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#
Examples#
Implementations#
MatSeqSELLSetPreallocation_SeqSELL in src/mat/impls/sell/seq/sell.c
Index of all Mat routines
Table of Contents for all manual pages
Index of all manual pages