3.4.2 Parallel Access Control
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3.4.2 Parallel Access Control

Given a sequential program with only a single thread, that single thread has full access to all of the program's resources. These resources are most often in-memory data structures, but can be CPUs, memory (including caches), I/O devices, computational accelerators, files, and much else besides.

The first parallel-access-control issue is whether the form of the access to a given resource depends on that resource's location. For example, in many message-passing environments, local-variable access is via expressions and assignments, while remote-variable access uses an entirely different syntax, usually involving messaging. The POSIX threads environment [Ope97], Structured Query Language (SQL) [Int92], and partitioned global address-space (PGAS) environments such as Universal Parallel C (UPC) [EGCD03] offer implicit access, while Message Passing Interface (MPI) [MPI08] offers explicit access because access to remote data requires explicit messaging.

The other parallel access-control issue is how threads coordinate access to the resources. This coordination is carried out by the very large number of synchronization mechanisms provided by various parallel languages and environments, including message passing, locking, transactions, reference counting, explicit timing, shared atomic variables, and data ownership. Many traditional parallel-programming concerns such as deadlock, livelock, and transaction rollback stem from this coordination. This framework can be elaborated to include comparisons of these synchronization mechanisms, for example locking vs. transactional memory [MMW07], but such elaboration is beyond the scope of this section.