My system just won't use SWAP

Question

I have everything configured. My swap is 2gb of size, swappiness is at 40, fstab is fine.
When I start running out of RAM memory, the system just becomes totally unresponsive, and yet the swap utilisation stays at 0%.
$ free
total used free shared buffers cached
Mem: 4052008 2746176 1305832 0 126492 965644
-/+ buffers/cache: 1654040 2397968
Swap: 2184188 0 2184188
# /etc/fstab: static file system information
# <file system> <dir> <type> <options> <dump> <pass>
#devpts /dev/pts devpts defaults 0 0
#shm /dev/shm tmpfs nodev,nosuid 0 0
tmpfs /tmp tmpfs nodev,nosuid 0 0
/dev/sr0 /media/dvd auto ro,user,noauto,unhide 0 0
/dev/sda1 /mnt/Disk_D ext4 defaults,noatime 0 2
/dev/sda2 / ext4 defaults,user_xattr,noatime 0 1
/dev/sda3 /mnt/win ntfs-3g defaults,noatime,locale=en_US.utf8 0 0
/dev/sda5 none swap defaults 0 0
I am on systemd now, but it was the case since always, even before I moved.
Does anyone have any idea what could be wrong?

Answer 1

This is my take, which might not describe reality properly:
Low swappiness favours keeping programs in memory and dropping cache, high swappiness means swapping unused (least recently used) portions of programs in favour of cache. Programs can work properly with a big part of them swapped out. They also need cache. If the part of the program you want to access got swapped out or the program doesn't have the stuff it wants to use in cache, it needs to use the disk, hence one perceives a greater delay.
When you run the script, it continuously asks for more and more memory, it creates great memory pressure, so caches have to be dropped and least recently used parts of programs swapped out. Dropping cache is instantaneous, swapping out needs writing to the disk. Heavy swapping causes lack of responsiveness (light swapping out shouldn't be perceivable). How much swapping out occurs when running the script depends on how much the kernel deems viable for swapping out - the longer the system runs and the greater the amount of rarely used memory, the greater the amount that can be swapped out.
The output of your "free" shows more than 1 GB free - the kernel shouldn't worry about swapping out or dropping cache at that point, whatever the swappiness.
I prefer to have swappiness set to at least 60, so that unused portions of programs get slowly swapped out while my caches grow (my X takes 33 MB of my swap and 25 MB of my RAM at the moment).