[petsc-users] Nesting splits based on IS

Sun Apr 18 13:38:01 CDT 2021

On Sun, Apr 18, 2021 at 2:20 PM Karin&NiKo <niko.karin at gmail.com> wrote:

> Dear Matt and Barry,
>
> Unfortunately DM is not used in our PETSc interface,which is essentially
> based on  the assembly matrix.
> As Matt mentioned, the trick could be to define a DMShell in order to
> expose the splits we internally build based on IS.
> If I understand well, I have to play around with
> DMShellSetCreateFieldDecomposition. Unfortunately, I do not find tests
> using this method. Am I missing something ?
>

I would not use that function. I would use DMCreateSubDM().

  Thanks,

    Matt

> Perhaps a good starting point would be to write a test
> like src/ksp/ksp/examples/tutorials/ex43.c in which one would consider the
> algebraic data to be known (the matrix and the IS) and would set up a
> DMShell  (with an appropriate call to DMShellSetCreateFieldDecomposition).
> Does that make sense?
>
> Thanks,
> Nicolas
>
> Le dim. 18 avr. 2021 à 15:54, Matthew Knepley <knepley at gmail.com> a
> écrit :
>
>> On Sat, Apr 17, 2021 at 6:13 PM Barry Smith <bsmith at petsc.dev> wrote:
>>
>>>
>>>   So you would like to be able to create three IS in your code and
>>> attach them with names to the PC.  Then have -pc_fieldsplit_XXX_fields be
>>> able to utilize the attached IS by name and use them to define the blocks.
>>>
>>>
>>>   This is all doable and could be added to PCFIELDSPLIT without too much
>>> code, new code. The code would be largely like
>>> PCFieldSplitSetRuntimeSplits_Private.
>>>
>>>    The recursive part may also be doable but I think your syntax below
>>> is not exactly right. You would need something like
>>>
>>> -fieldsplit_0_pc_type fieldsplit   // split the first PC into a
>>> fieldsplit
>>>      -fieldsplit_0_pc_fieldsplit_0_fields xxx
>>> -fieldsplit_0_fieldsplit_0_pc_type jacobi
>>>      -fieldsplit_0_pc_fieldsplit_1_fields yyy
>>>      etc
>>>
>>> this would split the first field into two fields and use jacobi on the
>>> first field.
>>>
>>> The problem is what to use for labelling the xxx and the yyy?
>>>
>>> I think one could achieve this by having the PCFIELDPLIT attach to each
>>> of its split PCs the appropriate modified IS with names attached to them.
>>> There are two cases,
>>>
>>>   when building the split uses first all the entries from fieldsplit_v_,
>>> then from fieldsplit_p_ then the new ISs it needs to attach to the first
>>> split PC are two sets of integers the first from 0 to the len(v)-1 and the
>>> second from len(v) to len(v)+len(p)-1.
>>>
>>>   when building the split it interlaces the indices from v and p
>>> (interlacing only make sense if the size of v and p is the same). Then the
>>> new v would be {0,2,4,...} and p would be {1,3,...}.
>>>
>>>   If you are ambitious and would like to add this to fieldsplit.c we'd
>>> be very happy to receive an MR. It might even lead to allowing us to simply
>>> how the PCFIELDPLIT interacts with DMs. If all the split type, stride,
>>> named, etc are handle in a single consistent manner.
>>>
>>
>> Barry, this is already working with DMs, which I think is the right way
>> to do this.
>>
>> Here is the code:
>>
>>
>> https://gitlab.com/petsc/petsc/-/blob/main/src/ksp/pc/impls/fieldsplit/fieldsplit.c#L420
>>
>> The DM must respond to DMCreateSubDM(). The interface is that the call
>> provides a list of fields [f_0, f_1, ...]
>> and the DM returns an IS for that combination and a subdm for it. The
>> subdm part is what allows you to do this
>> recursively, since you make the same call on the subdm.
>>
>> If you use a PetscSection to keep track of the data layout, the code to
>> do field selection is already done:
>>
>>   https://gitlab.com/petsc/petsc/-/blob/main/src/dm/interface/dmi.c#L105
>>
>> which can just be called from the DMCreateSubDM() implementation.
>>
>>   Thanks,
>>
>>      Matt
>>
>>
>>>   Barry
>>>
>>>
>>>
>>> > On Apr 17, 2021, at 11:53 AM, Karin&NiKo <niko.karin at gmail.com> wrote:
>>> >
>>> > Dear PETSc users,
>>> >
>>> > I use the fieldsplit PC in an application where the splits are
>>> programmatically defined by IS using PCFieldSplitSetIS. Then the user can
>>> specify its own PC at runtime using PETSc options.
>>> > My question : is it possible to define nested splits in this case as
>>> it can be done with strided splits (see snes/examples/tutorials/ex19.c with
>>> test suffix fieldsplit_4).
>>> >
>>> > In order to be perfectly clear : let's say I have a 3 fields problem :
>>> velocity (v split), pressure (p split) and temperature (t split).
>>> > What I would like to do is something like the following but it fails :
>>> >
>>> > -ksp_type fgmres
>>> > -pc_fieldsplit_type multiplicative
>>> > -pc_type fieldsplit    -pc_fieldsplit_0_fields fieldsplit_v_,
>>> fieldsplit_p_    -pc_fieldsplit_1_fields fieldsplit_t_
>>> >
>>> > -prefix_push  fieldsplit_0_
>>> > -ksp_type fgmres
>>> > -pc_fieldsplit_schur_factorization_type upper
>>> > -pc_type fieldsplit
>>> > -pc_fieldsplit_type schur
>>> > -pc_fieldsplit_schur_precondition a11
>>> > -prefix_pop
>>> >
>>> > -prefix_push  fieldsplit_1_
>>> > -ksp_type fgmres
>>> > -pc_type jacobi
>>> > -prefix_pop
>>> >
>>> > -prefix_push  fieldsplit_v_
>>> > -ksp_type fgmres
>>> > -pc_type gamg
>>> > -prefix_pop
>>> >
>>> > -prefix_push  fieldsplit_p_
>>> > -ksp_type fgmres
>>> > -pc_type jacobi
>>> > -prefix_pop
>>> >
>>> > I thank you for your help,
>>> > Nicolas
>>> >
>>> >
>>>
>>>
>>
>> --
>> 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/>
>>
>

-- 
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/>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20210418/b02ce0f1/attachment.html>