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# MatSeqSELLSetPreallocation
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 MatSeqSELLSetPreallocation(Mat B, PetscInt rlenmax, const PetscInt rlen[])
```
Collective


## Input Parameters

- ***B -*** The `MATSEQSELL` matrix
- ***rlenmax -*** number of nonzeros per row (same for all rows), ignored if `rlen` is provided
- ***rlen -*** array containing the number of nonzeros in the various rows (possibly different for each row) or `NULL`





## Notes
Specify the preallocated storage with either `rlenmax` or `rlen` (not both).
Set `rlenmax` = `PETSC_DEFAULT` and `rlen` = `NULL` for PETSc to control dynamic memory
allocation.

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.


## Developer Note
Use `rlenmax` of `MAT_SKIP_ALLOCATION` to not allocate any space for the matrix
entries or columns indices.

The maximum number of nonzeos in any row should be as accurate as possible.
If it is underestimated, you will get bad performance due to reallocation
(`MatSeqXSELLReallocateSELL()`).


## See Also
 `Mat`, `MATSEQSELL`, `MATSELL`, `MatCreate()`, `MatCreateSELL()`, `MatSetValues()`, `MatGetInfo()`

## Level
intermediate

## Location
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/sell/seq/sell.c.html#MatSeqSELLSetPreallocation">src/mat/impls/sell/seq/sell.c</A>

## Examples
<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/ksp/ksp/tutorials/ex2.c.html">src/ksp/ksp/tutorials/ex2.c</A><BR>

## Implementations

<A HREF="PETSC_DOC_OUT_ROOT_PLACEHOLDER/src/mat/impls/sell/seq/sell.c.html#MatSeqSELLSetPreallocation_SeqSELL">MatSeqSELLSetPreallocation_SeqSELL in src/mat/impls/sell/seq/sell.c</A><BR>


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[Edit on GitLab](https://gitlab.com/petsc/petsc/-/edit/release/src/mat/impls/sell/seq/sell.c)


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