Effect and Restrictions

Effect:

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.