- We distinguish between the locally held physical segment of an array
and the surrounding ghost region, which is used to cache local copies
of remote elements. The effect of this operation
is to overwrite a portion of the ghost region--a halo of extent
defined by the wlo, whi vectors of the constructor--with
values from processes holding the corresponding elements in their physical
segments. The operation is visualized in figure 3.6.
If the value of the mode element for a dimension is EDGE,
ghost cells past the extreme ends of the array range are not updated
by the the write-halo operation. If the value is CYCL, those
cells are updated assuming cyclic wraparound.
If the value is NONE, there is no updating at all of the ghost
cells associated with this dimension.
- Containment restrictions:
- The source array must be fully contained in the active progess group.
- Replicated data:
- If the array has replicated mapping, values for
individual copies of the ghost cell are generally taken from the nearest
copy of the corresponding physical array element. The definition of
``nearest'' is implementation dependent. This schedule does not implement
a broadcast--consistent replication of copies in the destination array
relies on consistency of copies of the source array.