[petsc-users] GAMG for the unsymmetrical matrix

[Mark Adams]

On Tue, Apr 4, 2017 at 10:10 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>> Does this mean that GAMG works for the symmetrical matrix only?
>
>   No, it means that for non symmetric nonzero structure you need the extra flag. So use the extra flag. The reason we don't always use the flag is because it adds extra cost and isn't needed if the matrix already has a symmetric nonzero structure.

BTW, if you have symmetric non-zero structure you can just set
-pc_gamg_threshold -1.0', note the "or" in the message.

If you want to mess with the threshold then you need to use the
symmetrized flag.