#include "petscmat.h" PetscErrorCode MatMatMult(Mat A,Mat B,MatReuse scall,PetscReal fill,Mat *C)Neighbor-wise Collective on Mat
|A||- the left matrix|
|B||- the right matrix|
|scall||- either MAT_INITIAL_MATRIX or MAT_REUSE_MATRIX|
|fill||- expected fill as ratio of nnz(C)/(nnz(A) + nnz(B)), use PETSC_DEFAULT if you do not have a good estimate if the result is a dense matrix this is irrelevent|
MAT_REUSE_MATRIX can only be used if the matrices A and B have the same nonzero pattern as in the previous call and C was obtained from a previous call to this function with MAT_INITIAL_MATRIX.
To determine the correct fill value, run with -info and search for the string "Fill ratio" to see the value actually needed.
If you have many matrices with the same non-zero structure to multiply, you should use MatProductCreate()/MatProductSymbolic(C)/ReplaceMats(), and call MatProductNumeric() repeatedly.
In the special case where matrix B (and hence C) are dense you can create the correctly sized matrix C yourself and then call this routine with MAT_REUSE_MATRIX, rather than first having MatMatMult() create it for you. You can NEVER do this if the matrix C is sparse.