Why does Out of memory: Kill process... occur even though there is free memory?

Question

MySQL

Various processes are regularly killed on the server due to lack of memory, for example, such entries in the system log:

Out of memory: Kill process 29475 (mysqld) score 80 or sacrifice child
...
Out of memory: Kill process 32263 (php-fpm7.0) score 8 or sacrifice child
...
Out of memory: Kill process 31177 (memcached) score 11 or sacrifice child

(Moreover, if memcached is killed, it won't start by itself...)
At the same time, on the memory monitoring graph, the consumption fluctuates between 40-70%
. What is the reason? How to overcome? What can be done so that at least memcached is not killed or starts itself after the kill?
Ubuntu 16.04.2LTS

Answer 1

do a swap
https://www.digitalocean.com/community/tutorials/h...

Answer 2

although there is free memory?

just like that, rand(0, 100) == 1
There is not enough memory at peak times.
Monitors are collected every 1-5 minutes and peaks may not be visible there.