What is a ParticleTile, and what is its relationship to the do_tiling flag and to the tiles of a MultiFab?

[n8xm]

It appears to me that we use ParticleTile regardless of whether the do_tiling flag is set to true in the inputs file. E.g. in the Electromagnetic PIC tutorial, we initialize particle_tile on lines 114-115 of EMParticleContainerInit.cpp:

auto& particles = GetParticles(lev);
auto& particle_tile = particles[std::make_pair(mfi.index(), mfi.LocalTileIndex())];

Yet do_tiling is never set in the inputs file, which means AMReX should use the default value of False. Futhermore, according to the documentation on constructing ParticleContainers:

If tiling is turned off, then every grid has its own Array-of-Structs and Struct-of-Arrays... When tiling is enabled, then each tile gets its own Struct-of-Arrays and Array-of-Structs instead.

I am now wondering:

1. Does "tile" in the above sentence refer to a ParticleTile?
2. My impression is that the do_tiling flag influences where particle data is stored, but it does not influence whether we do logical tiling over the grids with MFIter, nor does it influence whether we use the ParticleTile class. Is this correct?
3. Given that we index into particles with std::make_pair(mfi.index(), mfi.LocalTileIndex()), I am assuming that there is exactly one ParticleTile associated with each tile that we iterate over using the MFIIter (denoted as the variable mfi) in the above code. Is my assumption correct?

[atmyers]

This sentence:

If tiling is turned off, then every grid has its own Array-of-Structs and Struct-of-Arrays... When tiling is enabled, then each tile gets its own Struct-of-Arrays and Array-of-Structs instead.

is wrong, or at least misleading. The underlying data structure always stores some number of ParticleTile objects per grid and level. When do_tiling = 0, there is always exactly one tile per grid, but the object that gets returned by the [] operator of ParticleLevel is still a ParticleTile. When do_tiling = 1, each grid can have multiple ParticleTileobjects, depending on thetile_size. Setting do_tiling = 0is equivalent to setting thetile_size` to be very large in each direction.

To answer your specific questions:

1. Yes, but I will change that sentence to make it more clear / correct.
2. Yes.
3. Yes.