[petsc-dev] FieldSplit fixes in 3.3
Hong Zhang
hzhang at mcs.anl.gov
Tue Aug 21 11:29:14 CDT 2012
Jed :
> 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.
I quit my merge and wait for someone else does it :-(
Hong
>
>
> 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.
>>>>
>>>
>>>
>>
>
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