Ubuntu – Swappiness for aggressive swapping

The difference is whether or not the "buffers" and "cached" memory is included in the "used" count.

Generally, Linux system memory is used by the kernel for two purposes: processes and file/network cache/buffers. If you look closely at the output of free, it is already shown:

             total       used       free     shared    buffers     cached
Mem:       3994908    3962396      32512          0     100852    1477032
-/+ buffers/cache:    2384512    1610396
Swap:      8000328      28468    7971860

If you add "buffers" and "cache", and then subtract that from the "used" column, you'll get the second line under "used" (the line that starts with -/+ buffers/cache), which shows about 2.3G (2384512) in use, which matches the reported memory in use that System Monitor is reporting (it is ignoring the buffers/cached because those will go away to make room for more processes).

Your grep against /proc/meminfo actually matches the first line's "free" column (32512 is close enough to 34536 -- it likely changed between the two command outputs).

Having checked sysctl -a | grep swappiness to ensure that swappiness is set as desired, and swapon to ensure that the swap partition is mounted, there remains a possibility that the swap partition has been improperly formatted for some reason.

This can be corrected by

sudo swapoff -a
sudo /sbin/mkswap /dev/hda8
sudo swapon -a

where /dev/hda8 should be replaced with the partition or swapfile that is suspected of being bad.

The sources for this answer include https://help.ubuntu.com/community/SwapFaq in the section "Why is my swap not being used?", and Swap not being used