[petsc-users] FIELDSPLIT fields

[Smith, Barry F.]

> On Sep 5, 2018, at 9:22 PM, Rossi, Simone <srossi at email.unc.edu> wrote:
> 
> Thanks for clarifying that.
> 
> So if I have my subdomain set as 
> 
> 0 —— 1 —— 2  —— 3 —— 4  —— 5 —— 6
>  | subdomain 1 | subdomain 2 | subdomain 1 |
> 
> Should I actually make sure that I define 3 separate subdomains?

   You don't have to. The degrees of freedom that define a subdomain do not have to be connected. So your first subdomain could be 0,1,2,5,6 and second subdomain 3,4.   For best convergence it is better if the subdomains are connected but they need not be for correctness. If it is a struggle to make them connected then don't worry about it.

   Barry



> Currently, my subdomains are defined  depending on a parameter of the system of PDEs I’m solving.
> 
> Thanks,
> 
> Simone 
> 
>> On Sep 5, 2018, at 12:37 PM, Fande Kong <fdkong.jd at gmail.com> wrote:
>> 
>> 
>> 
>> On Wed, Sep 5, 2018 at 9:54 AM Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
>> 
>>   2 should belong to one of the subdomains, either one is fine.
>> 
>>    Barry
>> 
>> 
>> > On Sep 5, 2018, at 10:46 AM, Rossi, Simone <srossi at email.unc.edu> wrote:
>> > 
>> > I’m trying to setup GASM, but I’m probably misunderstanding something.
>> > 
>> > If I have this mesh
>> > 
>> > 0 —— 1 —— 2 —— 3 —— 4
>> > subdomain 1  |   subdomain 2
>> > 
>> 
>> You may need to make a decision which subdomain ``2" belongs to. Most people just let the shared node go to the lower MPI rank. If so, in this example, ``2" belongs to the subdomain one. 
>> 
>> iis1 = {0, 1, 2}
>> ois  = {0, 1, 2, 3}
>> 
>> iis2 = {3, 4}
>> ois2 = {2, 3, 4}
>> 
>> You consider seriously to use GASM, I would suggest to partition  your problem (using ``hierach") in such a way that multi-rank subdomain is actually connected, otherwise you may end up having a deficient performance.  
>> 
>> 
>> Thanks,
>> 
>> Fande,
>> 
>> 
>>  
>> > I create an interior (no overlap) and an outer (with overlap) IS for both subdomains.
>> > 
>> > In my naive understanding
>> > 
>> > iis1  = {0, 1}
>> > ois1 = {0, 1, 2}
>> > 
>> > and
>> > 
>> > iis2  = {3, 4}
>> > ois2 = {2, 3, 4}
>> > 
>> > but then the node at the interface (node 2) does not belong to any interior IS. Should node 2 belong to both interior IS? Or should it belong only to one of the domains?
>> > 
>> > Thanks,
>> > Simone
>> > 
>> > 
>> > On Aug 15, 2018, at 22:11, Griffith, Boyce Eugene <boyceg at email.unc.edu> wrote:
>> > 
>> >> 
>> >> 
>> >>> On Aug 15, 2018, at 10:07 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
>> >>> 
>> >>> 
>> >>> Yes you can have "overlapping fields" with FIELDSPLIT but I don't think you can use FIELDSPLIT for your case. You seem to have a geometric decomposition into regions. ASM and GASM are intended for such decompositions. Fieldsplit is for multiple fields that each live across the entire domain.
>> >> 
>> >> Basically there is one field the lives on the entire domain, and another field that lives only on a subdomain.
>> >> 
>> >> Perhaps we could do GASM for the geometric split and FIELDSPLIT within the subdomain with the two fields.
>> >> 
>> >>> Barry
>> >>> 
>> >>> 
>> >>>> On Aug 15, 2018, at 7:42 PM, Griffith, Boyce Eugene <boyceg at email.unc.edu> wrote:
>> >>>> 
>> >>>> Is it permissible to have overlapping fields in FIELDSPLIT? We are specifically thinking about how to handle DOFs living on the interface between two regions.
>> >>>> 
>> >>>> Thanks!
>> >>>> 
>> >>>> — Boyce
>> >>> 
>> >> 
>> > 
>