Unexpected ghost cell data overwrite when copying MultiFab with overlapping boxes

[Afraid31]

I'm working with AMReX in 2D using cell-centered data. I have a MultiFab fi[1] with ghost cells, and I want to create an identical copy of it, including ghost cell data, into fi_p[1]. Both fi[1] and fi_p[1] share the same BoxArray, DistributionMapping, number of components, and number of ghost cells.

I used the following line to copy the data:

fi_p[1].copy(fi[1], 0, 0, fi[1].nComp(),
             IntVect(fi[1].nGrow()), IntVect(fi_p[1].nGrow()),
             geom[1].periodicity());

However, I noticed that when the BoxArray consists of two adjacent boxes, their ghost cells overlap. As a result, during the copy, the overlapping ghost regions in fi_p[1] seem to be populated with data from the ghost regions of neighboring FABs in fi[1], rather than being copied strictly from the corresponding FAB in fi[1].

That is, ghost data from the left FAB in fi[1] appears in the overlapping ghost region of the right FAB in fi_p[1], and vice versa.

My understanding was that MultiFab::copy() would just copy data box by box, including ghost regions, and not attempt to fill ghost regions via neighbor communication (similar to FillBoundary() behavior). But it seems like even a simple copy() operation triggers overlap communication between FABs.

Questions:

Is this expected behavior in MultiFab::copy()?

If I want to clone a MultiFab, including ghost cells, without any ghost-overlap filling or internal communication, what is the correct way to do it?

Would manually iterating over the FABs and using fab.copy() be the correct and safe alternative?

Thanks in advance!

[WeiqunZhang]

This is expected behavior. The copy function, which is equivalent to ParallelCopy, has been deprecated because it causes confusion. For your case, you can do

fi_p[1].LocalCopy(fi[1], 0, 0, fi[1].nComp(), fi[1].nGrowVect());

[WeiqunZhang]

If you use gcc and amrex's gnu make system, make ERROR_DEPRECATED=TRUE will add -Werror=deprecated to the compiler flag, and then your code will not compile.

P.S. Here is the doxygen documentation for LocalCopy. https://amrex-codes.github.io/amrex/doxygen/classamrex_1_1FabArray.html#a5086d98e3a63866d87afcd150a8170ef

[Afraid31]

Thank you very much for the clarification — that makes perfect sense now!

I wasn’t aware that copy() had been deprecated and that it implicitly performs communication across ghost cells. Using LocalCopy is exactly what I need here. I appreciate the explanation and the pointer to the documentation.

Thanks again for your help!