Scheduling

Abstract

The next facility to be discussed is the scheduling of process activity. The design of the virtual machine enables the software designer to separate considerations of process scheduling from those of process design. The processes think that they have sole use of their own processor and memory. In a one-process-per-processor setup, the processes can be thought of as running continuously, subject only to synchronisation requirements; in this case the scheduling problem does not arise. However, in a multiprocessing environment, the allocation of CPU/memory to the processes must be organised by agencies within the system. As a total concept, scheduling consists of deciding when the CPU/memory will be re-scheduled; how the CPU/memory will be re-scheduled; and which new process will be chosen. When and how to re-schedule have been discussed in the previous chapter. Here we will discuss how the system selects suitable candidate processes when a CPU/memory comes up for re-allocation – the which. —