Apparently. Just call ISSort() on the IS after forming it.
> On Jan 19, 2026, at 1:31 PM, neil liu <[email protected]> wrote:
>
> Dear Petsc developers and users,
>
> I am exploring Schur complement to reuse the inverse of A, for a block matrix
> [ A B; C D].
> Here A's size is much bigger than D. Therefore it is desirable to reuse the
> inverse of A. The code is listed below. It works well for 10k tetrahedra
> elements on 4 ranks . But it shows some error for a coarse mesh (260
> tetrahedra cells) on 4 ranks.
> Do i have to sort isNonPort and isPort?
> Thanks a lot,
> Xiaodong
> MatCreateSubMatrix(A, isNonPort, isNonPort, MAT_INITIAL_MATRIX, &A11);
>
> MatCreateSubMatrix(A, isNonPort, isPorts, MAT_INITIAL_MATRIX, &A12);
>
> MatCreateSubMatrix(A, isPorts, isNonPort, MAT_INITIAL_MATRIX, &A21);
>
> MatCreateSubMatrix(A, isPorts, isPorts, MAT_INITIAL_MATRIX, &A22);
>
> Mat S;
>
> MatCreateSchurComplement(A11, A11, A12, A21, A22, &S);
>
> MatSchurComplementSetKSP(S, kspA11);
>
> Mat Sdense;
>
> MatSchurComplementComputeExplicitOperator(S, &Sdense);
>
> MatAssemblyBegin(Sdense, MAT_FINAL_ASSEMBLY);
>
> MatAssemblyEnd(Sdense, MAT_FINAL_ASSEMBLY);
>
> [1]PETSC ERROR: Argument out of range
> [1]PETSC ERROR: Column entry number 2 (actual column 0) in row 74 is not
> sorted
> [1]PETSC ERROR: See
> https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!fXUit4Ume0pJ59Ww3NtLFk7wqiN1KvrU3kRM15Vb0xZ_2zuoO91UEuvSW3QPW4ieEuamNnC1uYRjn4xD126Bbzs$
>
> <https://urldefense.us/v3/__https://petsc.org/release/faq/__;!!G_uCfscf7eWS!az-Ao1BxQdotvJlqia0i_iZ5sCx1AZNlDZwT2d1YUzJ4TE_esLV_bwZ-lyIBrsVlK3JJu36cOohpqoohAStoSQ$>
> for trouble shooting.
> [1]PETSC ERROR: PETSc Release Version 3.23.3, May 30, 2025
> [1]PETSC ERROR: ./app with 4 MPI process(es) and PETSC_ARCH
> arch-linux-c-debug
> [1]PETSC ERROR: Configure options: -with-cc=gcc --with-fc=gfortran
> --with-cxx=g++ --download-fblaslapack --download-mpich
> --with-scalar-type=complex --with-debugging=yes --download-parmetis
> --download-metis --download-hdf5 --download-scalapack --download-mumps -
> [1]PETSC ERROR: #1 MatCreateSeqAIJWithArrays() at
> /Documents/petsc-3.23.3/src/mat/impls/aij/seq/aij.c:5275
> [1]PETSC ERROR: #2 MatMPIAIJGetLocalMat() at
> /Documents/petsc-3.23.3/src/mat/impls/aij/mpi/mpiaij.c:5246
> [1]PETSC ERROR: #3 MatConvert_MPIAIJ_MPIDense() at
> /Documents/petsc-3.23.3/src/mat/impls/dense/mpi/mpidense.c:1393
> [1]PETSC ERROR: #4 MatConvert() at
> /Documents/petsc-3.23.3/src/mat/interface/matrix.c:4485
> [1]PETSC ERROR: #5 MatSchurComplementComputeExplicitOperator() at
> /Documents/petsc-3.23.3/src/ksp/ksp/utils/schurm/schurm.c:518
>
>
>
