On Jan 1, 2014, at 11:09 AM, Song Gao <[email protected]> wrote:
> Dear all,
>
> Happy new year!
>
> I'm using the matrix-free method to solve NS equations. I call the
> SNESSetFunction to set the RHS function. I think SNES also uses that RHS
> function to evaluate the matrix vector product.
Yes, PETSc differences this function to evaluate the matrix vector product.
>
> But I want to set one function to evaluate the residual, and another
> different function to evaluate the matrix vector product.
Are you providing a function that
1) actually evaluates the matrix vector product or are you
2) providing a function that you want PETSc to difference to evaluate the
matrix vector product?
> How can I do that? Does MatMFFDSetFunction do this job?
For 2) yes, but if the function you provide is different than the function
provided with SNESSetFunction then the matrix-vector product obtained from
differencing it will not be “correct” Jacobian for the SNESSetFunction() you
are providing so I don’t see why you would do it.
For 1) you should use MatCreateShell() and
MatShellSetOperation(mat,MATOP_MULT, ….) and then pass that matrix to
SNESSetJacobian(), then PETSc will use that “matrix” to do its matrix-vector
products.
Barry
>
> Any suggestion is appreciated. Thank you.
>
>
> Song
