MatGetMultiProcBlock — PETSc 3.21.4 documentation

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# MatGetMultiProcBlock

Create multiple ‘parallel submatrices’ from a given `Mat`. Each submatrix can span multiple procs.

## Synopsis

    #include "petscmat.h"
    PetscErrorCode MatGetMultiProcBlock(Mat mat, MPI_Comm subComm, MatReuse scall, Mat *subMat)

Collective

## Input Parameters

- mat - the matrix

- subComm - the sub communicator obtained as if by `MPI_Comm_split(PetscObjectComm((PetscObject)mat))`

- scall - either `MAT_INITIAL_MATRIX` or `MAT_REUSE_MATRIX`

## Output Parameter

- subMat - parallel sub-matrices each spanning a given `subcomm`

## Notes

The submatrix partition across processors is dictated by `subComm` a communicator obtained by `MPI_comm_split()` or via `PetscSubcommCreate()`. The `subComm` is not restricted to be grouped with consecutive original MPI processes.

Due the `MPI_Comm_split()` usage, the parallel layout of the submatrices map directly to the layout of the original matrix [wrt the local row,col partitioning]. So the original ‘DiagonalMat’ naturally maps into the ‘DiagonalMat’ of the `subMat`, hence it is used directly from the `subMat`. However the offDiagMat looses some columns - and this is reconstructed with `MatSetValues()`

This is used by `PCBJACOBI` when a single block spans multiple MPI processes.

## See Also

Matrices, `Mat`, `MatCreateRedundantMatrix()`, `MatCreateSubMatrices()`, `PCBJACOBI`

## Level

advanced

## Location

src/mat/interface/matrix.c

## Implementations

MatGetMultiProcBlock_MPIAIJ() in src/mat/impls/aij/mpi/mpb_aij.c
MatGetMultiProcBlock_SeqAIJ() in src/mat/impls/aij/seq/aij.c
MatGetMultiProcBlock_MPIBAIJ() in src/mat/impls/baij/mpi/mpb_baij.c

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