Andreas,
I am trying to read from a mesh file and impose periodic boundary
> condition. This works out very well for small geometries in three spatial
> dimensions. However, if the read meshes tend to have more elements, the
> amount of time needed to create the mesh rises drastically. The time is
> spent within the routine triangulation.add_periodicity. Thus, I have
> created a minimal code example where cubes of side lengths 5, 20, and 50
> are created. For each cube, the periodic face pairs are identified and
> collected. Afterwards, triangulation.add_periodicity is executed and the
> needed time is measured using the Timer class. The output of the attached
> code example looks as follows:
>
> Starting Constructor with side length = 5
> Periodic faces collected: Size of periodicity_vector = 75
> Start triangulation.add_periodicity(periodicity_vector); after 0.018097
> seconds
> Done triangulation.add_periodicity(periodicity_vector); after 3.0626
> seconds
>
> Starting Constructor with side length = 20
> Periodic faces collected: Size of periodicity_vector = 1200
> Start triangulation.add_periodicity(periodicity_vector); after 2.10931
> seconds
> Done triangulation.add_periodicity(periodicity_vector); after 15749
> seconds
>
> Starting Constructor with side length = 50
> Periodic faces collected: Size of periodicity_vector = 7500
> Start triangulation.add_periodicity(periodicity_vector); after 298.734
> seconds
>
Were you running in Debug or Release mode? I only tried Release mode and
the timings weren't that terrible. I observed a runtime like three times
larger in that function than for creating the mesh for up to 100 elements
per side.
Still, there are some oppurtunities for improvement, see
https://github.com/dealii/dealii/pull/8665. With these changes, creating
the mesh and adding periodicity takes roughly about the same time.
Apart from that, it is much better to have a coarse mesh with few cells and
refine a few times than starting with a rather fine coarse mesh (if you can
do that).
Best,
Daniel
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