petsc-3.14.5 2021-03-03
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# MatSeqBAIJSetPreallocation

Sets the block size and expected nonzeros per row in the matrix. For good matrix assembly performance the user should preallocate the matrix storage by setting the parameter nz (or the array nnz). By setting these parameters accurately, performance during matrix assembly can be increased by more than a factor of 50.

### Synopsis

    #include "petscmat.h"
    PetscErrorCode  MatSeqBAIJSetPreallocation(Mat B,PetscInt bs,PetscInt nz,const PetscInt nnz[])

Collective

### Input Parameters

| B | - the 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 various block rows (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

### Notes

If the nnz parameter is given then the nz parameter is ignored

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.

The block AIJ format is fully compatible with standard Fortran 77 storage. That is, the stored row and column indices can begin at either one (as in Fortran) or zero. See the users' manual for details.

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 Users-Manual: ch_mat for details.

### See Also

MatCreate(), MatCreateSeqAIJ(), MatSetValues(), MatCreateBAIJ(), MatGetInfo()

### Level

intermediate

### Location

src/mat/impls/baij/seq/baij.c

### Examples

src/mat/tutorials/ex17.c.html
src/mat/tutorials/ex17f.F90.html
src/ksp/ksp/tutorials/ex59.c.html
src/snes/tutorials/ex48.c.html

### Implementations

MatSeqBAIJSetPreallocation_SeqBAIJ in src/mat/impls/baij/seq/baij.c
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