Cgroups

June 14, 2020 - Reading time: 2 minutes

Cgroups are a Linux kernel feature that limits, accounts for, and isolates the resource usage (CPU, memory, disk I/O, network, etc.) of a collection of processes.

Subsystems:

• blkio - measures and limits the amount of I/O for each group of processes
• cpu - monitors and keeps track of usage per group of processes
• cpuaccount - generates auto reports
• cpuset - used to pin a group of processes to one CPU
• devices - to control what a group of processes can do in a device
• freezer - to sigstop (stop / resume) all processes on a container
• memory - keeps track of how many 4K pages are used
• net_cls - tags network packages with specific identifiers (classid) like QoS
• net_prio - to dinamically set priorities
• huge_tlb - 2MB to 1GB memory pages.

For example, this is how the freezer system looks for the default cgroup (all processes)

root@twickenham:~# cat /sys/fs/cgroup/freezer/cgroup.procs  | wc -l
193

For a container:

root@twickenham:~# docker container ls
CONTAINER ID        IMAGE               COMMAND             CREATED             STATUS              PORTS               NAMES
69f525bf26f5        debian              "/bin/bash"         8 weeks ago         Up 2 hours                              testdebian

root@twickenham:~# cat /sys/fs/cgroup/freezer/docker/69f525bf26f5f4c16ab631ac96a16c9acbc5b5f69a924f63b4fe4fd16fd96657/cgroup.procs
1840
1939
1940

root@twickenham:~# ps aux | grep -e 1840 -e 1939 -e 1940                               root       1840  0.0  0.0   4032  3356 pts/0    Ss+  15:30   0:00 /bin/bash
root       1939  0.0  0.0  13084  1048 ?        Ss   15:41   0:00 nginx: master process nginx
systemd+   1940  0.0  0.0  13508  2692 ?        S    15:41   0:00 nginx: worker process
root       2077  0.0  0.0   6076   840 pts/0    S+   17:22   0:00 grep -e 1840 -e 1939 -e 1940

# Inside the container

root@69f525bf26f5:/# ps aux
USER        PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root          1  0.0  0.0   4032  3352 pts/0    Ss   14:30   0:00 /bin/bash
root         10  0.0  0.0  13084  1048 ?        Ss   14:41   0:00 nginx: master process
nginx        11  0.0  0.0  13508  2692 ?        S    14:41   0:00 nginx: worker process
root         12  0.0  0.0   7648  2788 pts/0    R+   16:10   0:00 ps aux