After the introduction of MemAvailable in the kernel v3.14 one can
estimate how much RAM memory is being "used" by how much it is left
before the system starts swapping.
That is the reason why I want to monitor memory usage, to know
how close the system is to swapping. Therefore I propose to
use MemAvailable to compute the percentage of "used" memory.
Theoretically, after this patch a 100% memory usage is a more
accurate description of "yeah, we need to swap from now on".
See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=34e431b0ae398fc54ea69ff85ec700722c9da773