Why is swap filled if RAM is free?

Question

linux

Good afternoon.
On Ubuntu 14.04 server, KVM
htop showed:
Mem: 1736/3953MB
Swp: 502/510MB
mysql ate 1700mb, after restarting mysql, Mem:1742/3953MB, swap turned out to be 10/510MB.
For what reason can RAM not be used, but go to swap?

Answer 1

Because the Swappiness value is set like this .
Is it worth changing is a good question. You can clear it once with swapoff and swapon.