Linux cgroups

External

• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/resource_management_guide/ch01
• https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/resource_management_guide/chap-introduction_to_control_groups
• https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt
• https://en.wikipedia.org/wiki/Cgroups
• http://man7.org/linux/man-pages/man7/cgroups.7.html
• https://docs.docker.com/config/containers/runmetrics/#control-groups

Internal

• Docker Concepts

Overview

cgroups is a Linux kernel feature that allows allocation of resources (CPU, system memory, network bandwidth, or a combination of these) among user-defined groups of processes running on the system. cgroups not only track groups of processes, but they also expose metrics about CPU, memory and block I/O usage.

cgroups are exposed through a pseudo-filesystem available at /sys/fs/cgroup (older systems expose it at /cgroup). The sub-directories of the cgroup pseudo-filesystem root correspond to different cgroups hierarchies: cpu, freezer, blkio.

This command returns a list of the cgroups that are mounted:

cat /proc/mounts | grep cgroup

The control groups subsystems known to the system are available in /proc/cgroups:

#subsys_name	hierarchy	num_cgroups	enabled
cpuset	6	13	1
cpu	4	89	1
cpuacct	4	89	1
memory	8	89	1
devices	3	83	1
freezer	10	13	1
net_cls	5	13	1
blkio	11	89	1
perf_event	2	13	1
hugetlb	9	13	1
pids	7	13	1
net_prio	5	13	1

cgroups are organized hierarchically, child cgroups inheriting certain attributes from their parent group. Many different hierarchies of cgroups can exist simultaneously on a system. Each hierarchy is attached to one or more subsystem, where a subsystem represents a single resource like CPU time or memory.

To figure out what cgroups a process belongs to, look at /proc/<pid>/cgroup: the cgroup is shown as a path relative to the root of the hierarchy mount point. "/" means the process has not been assigned to a group, while "/lxc/something" means the process is member of a container named "something".

cgroups can be configured via the cgconfig service.

cgroups Subsystems

These subsystems are also known as "controllers":

blkio

Sets limits on input/output access from and to block devices.

cpu

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/resource_management_guide/sec-cpu

Uses the scheduler to provide cgroup tasks access to the CPU. Usually, the access to CPU is scheduled using the CFS scheduler, and the control parameters make that obvious by using "cfs" in their name. The RT scheduler is also available.

cgroups can be used to control two things:

• the relative share of CPU time to be allocated to the tasks in the cgroup.
• ceiling enforcement: a hard limit of the amount of CPU a cgroup can utilize, to prevent the "noisy neighbor" problem, given the fact that arbitrary processes can use all available CPU if they are allowed to.

Controlling Relative Share of CPU

The relative share of CPU to be allocated to the tasks in a cgroup can be controlled with an integer value specified in the "cpu.shares" file of the cgroup. The integer value specifies the share of CPU time available to the tasks in the cgroup, relative to all other tasks being scheduled at the same time. For example, tasks in two cgroups that have cpu.shares set to 100 will receive equal CPU time, but tasks in a cgroup that has cpu.shares set to 200 receive twice the CPU time of tasks in a cgroup where cpu.shares is set to 100. The value specified in the cpu.shares file must be 2 or higher.

In case of a multi-core system, shares specified in "cpu.shares" are distributed across all CPU cores of the system.

As a consequence of how CFS works, is difficult to predict how much actually CPU an arbitrary task will get, because that depends on the actual process population on a node, and what they actually do.

cpuacct

https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/resource_management_guide/sec-cpuacct

https://docs.docker.com/config/containers/runmetrics/#cpu-metrics-cpuacctstat

Generates automatic reports on CPU resources. Statistics are maintained in "cpuacct.stat", which contains the CPU usage accumulated by the processes of the group, broken down into user and system time. The times are expressed in USER_HZ.

user

"user" time is the amount of time a process has direct control of the CPU, executing process code. Also see /proc/stat cpu.

system

"system" time is the time the kernel is executing system calls on behalf of the process. Also see /proc/stat cpu.

cpuset

Assigns individual CPUs and memory nodes to tasks in a cgroup.

memory

Memory metrics are found in the "memory" cgroup. To enable memory control group, add the following kernel command-line parameters:

cgroup_enable=memory swappacount=1

The metrics are available in "memory.stat".

More details:

https://docs.docker.com/config/containers/runmetrics/#metrics-from-cgroups-memory-cpu-block-io

net_cls

Tags network packets with a tag identifier (classid) that allow the Linux traffic controller (tc) to identify packets.

ns

The namespace subsystem.

Operations

The recommended location for cgroup hierarchies:

/sys/fs/cgroup