[petsc-users] Fwd: SNES with Fieldsplit + MatNest in parallel

[Matthew Knepley]

On Tue, Nov 9, 2021 at 4:24 AM Nicolas Barnafi <nicolas.barnafi at e.email>
wrote:

>
> I am not adept enough at FEniCS to look at the code and debug. However,
> I would make a very small problem for 2 processes, say two cells on each,
> and then print out is_0 and is_1. That should tell you if you have the
> right
> unknowns in each block. If so, then it seems like something is not correct
> in
> your assembly routine, which is probably better debugged on the FEniCS
> list.
> However, if they are not right, we can help get them right.
>
>   THanks,
>
>     Matt
> --
> 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/>
>
>
> Thank Matthew, I had been testing the indexes and they looked fine. In
> fact:
>
> Rank-0, is_0: [0 1 2] [633 634 635]
> Rank-0, is_1 [636 637 638] [729 730 731]
> Rank-1, is_0: [732 733 734] [1368 1369 1370]
> Rank-1, is_1 [1371 1372 1373] [1464 1465 1466]
>
> where is_1 should be much smaller, and it is indeed the case.
>

I am now completely confused. The way PCFIELDSPLIT works, when doing Schur
complements, is to take
in two ISes, which can be thought of as lists of vector rows. These two
ISes must partition [0, N) if N is the
global size of the problem. So

  1) The numbers you have above do not appear to be a partition

  2) The problem appears to be huge. Use a problem with 2-4 to debug.

  Thanks,

     Matt


> Thanks!
> NB
>
>
>

-- 
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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