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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 ranks.

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 ranks


## See Also
 [](ch_matrices), `Mat`, `MatCreateRedundantMatrix()`, `MatCreateSubMatrices()`, `PCBJACOBI`

## Level
advanced

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/interface/matrix.c.html#MatGetMultiProcBlock">src/mat/interface/matrix.c</A>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/aij/mpi/mpb_aij.c.html#MatGetMultiProcBlock_MPIAIJ">MatGetMultiProcBlock_MPIAIJ in src/mat/impls/aij/mpi/mpb_aij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/aij/seq/aij.c.html#MatGetMultiProcBlock_SeqAIJ">MatGetMultiProcBlock_SeqAIJ in src/mat/impls/aij/seq/aij.c</A><BR>
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/baij/mpi/mpb_baij.c.html#MatGetMultiProcBlock_MPIBAIJ">MatGetMultiProcBlock_MPIBAIJ in src/mat/impls/baij/mpi/mpb_baij.c</A><BR>


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