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5.2.1 Xnet Communication

Of the two types of PE-to-PE communication available on the MasPar, Xnet is the most efficient; Xnet can communicate data between neighboring PEs at the rate of 5.75 GBytes/sec, whereas the global router achieves only 325 Mbytes/sec (both figures are for a 4096 processor machine). However, Xnet communications are only possible for local or regular (same relative distance and direction) data communication; the global router handles all irregular or nonlocal communication between PEs.

Xnet connects each PE to its corresponding neighbor PE; neighbors are indicated using the directions shown in Figure 1.

Figure 1: Xnet.

Consider the following PE array of 16 processors:

Suppose variables sum and summand reside on all the processors. If the statement

is executed, the values of summand on y and z will be added to sum on processor x. This is executed simultaneously on all processors of the PE array. Xnet treats processors on opposite edges as neighbors. For example, y would be considered the neighbor of v in the east direction, since the grid is logically arranged as if it were on the surface of a torus.