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RIGHT ON SCHEDULE
The DG/UX 4.00 kernel includes a new scheduler based on the virtual processor abstraction originally described in 1976 by MIT's David P. Reed in his work, "Processor Multiplexing in a Layered Operating System." A virtual processor (VP) is an abstraction of a physical processor that allows the uniprocessor or multiprocessor nature of the underlying hardware to be hidden from the higher levels of the kernel.
An instance of the kernel has a fixed number of VPs that is more than the number of physical processors, but usually less than the number of processes wanting to execute. Thus, a two-level scheduling scheme is used nsee Figure 1, page 23). The lower level of scheduling multiplexes VPs onto physical processors so the VPs appear to be active entities that execute code. This short term scheduling is performed by the dispatcher. The higher level of scheduling multiplexes processes onto VPs so the processes may execute. This scheduling is performed by the medium term scheduler …