memory usage of a SeqAIJ matrix

[Barry Smith]

   You can also call MatGetInfo() with a second argument of  
MAT_GLOBAL_SUM.

    Barry

On Sep 15, 2009, at 2:07 PM, Bernardo Rocha wrote:

> Thanks a lot! =)
>
> 2009/9/15 Matthew Knepley <knepley at gmail.com>
> You can use the output of -ksp_view, which gives the matrix  
> information.
>
>    Matt
>
>
> On Tue, Sep 15, 2009 at 2:01 PM, Bernardo Rocha  
> <bernardosk at gmail.com> wrote:
> 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.
>
>
>
> -- 
> What most experimenters take for granted before they begin their  
> experiments is infinitely more interesting than any results to which  
> their experiments lead.
> -- Norbert Wiener
>