6.3. Using a swap space
An initialized swap space is taken into use with
swapon. This command tells the kernel that
the swap space can be used. The path to the swap space is given
as the argument, so to start swapping on a temporary swap file
one might use the following command.
Swap spaces can be used automatically by listing them in
/dev/hda8 none swap sw 0 0
/swapfile none swap sw 0 0
The startup scripts will run the command swapon
, which will start swapping on all the swap
spaces listed in /etc/fstab
command is usually used only when
extra swap is needed.
You can monitor the use of swap spaces with
free. It will tell the total amount of swap
total used free shared
Mem: 15152 14896 256 12404 2528
-/+ buffers: 12368 2784
Swap: 32452 6684 25768
The first line of output (Mem:
) shows the
physical memory. The total column does not show the physical
memory used by the kernel, which is usually about a megabyte.
The used column shows the amount of memory used (the second
line does not count buffers). The free column shows completely
unused memory. The shared column shows the amount of memory
shared by several processes; the more, the merrier. The buffers
column shows the current size of the disk buffer cache.
That last line (Swap:) shows similar
information for the swap spaces. If this line is all zeroes,
your swap space is not activated.
The same information is available via
top, or using the proc filesystem in file
/proc/meminfo. It is currently difficult
to get information on the use of a specific swap space.
A swap space can be removed from use with
swapoff. It is usually not necessary to do it,
except for temporary swap spaces. Any pages in use in the swap
space are swapped in first; if there is not sufficient physical
memory to hold them, they will then be swapped out (to some other
swap space). If there is not enough virtual memory to hold all
of the pages Linux will start to thrash; after a long while it
should recover, but meanwhile the system is unusable. You should
check (e.g., with free) that there is enough
free memory before removing a swap space from use.
All the swap spaces that are used automatically
with swapon -a can be removed from use
with swapoff -a; it looks at the file
/etc/fstab to find what to remove.
Any manually used swap spaces will remain in use.
Sometimes a lot of swap space can be in use even though
there is a lot of free physical memory. This can happen for
instance if at one point there is need to swap, but later a big
process that occupied much of the physical memory terminates
and frees the memory. The swapped-out data is not automatically
swapped in until it is needed, so the physical memory may remain
free for a long time. There is no need to worry about this,
but it can be comforting to know what is happening.