[pikrzyszto]

Remember that a lot of software not only relies on memory that the developer explicitly allocated but also on buffers and various caches, disk cache being an example. It goes a long way - kernel might refuse dropping disk cache when it considers it's too hot (ex. db machine.)

The question "how much free memory do I have" or "how much memory does this program use" is a very hard one, and you can't just read the "MemAvailable" column from /proc/meminfo or RSS/PSS/Virt from ps/pmap.