
> >
> >
>
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