Unused or Inactive memory is memory that hasn't been accessed recently. The kernel maintains LRU (least recently used) lists for most of its memory pages. The kernel memory management works on the assumption that the least recently used pages are least likely to be accessed soon. Under memory pressure, when the kernel needs to free some memory pages, it swaps out pages at the tail of the inactive anonymous LRU.

Cgroup limits and OOM scores allow to prioritize memory usage on a per-process and per-process group basis. madvise(2) syscall allows to prioritize memory usage within a process.