Effect:

 
Figure 5.1: Illustration of the effect of executing a write-halo schedule. 

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 5.1. Note that, so long as it fits in the ghost area allocated for the array, there is no restriction on the width of the halo region. In particular, the width of the halo region may be larger than the width of the neighbouring physical segment, in which case values will be fetched from next-nearest neighbours, and so on.

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.