Parallel Computer Architecture

7.2 Realizing Programming Models 473

■ Transaction ordering. Whereas a bus provides strong ordering properties

among transactions, in a network the ordering is quite weak. Even on a split-

transaction bus with multiple outstanding transactions, we could rely on the

serial arbitration for the address bus to provide a global order. Some networks

ensure that a sequence of transactions from a given source to a single destina-

tion will be seen in order at the destination; others will not even provide this

limited assurance. In either case, no node can perceive the global order. In

realizing programming models on large-scale machines, ordering constraint ...