This site runs on a 512MB DigitalOcean droplet. Every week or two the Linux kernel would kill MySQL for using too much RAM... and restarting it got tiring.
When I first start nginx, php-fpm and MySQL, the memory usage starts at 370MB and only goes upwards from there. htop (and journalctl) tells me that MySQL is the biggest offender, so let's start there.
After some googling one parameter always popped up in every guide: innodb_buffer_pool_size. MySQL docs state that it's 128MB by default. So I reduced it to 8MB to see what would happen. After all, I doubt the entirety of my posts on this site+Wordpress settings could ever reach 8MB, and even if it did, it's backed by an SSD, so who cares.
It made a huge difference. MySQL went from using 18% of my RAM (~92MB) to just 8% (43MB).
Some Googling led to A better way to run PHP, which advocates using pm = ondemand instead of pm = dynamic.
Before, php-fpm had 3 child processes taking up 18% of my RAM. Now, it has none, and it just spins them up on demand which makes a lot of sense.
I don't use php-fpm pools so I ignored that part.
390MB memory on fresh start -> 140MB!!
The system now uses ~140MB of RAM and I'm pretty proud of myself - after just two tweaks! Nobody cares about this site right now but if traffic picks up I'll just enable nginx microcaching.