[petsc-users] Near null space for a fieldsplit in petsc4py
Eric Chamberland
Eric.Chamberland at giref.ulaval.ca
Thu Jul 13 16:30:02 CDT 2023
Hi everyone,
I'm joining this discussion because we're testing a simple way to
simplify the current object naming and configuration system in PETSc.
The solution we're testing uses the same philosophy as the option
database keys: a "default" database.
We're creating a "default" PetscObjectList that gives access to a global
list of pairs: <string, PetscObjectList>.
With this, we can simply "name" and add all our PetscObjects to it.
At run time, we create an option that holds the name of the PetscObject
to be passed as a parameter to whatever we want to configure.
For example, let's say you want to pass an optional matrix named
"preSchur" to a specific PC. It could be configured like this:
Mat MyMat;
PetscObjectSetName(MyMat, "preSchur");
PetscObjectListAdd(gGlobalList, "preSchur", (PetscObject)MyMat); /* Here
MyMat is made available into the global object list with the key
"preSchur" */
Then, in a PC, you can retrieve the matrix by using both the options
database keys and the objects database. For example, if a PC looks for
options like:
-pc_fieldsplit_schur_precondition user
-pc_fieldsplit_schur_precondition_user_mat preSchur
It just needs to look if the key
"pc_fieldsplit_schur_precondition_user_mat" has been set, and if so,
retrieve the value, which is the name of the matrix to be retrieved from
the global list.
With this approach, all PCs, KSPs, and other objects can be given
objects at setup time (or later) with proper naming of the options and
objects.
Some key features of this approach include:
- If a user misspells an option, it will be visible in the unused options.
- Any new options to any PC or KSP do not need a new function to use
them; just a new option key is needed.
- All software that uses PETSc as a library can have a very "light"
interface to it: just name all objects and let users "configure" them!
- This is almost the same idea as "composing objects," which is already
used in many places in PETSc. It's simpler to use and really relaxes the
constraints on "when" you can configure a PC.
I think this is a promising approach to simplifying the object naming
and configuration system in PETSc. I'm interested in hearing your feedback.
Thanks,
Eric
On 2023-07-13 13:44, Matthew Knepley wrote:
> On Thu, Jul 13, 2023 at 5:33 AM Pierre Jolivet
> <pierre.jolivet at lip6.fr> wrote:
>
> Dear Nicolas,
>
>> On 13 Jul 2023, at 10:17 AM, TARDIEU Nicolas
>> <nicolas.tardieu at edf.fr> wrote:
>>
>> Dear Pierre,
>>
>> You are absolutely right. I was using a --with-debugging=0 (aka
>> release) install and this is definitely an error.
>> Once I used my debug install, I found the way to fix my problem.
>> The solution is in the attached script: I first need to extract
>> the correct block from the PC operator's MatNest and then append
>> the null space to it.
>> Anyway this is a bit tricky...
>
> Yep, it’s the same with all “nested” solvers, fieldsplit, ASM, MG,
> you name it.
> You first need the initial PCSetUp() so that the bare minimum is
> put in place, then you have to fetch things yourself and adapt it
> to your needs.
> We had a similar discussion with the MEF++ people last week, there
> is currently no way around this, AFAIK.
>
>
> Actually, I hated this as well, so I built a way around it _if_ you
> are using a DM to define the problem. Then
> you can set a "nullspace constructor" to make it if the field you are
> talking about is ever extracted. You use DMSetNullSpaceConstructor().
> I do this in SNES ex62 and ex69, and other examples.
>
> Thanks,
>
> Matt
>
> Thanks,
> Pierre
>
>> Regards,
>> Nicolas
>>
>> ------------------------------------------------------------------------
>> *De :*pierre.jolivet at lip6.fr <pierre.jolivet at lip6.fr>
>> *Envoyé :*mercredi 12 juillet 2023 19:52
>> *À :*TARDIEU Nicolas <nicolas.tardieu at edf.fr>
>> *Cc :*petsc-users at mcs.anl.gov <petsc-users at mcs.anl.gov>
>> *Objet :*Re: [petsc-users] Near null space for a fieldsplit in
>> petsc4py
>>
>> > On 12 Jul 2023, at 6:04 PM, TARDIEU Nicolas via petsc-users
>> <petsc-users at mcs.anl.gov> wrote:
>> >
>> > Dear PETSc team,
>> >
>> > In the attached example, I set up a block pc for a saddle-point
>> problem in petsc4py. The IS define the unknowns, namely some
>> physical quantity (phys) and a Lagrange multiplier (lags).
>> > I would like to attach a near null space to the physical block,
>> in order to get the best performance from an AMG pc.
>> > I have been trying hard, attaching it to the initial block, to
>> the IS but no matter what I am doing, when it comes to
>> "ksp_view", no near null space is attached to the matrix.
>> >
>> > Could you please help me figure out what I am doing wrong ?
>>
>> Are you using a double-precision 32-bit integers real build of PETSc?
>> Is it --with-debugging=0?
>> Because with my debug build, I get the following error (thus
>> explaining why it’s not attached to the KSP).
>> Traceback (most recent call last):
>> File "/Volumes/Data/Downloads/test/test_NullSpace.py", line 35,
>> in <module>
>> ns = NullSpace().create(True, [v], comm=comm)
>> ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> File "petsc4py/PETSc/Mat.pyx", line 5611, in
>> petsc4py.PETSc.NullSpace.create
>> petsc4py.PETSc.Error: error code 62
>> [0] MatNullSpaceCreate() at
>> /Volumes/Data/repositories/petsc/src/mat/interface/matnull.c:249
>> [0] Invalid argument
>> [0] Vector 0 must have 2-norm of 1.0, it is 22.3159
>>
>> Furthermore, if you set yourself the constant vector in the near
>> null-space, then the first argument of create() must be False,
>> otherwise, you’ll have twice the same vector, and you’ll end up
>> with another error (the vectors in the near null-space must be
>> orthonormal).
>> If things still don’t work after those couple of fixes, please
>> feel free to send an up-to-date reproducer.
>>
>> Thanks,
>> Pierre
>>
>> > Thanks,
>> > Nicolas
>> >
>> >
>> >
>> >
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>> interdite sauf autorisation expresse.
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>> interdit de le copier, de le faire suivre, de le divulguer ou
>> d'en utiliser tout ou partie. Si vous avez reçu ce Message par
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>> immédiatement l'expéditeur par retour du message.
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>> messagerie électronique arrivent en temps utile, sont sécurisées
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>> ____________________________________________________
>> This message and any attachments (the 'Message') are intended
>> solely for the addressees. The information contained in this
>> Message is confidential. Any use of information contained in this
>> Message not in accord with its purpose, any dissemination or
>> disclosure, either whole or partial, is prohibited except formal
>> approval.
>> If you are not the addressee, you may not copy, forward, disclose
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>> <test_NullSpace.py.gz>
>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which
> their experiments lead.
> -- Norbert Wiener
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/~knepley/>
--
Eric Chamberland, ing., M. Ing
Professionnel de recherche
GIREF/Université Laval
(418) 656-2131 poste 41 22 42
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