I could not find any in-ruby solution for this question, so here is a unix-solution.

def memory
  pid = Process.pid
  map = `pmap -d #{pid}`
  map.split("\n").last
end

Output

mapped: 17652K    writeable/private: 2180K    shared: 0K

Lesson learned
Use `open(file).each(seperator)` over `open(file).read.split(seperator)` if you want to optimize for memory (is slightly slower)