[petsc-users] nonzero structure setting and remembering

[Jed Brown]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120518/cfbfa87d/attachment.htm>