Swapping" is an old mechanism where whole process images were moved from memory

to disk to free memory for another process.

"Paging" is the deactivation of pages or portions of a process's memory. That is, the least

most recently used pages are moved to disk as necessary to create free memory.


In particular, "swapping out a process"
used to be simultaneously a scheduling operating and a memory management operation.

With paging, there is no explicit connection between how much of a program's virtual

memory is resident and whether that process is schedulable. A program in a tight loop

might only have one or two pages mapped with the majority of it being paged out. In

other words, the OS just grabs pages away from processes, and doesn't give them back

unless the process asks for them again (via a page fault).


With swapping, the OS guarantees that the program is resident before
scheduling it. The program may also explicitly manage swapping in areas of memory, by

asking the OS to swap the data in before the program uses it, and telling the OS it's done

afterwards. (Windows allows memory to be managed this way.)


Paging is preferred over swapping because it does a better job of keeping only the most

important data in memory. However, paging requires restartable instructions, so that an

instruction can be restarted after a page fault has been handled. You also need an MMU

or a TLB to handle the virtual-to-physical address conversions.


Swapping is one of the Unix mechanisms to accommodate the size limitation of memory

by moving entire processes to disk to reclaim memory.

Paging is another Unix mechanism to manage the limitations of memory. Unlike

swapping, where entire processes are moved in and out of memory, paging moves only

individual pages of processes to disk. Paging is not as serious a problem as swapping, as

the entire program does not have to reside in memory to run. A small amount of paging

may not noticeably affect the performance of a system.

However, the performance of a system may degrade rapidly as paging activity increases.

Swap space on disk is used to hold pages of memory that have been paged or swapped

out. A shortage of swap space may cause symptoms such as system hanging, poor

response times, and unsuccessful spawning of new processes.

Similar posts: best online casino