How to efficiently use server memory?

Question

System administration

There are a number of servers that are loaded on the processor, but idle on the memory.

An example is an application server, it has 8gb ram, but only 2GB is occupied.
The database server is the same, no more than two out of 8GB are occupied.
There is no processor in abundance, but memory - please.
The load on the server is about the

same. Is it possible to somehow put idle memory into action?
The application server is spinning en php-fpm + nginx + memcache
DB MariaDB (innoDB + myisam) It's trite to

shove all the data into the cache - we tried it. Efficiency decreases, in comparison with the database (the database is faster).
Give advice on where you can attach memory with a profit :)

Answer 1

Give advice on where you can attach memory with a profit :)

look at the yellow line in htop - the memory has long been disposed of correctly for caches
So don't touch anything

Answer 2

incorrectly shoved into the cache, the database, in principle, cannot be faster than the cache.