[petsc-users] nonzero structure setting and remembering

[Andrew Spott]

How do I read the nonzero structure (from an assembled matrix?)

-Andrew

On May 18, 2012, at 2:31 PM, Jed Brown wrote:

> On Fri, May 18, 2012 at 3:26 PM, Andrew Spott <andrew.spott at gmail.com> wrote:
> I have a couple of questions about how to deal with the nonzero structure.
> 
> 0)  Is the "nonzero structure" the same as "number of non zeros per row"? Or are they different?
> 
> Yes, for parallel *AIJ matrices, it's the number of nonzeros in the diagonal part and in the off-diagonal part. Preallocation can optionally also set the column indices, but it's not critical.
>  
> 
> 1)  How much of a speedup do you get for copying/creating/loading matrices when you set the number of non zeros per row beforehand? (if the nonzero structure is different from the number of non zeros per row, what about the nonzero structure)
> 
> a million
>  
> 
> 2)  If I have a matrix, how do I add to the nonzero structure, without actually adding nonzero values, do I just add "0.0"? (the matrix will have values there, but doesn't at the moment)
> 
> Mat*AIJSetPreallocation(), then reassemble.
>  
> 
> As usual, thanks for all the help
> 
> -Andrew
> 

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