Setting absolute limits on CPU for Docker containers

Question

I'm trying to set absolute limits on Docker container CPU usage. The CPU shares concept (docker run -c <shares>) is relative, but I would like to say something like "let this container use at most 20ms of CPU time every 100ms. The closest answer I can find is a hint from the mailing list on using cpu.cfs_quota_us and cpu.cfs_period_us. How does one use these settings when using docker run?

I don't have a strict requirement for either LXC-backed Docker (e.g. pre0.9) or later versions, just need to see an example of these settings being used--any links to relevant documentation or helpful blogs are very welcome too. I am currently using Ubuntu 12.04, and under /sys/fs/cgroup/cpu/docker I see these options:

$ ls /sys/fs/cgroup/cpu/docker
cgroup.clone_children  cpu.cfs_quota_us   cpu.stat
cgroup.event_control   cpu.rt_period_us   notify_on_release
cgroup.procs           cpu.rt_runtime_us  tasks
cpu.cfs_period_us      cpu.shares

Answer 1

I believe I've gotten this working. I had to restart my Docker daemon with --exec-driver=lxc as I could not find a way to pass cgroup arguments to libcontainer. This approach worked for me:

# Run with absolute limit
sudo docker run --lxc-conf="lxc.cgroup.cpu.cfs_quota_us=50000" -it ubuntu bash

The necessary CFS docs on bandwidth limiting are here.

I briefly confirmed with sysbench that this does seem to introduce an absolute limit, as shown below:

$ sudo docker run --lxc-conf="lxc.cgroup.cpu.cfs_quota_us=10000" --lxc-conf="lxc.cgroup.cpu.cfs_period_us=50000" -it ubuntu bash
root@302e651c0686:/# sysbench --test=cpu --num-threads=1 run
   <snip> 
   total time:                          90.5450s
$ sudo docker run --lxc-conf="lxc.cgroup.cpu.cfs_quota_us=20000" --lxc-conf="lxc.cgroup.cpu.cfs_period_us=50000" -it ubuntu bash
root@302e651c0686:/# sysbench --test=cpu --num-threads=1 run
   <snip> 
    total time:                          45.0423s