NAME | DESCRIPTION | ERRORS | NOTES | SEE ALSO | COLOPHON

CGROUPS(7)                Linux Programmer's Manual               CGROUPS(7)

NAME         top

       cgroups - Linux control groups

DESCRIPTION         top

       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 hier‐
       archy.  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 hierar‐
       chy:

           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 con‐
       figuration 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 CON‐
              FIG_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.  Hier‐
              archy 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 pro‐
              cesses 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 throt‐
              tling 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 guar‐
       anteed to be in order.  Nor is it guaranteed to be free of dupli‐
       cates.  (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 sim‐
       ply 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 noti‐
       fications 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 hier‐
       archies.

       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 con‐
          trollers 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 con‐
          troller has been removed.

       5. An improved mechanism for notification of empty cgroups is pro‐
          vided 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 automati‐
       cally 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 relation‐
       ship 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 con‐
       trollers are active until they are enabled through the cgroup.sub‐
       tree_control file, by writing the list of space-delimited names of
       the controllers, each preceded by '+' (to enable) or '-' (to dis‐
       able).  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 gen‐
       eral-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 (nonzom‐
       bie) processes, or 1, meaning that the cgroup contains member pro‐
       cesses.

       The cgroup.events file can be monitored, in order to receive notifi‐
       cation when a cgroup transitions between the populated and unpopu‐
       lated states (or vice versa).  When monitoring this file using
       inotify(7), transitions generate IN_MODIFY events, and when monitor‐
       ing the file using poll(2), transitions generate POLLPRI events.

       The cgroups v2 notify_on_release mechanism offers at least two advan‐
       tages 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, notifica‐
       tion 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 con‐
                 troller 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 hierar‐
                      chy;

                   b) the controller is bound to the cgroups v2 single uni‐
                      fied 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_dis‐
                 able 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 dif‐
              fers 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 hierar‐
                 chy ID in /proc/cgroups.  For the cgroups version 2 hierar‐
                 chy, 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 hier‐
                 archy.  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.

ERRORS         top

       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).

NOTES         top

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

SEE ALSO         top

       prlimit(1), systemd(1), systemd-cgls(1), systemd-cgtop(1), clone(2),
       ioprio_set(2), perf_event_open(2), setrlimit(2),
       cgroup_namespaces(7), cpuset(7), namespaces(7), sched(7),
       user_namespaces(7)

COLOPHON         top

       This page is part of release 4.13 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                            2017-09-15                       CGROUPS(7)

Pages that refer to this page: getrlimit(2)ioprio_set(2)poll(2)proc(5)sysfs(5)systemd.exec(5)cgroup_namespaces(7)cpuset(7)namespaces(7)sched(7)