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()).