gistfile1.sh

# let's get the ID of the container. Docker uses that ID
# to name things in the host to we can probably use it to
# find the cgroup created for the container
# under the parent docker cgroup
docker ps
CONTAINER ID        IMAGE               COMMAND       
a730051832e7        cirocosta/stress    "pid -n 300"  

 # Having the prefix in hands, let's search for it under the
 # mountpoint for cgroups in our system
 find  /sys/fs/cgroup/ -name "a730051832e7*"
 
/sys/fs/cgroup/cpu,cpuacct/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/cpuset/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/devices/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/pids/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/freezer/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/perf_event/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/blkio/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/memory/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/net_cls,net_prio/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/hugetlb/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959
/sys/fs/cgroup/systemd/docker/a730051832e7d776442b2e969e057660ad108a7d6e6a30569398ec660a75a959

# There they are! Docker creates a control group with the name
# being the exact ID of the container under all the subsystems.

# What can we discover from this inspection? We can look at the
# subsystem that we want to place contrainst on (PIDs), for instance:

 tree /sys/fs/cgroup/pids/docker/a7300518327d...
/sys/fs/cgroup/pids/docker/a73005183...
├── cgroup.clone_children
├── cgroup.procs
├── notify_on_release
├── pids.current
├── pids.events
├── pids.max
└── tasks

# Which means that, if we want to know how many PIDs are in use right
# now we can look at 'pids.current', to know the limits, 'pids.max' and
# to know which processes have been assigned to this control group,
# look at tasks. Lets do it:
cat /sys/fs/cgroup/pids/docker/a730...c660a75a959/tasks 
5329
5371
5372
5373
5374
5375
5376
5377
(...)
# continues until the 300th entry - as we have 300 processes in this container

# 300 pids
cat /sys/fs/cgroup/pids/docker/a730051832e7d7764...9/pids.current
300

# no max set
cat /sys/fs/cgroup/pids/docker/a730051832e7d77.../pids.max 
max