[petsc-dev] Unions of IS and MatNest

[Pierre Jolivet]

Hello Lawrence,
Any luck finding the appropriate patch?

Thanks,
Pierre

> On 20 Mar 2020, at 10:53 PM, Lawrence Mitchell <wence at gmx.li> wrote:
> 
> 
> On Fri, 20 Mar 2020 at 21:07, Pierre Jolivet <pierre.jolivet at enseeiht.fr <mailto:pierre.jolivet at enseeiht.fr>> wrote:
> 
> 
>> On 19 Mar 2020, at 7:56 PM, Matthew Knepley <knepley at gmail.com <mailto: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?
> 
> If you have a flat 3x3 matnest, the index set matching will not find a subset that is anything other than a 1-field split. I have a patch lying around that i think fixes this. I can try and dig it out on Monday. 
> 
> Lawrence

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