Parallel Computer Architecture

152 CHAPTER 3 Programming for Performance

reduce the message frequency. Explicitly initiated communication allows the

programmer greater flexibility in specifying the sizes of messages (recall the send

primitive described in Section 23.6). On the other hand, implicit communication

through read and write operations does not afford the program direct control, and

the system must take responsibility for coalescing the reads and writes into larger

messages if necessary.

Making messages larger is easy in applications that have regular data access and

communication patterns. For example, in the message-passing equation solver parti-

tioned into rows ...