Let's say you got no swap partition, but want to use some swap space.
Here's fast and simple way to do so without a reboot.
Create empty image file of desired size:
# SWAP_GB=2 # dd if=/dev/zero of=/swap bs=1024 count=$[$SWAP_GB*1024*1024]
Format it as swap space, do the permissions routine:
# mkswap /swap # chmod 0600 /swap
And then see the magic:
# free -g # swapon /swap # free -g
Don't forget to mount it at permanent basis:
# echo "/swap swap swap defaults 0 0" >> /etc/fstab
Not a single reboot was given. :)