Virtual memory The illusion that there is no restriction on program size because an entire process need not be in memory at the same time Thrashing Inefficient processing caused by constant page swapping Process states The conceptual stages through which a process moves as it is managed by the operating system 10.3 Process Management 333 The demand paging approach gives rise to the idea of virtual memory , the illusion that there are no restrictions on the size of a program (because the entire program is not necessarily in memory at the same time anyway). In all earlier memory management techniques we examined, the entire process had to be brought into memory as a continuous whole. We there-fore always had an upper bound on process size. Demand paging removes that restriction. However, virtual memory comes with lots of overhead during the execu-tion of a program. Before, once a program was loaded into memory, it was all there and ready to go. With the virtual memory approach, we constantly have
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