[petsc-users] MatCreateSubMatricesMPI strange behavior

[Alexis SALZMAN]

Hi,

Thanks for your answer, Mark. Perhaps MatCreateSubMatricesMPI is the 
only PETSc function that acts on a sub-communicator — I'm not sure — but 
it's clear that there's no ambiguity on that point. The first line of 
the documentation for that function states that it 'may live on 
subcomms'. This is confirmed by the 'src/mat/tests/ex183.c' test case. I 
used this test case to understand the function, which helped me with my 
code and the example I provided in my initial post. Unfortunately, in 
this example, the matrix from which the sub-matrices are extracted is 
dense, even though it uses a sparse structure. This does not clarify how 
to define sub-matrices when extracting from a sparse distributed matrix. 
Since my initial post, I have discovered that having more columns than 
rows can also result in the same error message.

So, my questions boil down to:

Can MatCreateSubMatricesMPI extract rectangular matrices from a square 
distributed sparse matrix?

If not, the fact that only square matrices can be extracted in this 
context should perhaps be mentioned in the documentation.

If so, I would be very grateful for any assistance in defining an IS 
pair in this context.

Regards

A.S.

Le 27/07/2025 à 00:15, Mark Adams a écrit :
> First, you can not mix communicators in PETSc calls in general 
> (ever?), but this error looks like you might be asking for a row from 
> the matrix that does not exist.
> You should start with a PETSc example code. Test it and modify it to 
> suit your needs.
>
> Good luck,
> Mark
>
> On Fri, Jul 25, 2025 at 9:31 AM Alexis SALZMAN 
> <alexis.salzman at ec-nantes.fr> wrote:
>
>     Hi,
>
>     As I am relatively new to Petsc, I may have misunderstood how to
>     use the
>     MatCreateSubMatricesMPI function. The attached code is tuned for
>     three
>     processes and extracts one matrix for each colour of a
>     subcommunicator
>     that has been created using the MPI_Comm_split function from an 
>     MPIAij
>     matrix. The following error message appears when the code is set
>     to its
>     default configuration (i.e. when a rectangular matrix is extracted
>     with
>     more rows than columns for colour 0):
>
>     [0]PETSC ERROR: --------------------- Error Message
>     --------------------------------------------------------------
>     [0]PETSC ERROR: Argument out of range
>     [0]PETSC ERROR: Column too large: col 4 max 3
>     [0]PETSC ERROR: See
>     https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!ZqH097BZ0G0O3WI7RWrwIKFNpyk0czSWEqfusAeTlgEygAffwpgBUzsLw1TIoGkjZ3mYG-NRQxxFoxU4y8EyY0ofiz9I43Qwe0w$
>     for trouble shooting.
>     [0]PETSC ERROR: Petsc Release Version 3.22.2, unknown
>
>     ... petsc git hash 2a89477b25f compiled on a dell i9 computer with
>     Gcc
>     14.3, mkl 2025.2, .....
>     [0]PETSC ERROR: #1 MatSetValues_SeqAIJ() at
>     ...petsc/src/mat/impls/aij/seq/aij.c:426
>     [0]PETSC ERROR: #2 MatSetValues() at
>     ...petsc/src/mat/interface/matrix.c:1543
>     [0]PETSC ERROR: #3 MatSetSeqMats_MPIAIJ() at
>     .../petsc/src/mat/impls/aij/mpi/mpiov.c:2965
>     [0]PETSC ERROR: #4 MatCreateSubMatricesMPI_MPIXAIJ() at
>     .../petsc/src/mat/impls/aij/mpi/mpiov.c:3163
>     [0]PETSC ERROR: #5 MatCreateSubMatricesMPI_MPIAIJ() at
>     .../petsc/src/mat/impls/aij/mpi/mpiov.c:3196
>     [0]PETSC ERROR: #6 MatCreateSubMatricesMPI() at
>     .../petsc/src/mat/interface/matrix.c:7293
>     [0]PETSC ERROR: #7 main() at sub.c:169
>
>     When the '-ok' option is selected, the code extracts a square
>     matrix for
>     colour 0, which runs smoothly in this case. Selecting the '-trans'
>     option swaps the row and column selection indices, providing a
>     transposed submatrix smoothly. For colour 1, which uses only one
>     process
>     and is therefore sequential, rectangular extraction is OK
>     regardless of
>     the shape.
>
>     Is this dependency on the shape expected? Have I missed an important
>     tuning step somewhere?
>
>     Thank you in advance for any clarification.
>
>     Regards
>
>     A.S.
>
>     P.S.: I'm sorry, but as I'm leaving my office for the following weeks
>     this evening, I won't be very responsive during this period.
>
>
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