[petsc-users] Null space correction depends on solver?

[Thomas Witkowski]

Am 28.08.2012 19:57, schrieb Jed Brown:
> On Tue, Aug 28, 2012 at 12:25 PM, Thomas Witkowski 
> <thomas.witkowski at tu-dresden.de 
> <mailto:thomas.witkowski at tu-dresden.de>> wrote:
>
>
>               GMRES uses left preconditioning by default which forces
>         the null space into the Krylov space at each iteration. FGMRES
>         uses right preconditioning and doesn't  do anything to the
>         null space because it is naturally in the Krylov space (since
>         the result with fgmres contains the constants this is why I
>         suspect the matrix actually doesn't have the null space you
>         think it has). You can try running -ksp_type gmres
>         -ksp_pc_side right      my guess is that you will not get near
>         zero for the dot.
>
>     Any idea how to debug this problem? Lets concentrate on the
>     gmres/no precon case first.
>
>
> Left preconditioning and anything using -pc_type none works correctly. 
> It is only the case of right preconditioning that is incorrect (and 
> needs to be fixed in PETSc).
Is the null space projected out at every iteration step? If so, can you 
give me a hint were I can find the corresponding line in source code? 
Thanks.

Thomas
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