petsc-3.10.5 2019-03-28
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# MatGetMultiProcBlock

Create multiple [bjacobi] 'parallel submatrices' from a given 'mat' object. Each submatrix can span multiple procs.

### Synopsis

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

Collective on Mat

### Input Parameters

| mat | - the matrix
| subcomm | - the subcommunicator obtained by com_split(comm)
| scall | - either MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX

### Output Parameter

subMat -'parallel submatrices each spans a given subcomm

### Notes

The submatrix partition across processors is dictated by 'subComm' a communicator obtained by com_split(comm). The comm_split is not restriced to be grouped with consecutive original ranks.

Due the 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()

### See Also

MatCreateSubMatrices()

### 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
Index of all Mat routines
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Index of all manual pages