Order of pairs is non uniform between Lattice sets about fluidx3d

[ProjectPhysX]

Hi @Meerkov,

this is the nature of the Esoteric-Pull streaming scheme, it loads/stores the DDFs in asymmetric locations, which solves the data-dependencies of the streaming to eliminate the 2nd copy of the grid in VRAM. This asymmetric storage pattern has no effect on the physics of the simulatuion. The location of every pair of opposing DDFs ((1,2), (3,4), ...) on the grid is irrelevant: individual pairs can be off-set by any number of cells in any direction, as long as they are consistently stored in swapped order in the same locations. The variant I have, where one DDF of every pair always is in the center cell, has best possible memory coalescence and is fastest.

I'd argue that i+1u (= DDF in opposite direction of i, see here Figure 1) is easier than rotate(i,2), although through loop unrolling there will not be additional integer computation for either.

Kind regards,
Moritz

[Meerkov]

There are 2 design choices mentioned here:

1. order for convenient 90 degree turns (currently in no particular order)
2. pull direction such that it always prefers to pull in preference order +X, +Y, +Z. (currently pulls with slight preference from +X, but not consistently).

Since neither impacts the speed of the algorithm, but they have other nice properties which can be used, I see it as a Feature Request, but I understand it would not be accepted as the purpose of these changes is non-obvious. This creates some useful properties for cubed sphere grids.