[petsc-users] preconditionning with a Matshell

[Jed Brown]

Nicolas Pozin <nicolas.pozin at inria.fr> writes:

> Hello everyone, 
>
> I'm trying to solve a system iteratively through conjugate gradient with jacobi preconditionning. 
> Thing is the system matrix is a matshell. 
> When I run the code matgetdiagonal returns an error : "MatGetDiagonal() line ......... /petsc/3.4.3/src/mat/interface/matrix.c Mat type shell" 
> Is matgetiagonal function enable to act on a matshell? 

MatShell is not magic -- it can't guess the diagonal without more
information or unreasonably high cost.  You have to implement
MatGetDiagonal to use Jacobi:

  MatShellSetOperation(mat,MATOP_GET_DIAGONAL,yourfunction)
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