[petsc-users] SNESSetFunction and MatMFFDSetFunction

[Barry Smith]

On Jan 1, 2014, at 11:09 AM, Song Gao <song.gao2 at mail.mcgill.ca> 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