.\" Copyright (C) 2015 Serge Hallyn .\" and Copyright (C) 2016 Michael Kerrisk .\" .\" %%%LICENSE_START(VERBATIM) .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" %%%LICENSE_END .\" .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 provides for grouping of processes and resource tracking and limitations for those groups. While several systems have been introduced to help in configuring and managing cgroups, the kernel's cgroup interface is provided through a pseudo-filesystem called cgroupfs. Grouping is implemented in the core cgroup kernel code, while resource tracking and limits are implemented in a set of per-resource-type subsystems (memory, CPU, and so on) which may be enabled as separate hierarchies, or joined into comounted hierarchies. Each hierarchy constitutes a separate mount of the cgroup filesystem, with the subsystems enabled in that hierarchy listed in the mount options. For each mounted hierarchy, the directory tree mirrors the control group hierarchy. Each control group is represented by a directory, with each of its child control cgroups represented as a child directory. For instance, .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 general, cgroup limits are hierarchical, meaning that the limits placed on .IR /user/joe cannot be exceeded by .IR /usr/joe/1.session . There are currently exceptions to this rule, but stricter adherence is a goal as cgroups are being largely reworked. In addition, cgroups can be mounted with no bound subsystem, in which case they serve only to track processes. An example of this is the .I name=systemd cgroup which is used by .BR systemd (1) to track services and user sessions. .\" .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 subsystem 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; these attributes may govern or propagate to child cgroups and their descendants in the hierarchy. .\" .SS Cgroups version 1 and version 2 The initial release of the cgroups implementation was in Linux 2.6.24. Over time, various cgroup subsystems have been added to allow the management of various types of resources. However, the development of these subsystems was largely uncoordinated, with the result that many inconsistencies arose between subsystems and management of the cgroup hierarchies became rather complex. (A longer description of these problems can be found in the kernel source file .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. .\" .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 To be available, a given cgroup subsystem must be compiled into the kernel. Since they are exposed through a virtual filesystem, subsystems must be mounted before they can be controlled. The usual place for this is under .IR /sys/fs/cgroup . If all the desired subsystems can be comounted, then one can do so with the following command: mount \-t cgroup \-o all cgroup /sys/fs/cgroup (One can achieve the same result by omitting .IR "\-o all" , since it is the default if subsystems are explicitly specified.) If multiple, separately mounted subsystems are desired, then this is usually done in per-subsystem subdirectories. This requires first mounting a tmpfs under .I /sys/fs/cgroup so that subdirectories can be created. For instance, one could mount .IR cpu , .IR memory , and .I devices cgroups as follows: .nf .in +4n mount \-t tmpfs \-o size=100000,mode=755 cgroups /sys/fs/cgroup for s in cpu memory devices; do mkdir /sys/fs/cgroup/$s mount \-t cgroup \-o $s $s /sys/fs/cgroup/$s done .in .fi Comounting subsystems has the effect that a process is in the same cgroup for all comounted subsystems. Separately mounting subsystems allows a process to be in cgroup .I /foo1 for one subsystem while being in .I /foo2/foo3 for another. .\" .SS Creating cgroups and moving processes The system begins with 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. On .BR fork (2), the new child is created as a member of the parent's cgroup, leading to implicit grouping of process hierarchies. Note: in the upcoming unified hierarchy, a new restriction is imposed such that 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. 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 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. For more changes, please see the .I Documentation/cgroup-v2.txt file in the kernel source. .\" .SS Cgroups version 1 subsystems Each of the cgroups version 1 subsystems is governed by a kernel configuration option (listed below). Additionally, the availability of the cgroups feature is governed by the .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 /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:subsystem-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 subsystems 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)