Support container CPU & memory limits

[jimsynz]

When running within a Docker container it is possible to get information about the CPU and memory limits from /sys/fs/cgroup.

For example /proc/meminfo shows the memory information of the host machine, however /sys/fs/cgroup/memory/memory.limit_in_bytes identifies exactly how much memory this container is allowed to allocate before being killed by the container runtime.

Correspondingly /proc/cpuinfo shows information about all host CPUs, and not the number of cores that the container is limited to.

Examples:

$ docker run --rm -it -m 128m --cpuset-cpus="0-0" alpine:latest cat /proc/meminfo | grep MemTotal
MemTotal:        8165020 kB
$ docker run --rm -it -m 128m --cpuset-cpus="0-0" alpine:latest cat /sys/fs/cgroup/memory/memory.limit_in_bytes
134217728
$ docker run --rm -it -m 128m --cpuset-cpus="0-0" alpine:latest cat /proc/cpuinfo | grep processor
processor	: 0
processor	: 1
$ docker run --rm -it -m 128m --cpuset-cpus="0-0" alpine:latest cat /sys/fs/cgroup/cpuset/cpuset.cpus
0

[rbtcollins]

Interesting overlap here with my effective_limits crate: I would happily consume container info from sysinfo if it has it, or some container specific abstraction as well.