swapmon will check the current swapusage and if more than SWAP_HIGH per-
cent of swapspace is in use it will create a new swapfile with a size of
size of SWAP_STEP percent of the current swapspace and activate it.  If
less than SWAP_LOW percent of swapspace is in use it will deactivate a
previously added swapspace file and remove it.

freebsd@nagilum.org