Horizontal

Syntax

horizontal[blocks]

Where blocks is number of blocks to partition into.

Semantics

This type will split up data horizontally into a number of blocks. If the split is uneven then the extra data will be distributed amongst the blocks in the most efficient way in order to keep the blocks a similar size. The figure below illustrates horizontally partioning an array into three blocks.

Communication

There are a number of different default communication rules associated with the one dimensional partitions, based on the assignment assigned variable:=assigning variable which are detailed below.

Assigned Variable	Assigning Variable	Semantics
single	partition	Gather
partition	single	Scatter
partition	partition	Local copy

As in the last row of the table , if the two partitions are the same type then a simple copy is performed. However, if they are different then an error will be generated as Mesham disallows differently typed partitions to be assigned to each other.

The programmer can also read and write to each element in the partitioned data directly. Either the global coordinates or the block ID its local coordinates can be supplied. This will deduce whether or not the block is on another process, issue any communication as required and complete in that single assignment or access. Because this completes in that expression rather than waiting for a synchronisation, non local data movement is potentially an expensive operation.

Dot operators

Horizontal blocks also support a variety of dot operators to provide meta data

Dot operation	Semantics
high	Largest global coordinate wrt a block in specific block dimension
low	Smallest global coordinate wrt a block in specific block dimension
top	Largest global coordinate in specific block dimension
localblocks	Number of blocks held on local process
localblockid[i]	Id number of ith local block

Since: Version 0.41b