[petsc-users] To store the RHS vector b of KSPSolve.

[Xuefei (Rebecca) Yuan]

Hello Matt,

On Jan 17, 2012, at 12:53 PM, Matthew Knepley wrote:

> On Tue, Jan 17, 2012 at 2:51 PM, Xuefei (Rebecca) Yuan <xyuan at lbl.gov> wrote:
> Hello all,
> 
> I have a piece of code that need to store the RHS vector b of the ksp object via vecview.
> 
> However, the routine is like:
> 
> 
>        ierr = DMMGSetSNESLocal(dmmg, FormFunctionLocal, FormJacobianLocal,0,0);CHKERRQ(ierr);
>        ierr = DMMGSolve(dmmg);CHKERRQ(ierr);
> 
> The rhs evaluation is in FormFunctionLocal(), how could store the rhs vector b?
> 
> 1) In your nonlinear problem, what do you mean by b?
> 

This b is refer to the linear problem's rhs, i.e., for the nonlinear problem F(u)=0 is the residual evaluation in FormFunctionLocal, and for the linear solver, Ax=b, this rhs vector b is -F(u^{k]), k is the iteration number for the nonlinear solver.

I want to separate the Jacobian matrix and the rhs vector from the very beginning for testing some linear solver:

******* start solving for time = 0.50000 at time step = 1******
  0 SNES Function norm 1.242539468950e-02
start saving jacobian
Linear solve converged due to CONVERGED_ITS iterations 1
  1 SNES Function norm 3.341795546738e-05
Linear solve converged due to CONVERGED_ITS iterations 1
  2 SNES Function norm 1.329755764187e-08
Linear solve converged due to CONVERGED_ITS iterations 1
  3 SNES Function norm 3.067585609727e-12
Nonlinear solve converged due to CONVERGED_FNORM_RELATIVE

The jacobian matrix was saved before calling the linear solver provided by PETSc, and I need to get the corresponding rhs vector b with this jacobian matrix.

> 2) Don't use DMMG, use petsc-dev with regular SNES and SNESSetDM()
> 
I will look into that later.

>    Matt
>  
Thanks very much!

Best regards,

Rebecca



> Thanks very much!
> 
> Cheers,
> 
> Rebecca
> 
> 
> 
> 
> 
> 
> -- 
> 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

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