2Control Group v2
   4October, 2015           Tejun Heo <>
   6This is the authoritative documentation on the design, interface and
   7conventions of cgroup v2.  It describes all userland-visible aspects
   8of cgroup including core and specific controller behaviors.  All
   9future changes must be reflected in this document.  Documentation for
  10v1 is available under Documentation/cgroup-v1/.
  141. Introduction
  15  1-1. Terminology
  16  1-2. What is cgroup?
  172. Basic Operations
  18  2-1. Mounting
  19  2-2. Organizing Processes
  20  2-3. [Un]populated Notification
  21  2-4. Controlling Controllers
  22    2-4-1. Enabling and Disabling
  23    2-4-2. Top-down Constraint
  24    2-4-3. No Internal Process Constraint
  25  2-5. Delegation
  26    2-5-1. Model of Delegation
  27    2-5-2. Delegation Containment
  28  2-6. Guidelines
  29    2-6-1. Organize Once and Control
  30    2-6-2. Avoid Name Collisions
  313. Resource Distribution Models
  32  3-1. Weights
  33  3-2. Limits
  34  3-3. Protections
  35  3-4. Allocations
  364. Interface Files
  37  4-1. Format
  38  4-2. Conventions
  39  4-3. Core Interface Files
  405. Controllers
  41  5-1. CPU
  42    5-1-1. CPU Interface Files
  43  5-2. Memory
  44    5-2-1. Memory Interface Files
  45    5-2-2. Usage Guidelines
  46    5-2-3. Memory Ownership
  47  5-3. IO
  48    5-3-1. IO Interface Files
  49    5-3-2. Writeback
  506. Namespace
  51  6-1. Basics
  52  6-2. The Root and Views
  53  6-3. Migration and setns(2)
  54  6-4. Interaction with Other Namespaces
  55P. Information on Kernel Programming
  56  P-1. Filesystem Support for Writeback
  57D. Deprecated v1 Core Features
  58R. Issues with v1 and Rationales for v2
  59  R-1. Multiple Hierarchies
  60  R-2. Thread Granularity
  61  R-3. Competition Between Inner Nodes and Threads
  62  R-4. Other Interface Issues
  63  R-5. Controller Issues and Remedies
  64    R-5-1. Memory
  671. Introduction
  691-1. Terminology
  71"cgroup" stands for "control group" and is never capitalized.  The
  72singular form is used to designate the whole feature and also as a
  73qualifier as in "cgroup controllers".  When explicitly referring to
  74multiple individual control groups, the plural form "cgroups" is used.
  771-2. What is cgroup?
  79cgroup is a mechanism to organize processes hierarchically and
  80distribute system resources along the hierarchy in a controlled and
  81configurable manner.
  83cgroup is largely composed of two parts - the core and controllers.
  84cgroup core is primarily responsible for hierarchically organizing
  85processes.  A cgroup controller is usually responsible for
  86distributing a specific type of system resource along the hierarchy
  87although there are utility controllers which serve purposes other than
  88resource distribution.
  90cgroups form a tree structure and every process in the system belongs
  91to one and only one cgroup.  All threads of a process belong to the
  92same cgroup.  On creation, all processes are put in the cgroup that
  93the parent process belongs to at the time.  A process can be migrated
  94to another cgroup.  Migration of a process doesn't affect already
  95existing descendant processes.
  97Following certain structural constraints, controllers may be enabled or
  98disabled selectively on a cgroup.  All controller behaviors are
  99hierarchical - if a controller is enabled on a cgroup, it affects all
 100processes which belong to the cgroups consisting the inclusive
 101sub-hierarchy of the cgroup.  When a controller is enabled on a nested
 102cgroup, it always restricts the resource distribution further.  The
 103restrictions set closer to the root in the hierarchy can not be
 104overridden from further away.
 1072. Basic Operations
 1092-1. Mounting
 111Unlike v1, cgroup v2 has only single hierarchy.  The cgroup v2
 112hierarchy can be mounted with the following mount command.
 114  # mount -t cgroup2 none $MOUNT_POINT
 116cgroup2 filesystem has the magic number 0x63677270 ("cgrp").  All
 117controllers which support v2 and are not bound to a v1 hierarchy are
 118automatically bound to the v2 hierarchy and show up at the root.
 119Controllers which are not in active use in the v2 hierarchy can be
 120bound to other hierarchies.  This allows mixing v2 hierarchy with the
 121legacy v1 multiple hierarchies in a fully backward compatible way.
 123A controller can be moved across hierarchies only after the controller
 124is no longer referenced in its current hierarchy.  Because per-cgroup
 125controller states are destroyed asynchronously and controllers may
 126have lingering references, a controller may not show up immediately on
 127the v2 hierarchy after the final umount of the previous hierarchy.
 128Similarly, a controller should be fully disabled to be moved out of
 129the unified hierarchy and it may take some time for the disabled
 130controller to become available for other hierarchies; furthermore, due
 131to inter-controller dependencies, other controllers may need to be
 132disabled too.
 134While useful for development and manual configurations, moving
 135controllers dynamically between the v2 and other hierarchies is
 136strongly discouraged for production use.  It is recommended to decide
 137the hierarchies and controller associations before starting using the
 138controllers after system boot.
 140During transition to v2, system management software might still
 141automount the v1 cgroup filesystem and so hijack all controllers
 142during boot, before manual intervention is possible. To make testing
 143and experimenting easier, the kernel parameter cgroup_no_v1= allows
 144disabling controllers in v1 and make them always available in v2.
 1472-2. Organizing Processes
 149Initially, only the root cgroup exists to which all processes belong.
 150A child cgroup can be created by creating a sub-directory.
 152  # mkdir $CGROUP_NAME
 154A given cgroup may have multiple child cgroups forming a tree
 155structure.  Each cgroup has a read-writable interface file
 156"cgroup.procs".  When read, it lists the PIDs of all processes which
 157belong to the cgroup one-per-line.  The PIDs are not ordered and the
 158same PID may show up more than once if the process got moved to
 159another cgroup and then back or the PID got recycled while reading.
 161A process can be migrated into a cgroup by writing its PID to the
 162target cgroup's "cgroup.procs" file.  Only one process can be migrated
 163on a single write(2) call.  If a process is composed of multiple
 164threads, writing the PID of any thread migrates all threads of the
 167When a process forks a child process, the new process is born into the
 168cgroup that the forking process belongs to at the time of the
 169operation.  After exit, a process stays associated with the cgroup
 170that it belonged to at the time of exit until it's reaped; however, a
 171zombie process does not appear in "cgroup.procs" and thus can't be
 172moved to another cgroup.
 174A cgroup which doesn't have any children or live processes can be
 175destroyed by removing the directory.  Note that a cgroup which doesn't
 176have any children and is associated only with zombie processes is
 177considered empty and can be removed.
 179  # rmdir $CGROUP_NAME
 181"/proc/$PID/cgroup" lists a process's cgroup membership.  If legacy
 182cgroup is in use in the system, this file may contain multiple lines,
 183one for each hierarchy.  The entry for cgroup v2 is always in the
 184format "0::$PATH".
 186  # cat /proc/842/cgroup
 187  ...
 188  0::/test-cgroup/test-cgroup-nested
 190If the process becomes a zombie and the cgroup it was associated with
 191is removed subsequently, " (deleted)" is appended to the path.
 193  # cat /proc/842/cgroup
 194  ...
 195  0::/test-cgroup/test-cgroup-nested (deleted)
 1982-3. [Un]populated Notification
 200Each non-root cgroup has a "" file which contains
 201"populated" field indicating whether the cgroup's sub-hierarchy has
 202live processes in it.  Its value is 0 if there is no live process in
 203the cgroup and its descendants; otherwise, 1.  poll and [id]notify
 204events are triggered when the value changes.  This can be used, for
 205example, to start a clean-up operation after all processes of a given
 206sub-hierarchy have exited.  The populated state updates and
 207notifications are recursive.  Consider the following sub-hierarchy
 208where the numbers in the parentheses represent the numbers of processes
 209in each cgroup.
 211  A(4) - B(0) - C(1)
 212              \ D(0)
 214A, B and C's "populated" fields would be 1 while D's 0.  After the one
 215process in C exits, B and C's "populated" fields would flip to "0" and
 216file modified events will be generated on the "" files of
 217both cgroups.
 2202-4. Controlling Controllers
 2222-4-1. Enabling and Disabling
 224Each cgroup has a "cgroup.controllers" file which lists all
 225controllers available for the cgroup to enable.
 227  # cat cgroup.controllers
 228  cpu io memory
 230No controller is enabled by default.  Controllers can be enabled and
 231disabled by writing to the "cgroup.subtree_control" file.
 233  # echo "+cpu +memory -io" > cgroup.subtree_control
 235Only controllers which are listed in "cgroup.controllers" can be
 236enabled.  When multiple operations are specified as above, either they
 237all succeed or fail.  If multiple operations on the same controller
 238are specified, the last one is effective.
 240Enabling a controller in a cgroup indicates that the distribution of
 241the target resource across its immediate children will be controlled.
 242Consider the following sub-hierarchy.  The enabled controllers are
 243listed in parentheses.
 245  A(cpu,memory) - B(memory) - C()
 246                            \ D()
 248As A has "cpu" and "memory" enabled, A will control the distribution
 249of CPU cycles and memory to its children, in this case, B.  As B has
 250"memory" enabled but not "CPU", C and D will compete freely on CPU
 251cycles but their division of memory available to B will be controlled.
 253As a controller regulates the distribution of the target resource to
 254the cgroup's children, enabling it creates the controller's interface
 255files in the child cgroups.  In the above example, enabling "cpu" on B
 256would create the "cpu." prefixed controller interface files in C and
 257D.  Likewise, disabling "memory" from B would remove the "memory."
 258prefixed controller interface files from C and D.  This means that the
 259controller interface files - anything which doesn't start with
 260"cgroup." are owned by the parent rather than the cgroup itself.
 2632-4-2. Top-down Constraint
 265Resources are distributed top-down and a cgroup can further distribute
 266a resource only if the resource has been distributed to it from the
 267parent.  This means that all non-root "cgroup.subtree_control" files
 268can only contain controllers which are enabled in the parent's
 269"cgroup.subtree_control" file.  A controller can be enabled only if
 270the parent has the controller enabled and a controller can't be
 271disabled if one or more children have it enabled.
 2742-4-3. No Internal Process Constraint
 276Non-root cgroups can only distribute resources to their children when
 277they don't have any processes of their own.  In other words, only
 278cgroups which don't contain any processes can have controllers enabled
 279in their "cgroup.subtree_control" files.
 281This guarantees that, when a controller is looking at the part of the
 282hierarchy which has it enabled, processes are always only on the
 283leaves.  This rules out situations where child cgroups compete against
 284internal processes of the parent.
 286The root cgroup is exempt from this restriction.  Root contains
 287processes and anonymous resource consumption which can't be associated
 288with any other cgroups and requires special treatment from most
 289controllers.  How resource consumption in the root cgroup is governed
 290is up to each controller.
 292Note that the restriction doesn't get in the way if there is no
 293enabled controller in the cgroup's "cgroup.subtree_control".  This is
 294important as otherwise it wouldn't be possible to create children of a
 295populated cgroup.  To control resource distribution of a cgroup, the
 296cgroup must create children and transfer all its processes to the
 297children before enabling controllers in its "cgroup.subtree_control"
 3012-5. Delegation
 3032-5-1. Model of Delegation
 305A cgroup can be delegated to a less privileged user by granting write
 306access of the directory and its "cgroup.procs" file to the user.  Note
 307that resource control interface files in a given directory control the
 308distribution of the parent's resources and thus must not be delegated
 309along with the directory.
 311Once delegated, the user can build sub-hierarchy under the directory,
 312organize processes as it sees fit and further distribute the resources
 313it received from the parent.  The limits and other settings of all
 314resource controllers are hierarchical and regardless of what happens
 315in the delegated sub-hierarchy, nothing can escape the resource
 316restrictions imposed by the parent.
 318Currently, cgroup doesn't impose any restrictions on the number of
 319cgroups in or nesting depth of a delegated sub-hierarchy; however,
 320this may be limited explicitly in the future.
 3232-5-2. Delegation Containment
 325A delegated sub-hierarchy is contained in the sense that processes
 326can't be moved into or out of the sub-hierarchy by the delegatee.  For
 327a process with a non-root euid to migrate a target process into a
 328cgroup by writing its PID to the "cgroup.procs" file, the following
 329conditions must be met.
 331- The writer's euid must match either uid or suid of the target process.
 333- The writer must have write access to the "cgroup.procs" file.
 335- The writer must have write access to the "cgroup.procs" file of the
 336  common ancestor of the source and destination cgroups.
 338The above three constraints ensure that while a delegatee may migrate
 339processes around freely in the delegated sub-hierarchy it can't pull
 340in from or push out to outside the sub-hierarchy.
 342For an example, let's assume cgroups C0 and C1 have been delegated to
 343user U0 who created C00, C01 under C0 and C10 under C1 as follows and
 344all processes under C0 and C1 belong to U0.
 346  ~~~~~~~~~~~~~ - C0 - C00
 347  ~ cgroup    ~      \ C01
 348  ~ hierarchy ~
 349  ~~~~~~~~~~~~~ - C1 - C10
 351Let's also say U0 wants to write the PID of a process which is
 352currently in C10 into "C00/cgroup.procs".  U0 has write access to the
 353file and uid match on the process; however, the common ancestor of the
 354source cgroup C10 and the destination cgroup C00 is above the points
 355of delegation and U0 would not have write access to its "cgroup.procs"
 356files and thus the write will be denied with -EACCES.
 3592-6. Guidelines
 3612-6-1. Organize Once and Control
 363Migrating a process across cgroups is a relatively expensive operation
 364and stateful resources such as memory are not moved together with the
 365process.  This is an explicit design decision as there often exist
 366inherent trade-offs between migration and various hot paths in terms
 367of synchronization cost.
 369As such, migrating processes across cgroups frequently as a means to
 370apply different resource restrictions is discouraged.  A workload
 371should be assigned to a cgroup according to the system's logical and
 372resource structure once on start-up.  Dynamic adjustments to resource
 373distribution can be made by changing controller configuration through
 374the interface files.
 3772-6-2. Avoid Name Collisions
 379Interface files for a cgroup and its children cgroups occupy the same
 380directory and it is possible to create children cgroups which collide
 381with interface files.
 383All cgroup core interface files are prefixed with "cgroup." and each
 384controller's interface files are prefixed with the controller name and
 385a dot.  A controller's name is composed of lower case alphabets and
 386'_'s but never begins with an '_' so it can be used as the prefix
 387character for collision avoidance.  Also, interface file names won't
 388start or end with terms which are often used in categorizing workloads
 389such as job, service, slice, unit or workload.
 391cgroup doesn't do anything to prevent name collisions and it's the
 392user's responsibility to avoid them.
 3953. Resource Distribution Models
 397cgroup controllers implement several resource distribution schemes
 398depending on the resource type and expected use cases.  This section
 399describes major schemes in use along with their expected behaviors.
 4023-1. Weights
 404A parent's resource is distributed by adding up the weights of all
 405active children and giving each the fraction matching the ratio of its
 406weight against the sum.  As only children which can make use of the
 407resource at the moment participate in the distribution, this is
 408work-conserving.  Due to the dynamic nature, this model is usually
 409used for stateless resources.
 411All weights are in the range [1, 10000] with the default at 100.  This
 412allows symmetric multiplicative biases in both directions at fine
 413enough granularity while staying in the intuitive range.
 415As long as the weight is in range, all configuration combinations are
 416valid and there is no reason to reject configuration changes or
 417process migrations.
 419"cpu.weight" proportionally distributes CPU cycles to active children
 420and is an example of this type.
 4233-2. Limits
 425A child can only consume upto the configured amount of the resource.
 426Limits can be over-committed - the sum of the limits of children can
 427exceed the amount of resource available to the parent.
 429Limits are in the range [0, max] and defaults to "max", which is noop.
 431As limits can be over-committed, all configuration combinations are
 432valid and there is no reason to reject configuration changes or
 433process migrations.
 435"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
 436on an IO device and is an example of this type.
 4393-3. Protections
 441A cgroup is protected to be allocated upto the configured amount of
 442the resource if the usages of all its ancestors are under their
 443protected levels.  Protections can be hard guarantees or best effort
 444soft boundaries.  Protections can also be over-committed in which case
 445only upto the amount available to the parent is protected among
 448Protections are in the range [0, max] and defaults to 0, which is
 451As protections can be over-committed, all configuration combinations
 452are valid and there is no reason to reject configuration changes or
 453process migrations.
 455"memory.low" implements best-effort memory protection and is an
 456example of this type.
 4593-4. Allocations
 461A cgroup is exclusively allocated a certain amount of a finite
 462resource.  Allocations can't be over-committed - the sum of the
 463allocations of children can not exceed the amount of resource
 464available to the parent.
 466Allocations are in the range [0, max] and defaults to 0, which is no
 469As allocations can't be over-committed, some configuration
 470combinations are invalid and should be rejected.  Also, if the
 471resource is mandatory for execution of processes, process migrations
 472may be rejected.
 474"cpu.rt.max" hard-allocates realtime slices and is an example of this
 4784. Interface Files
 4804-1. Format
 482All interface files should be in one of the following formats whenever
 485  New-line separated values
 486  (when only one value can be written at once)
 488        VAL0\n
 489        VAL1\n
 490        ...
 492  Space separated values
 493  (when read-only or multiple values can be written at once)
 495        VAL0 VAL1 ...\n
 497  Flat keyed
 499        KEY0 VAL0\n
 500        KEY1 VAL1\n
 501        ...
 503  Nested keyed
 505        KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
 506        KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
 507        ...
 509For a writable file, the format for writing should generally match
 510reading; however, controllers may allow omitting later fields or
 511implement restricted shortcuts for most common use cases.
 513For both flat and nested keyed files, only the values for a single key
 514can be written at a time.  For nested keyed files, the sub key pairs
 515may be specified in any order and not all pairs have to be specified.
 5184-2. Conventions
 520- Settings for a single feature should be contained in a single file.
 522- The root cgroup should be exempt from resource control and thus
 523  shouldn't have resource control interface files.  Also,
 524  informational files on the root cgroup which end up showing global
 525  information available elsewhere shouldn't exist.
 527- If a controller implements weight based resource distribution, its
 528  interface file should be named "weight" and have the range [1,
 529  10000] with 100 as the default.  The values are chosen to allow
 530  enough and symmetric bias in both directions while keeping it
 531  intuitive (the default is 100%).
 533- If a controller implements an absolute resource guarantee and/or
 534  limit, the interface files should be named "min" and "max"
 535  respectively.  If a controller implements best effort resource
 536  guarantee and/or limit, the interface files should be named "low"
 537  and "high" respectively.
 539  In the above four control files, the special token "max" should be
 540  used to represent upward infinity for both reading and writing.
 542- If a setting has a configurable default value and keyed specific
 543  overrides, the default entry should be keyed with "default" and
 544  appear as the first entry in the file.
 546  The default value can be updated by writing either "default $VAL" or
 547  "$VAL".
 549  When writing to update a specific override, "default" can be used as
 550  the value to indicate removal of the override.  Override entries
 551  with "default" as the value must not appear when read.
 553  For example, a setting which is keyed by major:minor device numbers
 554  with integer values may look like the following.
 556    # cat cgroup-example-interface-file
 557    default 150
 558    8:0 300
 560  The default value can be updated by
 562    # echo 125 > cgroup-example-interface-file
 564  or
 566    # echo "default 125" > cgroup-example-interface-file
 568  An override can be set by
 570    # echo "8:16 170" > cgroup-example-interface-file
 572  and cleared by
 574    # echo "8:0 default" > cgroup-example-interface-file
 575    # cat cgroup-example-interface-file
 576    default 125
 577    8:16 170
 579- For events which are not very high frequency, an interface file
 580  "events" should be created which lists event key value pairs.
 581  Whenever a notifiable event happens, file modified event should be
 582  generated on the file.
 5854-3. Core Interface Files
 587All cgroup core files are prefixed with "cgroup."
 589  cgroup.procs
 591        A read-write new-line separated values file which exists on
 592        all cgroups.
 594        When read, it lists the PIDs of all processes which belong to
 595        the cgroup one-per-line.  The PIDs are not ordered and the
 596        same PID may show up more than once if the process got moved
 597        to another cgroup and then back or the PID got recycled while
 598        reading.
 600        A PID can be written to migrate the process associated with
 601        the PID to the cgroup.  The writer should match all of the
 602        following conditions.
 604        - Its euid is either root or must match either uid or suid of
 605          the target process.
 607        - It must have write access to the "cgroup.procs" file.
 609        - It must have write access to the "cgroup.procs" file of the
 610          common ancestor of the source and destination cgroups.
 612        When delegating a sub-hierarchy, write access to this file
 613        should be granted along with the containing directory.
 615  cgroup.controllers
 617        A read-only space separated values file which exists on all
 618        cgroups.
 620        It shows space separated list of all controllers available to
 621        the cgroup.  The controllers are not ordered.
 623  cgroup.subtree_control
 625        A read-write space separated values file which exists on all
 626        cgroups.  Starts out empty.
 628        When read, it shows space separated list of the controllers
 629        which are enabled to control resource distribution from the
 630        cgroup to its children.
 632        Space separated list of controllers prefixed with '+' or '-'
 633        can be written to enable or disable controllers.  A controller
 634        name prefixed with '+' enables the controller and '-'
 635        disables.  If a controller appears more than once on the list,
 636        the last one is effective.  When multiple enable and disable
 637        operations are specified, either all succeed or all fail.
 641        A read-only flat-keyed file which exists on non-root cgroups.
 642        The following entries are defined.  Unless specified
 643        otherwise, a value change in this file generates a file
 644        modified event.
 646          populated
 648                1 if the cgroup or its descendants contains any live
 649                processes; otherwise, 0.
 6525. Controllers
 6545-1. CPU
 656[NOTE: The interface for the cpu controller hasn't been merged yet]
 658The "cpu" controllers regulates distribution of CPU cycles.  This
 659controller implements weight and absolute bandwidth limit models for
 660normal scheduling policy and absolute bandwidth allocation model for
 661realtime scheduling policy.
 6645-1-1. CPU Interface Files
 666All time durations are in microseconds.
 668  cpu.stat
 670        A read-only flat-keyed file which exists on non-root cgroups.
 672        It reports the following six stats.
 674          usage_usec
 675          user_usec
 676          system_usec
 677          nr_periods
 678          nr_throttled
 679          throttled_usec
 681  cpu.weight
 683        A read-write single value file which exists on non-root
 684        cgroups.  The default is "100".
 686        The weight in the range [1, 10000].
 688  cpu.max
 690        A read-write two value file which exists on non-root cgroups.
 691        The default is "max 100000".
 693        The maximum bandwidth limit.  It's in the following format.
 695          $MAX $PERIOD
 697        which indicates that the group may consume upto $MAX in each
 698        $PERIOD duration.  "max" for $MAX indicates no limit.  If only
 699        one number is written, $MAX is updated.
 701  cpu.rt.max
 703  [NOTE: The semantics of this file is still under discussion and the
 704   interface hasn't been merged yet]
 706        A read-write two value file which exists on all cgroups.
 707        The default is "0 100000".
 709        The maximum realtime runtime allocation.  Over-committing
 710        configurations are disallowed and process migrations are
 711        rejected if not enough bandwidth is available.  It's in the
 712        following format.
 714          $MAX $PERIOD
 716        which indicates that the group may consume upto $MAX in each
 717        $PERIOD duration.  If only one number is written, $MAX is
 718        updated.
 7215-2. Memory
 723The "memory" controller regulates distribution of memory.  Memory is
 724stateful and implements both limit and protection models.  Due to the
 725intertwining between memory usage and reclaim pressure and the
 726stateful nature of memory, the distribution model is relatively
 729While not completely water-tight, all major memory usages by a given
 730cgroup are tracked so that the total memory consumption can be
 731accounted and controlled to a reasonable extent.  Currently, the
 732following types of memory usages are tracked.
 734- Userland memory - page cache and anonymous memory.
 736- Kernel data structures such as dentries and inodes.
 738- TCP socket buffers.
 740The above list may expand in the future for better coverage.
 7435-2-1. Memory Interface Files
 745All memory amounts are in bytes.  If a value which is not aligned to
 746PAGE_SIZE is written, the value may be rounded up to the closest
 747PAGE_SIZE multiple when read back.
 749  memory.current
 751        A read-only single value file which exists on non-root
 752        cgroups.
 754        The total amount of memory currently being used by the cgroup
 755        and its descendants.
 757  memory.low
 759        A read-write single value file which exists on non-root
 760        cgroups.  The default is "0".
 762        Best-effort memory protection.  If the memory usages of a
 763        cgroup and all its ancestors are below their low boundaries,
 764        the cgroup's memory won't be reclaimed unless memory can be
 765        reclaimed from unprotected cgroups.
 767        Putting more memory than generally available under this
 768        protection is discouraged.
 770  memory.high
 772        A read-write single value file which exists on non-root
 773        cgroups.  The default is "max".
 775        Memory usage throttle limit.  This is the main mechanism to
 776        control memory usage of a cgroup.  If a cgroup's usage goes
 777        over the high boundary, the processes of the cgroup are
 778        throttled and put under heavy reclaim pressure.
 780        Going over the high limit never invokes the OOM killer and
 781        under extreme conditions the limit may be breached.
 783  memory.max
 785        A read-write single value file which exists on non-root
 786        cgroups.  The default is "max".
 788        Memory usage hard limit.  This is the final protection
 789        mechanism.  If a cgroup's memory usage reaches this limit and
 790        can't be reduced, the OOM killer is invoked in the cgroup.
 791        Under certain circumstances, the usage may go over the limit
 792        temporarily.
 794        This is the ultimate protection mechanism.  As long as the
 795        high limit is used and monitored properly, this limit's
 796        utility is limited to providing the final safety net.
 800        A read-only flat-keyed file which exists on non-root cgroups.
 801        The following entries are defined.  Unless specified
 802        otherwise, a value change in this file generates a file
 803        modified event.
 805          low
 807                The number of times the cgroup is reclaimed due to
 808                high memory pressure even though its usage is under
 809                the low boundary.  This usually indicates that the low
 810                boundary is over-committed.
 812          high
 814                The number of times processes of the cgroup are
 815                throttled and routed to perform direct memory reclaim
 816                because the high memory boundary was exceeded.  For a
 817                cgroup whose memory usage is capped by the high limit
 818                rather than global memory pressure, this event's
 819                occurrences are expected.
 821          max
 823                The number of times the cgroup's memory usage was
 824                about to go over the max boundary.  If direct reclaim
 825                fails to bring it down, the OOM killer is invoked.
 827          oom
 829                The number of times the OOM killer has been invoked in
 830                the cgroup.  This may not exactly match the number of
 831                processes killed but should generally be close.
 833  memory.stat
 835        A read-only flat-keyed file which exists on non-root cgroups.
 837        This breaks down the cgroup's memory footprint into different
 838        types of memory, type-specific details, and other information
 839        on the state and past events of the memory management system.
 841        All memory amounts are in bytes.
 843        The entries are ordered to be human readable, and new entries
 844        can show up in the middle. Don't rely on items remaining in a
 845        fixed position; use the keys to look up specific values!
 847          anon
 849                Amount of memory used in anonymous mappings such as
 850                brk(), sbrk(), and mmap(MAP_ANONYMOUS)
 852          file
 854                Amount of memory used to cache filesystem data,
 855                including tmpfs and shared memory.
 857          kernel_stack
 859                Amount of memory allocated to kernel stacks.
 861          slab
 863                Amount of memory used for storing in-kernel data
 864                structures.
 866          sock
 868                Amount of memory used in network transmission buffers
 870          file_mapped
 872                Amount of cached filesystem data mapped with mmap()
 874          file_dirty
 876                Amount of cached filesystem data that was modified but
 877                not yet written back to disk
 879          file_writeback
 881                Amount of cached filesystem data that was modified and
 882                is currently being written back to disk
 884          inactive_anon
 885          active_anon
 886          inactive_file
 887          active_file
 888          unevictable
 890                Amount of memory, swap-backed and filesystem-backed,
 891                on the internal memory management lists used by the
 892                page reclaim algorithm
 894          slab_reclaimable
 896                Part of "slab" that might be reclaimed, such as
 897                dentries and inodes.
 899          slab_unreclaimable
 901                Part of "slab" that cannot be reclaimed on memory
 902                pressure.
 904          pgfault
 906                Total number of page faults incurred
 908          pgmajfault
 910                Number of major page faults incurred
 912  memory.swap.current
 914        A read-only single value file which exists on non-root
 915        cgroups.
 917        The total amount of swap currently being used by the cgroup
 918        and its descendants.
 920  memory.swap.max
 922        A read-write single value file which exists on non-root
 923        cgroups.  The default is "max".
 925        Swap usage hard limit.  If a cgroup's swap usage reaches this
 926        limit, anonymous meomry of the cgroup will not be swapped out.
 9295-2-2. Usage Guidelines
 931"memory.high" is the main mechanism to control memory usage.
 932Over-committing on high limit (sum of high limits > available memory)
 933and letting global memory pressure to distribute memory according to
 934usage is a viable strategy.
 936Because breach of the high limit doesn't trigger the OOM killer but
 937throttles the offending cgroup, a management agent has ample
 938opportunities to monitor and take appropriate actions such as granting
 939more memory or terminating the workload.
 941Determining whether a cgroup has enough memory is not trivial as
 942memory usage doesn't indicate whether the workload can benefit from
 943more memory.  For example, a workload which writes data received from
 944network to a file can use all available memory but can also operate as
 945performant with a small amount of memory.  A measure of memory
 946pressure - how much the workload is being impacted due to lack of
 947memory - is necessary to determine whether a workload needs more
 948memory; unfortunately, memory pressure monitoring mechanism isn't
 949implemented yet.
 9525-2-3. Memory Ownership
 954A memory area is charged to the cgroup which instantiated it and stays
 955charged to the cgroup until the area is released.  Migrating a process
 956to a different cgroup doesn't move the memory usages that it
 957instantiated while in the previous cgroup to the new cgroup.
 959A memory area may be used by processes belonging to different cgroups.
 960To which cgroup the area will be charged is in-deterministic; however,
 961over time, the memory area is likely to end up in a cgroup which has
 962enough memory allowance to avoid high reclaim pressure.
 964If a cgroup sweeps a considerable amount of memory which is expected
 965to be accessed repeatedly by other cgroups, it may make sense to use
 966POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
 967belonging to the affected files to ensure correct memory ownership.
 9705-3. IO
 972The "io" controller regulates the distribution of IO resources.  This
 973controller implements both weight based and absolute bandwidth or IOPS
 974limit distribution; however, weight based distribution is available
 975only if cfq-iosched is in use and neither scheme is available for
 976blk-mq devices.
 9795-3-1. IO Interface Files
 981  io.stat
 983        A read-only nested-keyed file which exists on non-root
 984        cgroups.
 986        Lines are keyed by $MAJ:$MIN device numbers and not ordered.
 987        The following nested keys are defined.
 989          rbytes        Bytes read
 990          wbytes        Bytes written
 991          rios          Number of read IOs
 992          wios          Number of write IOs
 994        An example read output follows.
 996          8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353
 997          8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252
 999  io.weight
1001        A read-write flat-keyed file which exists on non-root cgroups.
1002        The default is "default 100".
1004        The first line is the default weight applied to devices
1005        without specific override.  The rest are overrides keyed by
1006        $MAJ:$MIN device numbers and not ordered.  The weights are in
1007        the range [1, 10000] and specifies the relative amount IO time
1008        the cgroup can use in relation to its siblings.
1010        The default weight can be updated by writing either "default
1011        $WEIGHT" or simply "$WEIGHT".  Overrides can be set by writing
1012        "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
1014        An example read output follows.
1016          default 100
1017          8:16 200
1018          8:0 50
1020  io.max
1022        A read-write nested-keyed file which exists on non-root
1023        cgroups.
1025        BPS and IOPS based IO limit.  Lines are keyed by $MAJ:$MIN
1026        device numbers and not ordered.  The following nested keys are
1027        defined.
1029          rbps          Max read bytes per second
1030          wbps          Max write bytes per second
1031          riops         Max read IO operations per second
1032          wiops         Max write IO operations per second
1034        When writing, any number of nested key-value pairs can be
1035        specified in any order.  "max" can be specified as the value
1036        to remove a specific limit.  If the same key is specified
1037        multiple times, the outcome is undefined.
1039        BPS and IOPS are measured in each IO direction and IOs are
1040        delayed if limit is reached.  Temporary bursts are allowed.
1042        Setting read limit at 2M BPS and write at 120 IOPS for 8:16.
1044          echo "8:16 rbps=2097152 wiops=120" > io.max
1046        Reading returns the following.
1048          8:16 rbps=2097152 wbps=max riops=max wiops=120
1050        Write IOPS limit can be removed by writing the following.
1052          echo "8:16 wiops=max" > io.max
1054        Reading now returns the following.
1056          8:16 rbps=2097152 wbps=max riops=max wiops=max
10595-3-2. Writeback
1061Page cache is dirtied through buffered writes and shared mmaps and
1062written asynchronously to the backing filesystem by the writeback
1063mechanism.  Writeback sits between the memory and IO domains and
1064regulates the proportion of dirty memory by balancing dirtying and
1065write IOs.
1067The io controller, in conjunction with the memory controller,
1068implements control of page cache writeback IOs.  The memory controller
1069defines the memory domain that dirty memory ratio is calculated and
1070maintained for and the io controller defines the io domain which
1071writes out dirty pages for the memory domain.  Both system-wide and
1072per-cgroup dirty memory states are examined and the more restrictive
1073of the two is enforced.
1075cgroup writeback requires explicit support from the underlying
1076filesystem.  Currently, cgroup writeback is implemented on ext2, ext4
1077and btrfs.  On other filesystems, all writeback IOs are attributed to
1078the root cgroup.
1080There are inherent differences in memory and writeback management
1081which affects how cgroup ownership is tracked.  Memory is tracked per
1082page while writeback per inode.  For the purpose of writeback, an
1083inode is assigned to a cgroup and all IO requests to write dirty pages
1084from the inode are attributed to that cgroup.
1086As cgroup ownership for memory is tracked per page, there can be pages
1087which are associated with different cgroups than the one the inode is
1088associated with.  These are called foreign pages.  The writeback
1089constantly keeps track of foreign pages and, if a particular foreign
1090cgroup becomes the majority over a certain period of time, switches
1091the ownership of the inode to that cgroup.
1093While this model is enough for most use cases where a given inode is
1094mostly dirtied by a single cgroup even when the main writing cgroup
1095changes over time, use cases where multiple cgroups write to a single
1096inode simultaneously are not supported well.  In such circumstances, a
1097significant portion of IOs are likely to be attributed incorrectly.
1098As memory controller assigns page ownership on the first use and
1099doesn't update it until the page is released, even if writeback
1100strictly follows page ownership, multiple cgroups dirtying overlapping
1101areas wouldn't work as expected.  It's recommended to avoid such usage
1104The sysctl knobs which affect writeback behavior are applied to cgroup
1105writeback as follows.
1107  vm.dirty_background_ratio
1108  vm.dirty_ratio
1110        These ratios apply the same to cgroup writeback with the
1111        amount of available memory capped by limits imposed by the
1112        memory controller and system-wide clean memory.
1114  vm.dirty_background_bytes
1115  vm.dirty_bytes
1117        For cgroup writeback, this is calculated into ratio against
1118        total available memory and applied the same way as
1119        vm.dirty[_background]_ratio.
11226. Namespace
11246-1. Basics
1126cgroup namespace provides a mechanism to virtualize the view of the
1127"/proc/$PID/cgroup" file and cgroup mounts.  The CLONE_NEWCGROUP clone
1128flag can be used with clone(2) and unshare(2) to create a new cgroup
1129namespace.  The process running inside the cgroup namespace will have
1130its "/proc/$PID/cgroup" output restricted to cgroupns root.  The
1131cgroupns root is the cgroup of the process at the time of creation of
1132the cgroup namespace.
1134Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
1135complete path of the cgroup of a process.  In a container setup where
1136a set of cgroups and namespaces are intended to isolate processes the
1137"/proc/$PID/cgroup" file may leak potential system level information
1138to the isolated processes.  For Example:
1140  # cat /proc/self/cgroup
1141  0::/batchjobs/container_id1
1143The path '/batchjobs/container_id1' can be considered as system-data
1144and undesirable to expose to the isolated processes.  cgroup namespace
1145can be used to restrict visibility of this path.  For example, before
1146creating a cgroup namespace, one would see:
1148  # ls -l /proc/self/ns/cgroup
1149  lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
1150  # cat /proc/self/cgroup
1151  0::/batchjobs/container_id1
1153After unsharing a new namespace, the view changes.
1155  # ls -l /proc/self/ns/cgroup
1156  lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
1157  # cat /proc/self/cgroup
1158  0::/
1160When some thread from a multi-threaded process unshares its cgroup
1161namespace, the new cgroupns gets applied to the entire process (all
1162the threads).  This is natural for the v2 hierarchy; however, for the
1163legacy hierarchies, this may be unexpected.
1165A cgroup namespace is alive as long as there are processes inside or
1166mounts pinning it.  When the last usage goes away, the cgroup
1167namespace is destroyed.  The cgroupns root and the actual cgroups
11716-2. The Root and Views
1173The 'cgroupns root' for a cgroup namespace is the cgroup in which the
1174process calling unshare(2) is running.  For example, if a process in
1175/batchjobs/container_id1 cgroup calls unshare, cgroup
1176/batchjobs/container_id1 becomes the cgroupns root.  For the
1177init_cgroup_ns, this is the real root ('/') cgroup.
1179The cgroupns root cgroup does not change even if the namespace creator
1180process later moves to a different cgroup.
1182  # ~/unshare -c # unshare cgroupns in some cgroup
1183  # cat /proc/self/cgroup
1184  0::/
1185  # mkdir sub_cgrp_1
1186  # echo 0 > sub_cgrp_1/cgroup.procs
1187  # cat /proc/self/cgroup
1188  0::/sub_cgrp_1
1190Each process gets its namespace-specific view of "/proc/$PID/cgroup"
1192Processes running inside the cgroup namespace will be able to see
1193cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
1194From within an unshared cgroupns:
1196  # sleep 100000 &
1197  [1] 7353
1198  # echo 7353 > sub_cgrp_1/cgroup.procs
1199  # cat /proc/7353/cgroup
1200  0::/sub_cgrp_1
1202From the initial cgroup namespace, the real cgroup path will be
1205  $ cat /proc/7353/cgroup
1206  0::/batchjobs/container_id1/sub_cgrp_1
1208From a sibling cgroup namespace (that is, a namespace rooted at a
1209different cgroup), the cgroup path relative to its own cgroup
1210namespace root will be shown.  For instance, if PID 7353's cgroup
1211namespace root is at '/batchjobs/container_id2', then it will see
1213  # cat /proc/7353/cgroup
1214  0::/../container_id2/sub_cgrp_1
1216Note that the relative path always starts with '/' to indicate that
1217its relative to the cgroup namespace root of the caller.
12206-3. Migration and setns(2)
1222Processes inside a cgroup namespace can move into and out of the
1223namespace root if they have proper access to external cgroups.  For
1224example, from inside a namespace with cgroupns root at
1225/batchjobs/container_id1, and assuming that the global hierarchy is
1226still accessible inside cgroupns:
1228  # cat /proc/7353/cgroup
1229  0::/sub_cgrp_1
1230  # echo 7353 > batchjobs/container_id2/cgroup.procs
1231  # cat /proc/7353/cgroup
1232  0::/../container_id2
1234Note that this kind of setup is not encouraged.  A task inside cgroup
1235namespace should only be exposed to its own cgroupns hierarchy.
1237setns(2) to another cgroup namespace is allowed when:
1239(a) the process has CAP_SYS_ADMIN against its current user namespace
1240(b) the process has CAP_SYS_ADMIN against the target cgroup
1241    namespace's userns
1243No implicit cgroup changes happen with attaching to another cgroup
1244namespace.  It is expected that the someone moves the attaching
1245process under the target cgroup namespace root.
12486-4. Interaction with Other Namespaces
1250Namespace specific cgroup hierarchy can be mounted by a process
1251running inside a non-init cgroup namespace.
1253  # mount -t cgroup2 none $MOUNT_POINT
1255This will mount the unified cgroup hierarchy with cgroupns root as the
1256filesystem root.  The process needs CAP_SYS_ADMIN against its user and
1257mount namespaces.
1259The virtualization of /proc/self/cgroup file combined with restricting
1260the view of cgroup hierarchy by namespace-private cgroupfs mount
1261provides a properly isolated cgroup view inside the container.
1264P. Information on Kernel Programming
1266This section contains kernel programming information in the areas
1267where interacting with cgroup is necessary.  cgroup core and
1268controllers are not covered.
1271P-1. Filesystem Support for Writeback
1273A filesystem can support cgroup writeback by updating
1274address_space_operations->writepage[s]() to annotate bio's using the
1275following two functions.
1277  wbc_init_bio(@wbc, @bio)
1279        Should be called for each bio carrying writeback data and
1280        associates the bio with the inode's owner cgroup.  Can be
1281        called anytime between bio allocation and submission.
1283  wbc_account_io(@wbc, @page, @bytes)
1285        Should be called for each data segment being written out.
1286        While this function doesn't care exactly when it's called
1287        during the writeback session, it's the easiest and most
1288        natural to call it as data segments are added to a bio.
1290With writeback bio's annotated, cgroup support can be enabled per
1291super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
1292selective disabling of cgroup writeback support which is helpful when
1293certain filesystem features, e.g. journaled data mode, are
1296wbc_init_bio() binds the specified bio to its cgroup.  Depending on
1297the configuration, the bio may be executed at a lower priority and if
1298the writeback session is holding shared resources, e.g. a journal
1299entry, may lead to priority inversion.  There is no one easy solution
1300for the problem.  Filesystems can try to work around specific problem
1301cases by skipping wbc_init_bio() or using bio_associate_blkcg()
1305D. Deprecated v1 Core Features
1307- Multiple hierarchies including named ones are not supported.
1309- All mount options and remounting are not supported.
1311- The "tasks" file is removed and "cgroup.procs" is not sorted.
1313- "cgroup.clone_children" is removed.
1315- /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" file
1316  at the root instead.
1319R. Issues with v1 and Rationales for v2
1321R-1. Multiple Hierarchies
1323cgroup v1 allowed an arbitrary number of hierarchies and each
1324hierarchy could host any number of controllers.  While this seemed to
1325provide a high level of flexibility, it wasn't useful in practice.
1327For example, as there is only one instance of each controller, utility
1328type controllers such as freezer which can be useful in all
1329hierarchies could only be used in one.  The issue is exacerbated by
1330the fact that controllers couldn't be moved to another hierarchy once
1331hierarchies were populated.  Another issue was that all controllers
1332bound to a hierarchy were forced to have exactly the same view of the
1333hierarchy.  It wasn't possible to vary the granularity depending on
1334the specific controller.
1336In practice, these issues heavily limited which controllers could be
1337put on the same hierarchy and most configurations resorted to putting
1338each controller on its own hierarchy.  Only closely related ones, such
1339as the cpu and cpuacct controllers, made sense to be put on the same
1340hierarchy.  This often meant that userland ended up managing multiple
1341similar hierarchies repeating the same steps on each hierarchy
1342whenever a hierarchy management operation was necessary.
1344Furthermore, support for multiple hierarchies came at a steep cost.
1345It greatly complicated cgroup core implementation but more importantly
1346the support for multiple hierarchies restricted how cgroup could be
1347used in general and what controllers was able to do.
1349There was no limit on how many hierarchies there might be, which meant
1350that a thread's cgroup membership couldn't be described in finite
1351length.  The key might contain any number of entries and was unlimited
1352in length, which made it highly awkward to manipulate and led to
1353addition of controllers which existed only to identify membership,
1354which in turn exacerbated the original problem of proliferating number
1355of hierarchies.
1357Also, as a controller couldn't have any expectation regarding the
1358topologies of hierarchies other controllers might be on, each
1359controller had to assume that all other controllers were attached to
1360completely orthogonal hierarchies.  This made it impossible, or at
1361least very cumbersome, for controllers to cooperate with each other.
1363In most use cases, putting controllers on hierarchies which are
1364completely orthogonal to each other isn't necessary.  What usually is
1365called for is the ability to have differing levels of granularity
1366depending on the specific controller.  In other words, hierarchy may
1367be collapsed from leaf towards root when viewed from specific
1368controllers.  For example, a given configuration might not care about
1369how memory is distributed beyond a certain level while still wanting
1370to control how CPU cycles are distributed.
1373R-2. Thread Granularity
1375cgroup v1 allowed threads of a process to belong to different cgroups.
1376This didn't make sense for some controllers and those controllers
1377ended up implementing different ways to ignore such situations but
1378much more importantly it blurred the line between API exposed to
1379individual applications and system management interface.
1381Generally, in-process knowledge is available only to the process
1382itself; thus, unlike service-level organization of processes,
1383categorizing threads of a process requires active participation from
1384the application which owns the target process.
1386cgroup v1 had an ambiguously defined delegation model which got abused
1387in combination with thread granularity.  cgroups were delegated to
1388individual applications so that they can create and manage their own
1389sub-hierarchies and control resource distributions along them.  This
1390effectively raised cgroup to the status of a syscall-like API exposed
1391to lay programs.
1393First of all, cgroup has a fundamentally inadequate interface to be
1394exposed this way.  For a process to access its own knobs, it has to
1395extract the path on the target hierarchy from /proc/self/cgroup,
1396construct the path by appending the name of the knob to the path, open
1397and then read and/or write to it.  This is not only extremely clunky
1398and unusual but also inherently racy.  There is no conventional way to
1399define transaction across the required steps and nothing can guarantee
1400that the process would actually be operating on its own sub-hierarchy.
1402cgroup controllers implemented a number of knobs which would never be
1403accepted as public APIs because they were just adding control knobs to
1404system-management pseudo filesystem.  cgroup ended up with interface
1405knobs which were not properly abstracted or refined and directly
1406revealed kernel internal details.  These knobs got exposed to
1407individual applications through the ill-defined delegation mechanism
1408effectively abusing cgroup as a shortcut to implementing public APIs
1409without going through the required scrutiny.
1411This was painful for both userland and kernel.  Userland ended up with
1412misbehaving and poorly abstracted interfaces and kernel exposing and
1413locked into constructs inadvertently.
1416R-3. Competition Between Inner Nodes and Threads
1418cgroup v1 allowed threads to be in any cgroups which created an
1419interesting problem where threads belonging to a parent cgroup and its
1420children cgroups competed for resources.  This was nasty as two
1421different types of entities competed and there was no obvious way to
1422settle it.  Different controllers did different things.
1424The cpu controller considered threads and cgroups as equivalents and
1425mapped nice levels to cgroup weights.  This worked for some cases but
1426fell flat when children wanted to be allocated specific ratios of CPU
1427cycles and the number of internal threads fluctuated - the ratios
1428constantly changed as the number of competing entities fluctuated.
1429There also were other issues.  The mapping from nice level to weight
1430wasn't obvious or universal, and there were various other knobs which
1431simply weren't available for threads.
1433The io controller implicitly created a hidden leaf node for each
1434cgroup to host the threads.  The hidden leaf had its own copies of all
1435the knobs with "leaf_" prefixed.  While this allowed equivalent
1436control over internal threads, it was with serious drawbacks.  It
1437always added an extra layer of nesting which wouldn't be necessary
1438otherwise, made the interface messy and significantly complicated the
1441The memory controller didn't have a way to control what happened
1442between internal tasks and child cgroups and the behavior was not
1443clearly defined.  There were attempts to add ad-hoc behaviors and
1444knobs to tailor the behavior to specific workloads which would have
1445led to problems extremely difficult to resolve in the long term.
1447Multiple controllers struggled with internal tasks and came up with
1448different ways to deal with it; unfortunately, all the approaches were
1449severely flawed and, furthermore, the widely different behaviors
1450made cgroup as a whole highly inconsistent.
1452This clearly is a problem which needs to be addressed from cgroup core
1453in a uniform way.
1456R-4. Other Interface Issues
1458cgroup v1 grew without oversight and developed a large number of
1459idiosyncrasies and inconsistencies.  One issue on the cgroup core side
1460was how an empty cgroup was notified - a userland helper binary was
1461forked and executed for each event.  The event delivery wasn't
1462recursive or delegatable.  The limitations of the mechanism also led
1463to in-kernel event delivery filtering mechanism further complicating
1464the interface.
1466Controller interfaces were problematic too.  An extreme example is
1467controllers completely ignoring hierarchical organization and treating
1468all cgroups as if they were all located directly under the root
1469cgroup.  Some controllers exposed a large amount of inconsistent
1470implementation details to userland.
1472There also was no consistency across controllers.  When a new cgroup
1473was created, some controllers defaulted to not imposing extra
1474restrictions while others disallowed any resource usage until
1475explicitly configured.  Configuration knobs for the same type of
1476control used widely differing naming schemes and formats.  Statistics
1477and information knobs were named arbitrarily and used different
1478formats and units even in the same controller.
1480cgroup v2 establishes common conventions where appropriate and updates
1481controllers so that they expose minimal and consistent interfaces.
1484R-5. Controller Issues and Remedies
1486R-5-1. Memory
1488The original lower boundary, the soft limit, is defined as a limit
1489that is per default unset.  As a result, the set of cgroups that
1490global reclaim prefers is opt-in, rather than opt-out.  The costs for
1491optimizing these mostly negative lookups are so high that the
1492implementation, despite its enormous size, does not even provide the
1493basic desirable behavior.  First off, the soft limit has no
1494hierarchical meaning.  All configured groups are organized in a global
1495rbtree and treated like equal peers, regardless where they are located
1496in the hierarchy.  This makes subtree delegation impossible.  Second,
1497the soft limit reclaim pass is so aggressive that it not just
1498introduces high allocation latencies into the system, but also impacts
1499system performance due to overreclaim, to the point where the feature
1500becomes self-defeating.
1502The memory.low boundary on the other hand is a top-down allocated
1503reserve.  A cgroup enjoys reclaim protection when it and all its
1504ancestors are below their low boundaries, which makes delegation of
1505subtrees possible.  Secondly, new cgroups have no reserve per default
1506and in the common case most cgroups are eligible for the preferred
1507reclaim pass.  This allows the new low boundary to be efficiently
1508implemented with just a minor addition to the generic reclaim code,
1509without the need for out-of-band data structures and reclaim passes.
1510Because the generic reclaim code considers all cgroups except for the
1511ones running low in the preferred first reclaim pass, overreclaim of
1512individual groups is eliminated as well, resulting in much better
1513overall workload performance.
1515The original high boundary, the hard limit, is defined as a strict
1516limit that can not budge, even if the OOM killer has to be called.
1517But this generally goes against the goal of making the most out of the
1518available memory.  The memory consumption of workloads varies during
1519runtime, and that requires users to overcommit.  But doing that with a
1520strict upper limit requires either a fairly accurate prediction of the
1521working set size or adding slack to the limit.  Since working set size
1522estimation is hard and error prone, and getting it wrong results in
1523OOM kills, most users tend to err on the side of a looser limit and
1524end up wasting precious resources.
1526The memory.high boundary on the other hand can be set much more
1527conservatively.  When hit, it throttles allocations by forcing them
1528into direct reclaim to work off the excess, but it never invokes the
1529OOM killer.  As a result, a high boundary that is chosen too
1530aggressively will not terminate the processes, but instead it will
1531lead to gradual performance degradation.  The user can monitor this
1532and make corrections until the minimal memory footprint that still
1533gives acceptable performance is found.
1535In extreme cases, with many concurrent allocations and a complete
1536breakdown of reclaim progress within the group, the high boundary can
1537be exceeded.  But even then it's mostly better to satisfy the
1538allocation from the slack available in other groups or the rest of the
1539system than killing the group.  Otherwise, memory.max is there to
1540limit this type of spillover and ultimately contain buggy or even
1541malicious applications.
1543Setting the original memory.limit_in_bytes below the current usage was
1544subject to a race condition, where concurrent charges could cause the
1545limit setting to fail. memory.max on the other hand will first set the
1546limit to prevent new charges, and then reclaim and OOM kill until the
1547new limit is met - or the task writing to memory.max is killed.
1549The combined memory+swap accounting and limiting is replaced by real
1550control over swap space.
1552The main argument for a combined memory+swap facility in the original
1553cgroup design was that global or parental pressure would always be
1554able to swap all anonymous memory of a child group, regardless of the
1555child's own (possibly untrusted) configuration.  However, untrusted
1556groups can sabotage swapping by other means - such as referencing its
1557anonymous memory in a tight loop - and an admin can not assume full
1558swappability when overcommitting untrusted jobs.
1560For trusted jobs, on the other hand, a combined counter is not an
1561intuitive userspace interface, and it flies in the face of the idea
1562that cgroup controllers should account and limit specific physical
1563resources.  Swap space is a resource like all others in the system,
1564and that's why unified hierarchy allows distributing it separately.