memory usage of a SeqAIJ matrix

[Bernardo Rocha]

Hi everyone,

I need to know the number of nonzero element of the matrix in an application
using PETSc. How can I do it? What is the best way to do it?

As far as I'm concerned with PETSc, running on a single processor, I'm using
the command line argument "-info" and then I get this information in some
line of the output that looks like this

[0] MatAssemblyEnd_SeqAIJ(): Matrix size: 5100 X 5100; storage space: 92706
unneeded,44994 used

then I simply get the number of used entries.

But when I have a large simulation, where the matrix does not fit into the
memory of one processor, I must use several processors. My question is how
to get the number of nonzero entries of the "global" matrix? I wrote a
simple python script to parse the output and sum the number of entries used
on each processor, but I found out that my calculations are wrong, I'm
having twice more nonzero elements (I tested against a tiny simulation on a
single processor). It seems that on the output I'm parsing I have two kinds
of informations about the entries used:

[0] MatAssemblyEnd_SeqAIJ(): Matrix size: 5100 X 5100; storage space: 92706
unneeded,44994 used

[0] MatAssemblyEnd_SeqAIJ(): Matrix size: 5100 X 5100; storage space: 0
unneeded,44994 used

That is, one that the "unneeded" field has some value and another that this
field is zero. Then I decided to discard the information where the field
"unneeded" is zero and finally the results matched perfectly with a single
processor case.

So, i would like to know (1) why do I have these lines with "0 unneeded" and
(2) if there is a more elegant way to measure this.

That's all!
Best regards,
Bernardo M. R.
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