[petsc-dev] Unions of IS and MatNest

[Pierre Jolivet]

> On 19 Mar 2020, at 7:56 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Fri, Mar 13, 2020 at 11:20 AM Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto:pierre.jolivet at enseeiht.fr>> wrote:
> 
> 
>> On 13 Mar 2020, at 2:36 PM, Matthew Knepley <knepley at gmail.com <mailto: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
> […]
> 
> I see now. If you call PCFieldsplitSetIS(), this overrides anything else. We do not even try to discover the split. If you want
> a split determined by command line arguments, you have to defer the split. It looks like it can get splits from the MatNest,
> so just do not call SetIS().

So, I’ve commented the three PCFieldSplitSetIS.
$ ./a.out -pc_type fieldsplit -pc_fieldsplit_0_fields 0,1 -pc_fieldsplit_1_fields 2 -pc_fieldsplit_block_size 3
is starting to give me something meaningful in the KSPView, i.e.:
[…]
  Split number 0 Fields  0, 1
[…]
  Split number 1 Fields  2
But it’s surrounded by a bunch of errors such as:
[0]PETSC ERROR: Arguments are incompatible
[0]PETSC ERROR: Could not find index set

Do I need some other command-line options?

Thanks,
Pierre

>   Thanks,
> 
>      Matt
>  
> 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/>
> 
> 
> -- 
> 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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