Linux uses swap space as "extra" memory for pages of application
memory that are not being actively used by the application but have
been modified (written to). The swap space size plus RAM size is the
total amount of virtual memory for the system. When most of the system's
real memory is in use, and there is a need for more, some data will be
moved into swap to free real RAM memory for use by applications or for
kernel use, such as for driver buffers, files, or network packets.
This is called swapping out. When the data that is in swap space needs
to be used it is swapped back in from swap space. The rate at which data
is swapped to and from one or more swap spaces can be monitored with the
'vmstat' command's swap-in (si) and swap-out (so) columns.
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