[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:

> For a symmetric matrix:
> I assume, I should compute the number of nonzeros per row just for the upper triangle?

Yes.

>
>     Am Montag, 16. April 2018, 14:23:19 MESZ hat Jed Brown <jed at jedbrown.org> Folgendes geschrieben:  
>  
>  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