GoKawiilFreeBSD 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 research in this field on my free time. If you find anything that’s particularly wrong, please comment it: sharing (knowledge) is caring!
RAM usage is hard to define
The whole point of Linux ate my RAM is explaining how unused RAM is wasted RAM. Just like the CPU cache will cache RAM contents because the CPU can access that quicker, the RAM will cache disk data to improve the user’s experience in the system. How that cache works is a bit more complicated, but before that, it’s important to understand how the kernel manages RAM.
Most modern operating systems have a Virtual Memory (VM) system. What it does is basically divide the physical memory into pages of (usually) 4KiB. Each page is then added to different queues, so that the kernel can juggle them around to make sure all the processes have their memory when they need and the whole system will keep working through moments of scarcity. For example: the swap memory. I never thought exactly how the Swap memory was used, except that it’s a space separate in disk that will store temporarily part of the RAM if needed. But in summary, when the OS sees allocated RAM that’s not being used too much, it will set it in a way that it can be stored in disk in case more memory is demanded. When those pages are requested again by the program that owns it, it will then get moved back into RAM.
Every OS has a different set of pages and rules for how to manage them. On FreeBSD, the types of page queues are:
#define PQ_NONE 255 #define PQ_INACTIVE 0 #define PQ_ACTIVE 1 #define PQ_LAUNDRY 2 #define PQ_UNSWAPPABLE 3 #define PQ_COUNT 4
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