Memory inside Linux containers

Or why don’t free and top work in a Linux container? Lately at Heroku, we have been trying to find the best way to expose memory usage and limits inside Linux containers. It would be easy to do it in a vendor-specific way: most container specific metrics are available at the cgroup filesystem via /path/to/cgroup/memory.stat, /path/to/cgroup/memory.usage_in_bytes, /path/to/cgroup/memory.limit_in_by