[petsc-users] odd SNES behavior

[Barry Smith]

On Mar 6, 2013, at 9:36 PM, "Mark F. Adams" <mark.adams at columbia.edu> wrote:

>> 
>> Presumbly when you call PCReset() it zeros out all the pointers. If you do not set the operators on the finest level again then it will use the "outer" operator on the smoothest level. Is this the problem?
> 
> This is not a problem but it should be a choice, right?  

> This app has an expensive nonlinear 'formFunction' but many apps will have fast matrix free apply-operator methods, in which case the current behavior with gamg is desired.  And having this choice being made by whether a PC is well integrated with PETSc (e.g., not hypre) is pretty awful semantics.
> 
> Is there an option that will make this happen or should I add one?  If so where should I add it? It should be in PC_MG I would think.
> 
> -pc_mg_levels_fine_use_op <false>
> -pc_mg_levels_fine_use_preop <true>

Mark,

   This same issue comes up in several preconditioners. We use the term True to indicate this (yes maybe not the best term)

Barrys-MacBook-Pro:petsc-dev barrysmith$ grep True include/petscpc.h
PETSC_EXTERN PetscErrorCode PCBJacobiSetUseTrueLocal(PC);   -pc_bjacobi_truelocal
PETSC_EXTERN PetscErrorCode PCKSPSetUseTrue(PC);                 -pc_ksp_true
PETSC_EXTERN PetscErrorCode PCCompositeSetUseTrue(PC);       -pc_composite_true

 There are possibly other PCs that could have this functionality but do not because we haven't realized it. It makes sense in any PC that internally has to apply the "full" (or part of the full) operator somewhere inside the application of the PC.

  So you could add PCMGSetUseTrue(PC) and -pc_mg_true  that would trigger setting the true (outer) operator inside the PCSetUp_MG() even if it was already set. You would still change/fix the current GAMG to always set the operators on the finest level using the pmat (which could be overwritten by the user with the PCMGSetUseTrue() or -pc_mg_true option.

  It may also make sense to move the use true flag and functionality up to the PC level (out of the subclasses) thus unifying its usage (or not)? PCSetUseTrue(), -pc_true.** 

  Suggestions to alternatives to "True" (that are concise yet descriptive) are also solicited.


   Barry

** note that in some complicated PCs there may be several different applications of the full (or part of the full) operator inside the application of the PC. Conceivably one may want to use True on some and not on others but we've never dealt with that.