Why does excessive disk usage occur?

Question

linux

Ubuntu 14.04 (Linux Mint), upgradable. 4Gb RAM and 8Gb swap. And the swap is actually used.
But when approaching the exhaustion of RAM, in addition to activating swap, terrible disk usage begins. Responsiveness drops to a minimum, up to several minutes. CPU level is ok.
What to do? Thank you.

Answer 1

swap and load disk

Answer 2

And you can find out what the computer is used for and what kind of percent?

Answer 3

As a result, the possible reasons are as follows:
But nothing specific...