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# MatCreateBAIJMKL
Creates a sparse parallel matrix in `MATBAIJMKL` format (block compressed row). 
## Synopsis
```
PetscErrorCode MatCreateBAIJMKL(MPI_Comm comm, PetscInt bs, PetscInt m, PetscInt n, PetscInt M, PetscInt N, PetscInt d_nz, const PetscInt d_nnz[], PetscInt o_nz, const PetscInt o_nnz[], Mat *A)
```
Collective


## Input Parameters

- ***comm -*** MPI communicator
- ***bs   -*** size of block, the blocks are ALWAYS square. One can use `MatSetBlockSizes()` to set a different row and column blocksize but the row
blocksize always defines the size of the blocks. The column blocksize sets the blocksize of the vectors obtained with `MatCreateVecs()`
- ***m -*** number of local rows (or `PETSC_DECIDE` to have calculated if `M` is given)
This value should be the same as the local size used in creating the
y vector for the matrix-vector product y = Ax.
- ***n -*** number of local columns (or `PETSC_DECIDE` to have calculated if `N` is given)
This value should be the same as the local size used in creating the
x vector for the matrix-vector product y = Ax.
- ***M -*** number of global rows (or `PETSC_DETERMINE` to have calculated if `m` is given)
- ***N -*** number of global columns (or `PETSC_DETERMINE` to have calculated if `n` is given)
- ***d_nz  -*** number of nonzero blocks per block row in diagonal portion of local
submatrix  (same for all local rows)
- ***d_nnz -*** array containing the number of nonzero blocks in the various block rows
of the in diagonal portion of the local (possibly different for each block
row) or `NULL`.  If you plan to factor the matrix you must leave room for the diagonal entry
and set it even if it is zero.
- ***o_nz  -*** number of nonzero blocks per block row in the off-diagonal portion of local
submatrix (same for all local rows).
- ***o_nnz -*** array containing the number of nonzero blocks in the various block rows of the
off-diagonal portion of the local submatrix (possibly different for
each block row) or `NULL`.



## Output Parameter

- ***A -*** the matrix



## Options Database Keys

- ***-mat_block_size -*** size of the blocks to use
- ***-mat_use_hash_table <fact> -*** set hash table factor





## Notes
It is recommended that one use the `MatCreate()`, `MatSetType()` and/or `MatSetFromOptions()`,
MatXXXXSetPreallocation() paradigm instead of this routine directly.
[MatXXXXSetPreallocation() is, for example, `MatSeqAIJSetPreallocation()`]

This type inherits from `MATBAIJ` and is largely identical, but uses sparse BLAS
routines from Intel MKL whenever possible.
`MatMult()`, `MatMultAdd()`, `MatMultTranspose()`, and `MatMultTransposeAdd()`
operations are currently supported.
If the installed version of MKL supports the "SpMV2" sparse
inspector-executor routines, then those are used by default.
Default PETSc kernels are used otherwise.
For good matrix assembly performance the user should preallocate the matrix
storage by setting the parameters `d_nz` (or `d_nnz`) and `o_nz` (or `o_nnz`).
By setting these parameters accurately, performance can be increased by more
than a factor of 50.

If the *_nnz parameter is given then the *_nz parameter is ignored

A nonzero block is any block that as 1 or more nonzeros in it

The user MUST specify either the local or global matrix dimensions
(possibly both).

If `PETSC_DECIDE` or  `PETSC_DETERMINE` is used for a particular argument on one processor
than it must be used on all processors that share the object for that argument.


## Storage Information
For a square global matrix we define each processor's diagonal portion
to be its local rows and the corresponding columns (a square submatrix);
each processor's off-diagonal portion encompasses the remainder of the
local matrix (a rectangular submatrix).

The user can specify preallocated storage for the diagonal part of
the local submatrix with either `d_nz` or `d_nnz` (not both).  Set
`d_nz` = `PETSC_DEFAULT` and `d_nnz` = `NULL` for PETSc to control dynamic
memory allocation.  Likewise, specify preallocated storage for the
off-diagonal part of the local submatrix with `o_nz` or `o_nnz` (not both).

Consider a processor that owns rows 3, 4 and 5 of a parallel matrix. In
the figure below we depict these three local rows and all columns (0-11).

```none
           0 1 2 3 4 5 6 7 8 9 10 11
          --------------------------
   row 3  |o o o d d d o o o o  o  o
   row 4  |o o o d d d o o o o  o  o
   row 5  |o o o d d d o o o o  o  o
          --------------------------
```


Thus, any entries in the d locations are stored in the d (diagonal)
submatrix, and any entries in the o locations are stored in the
o (off-diagonal) submatrix.  Note that the d and the o submatrices are
stored simply in the `MATSEQBAIJMKL` format for compressed row storage.

Now `d_nz` should indicate the number of block nonzeros per row in the d matrix,
and `o_nz` should indicate the number of block nonzeros per row in the o matrix.
In general, for PDE problems in which most nonzeros are near the diagonal,
one expects `d_nz` >> `o_nz`.


## See Also
 [](ch_matrices), `Mat`, `MATBAIJMKL`, `MATBAIJ`, `MatCreate()`, `MatCreateSeqBAIJMKL()`, `MatSetValues()`, `MatCreateBAIJMKL()`, `MatMPIBAIJSetPreallocation()`, `MatMPIBAIJSetPreallocationCSR()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/baij/mpi/baijmkl/mpibaijmkl.c.html#MatCreateBAIJMKL">src/mat/impls/baij/mpi/baijmkl/mpibaijmkl.c</A>


---
[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/baij/mpi/baijmkl/mpibaijmkl.c)


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