analyze preconditioned operator?

[Matt Funk]

Ok,

thanks, i think i finally got the main idea (the emphasize here being on 'i 
think' which might not mean much ...)

One other question though:
What do i declare ctx to be: PetscObject * ctx ?


Another question related to KSPComputeExplicitOperator:

I was trying to use KSPComputeExplicitOperator and i am having issues with it 
as well. My matrix is a sparse matrix of length ((2*73)^3) with a maximum of 
~30 entries per row.

I believe that in order to use KSPComputeExplicitOperator i need to set up a 
matrix, allocate the memory for it and then pass it to the routine.
>From what i gather on the reference page this matrix needs to be of type 
MatDense? (if so then i cannot use KSPComputeExplicitOperator due to the size 
of the resulting matrix)
However, it said on the website that when multiple procs are used it uses a 
sparse format.
So why cannot i not use a sparse format in serial?


thanks
matt


On Thursday 09 October 2008, Matthew Knepley wrote:
> On Thu, Oct 9, 2008 at 5:58 PM, Matt Funk <mafunk at nmsu.edu> wrote:
> > So then,
> >
> > is it then correct to say that this code "registers" this extra operation
> > (i.e. applying the preconditioner) with the matrix context such that
> > whenever a matrix operation involving this matrix is invoked (like
> > MatMult for example) the pc-applying fcn (i.e. myApply() ) is called
> > first?
>
> The idea here is to replace a given matrix A, which you are passing to
> SLEPc, with another matrix M, which is a shell matrix. When MatMult() is
> called on M, we call MatMult on A, and then PCApply on the result.
>
>   Matt
>
> > matt
> >
> > ps: sorry for being a little slow on the uptake here ...
> >
> > On Thursday 09 October 2008, Matthew Knepley wrote:
> >> On Thu, Oct 9, 2008 at 5:25 PM, Matt Funk <mafunk at nmsu.edu> wrote:
> >> > Hi Matt,
> >> >
> >> > so, the basic idea with this code is to apply the pc to each column
> >> > vector of the matrix? Is that right?
> >>
> >> No, that is what KSPGetExplicitOperator() does. This applies the
> >> operator in a matrix-free way.
> >>
> >> > Also, in your example: when is myApply actually invoked? I also looked
> >> > at the example listed under the MatShellSetOperation reference page.
> >>
> >> When MatMult(A) is called.
> >>
> >>   Matt
> >>
> >> > Is the function then actually internally called when
> >> > MatShellSetOperation is called, or when KSPSetOperators is called or
> >> > KSPSolve?
> >> >
> >> > The reason i am asking is that if it is called when KSPSolve called
> >> > then there is a problem because for the analysis i never call KSPSolve
> >> > directly.
> >> >
> >> > thanks
> >> > matt
> >> >
> >> > On Thursday 09 October 2008, Matthew Knepley wrote:
> >> >> On Thu, Oct 9, 2008 at 4:32 PM, Matt Funk <mafunk at nmsu.edu> wrote:
> >> >> > mmhh,
> >> >> >
> >> >> > i think i am missing something. Doesn't PCApply() apply the
> >> >> > preconditioner to a vector? So how would that work (easily) with a
> >> >> > matrix?
> >> >>
> >> >> You do not apply it to the matrix. Here is a skeleton (maybe has
> >> >> mistakes)
> >> >>
> >> >> void myApply(Mat A, Vec x, Vec y) {
> >> >>   MatShellGetContext(A, &ctx);
> >> >>   MatMult(ctx->M, x, ctx->work);
> >> >>   PCApply(ctx->pc, ctx->work, y);
> >> >> }
> >> >>
> >> >> MatShellSetOperation(A, MATOP_MULT, myApply)
> >> >>
> >> >>   Matt
> >> >>
> >> >> > matt
> >> >> >
> >> >> > On Thursday 09 October 2008, Matthew Knepley wrote:
> >> >> >> CApply().