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CHAPTER 5 File System Management

5.4 Swapping and Paging

SunOS uses virtual memory, so that disk area (swap space) is used as an extension of physical memory for temporary storage when the operating system tries to keep track of processes requiring more physical memory than what is available. When this happens the swap space is used for swapping and paging.

Paging is when individual memory segments, or pages, are moved to or from the swap area. When memory is low portions of a process (data areas, but not instructions which are available from local or remote file systems) are moved to free up memory space. Segments are chosen to be moved if they haven't been referenced recently. When the process next tries to reference this segment a page fault occurs and the process is suspended until the segment is returned to memory. A page fault is normally returned the first time a program is started, as it won't be in memory. It's then paged from the local or remote file system.

Swapping happens under a heavier work load. With swapping the kernel moves all segments belonging to a process to the swap area. The process is chosen if it's not expected to be run for a while. Before the process can run again it must be copied back into physical memory.

Unix System Administration - 8 AUG 1996
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