Parallel Computer Architecture

730 CHAPTER 9 Hardware/Software Trade-Offs

with less programming effort. Interestingly, in flat COMA systems, explicit migra-

tion and proper data (home) placement can still be moderately useful despite the

COMA nature. This is because ownership requests on writes still go to the directory,

which may be remote and does not migrate with the data, and thus can cause extra

traffic and contention even if only a single processor ever writes a page.

Commodity-oriented systems put more pressure on software not only to reduce

communication volume but also to orchestrate data access and communication care-

fully since the costs and/or granularities o