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.TH CGROUPS 7 2016-07-17 "Linux" "Linux Programmer's Manual"
.SH NAME
cgroups \- Linux control groups
.SH DESCRIPTION
Control cgroups, usually referred to as cgroups,
are a Linux kernel feature which 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).
.\"
.SS Terminology
A
.I cgroup
is a collection of processes that are bound to a set of
limits or parameters defined via the cgroup filesystem.

A
.I 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
.IR "resource controllers"
(or simply, controllers).

The cgroups for a controller are arranged in a
.IR 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.
.\"
.SS 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
.IR 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
.I "\-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.
.\"
.SS 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,
.IR /user/joe/1.session
represents control group
.IR 1.session ,
which is a child of cgroup
.IR joe ,
which is a child of
.IR /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
.I name=systemd
cgroup which is used by
.BR systemd (1)
to track services and user sessions.
.\"
.SS Tasks (threads) versus processes
In cgroups v1, a distinction is drawn between
.I processes
and
.IR 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,
.\" FIXME Add some text describing why this was a problem.
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).
.\"
.SS Mounting v1 controllers
The use of cgroups requires a kernel built with the
.BR 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 filesystem mounted at
.IR /sys/fs/cgroup .
Thus, one might mount the
.I cpu
controller as follows:

.nf
.in +4n
mount \-t cgroup \-o cpu none /sys/fs/cgroup/cpu
.in
.fi

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

.nf
.in +4n
mount \-t cgroup \-o cpu,cpuacct none /sys/fs/cgroup/cpu,cpuacct
.in
.fi

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
.I /foo1
for one controller while being in
.I /foo2/foo3
for another.

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

.nf
.in +4n
mount \-t cgroup \-o all cgroup /sys/fs/cgroup
.in
.fi

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

Note that on many systems, the v1 controllers are automatically mounted under
.IR /sys/fs/cgroup ;
in particular,
.BR systemd (1)
automatically creates such mount points.
.\"
.SS Creating cgroups and moving processes
A cgroup filesystem initially contains
a single root cgroup (per hierarchy), '/',
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.
Processes (threads) may be moved to this cgroup by writing
their PIDs (TIDs) into the cgroup's
.I cgroup.procs
.RI ( tasks )
file:

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

The same files can be read to obtain a list of the processes (threads)
that  are currently in
.IR cg1 .
When writing a PID into the
.IR cgroup.procs ,
all tasks in the process are moved into the new cgroup at once.

A child process created via
.BR fork (2)
inherits its parent's cgroup memberships.
A process's cgroup memberships are preserved across
.BR execve (2).

Note: in the upcoming unified hierarchy, a new restriction is imposed such
that processes may reside only in leaf cgroups.
For instance, if cgroup
.I /cg1/cg2
exists, then a process may reside in
.IR /cg1/cg2 ,
but not in
.IR /cg1 .
This is to avoid the current ambiguity in the delegation of resources
between processes in
.I /cg1
and its child cgroups.
The recommended workaround is to create a subdirectory called
.I leaf
for any non-leaf cgroup which should contain processes, and make sure not to
create child cgroups of it.
In the above example, processes which previously would have gone into
.I /cg1
would now go into
.IR /cg1/leaf .
This has the advantage of making explicit the relationship between processes in
.I /cg1/leaf
and
.IR /cg1 's
other children.
.\"
.SS 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.
.\"
.SS 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:
.TP 3
1. Processes only in leaf nodes
With the exception of the root cgroup, processes may reside only in leaf nodes.
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.
.TP
2. Active cgroups must be specified
The unified hierarchy presents two new files,
.IR cgroup.controllers
and
.IR cgroup.subtree_control .
When a cgroup
.I A/b
is created, its
.IR 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
.IR 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
.I freezer
controller is not enabled in
.IR /A/B ,
then it cannot be enabled in
.IR /A/B/C .
.TP
3. No "tasks" or "cgroup.clone_children" files
.TP
4. Empty cgroup notification
A new file,
.IR 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.
.PP
For more changes, please see the
.I Documentation/cgroup-v2.txt
file in the kernel source.
.\"
.SS 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
.BR CONFIG_CGROUPS
kernel configuration option.
.TP
.IR cpu " (since Linux 2.6.24; " \fBCONFIG_CGROUP_SCHED\fP )
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
.IR Documentation/scheduler/sched\-bwc.txt .
.TP
.IR cpuacct " (since Linux 2.6.24; " \fBCONFIG_CGROUP_CPUACCT\fP )
This provides accounting for CPU usage by groups of processes.

Further information can be found in the kernel source file
.IR Documentation/cgroup\-v1/cpuacct.txt .
.TP
.IR cpuset " (since Linux 2.6.24; " \fBCONFIG_CPUSETS\fP )
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
.IR Documentation/cgroup\-v1/cpusets.txt .
.TP
.IR memory " (since Linux 2.6.25; " \fBCONFIG_MEMCG\fP )
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
.IR Documentation/cgroup\-v1/memory.txt .
.TP
.IR devices " (since Linux 2.6.26; " \fBCONFIG_CGROUP_DEVICE\fP )
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
.IR Documentation/cgroup-v1/devices.txt .
.TP
.IR freezer " (since Linux 2.6.28; " \fBCONFIG_CGROUP_FREEZER\fP )
The
.IR freezer
cgroup can suspend and restore (resume) all processes in a cgroup.
Freezing a cgroup
.I /A
also causes its children, for example, processes in
.IR /A/B ,
to be frozen.

Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/freezer-subsystem.txt .
.TP
.IR net_cls " (since Linux 2.6.29; " \fBCONFIG_CGROUP_NET_CLASSID\fP )
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
.BR 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
.IR Documentation/cgroup-v1/net_cls.txt .
.TP
.IR blkio " (since Linux 2.6.33; " \fBCONFIG_BLK_CGROUP\fP )
The
.I 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
.IR Documentation/cgroup-v1/blkio-controller.txt .
.TP
.IR perf_event " (since Linux 2.6.39; " \fBCONFIG_CGROUP_PERF\fP )
This controller allows
.I perf
monitoring of the set of processes grouped in a cgroup.

Further information can be found in the kernel source file
.IR Documentation/perf-record.txt .
.TP
.IR net_prio " (since Linux 3.3; " \fBCONFIG_CGROUP_NET_PRIO\fP )
This allows priorities to be specified, per network interface, for cgroups.

Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/net_prio.txt .
.TP
.IR hugetlb " (since Linux 3.5; " \fBCONFIG_CGROUP_HUGETLB\fP )
This supports limiting the use of huge pages by cgroups.

Further information can be found in the kernel source file
.IR Documentation/cgroup-v1/hugetlb.txt .
.TP
.IR pids " (since Linux 4.3; " \fBCONFIG_CGROUP_PIDS\fP )
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
.IR Documentation/cgroup-v1/pids.txt .
.\"
.SS Release notification
A special file in each cgroup hierarchy,
.IR 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
.IR notify_on_release
file;
this value is inherited by newly created child cgroups.

A new feature in cgroups v2 is the
.I cgroup.populated
file.
This reads 0 if there are no processes in the cgroup or its descendants,
and 1 otherwise.
It can be watched for changes using
.BR inotify (7).
This allows user-space applications to efficiently watch cgroups
for autoremove conditions.
.\"
.SS /proc files
.TP
.IR /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:

.nf
.in +4n
#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
.in
.fi

The fields in this file are, from left to right:
.RS
.IP 1. 3
The name of the controller.
.IP 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:
.RS 5
.IP a) 3
the controller is not mounted on a cgroups v1 hierarchy;
.IP b)
the controller is bound to the cgroups v2 single unified hierarchy; or
.IP c)
the controller is disabled (see below).
.RE
.IP 3.
The number of control groups in this hierarchy using this controller.
.IP 4.
This field contains the value 1 if this controller is enabled,
or 0 if it has been disabled (via the
.IR cgroup_disable
kernel command-line boot parameter).
.RE
.TP
.IR /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:
.nf
.ft CW

    5:cpuacct,cpu,cpuset:/daemons
.ft
.fi
.IP
The colon-separated fields are, from left to right:
.RS
.IP 1. 3
For cgroups version 1 hierarchies,
this field contains a unique hierarchy ID number
that can be matched to a hierarchy ID in
.IR /proc/cgroups .
For the cgroups version 2 hierarchy, this field contains the value 0.
.IP 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.
.IP 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.
.RE
.SH ERRORS
The following errors can occur for
.BR mount (2):
.TP
.B EBUSY
An attempt to mount a cgroup version 1 filesystem specified neither the
.I name=
option (to mount a named hierarchy) nor a controller name (or
.IR all ).
.SH SEE ALSO
.BR prlimit (1),
.BR systemd (1),
.BR clone (2),
.BR ioprio_set (2),
.BR perf_event_open (2),
.BR setrlimit (2),
.BR cgroup_namespaces (7),
.BR cpuset (7),
.BR namespaces (7),
.BR sched (7),
.BR user_namespaces (7)