[petsc-users] preallocate space for matrices if only the global number of nonzeros is known

[Jed Brown]

Klaus Burkart <k_burkart at yahoo.com> writes:

> Hi, 
> How can I preallocate space for matrices (some symmetric, others
> asymmetric) if I have the global number of nonzeros (NNZ) but not the
> number of nonzeros per row? 

Unfortunately, this is not enough for efficient assembly.  It is more
efficient to over-estimate.

> I could compute the NNZ for the upper or lower part separately if this
> would be useful for symmetric matrices.
>
> I create the matrix this way:
>     MatCreate(PETSC_COMM_WORLD, &M);
>     MatSetSizes(M, PETSC_DECIDE, PETSC_DECIDE, N, N);
>     MatSetFromOptions(M);
> Klaus