[petsc-dev] composite vs. shell

[Jed Brown]

PCSetUp_Composite propagates the outer matrices to the inner PCs.

  while (next) {
    ierr = PCSetDM(next->pc,dm);CHKERRQ(ierr);
    ierr = PCSetOperators(next->pc,pc->mat,pc->pmat);CHKERRQ(ierr);
    next = next->next;
  }


You can get the desired behavior by calling PCSetUp before
PCCompositeGetPC.

Perhaps the code should be changed to only propagate the outer matrices
if they haven't been set yet.  But propagating matrices like this is a
poor abstraction in general because it conflates configuration with the
supply of problem information.  We don't want the FormJacobian in a time
integrator to need to reach into the SNES -> KSP -> PCComposite to set a
different operator; it doesn't even have enough information to do that.

Pierre Jolivet <pierre.jolivet at enseeiht.fr> writes:

> I’d like to use PCCOMPOSITE to define a preconditioner as the sum of two preconditioners with different operators.
> My current code works with a PCSHELL, but I don’t know how to setup the PCCOMPOSITE appropriately to get the same results.
> Attached is a reproducer (using complex scalars).
> $ ./SHELL_v_COMPOSITE -fSp S.bin -fMp Mp.bin -fLp Lp.bin -ksp_view -pc_type shell -ksp_type preonly 
> $ ./SHELL_v_COMPOSITE -fSp S.bin -fMp Mp.bin -fLp Lp.bin -ksp_view -ksp_type preonly -pc_type composite -pc_composite_type additive -prefix_push sub_0_ -pc_type ksp -ksp_ksp_type preonly -ksp_pc_type jacobi -prefix_pop -prefix_push sub_1_ -pc_type ksp -ksp_ksp_type preonly -ksp_pc_type jacobi -prefix_pop
> What looks weird to me is that for the PCSHELL, the operators for both PCs are of the correct size (36588 and 36814 nonzeros respectively), while for the PCCOMPOSITE, it looks like the same operator is duplicated (-ksp_view tells me that both have 36814 nonzeros which is not what I get with -mat_view ::ascii_info).
> Any idea on how to fix this?
>
> Thanks,
> Pierre