[petsc-users] PETSc (3.9.0) GAMG weak scaling test issue

[Mark Adams]

>
>
> I did not configured PETSc with ParMetis support. Should I?
>
> I figured it out when I tried to use "-pc_gamg_repartition". PETSc
> complained that it was not compiled with ParMetis support.
>

You need ParMetis, or some parallel mesh partitioner, configured to use
repartitioning. I would guess that "-pc_gamg_repartition" would not help
and might hurt, because it just does the coarse grids, not the fine grid.
But it is worth a try. Just configure with --download-parmetis

The problem is that you are using space filling curves on the background
grid and are getting empty processors. Right?  The mesh setup phase is not
super optimized, but your times

And you said in your attachment that you added the near null space, but
just the constant vector. I trust you mean the three translational rigid
body modes. That is the default and so you should not see any difference.
If you added one vector of all 1s then that would be bad. You also want the
rotational rigid body modes. Now, you are converging pretty well and if
your solution does not have much rotation in it the the rotational modes
are not needed, but they are required for optimality in general.
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