FreeBSD ate my ram!

Last month I posted about my journey migrating my site server from an old Ubuntu server to FreeBSD. Some people on Hacker News noticed that, when I showed the fastfetch result, I said I was confused with the RAM usage compared to btop and commented that fastfetch is probably more correct. I decided to enter that rabbit hole and try to understand why reporting free or used memory in a modern operating system is more complicated than it seems. Another user shared Linux ate my RAM, which provide a quick explanation for the same effect on Linux. And if you want a quick answer for FreeBSD too: the usage sometimes look off because the OS will cache everything it can from the disk into the RAM to improve overall performance, but that cache is volatile and will be freed in case it needs more memory. If you want a slightly longer answer, keep reading. But just a quick disclaimer before: I am not an expert in operating systems internals, especially FreeBSD. This is a writeup of weeks of…