[petsc-dev] FieldSplit fixes in 3.3
Jed Brown
jedbrown at mcs.anl.gov
Tue Aug 21 11:15:11 CDT 2012
Hong, as far as I can tell, none of these changes should be merged to
petsc-dev right now. If you go through with the merge, use hg diff -r
SHA1_OF_UPSTREAM_PETSC_DEV to make sure that none of this 3.3 stuff gets
merged. Otherwise, one of us can do the merge.
On Tue, Aug 21, 2012 at 11:08 AM, Hong Zhang <hzhang at mcs.anl.gov> wrote:
> Dmitry :
> I pushed a bugfix of mumps interface to 3.3. When merging it to petsc-dev,
> I get
>
> merging src/ksp/pc/impls/fieldsplit/fieldsplit.c failed!
>
> Hong
>
>
>>
>> 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/20120821/18bc8451/attachment.html>
More information about the petsc-dev
mailing list