Parallel Computer Architecture

3.6 Implications for Programming Models 187

the word being referenced (e.g., a cache block). The cost of communication is thus

amortized without the programmer having to worry about preserving the coherence

of the rest of the transferred data; the system takes this responsibility. In the latter

case,

however, replication and coherence are the programmer's responsibility; so on

a miss, the system fetches only the referenced word (otherwise, the burden on the

programmer may be too large).

In message-passing systems, local references incur no more overhead than on a

uniprocessor. Communication messages, however, are very flexible and therefor