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# MatSeqSBAIJSetPreallocation
Creates a sparse symmetric matrix in block AIJ (block compressed row) `MATSEQSBAIJ` format. For good matrix assembly performance the user should preallocate the matrix storage by setting the parameter `nz` (or the array `nnz`).
## Synopsis
```
#include "petscmat.h"
PetscErrorCode MatSeqSBAIJSetPreallocation(Mat B, PetscInt bs, PetscInt nz, const PetscInt nnz[])
```
Collective
## Input Parameters
- ***B -*** the symmetric matrix
- ***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()`
- ***nz -*** number of block nonzeros per block row (same for all rows)
- ***nnz -*** array containing the number of block nonzeros in the upper triangular plus
diagonal portion of each block (possibly different for each block row) or `NULL`
## Options Database Keys
- ***-mat_no_unroll -*** uses code that does not unroll the loops in the
block calculations (much slower)
- ***-mat_block_size -*** size of the blocks to use (only works if a negative bs is passed in
## Notes
Specify the preallocated storage with either `nz` or `nnz` (not both).
Set `nz` = `PETSC_DEFAULT` and `nnz` = `NULL` for PETSc to control dynamic memory
allocation. See [Sparse Matrices](sec_matsparse) for details.
You can call `MatGetInfo()` to get information on how effective the preallocation was;
for example the fields mallocs,nz_allocated,nz_used,nz_unneeded;
You can also run with the option `-info` and look for messages with the string
malloc in them to see if additional memory allocation was needed.
If the `nnz` parameter is given then the `nz` parameter is ignored
## See Also
[](ch_matrices), `Mat`, [Sparse Matrices](sec_matsparse), `MATSEQSBAIJ`, `MatCreate()`, `MatCreateSeqAIJ()`, `MatSetValues()`, `MatCreateSBAIJ()`
## Level
intermediate
## Location
src/mat/impls/sbaij/seq/sbaij.c
## Examples
src/ksp/ksp/tutorials/ex2.c
src/ksp/ksp/tutorials/ex59.c
src/mat/tutorials/ex17.c
src/mat/tutorials/ex17f.F90
src/snes/tutorials/ex48.c
## Implementations
MatSeqSBAIJSetPreallocation_SeqSBAIJ in src/mat/impls/sbaij/seq/sbaij.c
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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/sbaij/seq/sbaij.c)
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