# MatSeqSELLSetPreallocation
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 significantly. 
## 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)
-  rlen - array containing the number of nonzeros in the various rows
(possibly different for each row) or `NULL`




## Notes
If `rlen` is given then `rlenmax` is ignored.

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.  For large problems you MUST preallocate memory or you
will get TERRIBLE performance, see the users' manual chapter on matrices.

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.


## Developers 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()`).

.seealso: `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.html</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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