man cgroups (7): Linux control groups

DESCRIPTION

Control cgroups, usually referred to as cgroups, are a Linux kernel feature which provides for grouping of tasks and resource tracking and limitations for those groups. While several systems have been introduced to help in configuring and managing cgroups, the kernel's cgroup interface is provided through a pseudo-filesystem called cgroupfs. Task grouping is implemented in the core cgroup kernel code, while resource tracking and limits are implemented in a set of per-resource-type subsystems (memory, CPU, and so on) which may be enabled as separate hierarchies, or joined into comounted hierarchies.

Each hierarchy constitutes a separate mount of the cgroup filesystem, with the subsystems enabled in that hierarchy listed in the mount options. For each mounted hierarchy, the directory tree mirrors the control group hierarchy. Each control group is represented by a directory, with each of its child control cgroups represented as a child directory. For instance, /user/joe/1.session represents control group 1.session, which is a child of cgroup joe, which is a child of /user. Under each cgroup directory is a set of files which can be read or written to, reflecting resource limits and a few general cgroup properties.

In general, cgroup limits are hierarchical, meaning that the limits placed on /user/joe cannot be exceeded by /usr/joe/1.session. There are currently exceptions to this rule, but stricter adherence is a goal as cgroups are being largely reworked.

In addition, cgroups can be mounted with no bound subsystem, in which case they serve only to track processes. An example of this is the name=systemd cgroup which is used by systemd(1) to track services and user sessions.

Terminology

A cgroup is a collection of processes that are bound to a set of limits or parameters defined via the cgroup filesystem.

A subsystem is a kernel component that modifies the behavior of the processes in a cgroup. Various subsystems have been implemented, making it possible to do things such as limiting the amount of CPU time and memory available to a cgroup, accounting for the CPU time used by a cgroup, and freezing and resuming execution of the processes in a cgroup. Subsystems are sometimes also known as resource controllers (or simply, controllers).

The cgroups for a subsystem are arranged in a hierarchy. This hierarchy is defined by creating, removing, and renaming subdirectories within the cgroup filesystem. At each level of the hierarchy, attributes (e.g., limits) can be defined; these attributes may govern or propagate to child cgroups and and their descendants in the hierarchy.

Cgroups version 1 and version 2

The initial release of the cgroups implementation was in Linux 2.6.24. Over time, various cgroup subsystems have been added to allow the management of various types of resources. However, the development of these subsystems was largely uncoordinated, with the result that many inconsistencies arose between subsystems and management of the cgroup hierarchies became rather complex. (A longer description of these problems can be found in the kernel source file Documentation/cgroup-v2.txt.)

Because of the problems with the initial cgroups implementation, now known as cgroups version 1, starting in Linux 3.10, work began on a new, orthogonal implementation to remedy these problems. Initially marked experimental, and hidden behind the -o __DEVEL__sane_behavior mount option, the new version (cgroups version 2) was eventually made official with the release of Linux 4.5. Differences between the two versions are described in the text below.

Although cgroups v2 is intended as a replacement for cgroups v1, the older system continues to exist (and for compatibility reasons is unlikely to be removed). Currently, cgroups v2 implements only a subset of the controllers available in cgroups v1. The two systems are implemented so that both v1 controllers and v2 controllers can be mounted on the same system. Thus, for example, it is possible to use those controllers that are supported under version 2, while also using version 1 controllers where version 2 does not yet support those controllers.

Tasks versus processes

In cgroups v1, a distinction is drawn between processes and tasks. In this view, a process can consist of multiple tasks (more commonly called threads, from a user-space perspective). In cgroups v1, it is possible independently manipulate the cgroup memberships of the tasks in a process. Because this ability caused certain problems, the ability to independently manipulate the cgroup memberships of the tasks in a process has been removed in cgroups v2. Cgroups v2 allows manipulation of cgroup membership only for processes (which has the effect of changing the cgroup membership of all tasks in the process).

Mounting

To be available, a given cgroup subsystem must be compiled into the kernel. Since they are exposed through a virtual filesystem, subsystems must be mounted before they can be controlled. The usual place for this is under /sys/fs/cgroup. If all the desired subsystems can be comounted, then one can do so with the following command:

mount -t cgroup -o all cgroup /sys/fs/cgroup

(One can achieve the same result by omitting -o all, since it is the default if subsystems are explicitly specified.)

If multiple, separately mounted subsystems are desired, then this is usually done in per-subsystem subdirectories. This requires first mounting a tmpfs under /sys/fs/cgroup so that subdirectories can be created. For instance, one could mount cpu, memory, and devices cgroups as follows:

mount -t tmpfs -o size=100000,mode=755 cgroups /sys/fs/cgroup
for s in cpu memory devices; do
    mkdir /sys/fs/cgroup/$s
    mount -t cgroup -o $s $s /sys/fs/cgroup/$s
done

Comounting subsystems has the effect that a task is in the same cgroup for all comounted subsystems. Separately mounting subsystems allows a task to be in cgroup /foo1 for one subsystem while being in /foo2/foo3 for another.

Introspection

The list of subsystems compiled into the kernel can be seen in the file /proc/cgroups. The file /proc/pid/cgroup lists the task's current cgroup membership for each mounted hierarchy.

Creating cgroups and moving tasks

The system begins with a single root cgroup (per hierarchy), '/', which all tasks belong to. A new cgroup is created by creating a directory in the cgroup filesystem:

mkdir /sys/fs/cgroup/cpu/cg1

This creates a new empty cgroup. Tasks may be moved to this cgroup by writing their PIDs into the cgroup's cgroup.procs or tasks (deprecated) file:

echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs

The same file can be read to obtain a list of the processes currently in cg1. By using the cgroup.procs file instead of the tasks file, all tasks in the thread group are moved into the new cgroup at once.

On fork(2), the new child is created as a member of the parent's cgroup, leading to implicit grouping of process hierarchies.

Note: in the upcoming unified hierarchy, a new restriction is imposed such that tasks may only exist in leaf cgroups. For instance, if cgroup /cg1/cg2 exists, then a task may exist in /cg1/cg2, but not in /cg1. This is to avoid the current ambiguity in the delegation of resources between tasks in /cg1 and its child cgroups. The recommended workaround is to create a subdirectory called leaf for any non-leaf cgroup which should contain tasks, and make sure not to create child cgroups of it. In the above example, tasks which previously would have gone into /cg1 would now go into /cg1/leaf. This has the advantage of making explicit the relationship between tasks in /cg1/leaf and /cg1's other children.

Removing cgroups

To remove a cgroup, it must first have no child cgroups and contain no tasks. So long as that is the case, one can simply remove the corresponding directory pathname. Note that files in a cgroup directory cannot and need not be removed.

A special file in each cgroup hierarchy, release_agent, can be used to register a program to handle cgroups which become newly empty. The program will be called each time a cgroup marked for autoremove becomes empty and childless. The cgroup path will be provided as the first command-line argument. The cgroup must be marked as eligible for autoremove by writing '1' into its notify_on_release file; this value is inherited by newly created child cgroups.

A new feature in cgroups v2 is the cgroup.populated file. This reads 0 if there are no tasks in the cgroup or its descendants, and 1 otherwise. It can be watched for changes using inotify(7). This allows user-space applications to efficiently watch cgroups for autoremove conditions.

Cgroups version 2

In cgroups v2, all mounted controllers reside in a single unified hierarchy. While (different) controllers may be simultaneously mounted under the v1 and v2 hierarchies, it is not possible to mount the same controller simultaneously under both the v1 and the v2 hierarchies.

The new behaviors in cgroups v2 are summarized below:

1. Tasks only in leaf nodes: With the exception of the root cgroup, tasks may only reside in leaf nodes. This avoids the need to decide how to partition resources between tasks which are members of cgroup A and tasks in child cgroups of A.
2. Active cgroups must be specified: The unified hierarchy presents two new files, cgroup.controllers and cgroup.subtree_control. When a cgroup A/b is created, its cgroup.controllers file contains the list of controllers which were active in its parent, A. This is the list of controllers which are available to this cgroup. No controllers are active until they are enabled through the cgroup.subtree_control file, by writing the list of space-delimited names of the controllers, each preceded by '+' (to enable) or '-' (to disable). If the freezer controller is not enabled in /A/B, then it cannot be enabled in /A/B/C.
3. No "tasks" or "cgroup.clone_children" files
4. Empty cgroup notification: A new file, cgroup.populated, under each cgroup contains '0' when the cgroup is empty, and 1 when it is populated. It therefore may be watched to detect when a cgroup becomes (non-)empty. This replaces the original notify-on-release mechanism.
For more changes, please see the Documentation/cgroups/unified-hierarchy file in the kernel source.

Cgroups version 1 subsystems

Each of the cgroups version 1 subsystems is governed by a kernel configuration option (listed below). Additionally, the availability of the cgroups feature is governed by the CONFIG_CGROUPS kernel configuration option.

cpu (since Linux 2.6.24; CONFIG_CGROUP_SCHED)

Cgroups can be guaranteed a minimum number of "CPU shares" when a system is busy. This does not limit a cgroup's CPU usage if the CPUs are not busy.

Further information can be found in the kernel source file Documentation/scheduler/sched-bwc.txt.

cpuacct (since Linux 2.6.24; CONFIG_CGROUP_CPUACCT)

This provides accounting for CPU usage by groups of tasks.

Further information can be found in the kernel source file Documentation/cgroup-v1/cpuacct.txt.

cpuset (since Linux 2.6.24; CONFIG_CPUSETS)

This cgroup can be used to bind the tasks in a cgroup to a specified set of CPUs and NUMA nodes.

Further information can be found in the kernel source file Documentation/cgroup-v1/cpusets.txt.

memory (since Linux 2.6.25; CONFIG_MEMCG)

The memory controller supports reporting and limiting of process memory, kernel memory, and swap used by cgroups.

Further information can be found in the kernel source file Documentation/cgroup-v1/memory.txt.

devices (since Linux 2.6.26; CONFIG_CGROUP_DEVICE)

This supports controlling which tasks may create (mknod) devices as well as open them for reading or writing. The policies may be specified as whitelists and blacklists. Hierarchy is enforced, so new rules must not violate existing rules for the target or ancestor cgroups.

Further information can be found in the kernel source file Documentation/cgroup-v1/devices.txt.

freezer (since Linux 2.6.28; CONFIG_CGROUP_FREEZER)

The freezer cgroup can suspend and restore (resume) all tasks in a cgroup. Freezing a cgroup /A also causes its children, for example, tasks in /A/B, to be frozen.

Further information can be found in the kernel source file Documentation/cgroup-v1/freezer-subsystem.txt.

net_cls (since Linux 2.6.29; CONFIG_CGROUP_NET_CLASSID)

This places a classid, specified for the cgroup, on network packets created by a cgroup. These classids can then be used in firewall rules, as well as used to shape traffic using tc(8). This only applies to packets leaving the cgroup, not to traffic arriving at the cgroup.

Further information can be found in the kernel source file Documentation/cgroup-v1/net_cls.txt.

blkio (since Linux 2.6.33; CONFIG_BLK_CGROUP)

The blkio cgroup controls and limits access to specified block devices by applying IO control in the form of throttling and upper limits against leaf nodes and intermediate nodes in the storage hierarchy.

Two policies are available. The first is a proportional-weight time-based division of disk implemented with CFQ. This is in effect for leaf nodes using CFQ. The second is a throttling policy which specifies upper I/O rate limits on a device.

Further information can be found in the kernel source file Documentation/cgroup-v1/blkio-controller.txt.

perf_event (since Linux 2.6.39; CONFIG_CGROUP_PERF)

This controller allows perf monitoring of the set of processes grouped in a cgroup.

Further information can be found in the kernel source file Documentation/perf-record.txt.

net_prio (since Linux 3.3; CONFIG_CGROUP_NET_PRIO)

This allows priorities to be specified, per network interface, for cgroups.

Further information can be found in the kernel source file Documentation/cgroup-v1/net_prio.txt.

hugetlb (since Linux 3.5; CONFIG_CGROUP_HUGETLB)

This supports limiting the use of huge pages by cgroups.

Further information can be found in the kernel source file Documentation/cgroup-v1/hugetlb.txt.

pids (since Linux 4.3; CONFIG_CGROUP_PIDS)

This controller permits limiting the number of process that may be created in a cgroup (and its descendants).

Further information can be found in the kernel source file Documentation/cgroup-v1/pids.txt.

/proc files

/proc/cgroups (since Linux 2.6.24)

This file contains information about the controllers that are available on the system. An example of the contents of this file (reformatted for readability) is the following:

#subsys_name     hierarchy        num_cgroups      enabled
cpuset           4                1                1
cpu              8                1                1
cpuacct          8                1                1
blkio            6                1                1
memory           3                1                1
devices          10               84               1
freezer          7                1                1
net_cls          9                1                1
perf_event       5                1                1
net_prio         9                1                1
hugetlb          0                1                0
pids             2                1                1

The fields in this file are, from left to right:

1.

The name of the controller.

2.

The unique ID of the cgroup hierarchy on which this controller is mounted. If multiple cgroups v1 controllers are bound to the same hierarchy, then each will show the same hierarchy ID in this field. The value in this field will be 0 if:

a): the controller is not mounted on a cgroups v1 hierarchy;
b): the controller is bound to the cgroups v2 single unified hierarchy; or
c): the controller is disabled (see below).

3.

The number of control groups in this hierarchy using this controller.

4.

This field contains the value 1 if this controller is enabled, or 0 if it has been disabled (via the cgroup_disable kernel command-line boot parameter).

/proc/[pid]/cgroup (since Linux 2.6.24)

This file describes control groups to which the process with the corresponding PID belongs. The displayed information differs for cgroups version 1 and version 2 hierarchies.

For each cgroup hierarchy of which the process is a member, there is one entry containing three colon-separated fields of the form:

hierarchy-ID:subsystem-list:cgroup-path

For example:

    5:cpuacct,cpu,cpuset:/daemons

The colon-separated fields are, from left to right:

1.: For cgroups version 1 hierarchies, this field contains a unique hierarchy ID number that can be matched to a hierarchy ID in /proc/cgroups. For the cgroups version 2 hierarchy, this field contains the value 0.
2.: For cgroups version 1 hierarchies, this field contains a comma-separated list of the subsystems bound to the hierarchy. For the cgroups version 2 hierarchy, this field is empty.
3.: This field contains the pathname of the control group in the hierarchy to which the process belongs. This pathname is relative to the mount point of the hierarchy.

COLOPHON

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