On Mon, Aug 20, 2012 at 8:36 AM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> On Mon, Aug 20, 2012 at 4:02 AM, Dmitry Karpeev <karpeev at mcs.anl.gov>wrote: > >> >> >> On Sun, Aug 19, 2012 at 1:17 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote: >> >>> On Sun, Aug 19, 2012 at 12:48 PM, Dmitry Karpeev <karpeev at >>> mcs.anl.gov>wrote: >>> >>>> The main reason for this being a patch in 3.3 is that recursive >>>> FieldSplit is broken there (which is why I couldn't enable it in >>>> libMesh/Moose correctly). >>> >>> >>> The current interface doesn't explicitly support all the ways to get >>> information into the split, but several people have worked around the >>> limitations to get the necessary information in there. Having you and Matt >>> changing things to rely on mutually incompatible side-effects does not >>> help. >>> >> I don't think the present fix is about the API or working through side >> effects (nullspace stuff excluded and partially backed out here: >> http://petsc.cs.iit.edu/petsc/releases/petsc-3.3/rev/1723d4624521) >> > > What do you call this external DM futzing? > It's no different than holding onto the ISs set in PCFieldSplitSetDefaults() or directly via PCFieldSplitSetIS() until they can be used in PCSetUp_FieldSplit(). I don't see how this can be avoided here or in the future, unless we do something to fundamentally change the set up process. > > >> The current interface assumes that the A00 solver and the Schur inner >> solver are to be set up identically >> > > If they are being set up identically, they should literally be the same > object. This is often the biggest setup cost in the whole problem (e.g. > AMG). > I agree, but that's available only in petsc-dev, not in petsc-3.3. The duplicate setup has been occurring all along, now it is actually consistent with the DM being forwarded to the inner solver. > > >> (something we relaxed in petsc-dev). In order to use fieldsplit on these >> solvers the DM for the corresponding split must be forwarded to both of >> these A00 solvers. It was being set only on the "outer" A00 solver. >> Partly this is an artifact of our "split" setup process: some of it occurs >> in PCFieldSplitSetDefaults(), some later in PCSetUp_FieldSplit(). Some >> objects obtained in PCFieldSplitSetDefaults() need to be cached until >> PCSetUp_FieldSplit(), and the splits' DMs are among those. I now put in >> proper reference counting for them here: >> http://petsc.cs.iit.edu/petsc/releases/petsc-3.3/rev/188af9799779 >> >> >>> Maybe we can reach some agreement on the proper way to do things. >>> >> That's fine with me. The reason I wanted to fix this particular problem >> now is that some dependent packages (e.g., Moose) only rely on release >> versions of petsc and would not be able to use this functionality properly >> for quite some time. >> > > Why do packages do this? Because petsc-dev is sometimes unstable. > > Now what happens when code that is pushed to the release changes behavior > and uses uninitialized memory? The release becomes unstable. > > Even worse, our nightly testing is targeted at petsc-dev because the > expectation is that only trivial and well-tested bug-fix patches are pushed > to release. If we are changing that model by introducing significant > changes, we MUST have nightly tests for the release, we have to actually > look at the results, and I would be strongly in favor of switching from > 3.3p3 numbering to 3.3.1. > > In all cases, these subminor or patch releases MUST be binary and > source-level backward compatible. > > >> Let's fix that "down the line". Setting a DM should never force it to be >>> used or cause an error due to unsupported operation in a case where not >>> having a DM is also acceptable. >>> >> Currently the inner and outer A00 solvers are essentially identified >> (duplicated), so they should be set up identically, including the DM. >> Especially when that DM defines a recursive split. This is in part >> because there is no way to configure those two differently programmatically >> or from the command line. So if the outer A00 is using >> -fieldsplit_0_pc_type_fieldsplit, the inner A00 will as well. >> However, the DM on the inner A00 will be rather different (or absent) and >> produce splits incompatible with the rest of the options (e.g., a single >> default split, while -fieldsplit_0_pc_fieldsplit_type schur). >> >> Incidentally, this raises this question down the road: in petsc-dev the >> inner and outer A00 can be configure separately using different prefixes, >> but how should the inner A00 DM be configured? >> > > The KSPs should only be different if the solver is different. Having them > separate, but using the same DM is one reason I was not wild about having > physics in the DM, but I don't see a good way to plumb in the extra > information. In any case, we need to come up with a solution before pushing > code. > > >> Okay, I'll back out that part of the second patch, since A11's >>>> nullspace is assumed to be meant for S. I'm not sure what's incorrect >>>> about the comment, though: if a vector is in the A11's kernel, but not in >>>> S's, I call it a "false positive". This terminology may be wanting, but >>>> what's incorrect about it? >>>> >>> >>> "force an inconsistent rhs" >>> >> The KSP solve will fail only if the search space X is contracted to the >> point where the rhs b is not in the range AX, hence, >> inconsistent. >> > > Okay, I just would have said that it changes the operator. Also, the > Krylov methods are not necessarily correct if the matrix does not have the > stated null space (since it's expected that the matrix has the null space > and only the preconditioner needs to be filtered). > > >> I backed out this hunk, however. >> >>> >>> S may well be nonsingular. >>> >>> >>>> >>>> >>>>> I think that's confusing and we need a specific API for it, but if >>>>> people are using it that way, we shouldn't change it in 3.3. >>>>> >>>>> I'd prefer to backout the first patch, split it into separate pieces, >>>>> and review each before pushing. I think it's a potentially big behavior >>>>> change for 3.3. >>>>> >>>> Which pieces of the first patch do you think are too big for 3.3? The >>>> splits' and Schur KSPs have to be set up (maybe not all in every situation) >>>> in order for recursive splitting to work. Should we declare that >>>> capability unavailable for 3.3? I can eliminate the unneeded KSPSetUp() >>>> calls. >>>> >>> >>> 1. I hate those hacky dm reference-non-references. >>> >>> 2. I don't want unused KSPs to be set up. (We should eventually fix the >>> model so they don't exist...) >>> >>> 3. Two of the three IncrementTabLevels that you introduced are not >>> necessary because the KSP's tab level was incremented before getting the >>> PC. >>> >> Where? >> > > ierr = > KSPCreate(((PetscObject)pc)->comm,&jac->kspschur);CHKERRQ(ierr); > ierr = > PetscLogObjectParent((PetscObject)pc,(PetscObject)jac->kspschur);CHKERRQ(ierr); > ierr = > PetscObjectIncrementTabLevel((PetscObject)jac->kspschur,(PetscObject)pc,1);CHKERRQ(ierr); > + { > + PC pcschur; > + ierr = KSPGetPC(jac->kspschur, &pcschur); CHKERRQ(ierr); > + ierr = > PetscObjectIncrementTabLevel((PetscObject)pcschur,(PetscObject)pc,1);CHKERRQ(ierr); > + } > > > ierr = > KSPCreate(((PetscObject)pc)->comm,&ilink->ksp);CHKERRQ(ierr); > ierr = > PetscObjectIncrementTabLevel((PetscObject)ilink->ksp,(PetscObject)pc,1);CHKERRQ(ierr); > + { > + PC ilinkpc; > + ierr = KSPGetPC(ilink->ksp, &ilinkpc); CHKERRQ(ierr); > + ierr = > PetscObjectIncrementTabLevel((PetscObject)ilinkpc,(PetscObject)pc,1);CHKERRQ(ierr); > + } > > > This is the pattern used everywhere else in PETSc so that sub-pcs always > have the correct tab level. > -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20120821/9ca22aad/attachment.html>
