Virtual memeory and Job Scheduling

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Virtual memory

Another area of operating-system research has been the design of virtual memory. Virtual memory is a scheme that gives users the illusion of working with a large block of contiguous memory space (perhaps even larger than real memory), when in actuality most of their work is on auxiliary storage (disk). Fixed-size blocks (pages) or variable-size blocks (segments) of the job are read into main memory as needed. Questions such as how much actual main memory space to allocate to users and which page should be returned to disk ("swapped out") to make room for an incoming page must be addressed in order for the system to execute jobs efficiently. Some virtual memory issues must be continually reexamined; for example, the optimal page size may change as main memory becomes larger and quicker.

Job scheduling

The allocation of system resources to various tasks, known as job scheduling, is a major assignment of the operating system. The system maintains prioritized queues of jobs waiting for CPU time and must decide which job to take from which queue and how much time to allocate to it, so that all jobs are completed in a fair and timely manner.