According to Bershad, Presto is a user-level thread management system originally implemented on top of Sequent’s DYNIX operating system, but later ported to DEC workstations. DYNIX provides a Presto program with a fixed number of UNIX processes that share memory. The Presto run-time system treats these processes as virtual processors and schedules the user’s threads among them. Presto’s thread interface is nearly identical to Windows NT’s. Presto is distinguished from most other thread systems in that it is structured for flexibility. Presto is easy to adapt to application-specific needs because it presents a uniform object-oriented interface to threads, synchronization, and scheduling. The object-oriented design of Presto encourages multiple implementations of the thread management functions and so offers the flexibility to efficiently accommodate differing parallel programming needs. Presto has been tuned to perform well on a multiprocessor. Presto does not provide true two-level scheduling. The base operating system, schedules the underlying virtual processors (qtd. inAnderson16).
According to Halstead, Multilisp demonstrates how thread support can be integrated into a programming language in order to simplify writing parallel programs. In Multilisp, a multiprocessor extension to LISP, the basic concurrency mechanism is the future. The future operator can be included in any Multilisp expression to spawn a new thread which computes the value of the expression in parallel. Once the value has been computed, the future resolves to that value. In the meantime, any thread that tries to use the future’s value in an expression automatically blocks until the future is resolved. With Multilisp, the programmer does not need to include any synchronization code beyond the future operator; the Multilisp interpreter keeps track of which futures remain unresolved (qtd. inAnderson17).
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