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 controller 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. The limits, control, and accounting provided by cgroups generally have effect throughout the subhierarchy underneath the cgroup where the attributes are defined. Thus, for example, the limits placed on a cgroup at a higher level in the hierarchy cannot be exceeded by descendant cgroups.

Cgroups version 1 and version 2

The initial release of the cgroups implementation was in Linux 2.6.24. Over time, various cgroup controllers have been added to allow the management of various types of resources. However, the development of these controllers was largely uncoordinated, with the result that many inconsistencies arose between controllers and management of the cgroup hierarchies became rather complex. A longer description of these problems can be found in the kernel source file Documentation/admin−guide/cgroup−v2.rst (or Documentation/cgroup−v2.txt in Linux 4.17 and earlier).

Because of the problems with the initial cgroups implementation (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. The file cgroup.sane_behavior, present in cgroups v1, is a relic of this mount option. The file always reports "0" and is only retained for backward compatibility.

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. The only restriction here is that a controller can't be simultaneously employed in both a cgroups v1 hierarchy and in the cgroups v2 hierarchy.

Tasks (threads) 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, and called such in the remainder of this man page). In cgroups v1, it is possible to independently manipulate the cgroup memberships of the threads in a process.

The cgroups v1 ability to split threads across different cgroups caused problems in some cases. For example, it made no sense for the memory controller, since all of the threads of a process share a single address space. Because of these problems, the ability to independently manipulate the cgroup memberships of the threads in a process was removed in the initial cgroups v2 implementation, and subsequently restored in a more limited form (see the discussion of "thread mode" below).

Mounting v1 controllers

The use of cgroups requires a kernel built with the CONFIG_CGROUP option. In addition, each of the v1 controllers has an associated configuration option that must be set in order to employ that controller.

In order to use a v1 controller, it must be mounted against a cgroup filesystem. The usual place for such mounts is under a tmpfs(5) filesystem mounted at /sys/fs/cgroup. Thus, one might mount the cpu controller as follows:

It is possible to comount multiple controllers against the same hierarchy. For example, here the cpu and cpuacct controllers are comounted against a single hierarchy:

Comounting controllers has the effect that a process is in the same cgroup for all of the comounted controllers. Separately mounting controllers allows a process to be in cgroup /foo1 for one controller while being in /foo2/foo3 for another.

It is possible to comount all v1 controllers against the same hierarchy:

(One can achieve the same result by omitting −o all, since it is the default if no controllers are explicitly specified.)

It is not possible to mount the same controller against multiple cgroup hierarchies. For example, it is not possible to mount both the cpu and cpuacct controllers against one hierarchy, and to mount the cpu controller alone against another hierarchy. It is possible to create multiple mount points with exactly the same set of comounted controllers. However, in this case all that results is multiple mount points providing a view of the same hierarchy.

Note that on many systems, the v1 controllers are automatically mounted under /sys/fs/cgroup; in particular, systemd(1) automatically creates such mount points.

Unmounting v1 controllers

A mounted cgroup filesystem can be unmounted using the umount(8) command, as in the following example:

But note well: a cgroup filesystem is unmounted only if it is not busy, that is, it has no child cgroups. If this is not the case, then the only effect of the umount(8) is to make the mount invisible. Thus, to ensure that the mount point is really removed, one must first remove all child cgroups, which in turn can be done only after all member processes have been moved from those cgroups to the root cgroup.

Cgroups version 1 controllers

Each of the cgroups version 1 controllers 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.

Creating cgroups and moving processes

A cgroup filesystem initially contains a single root cgroup, '/', which all processes belong to. A new cgroup is created by creating a directory in the cgroup filesystem:

This creates a new empty cgroup.

A process may be moved to this cgroup by writing its PID into the cgroup's cgroup.procs file:

Only one PID at a time should be written to this file.

Writing the value 0 to a cgroup.procs file causes the writing process to be moved to the corresponding cgroup.

When writing a PID into the cgroup.procs, all threads in the process are moved into the new cgroup at once.

Within a hierarchy, a process can be a member of exactly one cgroup. Writing a process's PID to a cgroup.procs file automatically removes it from the cgroup of which it was previously a member.

The cgroup.procs file can be read to obtain a list of the processes that are members of a cgroup. The returned list of PIDs is not guaranteed to be in order. Nor is it guaranteed to be free of duplicates. (For example, a PID may be recycled while reading from the list.)

In cgroups v1, an individual thread can be moved to another cgroup by writing its thread ID (i.e., the kernel thread ID returned by clone(2) and gettid(2)) to the tasks file in a cgroup directory. This file can be read to discover the set of threads that are members of the cgroup.

Removing cgroups

To remove a cgroup, it must first have no child cgroups and contain no (nonzombie) processes. 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.

Cgroups v1 release notification

Two files can be used to determine whether the kernel provides notifications when a cgroup becomes empty. A cgroup is considered to be empty when it contains no child cgroups and no member processes.

A special file in the root directory of each cgroup hierarchy, release_agent, can be used to register the pathname of a program that may be invoked when a cgroup in the hierarchy becomes empty. The pathname of the newly empty cgroup (relative to the cgroup mount point) is provided as the sole command-line argument when the release_agent program is invoked. The release_agent program might remove the cgroup directory, or perhaps repopulate it with a process.

The default value of the release_agent file is empty, meaning that no release agent is invoked.

The content of the release_agent file can also be specified via a mount option when the cgroup filesystem is mounted:

Whether or not the release_agent program is invoked when a particular cgroup becomes empty is determined by the value in the notify_on_release file in the corresponding cgroup directory. If this file contains the value 0, then the release_agent program is not invoked. If it contains the value 1, the release_agent program is invoked. The default value for this file in the root cgroup is 0. At the time when a new cgroup is created, the value in this file is inherited from the corresponding file in the parent cgroup.

Cgroup v1 named hierarchies

In cgroups v1, it is possible to mount a cgroup hierarchy that has no attached controllers:

Multiple instances of such hierarchies can be mounted; each hierarchy must have a unique name. The only purpose of such hierarchies is to track processes. (See the discussion of release notification below.) An example of this is the name=systemd cgroup hierarchy that is used by systemd(1) to track services and user sessions.

Since Linux 5.0, the cgroup_no_v1 kernel boot option (described below) can be used to disable cgroup v1 named hierarchies, by specifying cgroup_no_v1=named.

Cgroups v2 unified hierarchy

In cgroups v1, the ability to mount different controllers against different hierarchies was intended to allow great flexibility for application design. In practice, though, the flexibility turned out to be less useful than expected, and in many cases added complexity. Therefore, in cgroups v2, all available controllers are mounted against a single hierarchy. The available controllers are automatically mounted, meaning that it is not necessary (or possible) to specify the controllers when mounting the cgroup v2 filesystem using a command such as the following:

A cgroup v2 controller is available only if it is not currently in use via a mount against a cgroup v1 hierarchy. Or, to put things another way, it is not possible to employ the same controller against both a v1 hierarchy and the unified v2 hierarchy. This means that it may be necessary first to unmount a v1 controller (as described above) before that controller is available in v2. Since systemd(1) makes heavy use of some v1 controllers by default, it can in some cases be simpler to boot the system with selected v1 controllers disabled. To do this, specify the cgroup_no_v1=list option on the kernel boot command line; list is a comma-separated list of the names of the controllers to disable, or the word all to disable all v1 controllers. (This situation is correctly handled by systemd(1), which falls back to operating without the specified controllers.)

Note that on many modern systems, systemd(1) automatically mounts the cgroup2 filesystem at /sys/fs/cgroup/unified during the boot process.

Cgroups v2 mount options

The following options (mount −o) can be specified when mounting the group v2 filesystem:

Cgroups v2 controllers

The following controllers, documented in the kernel source file Documentation/admin−guide/cgroup−v2.rst (or Documentation/cgroup−v2.txt in Linux 4.17 and earlier), are supported in cgroups version 2:

There is no direct equivalent of the net_cls and net_prio controllers from cgroups version 1. Instead, support has been added to iptables(8) to allow eBPF filters that hook on cgroup v2 pathnames to make decisions about network traffic on a per-cgroup basis.

The v2 devices controller provides no interface files; instead, device control is gated by attaching an eBPF (BPF_CGROUP_DEVICE) program to a v2 cgroup.

Cgroups v2 subtree control

Each cgroup in the v2 hierarchy contains the following two files:

Because the list of controllers in cgroup.subtree_control is a subset of those cgroup.controllers, a controller that has been disabled in one cgroup in the hierarchy can never be re-enabled in the subtree below that cgroup.

A cgroup's cgroup.subtree_control file determines the set of controllers that are exercised in the child cgroups. When a controller (e.g., pids) is present in the cgroup.subtree_control file of a parent cgroup, then the corresponding controller-interface files (e.g., pids.max) are automatically created in the children of that cgroup and can be used to exert resource control in the child cgroups.

Cgroups v2 "no internal processes" rule

Cgroups v2 enforces a so-called "no internal processes" rule. Roughly speaking, this rule means that, with the exception of the root cgroup, processes may reside only in leaf nodes (cgroups that do not themselves contain child cgroups). This avoids the need to decide how to partition resources between processes which are members of cgroup A and processes in child cgroups of A.

For instance, if cgroup /cg1/cg2 exists, then a process may reside in /cg1/cg2, but not in /cg1. This is to avoid an ambiguity in cgroups v1 with respect to the delegation of resources between processes in /cg1 and its child cgroups. The recommended approach in cgroups v2 is to create a subdirectory called leaf for any nonleaf cgroup which should contain processes, but no child cgroups. Thus, processes which previously would have gone into /cg1 would now go into /cg1/leaf. This has the advantage of making explicit the relationship between processes in /cg1/leaf and /cg1's other children.

The "no internal processes" rule is in fact more subtle than stated above. More precisely, the rule is that a (nonroot) cgroup can't both (1) have member processes, and (2) distribute resources into child cgroups—that is, have a nonempty cgroup.subtree_control file. Thus, it is possible for a cgroup to have both member processes and child cgroups, but before controllers can be enabled for that cgroup, the member processes must be moved out of the cgroup (e.g., perhaps into the child cgroups).

With the Linux 4.14 addition of "thread mode" (described below), the "no internal processes" rule has been relaxed in some cases.

Cgroups v2 cgroup.events file

Each nonroot cgroup in the v2 hierarchy contains a read-only file, cgroup.events, whose contents are key-value pairs (delimited by newline characters, with the key and value separated by spaces) providing state information about the cgroup:

The following keys may appear in this file:

The cgroup.events file can be monitored, in order to receive notification when the value of one of its keys changes. Such monitoring can be done using inotify(7), which notifies changes as IN_MODIFY events, or poll(2), which notifies changes by returning the POLLPRI and POLLERR bits in the revents field.

Cgroup v2 release notification

Cgroups v2 provides a new mechanism for obtaining notification when a cgroup becomes empty. The cgroups v1 release_agent and notify_on_release files are removed, and replaced by the populated key in the cgroup.events file. This key either has the value 0, meaning that the cgroup (and its descendants) contain no (nonzombie) member processes, or 1, meaning that the cgroup (or one of its descendants) contains member processes.

The cgroups v2 release-notification mechanism offers the following advantages over the cgroups v1 release_agent mechanism:

Cgroups v2 cgroup.stat file

Each cgroup in the v2 hierarchy contains a read-only cgroup.stat file (first introduced in Linux 4.14) that consists of lines containing key-value pairs. The following keys currently appear in this file:

Limiting the number of descendant cgroups

Each cgroup in the v2 hierarchy contains the following files, which can be used to view and set limits on the number of descendant cgroups under that cgroup:

Cgroups v2 delegation: nsdelegate and cgroup namespaces

Starting with Linux 4.13, there is a second way to perform cgroup delegation in the cgroups v2 hierarchy. This is done by mounting or remounting the cgroup v2 filesystem with the nsdelegate mount option. For example, if the cgroup v2 filesystem has already been mounted, we can remount it with the nsdelegate option as follows:

The effect of this mount option is to cause cgroup namespaces to automatically become delegation boundaries. More specifically, the following restrictions apply for processes inside the cgroup namespace:

The ability to define cgroup namespaces as delegation boundaries makes cgroup namespaces more useful. To understand why, suppose that we already have one cgroup hierarchy that has been delegated to a nonprivileged user, cecilia, using the older delegation technique described above. Suppose further that cecilia wanted to further delegate a subhierarchy under the existing delegated hierarchy. (For example, the delegated hierarchy might be associated with an unprivileged container run by cecilia.) Even if a cgroup namespace was employed, because both hierarchies are owned by the unprivileged user cecilia, the following illegitimate actions could be performed:

Employing the nsdelegate mount option prevents both of these possibilities.

The nsdelegate mount option only has an effect when performed in the initial mount namespace; in other mount namespaces, the option is silently ignored.

	Note
	On some systems, systemd(1) automatically mounts the cgroup v2 filesystem. In order to experiment with the nsdelegate operation, it may be useful to boot the kernel with the following command-line options:

These options cause the kernel to boot with the cgroups v1 controllers disabled (meaning that the controllers are available in the v2 hierarchy), and tells systemd(1) not to mount and use the cgroup v2 hierarchy, so that the v2 hierarchy can be manually mounted with the desired options after boot-up.

Cgroup delegation containment rules

Some delegation containment rules ensure that the delegatee can move processes between cgroups within the delegated subtree, but can't move processes from outside the delegated subtree into the subtree or vice versa. A nonprivileged process (i.e., the delegatee) can write the PID of a "target" process into a cgroup.procs file only if all of the following are true:

	Note
	one consequence of these delegation containment rules is that the unprivileged delegatee can't place the first process into the delegated subtree; instead, the delegater must place the first process (a process owned by the delegatee) into the delegated subtree.

Threaded versus domain controllers

With the addition of threads mode, cgroups v2 now distinguishes two types of resource controllers:

Creating a threaded subtree

There are two pathways that lead to the creation of a threaded subtree. The first pathway proceeds as follows:

The second way of creating a threaded subtree is as follows:

One of the consequences of the above pathways to creating a threaded subtree is that the threaded root cgroup can be a parent only to threaded (and domain invalid) cgroups. The threaded root cgroup can't be a parent of a domain cgroups, and a threaded cgroup can't have a sibling that is a domain cgroup.

Using a threaded subtree

Within a threaded subtree, threaded controllers can be enabled in each subgroup whose type has been changed to threaded; upon doing so, the corresponding controller interface files appear in the children of that cgroup.

A process can be moved into a threaded subtree by writing its PID to the cgroup.procs file in one of the cgroups inside the tree. This has the effect of making all of the threads in the process members of the corresponding cgroup and makes the process a member of the threaded subtree. The threads of the process can then be spread across the threaded subtree by writing their thread IDs (see gettid(2)) to the cgroup.threads files in different cgroups inside the subtree. The threads of a process must all reside in the same threaded subtree.

As with writing to cgroup.procs, some containment rules apply when writing to the cgroup.threads file:

The cgroup.threads file is present in each cgroup (including domain cgroups) and can be read in order to discover the set of threads that is present in the cgroup. The set of thread IDs obtained when reading this file is not guaranteed to be ordered or free of duplicates.

The cgroup.procs file in the threaded root shows the PIDs of all processes that are members of the threaded subtree. The cgroup.procs files in the other cgroups in the subtree are not readable.

Domain controllers can't be enabled in a threaded subtree; no controller-interface files appear inside the cgroups underneath the threaded root. From the point of view of a domain controller, threaded subtrees are invisible: a multithreaded process inside a threaded subtree appears to a domain controller as a process that resides in the threaded root cgroup.

Within a threaded subtree, the "no internal processes" rule does not apply: a cgroup can both contain member processes (or thread) and exercise controllers on child cgroups.

Rules for writing to cgroup.type and creating threaded subtrees

A number of rules apply when writing to the cgroup.type file:

There are also some constraints that must be satisfied in order to create a threaded subtree rooted at the cgroup x:

If any of the above constraints is violated, then an attempt to write "threaded" to a cgroup.type file fails with the error ENOTSUP.

The "domain threaded" cgroup type

According to the pathways described above, the type of a cgroup can change to domain threaded in either of the following cases:

A domain threaded cgroup, x, can revert to the type domain if the above conditions no longer hold true—that is, if all threaded child cgroups of x are removed and either x no longer has threaded controllers enabled or no longer has member processes.

When a domain threaded cgroup x reverts to the type domain:

Exceptions for the root cgroup

The root cgroup of the v2 hierarchy is treated exceptionally: it can be the parent of both domain and threaded cgroups. If the string "threaded" is written to the cgroup.type file of one of the children of the root cgroup, then

Note that in this case, there is no cgroup whose type becomes domain threaded. (Notionally, the root cgroup can be considered as the threaded root for the cgroup whose type was changed to threaded.)

The aim of this exceptional treatment for the root cgroup is to allow a threaded cgroup that employs the cpu controller to be placed as high as possible in the hierarchy, so as to minimize the (small) cost of traversing the cgroup hierarchy.

The cgroups v2 "cpu" controller and realtime threads

As at Linux 4.19, the cgroups v2 cpu controller does not support control of realtime threads (specifically threads scheduled under any of the policies SCHED_FIFO, SCHED_RR, described SCHED_DEADLINE; see sched(7)). Therefore, the cpu controller can be enabled in the root cgroup only if all realtime threads are in the root cgroup. (If there are realtime threads in nonroot cgroups, then a write(2) of the string "+cpu" to the cgroup.subtree_control file fails with the error EINVAL.)

On some systems, systemd(1) places certain realtime threads in nonroot cgroups in the v2 hierarchy. On such systems, these threads must first be moved to the root cgroup before the cpu controller can be enabled.

/proc files

/sys/kernel/cgroup files