[petsc-users] Setting initial guess for KSP solver

[Jed Brown]

On Wed, Sep 26, 2012 at 7:43 PM, Jinquan Zhong <jzhong at scsolutions.com>wrote:

>  Jed,****
>
> ** **
>
> So I need to assembled x before calling KSPSetInitialGuessNonzero().  KSPSolve
> will take the assembled x as its initial guess.  Correct?
>

Yes


> That is****
>
> ** **
>
> 129:   KSPSetFromOptions <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetFromOptions.html#KSPSetFromOptions>(ksp);****
>
> ** **
>
> 131:   if (nonzeroguess) {****
>
> 132:     PetscScalar <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PetscScalar.html#PetscScalar> p = .5;****
>
> 133:     VecSet <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecSet.html#VecSet>(x,p);****
>
> 134:     KSPSetInitialGuessNonzero <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetInitialGuessNonzero.html#KSPSetInitialGuessNonzero>(ksp,PETSC_TRUE <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Sys/PETSC_TRUE.html#PETSC_TRUE>);****
>
> 135:   }****
>
> 136: ****
>
> 137:   /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ****
>
> 138:                       Solve the linear system****
>
> 139:      - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - */****
>
> 140:   /* ****
>
> 141:      Solve linear system****
>
> 142:   */****
>
> 143:   KSPSolve <http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSolve.html#KSPSolve>(ksp,b,x);****
>
> ** **
>
> ** **
>
> I don’t think the manual specifies this.  I only read****
>
> ** **
>
> “Tells the iterative solver that the initial guess is nonzero; otherwise
> KSP<http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSP.html#KSP>assumes the initial guess is to be zero (and thus zeros it out before
> solving).”****
>
> ** **
>
> Thanks,****
>
> ** **
>
> Jinquan****
>
> ** **
>
> From: petsc-users-bounces at mcs.anl.gov [mailto:
> petsc-users-bounces at mcs.anl.gov] On Behalf Of Jed Brown
> Sent: Wednesday, September 26, 2012 5:37 PM
> To: PETSc users list
>
> *Subject:* Re: [petsc-users] Setting initial guess for KSP solver****
>
> ** **
>
> On Wed, Sep 26, 2012 at 7:33 PM, Jinquan Zhong <jzhong at scsolutions.com>
> wrote:****
>
>         PetscErrorCode  KSPSetInitialGuessNonzero(KSP ksp,PetscBool  flg)*
> ***
>
>                 ****
>
> does not provide the option to input x0 though.****
>
>  ** **
>
> It causes the vector X that you pass to KSPSolve to be used as the initial
> guess. Did you read the man page?****
>
> ** **
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/KSP/KSPSetInitialGuessNonzero.html
> ****
>
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