> On Feb 18, 2020, at 8:26 AM, Yuyun Yang <[email protected]> wrote:
>
> Thanks. Also, when using KSP, would the syntax be KSPSetOperators(ksp,A,A)?
> Since you mentioned preconditioners are not generally used for matrix-free
> operators, I wasn’t sure whether I should still put “A” in the Pmat field.
>
> Is it still possible to use TS in conjunction with the matrix-free operator?
> I’d like to create a simple test case that solves the 1d heat equation
> implicitly with variable coefficients, but didn’t know how the time stepping
> can be set up.
On Feb 15, 2020, at 9:42 PM you asked about "(for example, when doing
backward Euler)"
on Saturday, February 15, 2020 10:02 PM I suggested you start with the
example src/ts/examples/tests/ex22.c I outlined how you could change it to be
matrix free. The example clearly uses TS
Now three days later you are asking about how time-stepping can be set up
with a matrix-free operator? If you are going to ignore answers we provide to
your questions maybe we won't bother answering in the future.
>
> Thanks,
> Yuyun
>
> From: Matthew Knepley <[email protected]>
> Date: Tuesday, February 18, 2020 at 9:23 PM
> To: Yuyun Yang <[email protected]>
> Cc: "Smith, Barry F." <[email protected]>, "[email protected]"
> <[email protected]>
> Subject: Re: [petsc-users] Matrix-free method in PETSc
>
> On Tue, Feb 18, 2020 at 8:20 AM Yuyun Yang <[email protected]> wrote:
> Thanks for the clarification.
>
> Got one more question: if I have variable coefficients, my stencil will be
> updated at every time step, so will the coefficients in myMatMult. In that
> case, is it necessary to destroy the shell matrix and create it all over
> again, or can I use it as it is, only calling the stencil update function,
> assuming the result will be passed into the matrix operation automatically?
>
> You update the information in the context associated with the shell matrix.
> No need to destroy it.
>
> Thanks,
>
> Matt
>
> Thanks,
> Yuyun
>
> On 2/18/20, 7:34 AM, "Smith, Barry F." <[email protected]> wrote:
>
>
>
> > On Feb 17, 2020, at 7:56 AM, Yuyun Yang <[email protected]> wrote:
> >
> > Hello,
> >
> > I actually have a question about the usage of DMDA since I'm quite new
> to this. I wonder if the DMDA suite of functions can be directly called on
> vectors created from VecCreate?
>
> Yes, but you have to make sure the ones you create have the same sizes
> and parallel layouts. Generally best to get them from the DMDA or
> VecDuplicate() than the hassle of figuring out sizes.
>
> > Or the vectors have to be formed by DMDACreateGlobalVector? I'm also
> not sure about what the dof and stencil width arguments do.
> >
> > I'm still unsure about the usage of MatCreateShell and
> MatShellSetOperation, since it seems that MyMatMult should still have 3
> inputs just like MatMult (the matrix and two vectors). Since I'm not forming
> the matrix, does that mean the matrix input is meaningless but still needs to
> exist for the sake of this format?
>
> Well the matrix input is your shell matrix so it likely has
> information you need to do your multiply routine. MatShellGetContext() (No
> you do not want to put your information about the matrix stencil inside
> global variables!)
>
>
> >
> > After I create such a shell matrix, can I use it like a regular matrix
> in KSP and utilize preconditioners?
> >
> > Thanks!
> > Yuyun
> > From: petsc-users <[email protected]> on behalf of Yuyun
> Yang <[email protected]>
> > Sent: Sunday, February 16, 2020 3:12 AM
> > To: Smith, Barry F. <[email protected]>
> > Cc: [email protected] <[email protected]>
> > Subject: Re: [petsc-users] Matrix-free method in PETSc
> >
> > Thank you, that is very helpful information indeed! I will try it and
> send you my code when it works.
> >
> > Best regards,
> > Yuyun
> > From: Smith, Barry F. <[email protected]>
> > Sent: Saturday, February 15, 2020 10:02 PM
> > To: Yuyun Yang <[email protected]>
> > Cc: [email protected] <[email protected]>
> > Subject: Re: [petsc-users] Matrix-free method in PETSc
> >
> > Yuyun,
> >
> > If you are speaking about using a finite difference stencil on a
> structured grid where you provide the Jacobian vector products yourself by
> looping over the grid doing the stencil operation we unfortunately do not
> have exactly that kind of example.
> >
> > But it is actually not difficult. I suggest starting with
> src/ts/examples/tests/ex22.c It computes the sparse matrix explicitly with
> FormIJacobian()
> >
> > What you need to do is instead in main() use MatCreateShell() and
> MatShellSetOperation(,MATOP_MULT,(void (*)(void))MyMatMult) then provide the
> routine MyMatMult() to do your stencil based matrix free product; note that
> you can create this new routine by taking the structure of IFunction() and
> reorganizing it to do the Jacobian product instead. You will need to get the
> information about the shell matrix size on each process by calling
> DMDAGetCorners().
> >
> > You will then remove the explicit computation of the Jacobian, and
> also remove the Event stuff since you don't need it.
> >
> > Extending to 2 and 3d is straight forward.
> >
> > Any questions let us know.
> >
> > Barry
> >
> > If you like this would make a great merge request with your code to
> improve our examples.
> >
> >
> > > On Feb 15, 2020, at 9:42 PM, Yuyun Yang <[email protected]> wrote:
> > >
> > > Hello team,
> > >
> > > I wanted to apply the Krylov subspace method to a matrix-free
> implementation of a stencil, such that the iterative method acts on the
> operation without ever constructing the matrix explicitly (for example, when
> doing backward Euler).
> > >
> > > I'm not sure whether there is already an example for that somewhere.
> If so, could you point me to a relevant example?
> > >
> > > Thank you!
> > >
> > > Best regards,
> > > Yuyun
>
>
>
>
>
> --
> 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/
