A different kind of lagged preconditioner

[Barry Smith]

   Just use VecCopy() for the times you skip the application of the  
preconditioner; if you do nothing for those times, it will fail as it  
does.

   As Matt says, you need to use KSPFGMRES if you use a different  
preconditioner on different iterations.

    BTW: your plan will suck.

    Barry

On Mar 23, 2009, at 6:41 PM, Rafael Santos Coelho wrote:

> Well, let me explain myself again. I've implemented a serial matrix- 
> free uniparametric LU-SGS preconditioner for non-linear problems  
> using the PETSc PCSHELL module. It's matrix-free in the sense that  
> whenever the jacobian matrix (and by that I mean the L, D and U  
> factors of the jacobian matrix) is required within the linear solver  
> in the form of a matrix-vector product, I resort to a finite- 
> difference formula in order to approximate the jacobian entries. So  
> the jacobian matrix is never actually formed. To use my  
> preconditioner, I just have to set the "-user_precond" command-line  
> option along with "-snes_mf".
>
> Now, the cost to apply that preconditioner obviously grows according  
> to the problem dimensions, so it is built and applied within the  
> linear solver to form the vectors of the Krylov subspace basis in  
> each linear iteration. I want to change that, and only have it  
> applied every "p" linear iterations to make it less costlier.
>
> Rafael