[petsc-dev] funny mg behavior

[Mark F. Adams]

I'm finding what looks like strange behavior with respect to setting the number of pre/post smooths in PCMG.  I setup my PCMG with a line:

ierr = PCMGGetSmoother( a_pc, lidx, &smoother ); CHKERRQ(ierr);

 [ set it up ... ]

It seems that this method has a side effect of setting the use-same-smoother-for-pre-and-post state, or this is default.

The problem is that if I try to set the number of smoothing steps with code:

  ierr = PCMGSetNumberSmoothDown(a_pc,2); CHKERRQ(ierr);
  ierr = PCMGSetNumberSmoothUp(a_pc,2); CHKERRQ(ierr);

or command lines:  -pc_mg_smoothdown 2 -pc_mg_smoothup 2

it unsets the use-same-smoother-for-pre-and-post state.  Then I get GMRES/ILU as my up (post) smoother (ie, a default solver), with my carefully crafted smoother only used for pre smoothing.  Not what I want.

Am I missing something or did a hole form in PETSc's logic at some point?  Perhaps there should be a PCMGSetNumberSmooth and '-pc_mg_smooth n' option that does not have this side effect?

Mark