Hello Wolfgang,
I went through the functions again and would be grateful if you could tell
me if I have got understood anything wrong here:
In file p4est_wrappers.cc from line 46:
1. As before, I would like to iterate over all the ghost layers to identify
the cells over which I need to exchange the indices. The function that I
think I would need to modify would be the
compute_vertices_with_ghost_neighbors() which calls the p4est_iterate
function. Considering one of the callbacks of this function,
find_ghosts_corner() which takes the sides (of type dealii::internal::p4est
::iter<dim>::corner_side) variable and checks if any of the sides of that
corner belong to a ghost cell using the cell_from_quad function.
In this function, there are two for loops (line 64 to 74 and line 84 to
97). First loop returns the ghost cell and gets the subdomain_id of that
cell into the sc_array subids and the second loop adds this subdomain_id
information to the vertices_with_ghost_neighbors.
I was wondering if it possible to also do something like this in the first
loop as well:
for (j = 0; j < nsubs; j++)
{
(*vertices_with_ghost_neighbors)[cell->vertex_index(
sides[i].corner)]
.insert (subdomain_ids[j]);
}
so that even the ghost cells to the first layer of ghost cells are added to
the vertices_with_ghost_neighbors. Would this work ? Is there a better way ?
Thank you,
Pratik.
On Thursday, April 12, 2018 at 1:57:32 AM UTC-4, Wolfgang Bangerth wrote:
>
>
> > 1. I tried out the p4est_ghost_expand as Wolfgang suggested, but it
> gives me
> > an error because for some reason I get
> > if(cell->is_ghost){cell->user_flag_set()==true} after the calls
> > communicate_dof_indices_on_marked_cells() in dof_handler_policy.cc line
> 3875.
> > I believe the function communicate_dof_indices_on_marked_cells() in line
> 3492
> > in the same file exchanges the dof indices (sending the local cells that
> are
> > ghost somewhere else and receiving its ghost cells from other procs) but
> the
> > unpack function should identify all the cells marked as ghost and
> receive the
> > corresponding indices from the pack function, right ? Or have I
> misunderstood
> > something here ?
>
> Hm, good point. When we decide who each process needs to send the locally
> owned DoF indices to (so that they can be set on ghost cells on the
> receiving
> process), we determine the cells in question by asking whether one of
> their
> vertices is adjacent to a ghost cell -- in other words, whether a cell is
> at
> the boundary of the locally owned subdomain.
>
> But if you have more than one layer of ghost cells, then this is not
> enough. I
> think you need to extend the logic in a way where each process sends a
> list of
> its ghost cells to the owning process of these cells, and then gets the
> information back for exactly those cells. That may be a little bit of
> work,
> but I don't think it's going to be terribly difficult to implement. Let us
> know if you need help with this.
>
>
> > 2. All places in example 40 (my reference) for the matrix and rhs
> assembly,
> > you use the locally_relevant_dofs and locally_owned_dofs. As you say in
> the
> > Glossary
> > <
> http://dealii.org/developer/doxygen/deal.II/DEALGlossary.html#GlossParallelScaling>,
>
>
> > for locally_active_dofs
> >
> > " Since degrees of freedom are owned by only one processor, degrees of
> freedom
> > on interfaces between cells owned by different processors may be owned
> by one
> > or the other, so not all degrees of freedom on a locally owned cell are
> also
> > locally owned degrees of freedom."
> >
> > And hence you assemble your local matrices with the locally_owned_dofs.
> But
> > for the overlapped decomposition, I need to assemble matrices including
> the
> > "ghost cells" . In my case, they wouldn't be ghost cells but locally
> owned
> > cells but duplicated between logically overlapping processes. I am not
> really
> > sure how to go about doing this.
>
> This is what I tried to explain in my previous email: on each process, you
> need a separate enumeration of all degrees of freedom that live on that
> process (either on locally owned or ghost cells). This enumeration must be
> independent of the "global" enumeration we typically use in deal.II which
> uniquely splits up all DoF indices among the processors.
>
> Best
> W.
>
> --
> ------------------------------------------------------------------------
> Wolfgang Bangerth email: bang...@colostate.edu
> <javascript:>
> www: http://www.math.colostate.edu/~bangerth/
>
>
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