[petsc-users] Composite shell preconditiner

[Alexander Grayver]

Hello petsc team!

I would like to apply some sort of correction during Krylov process 
every N iterations.
So I decided to use composite preconditioner like that:

-pc_type composite -pc_composite_pcs shell,sor/icc/gamg

I've started to test with the simplest shell preconditioner that does 
nothing:

subroutine ShellPCApply(pc,x,y,ierr)
   implicit none

#include "finclude/petscsys.h"
#include "finclude/petscvec.h"

   PC,intent(in)        :: pc
   Vec,intent(in)       :: x
   Vec,intent(inout)    :: y

   call VecCopy(x,y,ierr);CHKERRQ(ierr)
endsubroutine ShellPCApply

And it turned out that this influences convergence by itself. In case:
-pc_type composite -pc_composite_pcs shell,sor
---->
Linear solve converged due to CONVERGED_RTOL iterations 125

whereas
-pc_type composite -pc_composite_pcs sor
----->
Linear solve converged due to CONVERGED_RTOL iterations 35

Why does it happen? What is the right way to go then?
Thanks in advance.

Just in case, here is -ksp_view for -pc_composite_pcs shell,sor :

PC Object:() 1 MPI processes
   type: composite
   Composite PC type - ADDITIVE
   PCs on composite preconditioner follow
   ---------------------------------
     PC Object:    (sub_0_)     1 MPI processes
       type: shell
         Shell: no name
       linear system matrix = precond matrix:
       Matrix Object:       1 MPI processes
         type: seqaij
         rows=59049, cols=59049
         total: nonzeros=741555, allocated nonzeros=767637
         total number of mallocs used during MatSetValues calls =0
           not using I-node routines
     PC Object:    (sub_1_)     1 MPI processes
       type: sor
         SOR: type = local_symmetric, iterations = 1, local iterations = 
1, omega = 1
       linear system matrix = precond matrix:
       Matrix Object:       1 MPI processes
         type: seqaij
         rows=59049, cols=59049
         total: nonzeros=741555, allocated nonzeros=767637
         total number of mallocs used during MatSetValues calls =0
           not using I-node routines
   ---------------------------------
   linear system matrix = precond matrix:
   Matrix Object:   1 MPI processes
     type: seqaij
     rows=59049, cols=59049
     total: nonzeros=741555, allocated nonzeros=767637
     total number of mallocs used during MatSetValues calls =0
       not using I-node routines

-- 
Regards,
Alexander