[petsc-users] zeros in sparse aij

[Xiangdong Liang]

On Wed, Dec 7, 2011 at 12:07 PM, Satish Balay <balay at mcs.anl.gov> wrote:
> On Wed, 7 Dec 2011, Xiangdong Liang wrote:
>
>> >> >> Matrix Object:
>> >> >> type=mpiaij, rows=10, cols=10
>> >> >> total: nonzeros=20, allocated nonzeros=40
>> >> >>   not using I-node (on process 0) routines
>> >> >>
>> >> >> Is there a way to have nonzeros=10, and allocated nonzeros=10, since
>> >> >> the number of real nonzeros=10? Thanks.
>> >> >
>> >> >   Those "extra" nonzeros are not actually in the sparse matrix so they
>> >> > do not cause extra multiples. You can safely ignore them.
>> >>
>> >> I am sorry I am still not clear.   Do "extra" nonzeros refer to
>> >> “nonzeros=20” or "allocated nonzeros=40"? With or without
>> >> MatSetOption(A,MAT_IGNORE_ZERO_ENTRIES,PETSC_TRUE), I see the same
>> >> mat_view_info. How can I see the differences in the spare matrix
>> >> generated With/Without this option? Thanks.
>> >
>> >
>> > If there are 2 nonzeros per row, there are 20 nonzeros not 10 in the matrix.
>> > According to
>> > the output, you allocated 40 nonzeros for the matrix.
>>
>> Now, I understand the "allocated nonzeros=40". How can I bring the
>> other one " total nonzero=20" down to " total nonzero=10", since 10
>> out of 20 values are 0.0? Thanks.
>
> As indicated call "MatSetOption(A,MAT_IGNORE_ZERO_ENTRIES,PETSC_TRUE)
> - before any calls to MatSetValue/s()

Great! It works now. I should pay attention to the order. Thank you.

With mat_view_info, now it looks like:
Matrix Object:
  type=mpiaij, rows=10, cols=10
  total: nonzeros=10, allocated nonzeros=40
    not using I-node (on process 0) routines




>
> And verify with MatView()
>
> Satish