CGROUPS(7) Linux Programmer's Manual CGROUPS(7)
cgroups - Linux control groups
Control cgroups, usually referred to as cgroups, are a Linux kernel
feature which allow processes to be organized into hierarchical
groups whose usage of various types of resources can then be limited
and monitored. The kernel's cgroup interface is provided through a
pseudo-filesystem called cgroupfs. 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).
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/cgroup-v2.txt.)
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.
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.
Cgroups version 1
Under cgroups v1, each controller may be mounted against a separate
cgroup filesystem that provides its own hierarchical organization of
the processes on the system. It is also possible comount multiple
(or even all) cgroups v1 controllers against the same cgroup
filesystem, meaning that the comounted controllers manage the same
hierarchical organization of processes.
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 addition, in cgroups v1, cgroups can be mounted with no bound
controller, in which case they serve only to track processes. (See
the discussion of release notification below.) An example of this is
the name=systemd cgroup which is used by systemd(1) to track services
and user sessions.
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. Because this ability caused certain problems, the ability
to independently manipulate the cgroup memberships of the threads 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 threads in the
process).
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:
mount -t cgroup -o cpu none /sys/fs/cgroup/cpu
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:
mount -t cgroup -o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct
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:
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 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.
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.
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. For further information, see
Documentation/scheduler/sched-design-CFS.txt.
In Linux 3.2, this controller was extended to provide CPU
"bandwidth" control. If the kernel is configured with
CONFIG_CFS_BANDWIDTH, then within each scheduling period
(defined via a file in the cgroup directory), it is possible
to define an upper limit on the CPU time allocated to the
processes in a cgroup. This upper limit applies even if there
is no other competition for the CPU. 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 processes.
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 processes 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 processes 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
processes in a cgroup. Freezing a cgroup /A also causes its
children, for example, processes 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 applies only 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
tools/perf/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.
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:
mkdir /sys/fs/cgroup/cpu/cg1
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:
echo $$ > /sys/fs/cgroup/cpu/cg1/cgroup.procs
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 (but not cgroups v2), 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. This file is not present in cgroup v2
directories.
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 with a process.
The default value of the release_agent file is empty, meaning that no
release agent is invoked.
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.
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 here, and in some
cases elaborated in the following subsections.
1. Cgroups v2 provides a unified hierarchy against which all
controllers are mounted.
2. "Internal" processes are not permitted. With the exception of the
root cgroup, processes may reside only in leaf nodes (cgroups that
do not themselves contain child cgroups).
3. Active cgroups must be specified via the files cgroup.controllers
and cgroup.subtree_control.
4. The tasks file has been removed. In addition, the
cgroup.clone_children file that is employed by the cpuset
controller has been removed.
5. An improved mechanism for notification of empty cgroups is
provided by the cgroup.events file.
For more changes, see the Documentation/cgroup-v2.txt file in the
kernel source.
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 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:
mount -t cgroup2 none /mnt/cgroup2
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.
Cgroups v2 "no internal processes" rule
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.
Cgroups v2 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.
Cgroups v2 cgroup.events file
With cgroups v2, a new mechanism is provided to obtain notification
about when a cgroup becomes empty. The cgroups v1 release_agent and
notify_on_release files are removed, and replaced by a new, more
general-purpose file, cgroup.events. This file contains key-value
pairs (delimited by newline characters, with the key and value
separated by spaces) that identify events or state for a cgroup.
Currently, only one key appears in this file, populated, which has
either the value 0, meaning that the cgroup (and its descendants)
contain no (nonzombie) processes, or 1, meaning that the cgroup
contains member processes.
The cgroup.events file can be monitored, in order to receive
notification when a cgroup transitions between the populated and
unpopulated states (or vice versa). When monitoring this file using
inotify(7), transitions generate IN_MODIFY events, and when
monitoring the file using poll(2), transitions generate POLLPRI
events.
The cgroups v2 notify_on_release mechanism offers at least two
advantages over the cgroups v1 release_agent mechanism. First, it
allows for cheaper notification, since a single process can monitor
multiple cgroup.events files. By contrast, the cgroups v1 mechanism
requires the creation of a process for each notification. Second,
notification can be delegated to a process that lives inside a
container associated with the newly empty cgroup.
/proc files
/proc/cgroups (since Linux 2.6.24)
This file contains information about the controllers that are
compiled into the kernel. 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:controller-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 controllers 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.
The following errors can occur for mount(2):
EBUSY An attempt to mount a cgroup version 1 filesystem specified
neither the name= option (to mount a named hierarchy) nor a
controller name (or all).
A child process created via fork(2) inherits its parent's cgroup
memberships. A process's cgroup memberships are preserved across
execve(2).
prlimit(1), systemd(1), clone(2), ioprio_set(2), perf_event_open(2),
setrlimit(2), cgroup_namespaces(7), cpuset(7), namespaces(7),
sched(7), user_namespaces(7)
This page is part of release 4.12 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2016-12-12 CGROUPS(7)
Pages that refer to this page: getrlimit(2), ioprio_set(2), poll(2), proc(5), systemd.exec(5), cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7)