[petsc-users] Question about setting block size for arbitrary Mat formats

[Matthew Knepley]

On Wed, Oct 27, 2021 at 12:25 PM Samuel Estes <samuelestes91 at gmail.com>
wrote:

> Hi,
>
> I am solving a linear system in which the matrix has some block structure.
> We will ultimately use the BAIJ format but for now we are just using the
> default CSR and would like to play with different formats to compare
> performance for our problem. Currently, I call MatSetBlockSize so that I
> can then use MatSetValuesBlocked and MatSetValuesBlockedLocal.
>
> My question is: in the absence of specifying one of the blocked formats,
> does setting the block size with MatSetBlockSize have any real effect on
> performance? My understanding is that it is really just useful from a
> programming perspective in that it allows you to set/access Mat values in
> blocks which is often a natural way to do things. Obviously changing the
> actual format to have a blocked structure could make a difference but I
> just want to check if there's anything else going on under the hood with
> the block size when the matrix is in AIJ format.
>

There are no performance gains with MatMult I think. There is some
specialized code for inverting blocks, but it does not sound like you would
be using that.

  Thanks,

     Matt


> Thanks!
>
> Sam
>


-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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