[petsc-dev] Unions of IS and MatNest

[Pierre Jolivet]

> On 13 Mar 2020, at 2:36 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Fri, Mar 13, 2020 at 3:19 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto:pierre.jolivet at enseeiht.fr>> wrote:
> 
> 
>> On 12 Mar 2020, at 11:40 PM, Matthew Knepley <knepley at gmail.com <mailto:knepley at gmail.com>> wrote:
>> 
>> On Thu, Mar 12, 2020 at 5:59 PM Jed Brown <jed at jedbrown.org <mailto:jed at jedbrown.org>> wrote:
>> Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto:pierre.jolivet at enseeiht.fr>> writes:
>> 
>> > Hello,
>> > Has there been any follow-up on this https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html> <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/024020.html>>?
>> > Given a 3x3 MatNest A = [A_00,0,0 ; 0,A_11,0 ; 0,0,A_22], I’d like to setup a two-way fieldsplit coupling [A_00,0 ; 0,A_11] and [A_22] but I can’t figure out the proper options.
>> 
>> Are you looking for a Schur split or additive/multiplicative?
> 
> Don’t know yet which will perform best, do you have a specific solution in mind for one scenario or the other?
> I was mostly wondering if it was possible in a general context, not taking -pc_fieldsplit_type into account.
> 
>> -pc_fieldsplit_field_0 0,1 -pc_fieldsplit_field_1 2 -pc_fieldsplit_type schur
> 
> These flags, used with my .cpp, yield:
> [0]PETSC ERROR: Arguments are incompatible
> [0]PETSC ERROR: To use Schur complement preconditioner you must have exactly 2 fields
> If I use -pc_fieldsplit_%d_fields <a,b,..> as advocated in the manual (instead of -pc_fieldsplit_field_%d as you suggested), I get the same error.
> 
> Okay it is  -pc_fieldsplit_%d_fields. When you use this, how many fields does it think you have?

Three.
In case it’s not clear, the MWE is at the bottom of my first email (nest.cpp).
$ mpicxx nest.cpp -I$PETSC_DIR/$PETSC_ARCH/include -I$PETSC_DIR/include -L$PETSC_DIR/$PETSC_ARCH/lib -lpetsc
$ ./a.out -pc_type fieldsplit -pc_fieldsplit_0_fields 0,1 -pc_fieldsplit_1_fields 2 -pc_fieldsplit_type schur
[…]
[0]PETSC ERROR: Arguments are incompatible
[0]PETSC ERROR: To use Schur complement preconditioner you must have exactly 2 fields
[…]
    Split info:
    Split number 0 Defined by IS
    Split number 1 Defined by IS
    Split number 2 Defined by IS
[…]

Thanks,
Pierre

>    Matt
>> I believe.
>> 
>>    Matt
>>  
>> > Jed, in this answer https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html><https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html <https://lists.mcs.anl.gov/pipermail/petsc-users/2015-January/023993.html>>, you recommend not to use MatNest. What would you recommend instead?
>> 
>> See src/snes/examples/tutorials/ex28.c for my preferred approach.
> 
> I’m not using DM, so in terms of Mat, I guess this example shows that I pretty much need to reimplement the field splitting myself?
> 
> Thanks,
> Pierre
> 
>> -- 
>> 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/>
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