[petsc-users] Nesting splits based on IS

[Karin&NiKo]

Thank you Pierre and Matt. In fact, I see how to do it programmatically but
I would like to give the user the capability to do it from the command-line
options juste it can be done with strided splits.
@Matt : can you please elaborate a little bit on the solution you proposed
please ?

Thanks for your precious help,
Nicolas

Le sam. 17 avr. 2021 à 20:07, Matthew Knepley <knepley at gmail.com> a écrit :

> On Sat, Apr 17, 2021 at 1:14 PM Pierre Jolivet <pierre at joliv.et> wrote:
>
>> Dear Nicolas,
>> Here is how I would do it:
>> - if your Pmat type is MATNEST, that’s quite easy, see
>> https://www.mcs.anl.gov/petsc/petsc-current/src/ksp/ksp/tutorials/ex81.c.html
>> - otherwise, you’ll need to first define a 2-way field splitting (with
>> v+p and t), then fetch the appropriate (0,0) block with
>> PCFieldSplitGetSubKSP(), and define an inner 2-way field splitting (with v
>> and p).
>>
>
> Or you can define a DMShell that gives back these splits for the fields.
> That is how Firedrake does it.
>
>   Thanks,
>
>      Matt
>
>
>> Thanks,
>> Pierre
>>
>> On 17 Apr 2021, at 6:53 PM, 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/>
>
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