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.
>
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