Control Group v2
  
  October, 2015		Tejun Heo <tj@kernel.org>
  
  This is the authoritative documentation on the design, interface and
  conventions of cgroup v2.  It describes all userland-visible aspects
  of cgroup including core and specific controller behaviors.  All
  future changes must be reflected in this document.  Documentation for

  v1 is available under Documentation/cgroup-v1/.

  
  CONTENTS
  
  1. Introduction
    1-1. Terminology
    1-2. What is cgroup?
  2. Basic Operations
    2-1. Mounting
    2-2. Organizing Processes
    2-3. [Un]populated Notification
    2-4. Controlling Controllers
      2-4-1. Enabling and Disabling
      2-4-2. Top-down Constraint
      2-4-3. No Internal Process Constraint
    2-5. Delegation
      2-5-1. Model of Delegation
      2-5-2. Delegation Containment
    2-6. Guidelines
      2-6-1. Organize Once and Control
      2-6-2. Avoid Name Collisions
  3. Resource Distribution Models
    3-1. Weights
    3-2. Limits
    3-3. Protections
    3-4. Allocations
  4. Interface Files
    4-1. Format
    4-2. Conventions
    4-3. Core Interface Files
  5. Controllers
    5-1. CPU
      5-1-1. CPU Interface Files
    5-2. Memory
      5-2-1. Memory Interface Files
      5-2-2. Usage Guidelines
      5-2-3. Memory Ownership
    5-3. IO
      5-3-1. IO Interface Files
      5-3-2. Writeback

  6. Namespace
    6-1. Basics
    6-2. The Root and Views
    6-3. Migration and setns(2)
    6-4. Interaction with Other Namespaces

  P. Information on Kernel Programming
    P-1. Filesystem Support for Writeback
  D. Deprecated v1 Core Features
  R. Issues with v1 and Rationales for v2
    R-1. Multiple Hierarchies
    R-2. Thread Granularity
    R-3. Competition Between Inner Nodes and Threads
    R-4. Other Interface Issues
    R-5. Controller Issues and Remedies
      R-5-1. Memory
  
  
  1. Introduction
  
  1-1. Terminology
  
  "cgroup" stands for "control group" and is never capitalized.  The
  singular form is used to designate the whole feature and also as a
  qualifier as in "cgroup controllers".  When explicitly referring to
  multiple individual control groups, the plural form "cgroups" is used.
  
  
  1-2. What is cgroup?
  
  cgroup is a mechanism to organize processes hierarchically and
  distribute system resources along the hierarchy in a controlled and
  configurable manner.
  
  cgroup is largely composed of two parts - the core and controllers.
  cgroup core is primarily responsible for hierarchically organizing
  processes.  A cgroup controller is usually responsible for
  distributing a specific type of system resource along the hierarchy
  although there are utility controllers which serve purposes other than
  resource distribution.
  
  cgroups form a tree structure and every process in the system belongs
  to one and only one cgroup.  All threads of a process belong to the
  same cgroup.  On creation, all processes are put in the cgroup that
  the parent process belongs to at the time.  A process can be migrated
  to another cgroup.  Migration of a process doesn't affect already
  existing descendant processes.
  
  Following certain structural constraints, controllers may be enabled or
  disabled selectively on a cgroup.  All controller behaviors are
  hierarchical - if a controller is enabled on a cgroup, it affects all
  processes which belong to the cgroups consisting the inclusive
  sub-hierarchy of the cgroup.  When a controller is enabled on a nested
  cgroup, it always restricts the resource distribution further.  The
  restrictions set closer to the root in the hierarchy can not be
  overridden from further away.
  
  
  2. Basic Operations
  
  2-1. Mounting
  
  Unlike v1, cgroup v2 has only single hierarchy.  The cgroup v2
  hierarchy can be mounted with the following mount command.
  
    # mount -t cgroup2 none $MOUNT_POINT
  
  cgroup2 filesystem has the magic number 0x63677270 ("cgrp").  All
  controllers which support v2 and are not bound to a v1 hierarchy are
  automatically bound to the v2 hierarchy and show up at the root.
  Controllers which are not in active use in the v2 hierarchy can be
  bound to other hierarchies.  This allows mixing v2 hierarchy with the
  legacy v1 multiple hierarchies in a fully backward compatible way.
  
  A controller can be moved across hierarchies only after the controller
  is no longer referenced in its current hierarchy.  Because per-cgroup
  controller states are destroyed asynchronously and controllers may
  have lingering references, a controller may not show up immediately on
  the v2 hierarchy after the final umount of the previous hierarchy.
  Similarly, a controller should be fully disabled to be moved out of
  the unified hierarchy and it may take some time for the disabled
  controller to become available for other hierarchies; furthermore, due
  to inter-controller dependencies, other controllers may need to be
  disabled too.
  
  While useful for development and manual configurations, moving
  controllers dynamically between the v2 and other hierarchies is
  strongly discouraged for production use.  It is recommended to decide
  the hierarchies and controller associations before starting using the
  controllers after system boot.

  During transition to v2, system management software might still
  automount the v1 cgroup filesystem and so hijack all controllers
  during boot, before manual intervention is possible. To make testing
  and experimenting easier, the kernel parameter cgroup_no_v1= allows
  disabling controllers in v1 and make them always available in v2.

  
  2-2. Organizing Processes
  
  Initially, only the root cgroup exists to which all processes belong.
  A child cgroup can be created by creating a sub-directory.
  
    # mkdir $CGROUP_NAME
  
  A given cgroup may have multiple child cgroups forming a tree
  structure.  Each cgroup has a read-writable interface file
  "cgroup.procs".  When read, it lists the PIDs of all processes which
  belong to the cgroup one-per-line.  The PIDs are not ordered and the
  same PID may show up more than once if the process got moved to
  another cgroup and then back or the PID got recycled while reading.
  
  A process can be migrated into a cgroup by writing its PID to the
  target cgroup's "cgroup.procs" file.  Only one process can be migrated
  on a single write(2) call.  If a process is composed of multiple
  threads, writing the PID of any thread migrates all threads of the
  process.
  
  When a process forks a child process, the new process is born into the
  cgroup that the forking process belongs to at the time of the
  operation.  After exit, a process stays associated with the cgroup
  that it belonged to at the time of exit until it's reaped; however, a
  zombie process does not appear in "cgroup.procs" and thus can't be
  moved to another cgroup.
  
  A cgroup which doesn't have any children or live processes can be
  destroyed by removing the directory.  Note that a cgroup which doesn't
  have any children and is associated only with zombie processes is
  considered empty and can be removed.
  
    # rmdir $CGROUP_NAME
  
  "/proc/$PID/cgroup" lists a process's cgroup membership.  If legacy
  cgroup is in use in the system, this file may contain multiple lines,
  one for each hierarchy.  The entry for cgroup v2 is always in the
  format "0::$PATH".
  
    # cat /proc/842/cgroup
    ...
    0::/test-cgroup/test-cgroup-nested
  
  If the process becomes a zombie and the cgroup it was associated with
  is removed subsequently, " (deleted)" is appended to the path.
  
    # cat /proc/842/cgroup
    ...
    0::/test-cgroup/test-cgroup-nested (deleted)
  
  
  2-3. [Un]populated Notification
  
  Each non-root cgroup has a "cgroup.events" file which contains
  "populated" field indicating whether the cgroup's sub-hierarchy has
  live processes in it.  Its value is 0 if there is no live process in
  the cgroup and its descendants; otherwise, 1.  poll and [id]notify
  events are triggered when the value changes.  This can be used, for
  example, to start a clean-up operation after all processes of a given
  sub-hierarchy have exited.  The populated state updates and
  notifications are recursive.  Consider the following sub-hierarchy
  where the numbers in the parentheses represent the numbers of processes
  in each cgroup.
  
    A(4) - B(0) - C(1)
                \ D(0)
  
  A, B and C's "populated" fields would be 1 while D's 0.  After the one
  process in C exits, B and C's "populated" fields would flip to "0" and
  file modified events will be generated on the "cgroup.events" files of
  both cgroups.
  
  
  2-4. Controlling Controllers
  
  2-4-1. Enabling and Disabling
  
  Each cgroup has a "cgroup.controllers" file which lists all
  controllers available for the cgroup to enable.
  
    # cat cgroup.controllers
    cpu io memory
  
  No controller is enabled by default.  Controllers can be enabled and
  disabled by writing to the "cgroup.subtree_control" file.
  
    # echo "+cpu +memory -io" > cgroup.subtree_control
  
  Only controllers which are listed in "cgroup.controllers" can be
  enabled.  When multiple operations are specified as above, either they
  all succeed or fail.  If multiple operations on the same controller
  are specified, the last one is effective.
  
  Enabling a controller in a cgroup indicates that the distribution of
  the target resource across its immediate children will be controlled.
  Consider the following sub-hierarchy.  The enabled controllers are
  listed in parentheses.
  
    A(cpu,memory) - B(memory) - C()
                              \ D()
  
  As A has "cpu" and "memory" enabled, A will control the distribution
  of CPU cycles and memory to its children, in this case, B.  As B has
  "memory" enabled but not "CPU", C and D will compete freely on CPU
  cycles but their division of memory available to B will be controlled.
  
  As a controller regulates the distribution of the target resource to
  the cgroup's children, enabling it creates the controller's interface
  files in the child cgroups.  In the above example, enabling "cpu" on B
  would create the "cpu." prefixed controller interface files in C and
  D.  Likewise, disabling "memory" from B would remove the "memory."
  prefixed controller interface files from C and D.  This means that the
  controller interface files - anything which doesn't start with
  "cgroup." are owned by the parent rather than the cgroup itself.
  
  
  2-4-2. Top-down Constraint
  
  Resources are distributed top-down and a cgroup can further distribute
  a resource only if the resource has been distributed to it from the
  parent.  This means that all non-root "cgroup.subtree_control" files
  can only contain controllers which are enabled in the parent's
  "cgroup.subtree_control" file.  A controller can be enabled only if
  the parent has the controller enabled and a controller can't be
  disabled if one or more children have it enabled.
  
  
  2-4-3. No Internal Process Constraint
  
  Non-root cgroups can only distribute resources to their children when
  they don't have any processes of their own.  In other words, only
  cgroups which don't contain any processes can have controllers enabled
  in their "cgroup.subtree_control" files.
  
  This guarantees that, when a controller is looking at the part of the
  hierarchy which has it enabled, processes are always only on the
  leaves.  This rules out situations where child cgroups compete against
  internal processes of the parent.
  
  The root cgroup is exempt from this restriction.  Root contains
  processes and anonymous resource consumption which can't be associated
  with any other cgroups and requires special treatment from most
  controllers.  How resource consumption in the root cgroup is governed
  is up to each controller.
  
  Note that the restriction doesn't get in the way if there is no
  enabled controller in the cgroup's "cgroup.subtree_control".  This is
  important as otherwise it wouldn't be possible to create children of a
  populated cgroup.  To control resource distribution of a cgroup, the
  cgroup must create children and transfer all its processes to the
  children before enabling controllers in its "cgroup.subtree_control"
  file.
  
  
  2-5. Delegation
  
  2-5-1. Model of Delegation
  
  A cgroup can be delegated to a less privileged user by granting write
  access of the directory and its "cgroup.procs" file to the user.  Note
  that resource control interface files in a given directory control the
  distribution of the parent's resources and thus must not be delegated
  along with the directory.
  
  Once delegated, the user can build sub-hierarchy under the directory,
  organize processes as it sees fit and further distribute the resources
  it received from the parent.  The limits and other settings of all
  resource controllers are hierarchical and regardless of what happens
  in the delegated sub-hierarchy, nothing can escape the resource
  restrictions imposed by the parent.
  
  Currently, cgroup doesn't impose any restrictions on the number of
  cgroups in or nesting depth of a delegated sub-hierarchy; however,
  this may be limited explicitly in the future.
  
  
  2-5-2. Delegation Containment
  
  A delegated sub-hierarchy is contained in the sense that processes
  can't be moved into or out of the sub-hierarchy by the delegatee.  For
  a process with a non-root euid to migrate a target process into a
  cgroup by writing its PID to the "cgroup.procs" file, the following
  conditions must be met.
  
  - The writer's euid must match either uid or suid of the target process.
  
  - The writer must have write access to the "cgroup.procs" file.
  
  - The writer must have write access to the "cgroup.procs" file of the
    common ancestor of the source and destination cgroups.
  
  The above three constraints ensure that while a delegatee may migrate
  processes around freely in the delegated sub-hierarchy it can't pull
  in from or push out to outside the sub-hierarchy.
  
  For an example, let's assume cgroups C0 and C1 have been delegated to
  user U0 who created C00, C01 under C0 and C10 under C1 as follows and
  all processes under C0 and C1 belong to U0.
  
    ~~~~~~~~~~~~~ - C0 - C00
    ~ cgroup    ~      \ C01
    ~ hierarchy ~
    ~~~~~~~~~~~~~ - C1 - C10
  
  Let's also say U0 wants to write the PID of a process which is
  currently in C10 into "C00/cgroup.procs".  U0 has write access to the
  file and uid match on the process; however, the common ancestor of the
  source cgroup C10 and the destination cgroup C00 is above the points
  of delegation and U0 would not have write access to its "cgroup.procs"
  files and thus the write will be denied with -EACCES.
  
  
  2-6. Guidelines
  
  2-6-1. Organize Once and Control
  
  Migrating a process across cgroups is a relatively expensive operation
  and stateful resources such as memory are not moved together with the
  process.  This is an explicit design decision as there often exist
  inherent trade-offs between migration and various hot paths in terms
  of synchronization cost.
  
  As such, migrating processes across cgroups frequently as a means to
  apply different resource restrictions is discouraged.  A workload
  should be assigned to a cgroup according to the system's logical and
  resource structure once on start-up.  Dynamic adjustments to resource
  distribution can be made by changing controller configuration through
  the interface files.
  
  
  2-6-2. Avoid Name Collisions
  
  Interface files for a cgroup and its children cgroups occupy the same
  directory and it is possible to create children cgroups which collide
  with interface files.
  
  All cgroup core interface files are prefixed with "cgroup." and each
  controller's interface files are prefixed with the controller name and
  a dot.  A controller's name is composed of lower case alphabets and
  '_'s but never begins with an '_' so it can be used as the prefix
  character for collision avoidance.  Also, interface file names won't
  start or end with terms which are often used in categorizing workloads
  such as job, service, slice, unit or workload.
  
  cgroup doesn't do anything to prevent name collisions and it's the
  user's responsibility to avoid them.
  
  
  3. Resource Distribution Models
  
  cgroup controllers implement several resource distribution schemes
  depending on the resource type and expected use cases.  This section
  describes major schemes in use along with their expected behaviors.
  
  
  3-1. Weights
  
  A parent's resource is distributed by adding up the weights of all
  active children and giving each the fraction matching the ratio of its
  weight against the sum.  As only children which can make use of the
  resource at the moment participate in the distribution, this is
  work-conserving.  Due to the dynamic nature, this model is usually
  used for stateless resources.
  
  All weights are in the range [1, 10000] with the default at 100.  This
  allows symmetric multiplicative biases in both directions at fine
  enough granularity while staying in the intuitive range.
  
  As long as the weight is in range, all configuration combinations are
  valid and there is no reason to reject configuration changes or
  process migrations.
  
  "cpu.weight" proportionally distributes CPU cycles to active children
  and is an example of this type.
  
  
  3-2. Limits
  
  A child can only consume upto the configured amount of the resource.
  Limits can be over-committed - the sum of the limits of children can
  exceed the amount of resource available to the parent.
  
  Limits are in the range [0, max] and defaults to "max", which is noop.
  
  As limits can be over-committed, all configuration combinations are
  valid and there is no reason to reject configuration changes or
  process migrations.
  
  "io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
  on an IO device and is an example of this type.
  
  
  3-3. Protections
  
  A cgroup is protected to be allocated upto the configured amount of
  the resource if the usages of all its ancestors are under their
  protected levels.  Protections can be hard guarantees or best effort
  soft boundaries.  Protections can also be over-committed in which case
  only upto the amount available to the parent is protected among
  children.
  
  Protections are in the range [0, max] and defaults to 0, which is
  noop.
  
  As protections can be over-committed, all configuration combinations
  are valid and there is no reason to reject configuration changes or
  process migrations.
  
  "memory.low" implements best-effort memory protection and is an
  example of this type.
  
  
  3-4. Allocations
  
  A cgroup is exclusively allocated a certain amount of a finite
  resource.  Allocations can't be over-committed - the sum of the
  allocations of children can not exceed the amount of resource
  available to the parent.
  
  Allocations are in the range [0, max] and defaults to 0, which is no
  resource.
  
  As allocations can't be over-committed, some configuration
  combinations are invalid and should be rejected.  Also, if the
  resource is mandatory for execution of processes, process migrations
  may be rejected.
  
  "cpu.rt.max" hard-allocates realtime slices and is an example of this
  type.
  
  
  4. Interface Files
  
  4-1. Format
  
  All interface files should be in one of the following formats whenever
  possible.
  
    New-line separated values
    (when only one value can be written at once)
  
  	VAL0
  
  	VAL1
  
  	...
  
    Space separated values
    (when read-only or multiple values can be written at once)
  
  	VAL0 VAL1 ...
  
  
    Flat keyed
  
  	KEY0 VAL0
  
  	KEY1 VAL1
  
  	...
  
    Nested keyed
  
  	KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
  	KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
  	...
  
  For a writable file, the format for writing should generally match
  reading; however, controllers may allow omitting later fields or
  implement restricted shortcuts for most common use cases.
  
  For both flat and nested keyed files, only the values for a single key
  can be written at a time.  For nested keyed files, the sub key pairs
  may be specified in any order and not all pairs have to be specified.
  
  
  4-2. Conventions
  
  - Settings for a single feature should be contained in a single file.
  
  - The root cgroup should be exempt from resource control and thus
    shouldn't have resource control interface files.  Also,
    informational files on the root cgroup which end up showing global
    information available elsewhere shouldn't exist.
  
  - If a controller implements weight based resource distribution, its
    interface file should be named "weight" and have the range [1,
    10000] with 100 as the default.  The values are chosen to allow
    enough and symmetric bias in both directions while keeping it
    intuitive (the default is 100%).
  
  - If a controller implements an absolute resource guarantee and/or
    limit, the interface files should be named "min" and "max"
    respectively.  If a controller implements best effort resource
    guarantee and/or limit, the interface files should be named "low"
    and "high" respectively.
  
    In the above four control files, the special token "max" should be
    used to represent upward infinity for both reading and writing.
  
  - If a setting has a configurable default value and keyed specific
    overrides, the default entry should be keyed with "default" and
    appear as the first entry in the file.
  
    The default value can be updated by writing either "default $VAL" or
    "$VAL".
  
    When writing to update a specific override, "default" can be used as
    the value to indicate removal of the override.  Override entries
    with "default" as the value must not appear when read.
  
    For example, a setting which is keyed by major:minor device numbers
    with integer values may look like the following.
  
      # cat cgroup-example-interface-file
      default 150
      8:0 300
  
    The default value can be updated by
  
      # echo 125 > cgroup-example-interface-file
  
    or
  
      # echo "default 125" > cgroup-example-interface-file
  
    An override can be set by
  
      # echo "8:16 170" > cgroup-example-interface-file
  
    and cleared by
  
      # echo "8:0 default" > cgroup-example-interface-file
      # cat cgroup-example-interface-file
      default 125
      8:16 170
  
  - For events which are not very high frequency, an interface file
    "events" should be created which lists event key value pairs.
    Whenever a notifiable event happens, file modified event should be
    generated on the file.
  
  
  4-3. Core Interface Files
  
  All cgroup core files are prefixed with "cgroup."
  
    cgroup.procs
  
  	A read-write new-line separated values file which exists on
  	all cgroups.
  
  	When read, it lists the PIDs of all processes which belong to
  	the cgroup one-per-line.  The PIDs are not ordered and the
  	same PID may show up more than once if the process got moved
  	to another cgroup and then back or the PID got recycled while
  	reading.
  
  	A PID can be written to migrate the process associated with
  	the PID to the cgroup.  The writer should match all of the
  	following conditions.
  
  	- Its euid is either root or must match either uid or suid of
            the target process.
  
  	- It must have write access to the "cgroup.procs" file.
  
  	- It must have write access to the "cgroup.procs" file of the
  	  common ancestor of the source and destination cgroups.
  
  	When delegating a sub-hierarchy, write access to this file
  	should be granted along with the containing directory.
  
    cgroup.controllers
  
  	A read-only space separated values file which exists on all
  	cgroups.
  
  	It shows space separated list of all controllers available to
  	the cgroup.  The controllers are not ordered.
  
    cgroup.subtree_control
  
  	A read-write space separated values file which exists on all
  	cgroups.  Starts out empty.
  
  	When read, it shows space separated list of the controllers
  	which are enabled to control resource distribution from the
  	cgroup to its children.
  
  	Space separated list of controllers prefixed with '+' or '-'
  	can be written to enable or disable controllers.  A controller
  	name prefixed with '+' enables the controller and '-'
  	disables.  If a controller appears more than once on the list,
  	the last one is effective.  When multiple enable and disable
  	operations are specified, either all succeed or all fail.
  
    cgroup.events
  
  	A read-only flat-keyed file which exists on non-root cgroups.
  	The following entries are defined.  Unless specified
  	otherwise, a value change in this file generates a file
  	modified event.
  
  	  populated
  
  		1 if the cgroup or its descendants contains any live
  		processes; otherwise, 0.
  
  
  5. Controllers
  
  5-1. CPU
  
  [NOTE: The interface for the cpu controller hasn't been merged yet]
  
  The "cpu" controllers regulates distribution of CPU cycles.  This
  controller implements weight and absolute bandwidth limit models for
  normal scheduling policy and absolute bandwidth allocation model for
  realtime scheduling policy.
  
  
  5-1-1. CPU Interface Files
  
  All time durations are in microseconds.
  
    cpu.stat
  
  	A read-only flat-keyed file which exists on non-root cgroups.
  
  	It reports the following six stats.
  
  	  usage_usec
  	  user_usec
  	  system_usec
  	  nr_periods
  	  nr_throttled
  	  throttled_usec
  
    cpu.weight
  
  	A read-write single value file which exists on non-root
  	cgroups.  The default is "100".
  
  	The weight in the range [1, 10000].
  
    cpu.max
  
  	A read-write two value file which exists on non-root cgroups.
  	The default is "max 100000".
  
  	The maximum bandwidth limit.  It's in the following format.
  
  	  $MAX $PERIOD
  
  	which indicates that the group may consume upto $MAX in each
  	$PERIOD duration.  "max" for $MAX indicates no limit.  If only
  	one number is written, $MAX is updated.
  
    cpu.rt.max
  
    [NOTE: The semantics of this file is still under discussion and the
     interface hasn't been merged yet]
  
  	A read-write two value file which exists on all cgroups.
  	The default is "0 100000".
  
  	The maximum realtime runtime allocation.  Over-committing
  	configurations are disallowed and process migrations are
  	rejected if not enough bandwidth is available.  It's in the
  	following format.
  
  	  $MAX $PERIOD
  
  	which indicates that the group may consume upto $MAX in each
  	$PERIOD duration.  If only one number is written, $MAX is
  	updated.
  
  
  5-2. Memory
  
  The "memory" controller regulates distribution of memory.  Memory is
  stateful and implements both limit and protection models.  Due to the
  intertwining between memory usage and reclaim pressure and the
  stateful nature of memory, the distribution model is relatively
  complex.
  
  While not completely water-tight, all major memory usages by a given
  cgroup are tracked so that the total memory consumption can be
  accounted and controlled to a reasonable extent.  Currently, the
  following types of memory usages are tracked.
  
  - Userland memory - page cache and anonymous memory.
  
  - Kernel data structures such as dentries and inodes.
  
  - TCP socket buffers.
  
  The above list may expand in the future for better coverage.
  
  
  5-2-1. Memory Interface Files
  
  All memory amounts are in bytes.  If a value which is not aligned to
  PAGE_SIZE is written, the value may be rounded up to the closest
  PAGE_SIZE multiple when read back.
  
    memory.current
  
  	A read-only single value file which exists on non-root
  	cgroups.
  
  	The total amount of memory currently being used by the cgroup
  	and its descendants.
  
    memory.low
  
  	A read-write single value file which exists on non-root
  	cgroups.  The default is "0".
  
  	Best-effort memory protection.  If the memory usages of a
  	cgroup and all its ancestors are below their low boundaries,
  	the cgroup's memory won't be reclaimed unless memory can be
  	reclaimed from unprotected cgroups.
  
  	Putting more memory than generally available under this
  	protection is discouraged.
  
    memory.high
  
  	A read-write single value file which exists on non-root
  	cgroups.  The default is "max".
  
  	Memory usage throttle limit.  This is the main mechanism to
  	control memory usage of a cgroup.  If a cgroup's usage goes
  	over the high boundary, the processes of the cgroup are
  	throttled and put under heavy reclaim pressure.
  
  	Going over the high limit never invokes the OOM killer and
  	under extreme conditions the limit may be breached.
  
    memory.max
  
  	A read-write single value file which exists on non-root
  	cgroups.  The default is "max".
  
  	Memory usage hard limit.  This is the final protection
  	mechanism.  If a cgroup's memory usage reaches this limit and
  	can't be reduced, the OOM killer is invoked in the cgroup.
  	Under certain circumstances, the usage may go over the limit
  	temporarily.
  
  	This is the ultimate protection mechanism.  As long as the
  	high limit is used and monitored properly, this limit's
  	utility is limited to providing the final safety net.
  
    memory.events
  
  	A read-only flat-keyed file which exists on non-root cgroups.
  	The following entries are defined.  Unless specified
  	otherwise, a value change in this file generates a file
  	modified event.
  
  	  low
  
  		The number of times the cgroup is reclaimed due to
  		high memory pressure even though its usage is under
  		the low boundary.  This usually indicates that the low
  		boundary is over-committed.
  
  	  high
  
  		The number of times processes of the cgroup are
  		throttled and routed to perform direct memory reclaim
  		because the high memory boundary was exceeded.  For a
  		cgroup whose memory usage is capped by the high limit
  		rather than global memory pressure, this event's
  		occurrences are expected.
  
  	  max
  
  		The number of times the cgroup's memory usage was
  		about to go over the max boundary.  If direct reclaim
  		fails to bring it down, the OOM killer is invoked.
  
  	  oom
  
  		The number of times the OOM killer has been invoked in
  		the cgroup.  This may not exactly match the number of
  		processes killed but should generally be close.

    memory.stat
  
  	A read-only flat-keyed file which exists on non-root cgroups.
  
  	This breaks down the cgroup's memory footprint into different
  	types of memory, type-specific details, and other information
  	on the state and past events of the memory management system.
  
  	All memory amounts are in bytes.
  
  	The entries are ordered to be human readable, and new entries
  	can show up in the middle. Don't rely on items remaining in a
  	fixed position; use the keys to look up specific values!
  
  	  anon
  
  		Amount of memory used in anonymous mappings such as
  		brk(), sbrk(), and mmap(MAP_ANONYMOUS)
  
  	  file
  
  		Amount of memory used to cache filesystem data,
  		including tmpfs and shared memory.

  	  kernel_stack
  
  		Amount of memory allocated to kernel stacks.

  	  slab
  
  		Amount of memory used for storing in-kernel data
  		structures.

  	  sock
  
  		Amount of memory used in network transmission buffers

  	  file_mapped
  
  		Amount of cached filesystem data mapped with mmap()
  
  	  file_dirty
  
  		Amount of cached filesystem data that was modified but
  		not yet written back to disk
  
  	  file_writeback
  
  		Amount of cached filesystem data that was modified and
  		is currently being written back to disk
  
  	  inactive_anon
  	  active_anon
  	  inactive_file
  	  active_file
  	  unevictable
  
  		Amount of memory, swap-backed and filesystem-backed,
  		on the internal memory management lists used by the
  		page reclaim algorithm

  	  slab_reclaimable
  
  		Part of "slab" that might be reclaimed, such as
  		dentries and inodes.
  
  	  slab_unreclaimable
  
  		Part of "slab" that cannot be reclaimed on memory
  		pressure.

  	  pgfault
  
  		Total number of page faults incurred
  
  	  pgmajfault
  
  		Number of major page faults incurred

    memory.swap.current
  
  	A read-only single value file which exists on non-root
  	cgroups.
  
  	The total amount of swap currently being used by the cgroup
  	and its descendants.
  
    memory.swap.max
  
  	A read-write single value file which exists on non-root
  	cgroups.  The default is "max".
  
  	Swap usage hard limit.  If a cgroup's swap usage reaches this
  	limit, anonymous meomry of the cgroup will not be swapped out.

  5-2-2. Usage Guidelines

  
  "memory.high" is the main mechanism to control memory usage.
  Over-committing on high limit (sum of high limits > available memory)
  and letting global memory pressure to distribute memory according to
  usage is a viable strategy.
  
  Because breach of the high limit doesn't trigger the OOM killer but
  throttles the offending cgroup, a management agent has ample
  opportunities to monitor and take appropriate actions such as granting
  more memory or terminating the workload.
  
  Determining whether a cgroup has enough memory is not trivial as
  memory usage doesn't indicate whether the workload can benefit from
  more memory.  For example, a workload which writes data received from
  network to a file can use all available memory but can also operate as
  performant with a small amount of memory.  A measure of memory
  pressure - how much the workload is being impacted due to lack of
  memory - is necessary to determine whether a workload needs more
  memory; unfortunately, memory pressure monitoring mechanism isn't
  implemented yet.
  
  
  5-2-3. Memory Ownership
  
  A memory area is charged to the cgroup which instantiated it and stays
  charged to the cgroup until the area is released.  Migrating a process
  to a different cgroup doesn't move the memory usages that it
  instantiated while in the previous cgroup to the new cgroup.
  
  A memory area may be used by processes belonging to different cgroups.
  To which cgroup the area will be charged is in-deterministic; however,
  over time, the memory area is likely to end up in a cgroup which has
  enough memory allowance to avoid high reclaim pressure.
  
  If a cgroup sweeps a considerable amount of memory which is expected
  to be accessed repeatedly by other cgroups, it may make sense to use
  POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
  belonging to the affected files to ensure correct memory ownership.
  
  
  5-3. IO
  
  The "io" controller regulates the distribution of IO resources.  This
  controller implements both weight based and absolute bandwidth or IOPS
  limit distribution; however, weight based distribution is available
  only if cfq-iosched is in use and neither scheme is available for
  blk-mq devices.
  
  
  5-3-1. IO Interface Files
  
    io.stat
  
  	A read-only nested-keyed file which exists on non-root
  	cgroups.
  
  	Lines are keyed by $MAJ:$MIN device numbers and not ordered.
  	The following nested keys are defined.
  
  	  rbytes	Bytes read
  	  wbytes	Bytes written
  	  rios		Number of read IOs
  	  wios		Number of write IOs
  
  	An example read output follows.
  
  	  8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353
  	  8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252
  
    io.weight
  
  	A read-write flat-keyed file which exists on non-root cgroups.
  	The default is "default 100".
  
  	The first line is the default weight applied to devices
  	without specific override.  The rest are overrides keyed by
  	$MAJ:$MIN device numbers and not ordered.  The weights are in
  	the range [1, 10000] and specifies the relative amount IO time
  	the cgroup can use in relation to its siblings.
  
  	The default weight can be updated by writing either "default
  	$WEIGHT" or simply "$WEIGHT".  Overrides can be set by writing
  	"$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
  
  	An example read output follows.
  
  	  default 100
  	  8:16 200
  	  8:0 50
  
    io.max
  
  	A read-write nested-keyed file which exists on non-root
  	cgroups.
  
  	BPS and IOPS based IO limit.  Lines are keyed by $MAJ:$MIN
  	device numbers and not ordered.  The following nested keys are
  	defined.
  
  	  rbps		Max read bytes per second
  	  wbps		Max write bytes per second
  	  riops		Max read IO operations per second
  	  wiops		Max write IO operations per second
  
  	When writing, any number of nested key-value pairs can be
  	specified in any order.  "max" can be specified as the value
  	to remove a specific limit.  If the same key is specified
  	multiple times, the outcome is undefined.
  
  	BPS and IOPS are measured in each IO direction and IOs are
  	delayed if limit is reached.  Temporary bursts are allowed.
  
  	Setting read limit at 2M BPS and write at 120 IOPS for 8:16.
  
  	  echo "8:16 rbps=2097152 wiops=120" > io.max
  
  	Reading returns the following.
  
  	  8:16 rbps=2097152 wbps=max riops=max wiops=120
  
  	Write IOPS limit can be removed by writing the following.
  
  	  echo "8:16 wiops=max" > io.max
  
  	Reading now returns the following.
  
  	  8:16 rbps=2097152 wbps=max riops=max wiops=max
  
  
  5-3-2. Writeback
  
  Page cache is dirtied through buffered writes and shared mmaps and
  written asynchronously to the backing filesystem by the writeback
  mechanism.  Writeback sits between the memory and IO domains and
  regulates the proportion of dirty memory by balancing dirtying and
  write IOs.
  
  The io controller, in conjunction with the memory controller,
  implements control of page cache writeback IOs.  The memory controller
  defines the memory domain that dirty memory ratio is calculated and
  maintained for and the io controller defines the io domain which
  writes out dirty pages for the memory domain.  Both system-wide and
  per-cgroup dirty memory states are examined and the more restrictive
  of the two is enforced.
  
  cgroup writeback requires explicit support from the underlying
  filesystem.  Currently, cgroup writeback is implemented on ext2, ext4
  and btrfs.  On other filesystems, all writeback IOs are attributed to
  the root cgroup.
  
  There are inherent differences in memory and writeback management
  which affects how cgroup ownership is tracked.  Memory is tracked per
  page while writeback per inode.  For the purpose of writeback, an
  inode is assigned to a cgroup and all IO requests to write dirty pages
  from the inode are attributed to that cgroup.
  
  As cgroup ownership for memory is tracked per page, there can be pages
  which are associated with different cgroups than the one the inode is
  associated with.  These are called foreign pages.  The writeback
  constantly keeps track of foreign pages and, if a particular foreign
  cgroup becomes the majority over a certain period of time, switches
  the ownership of the inode to that cgroup.
  
  While this model is enough for most use cases where a given inode is
  mostly dirtied by a single cgroup even when the main writing cgroup
  changes over time, use cases where multiple cgroups write to a single
  inode simultaneously are not supported well.  In such circumstances, a
  significant portion of IOs are likely to be attributed incorrectly.
  As memory controller assigns page ownership on the first use and
  doesn't update it until the page is released, even if writeback
  strictly follows page ownership, multiple cgroups dirtying overlapping
  areas wouldn't work as expected.  It's recommended to avoid such usage
  patterns.
  
  The sysctl knobs which affect writeback behavior are applied to cgroup
  writeback as follows.
  
    vm.dirty_background_ratio
    vm.dirty_ratio
  
  	These ratios apply the same to cgroup writeback with the
  	amount of available memory capped by limits imposed by the
  	memory controller and system-wide clean memory.
  
    vm.dirty_background_bytes
    vm.dirty_bytes
  
  	For cgroup writeback, this is calculated into ratio against
  	total available memory and applied the same way as
  	vm.dirty[_background]_ratio.

  6. Namespace
  
  6-1. Basics
  
  cgroup namespace provides a mechanism to virtualize the view of the
  "/proc/$PID/cgroup" file and cgroup mounts.  The CLONE_NEWCGROUP clone
  flag can be used with clone(2) and unshare(2) to create a new cgroup
  namespace.  The process running inside the cgroup namespace will have
  its "/proc/$PID/cgroup" output restricted to cgroupns root.  The
  cgroupns root is the cgroup of the process at the time of creation of
  the cgroup namespace.
  
  Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
  complete path of the cgroup of a process.  In a container setup where
  a set of cgroups and namespaces are intended to isolate processes the
  "/proc/$PID/cgroup" file may leak potential system level information
  to the isolated processes.  For Example:
  
    # cat /proc/self/cgroup
    0::/batchjobs/container_id1
  
  The path '/batchjobs/container_id1' can be considered as system-data
  and undesirable to expose to the isolated processes.  cgroup namespace
  can be used to restrict visibility of this path.  For example, before
  creating a cgroup namespace, one would see:
  
    # ls -l /proc/self/ns/cgroup
    lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
    # cat /proc/self/cgroup
    0::/batchjobs/container_id1
  
  After unsharing a new namespace, the view changes.
  
    # ls -l /proc/self/ns/cgroup
    lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
    # cat /proc/self/cgroup
    0::/
  
  When some thread from a multi-threaded process unshares its cgroup
  namespace, the new cgroupns gets applied to the entire process (all
  the threads).  This is natural for the v2 hierarchy; however, for the
  legacy hierarchies, this may be unexpected.
  
  A cgroup namespace is alive as long as there are processes inside or
  mounts pinning it.  When the last usage goes away, the cgroup
  namespace is destroyed.  The cgroupns root and the actual cgroups
  remain.
  
  
  6-2. The Root and Views
  
  The 'cgroupns root' for a cgroup namespace is the cgroup in which the
  process calling unshare(2) is running.  For example, if a process in
  /batchjobs/container_id1 cgroup calls unshare, cgroup
  /batchjobs/container_id1 becomes the cgroupns root.  For the
  init_cgroup_ns, this is the real root ('/') cgroup.
  
  The cgroupns root cgroup does not change even if the namespace creator
  process later moves to a different cgroup.
  
    # ~/unshare -c # unshare cgroupns in some cgroup
    # cat /proc/self/cgroup
    0::/
    # mkdir sub_cgrp_1
    # echo 0 > sub_cgrp_1/cgroup.procs
    # cat /proc/self/cgroup
    0::/sub_cgrp_1
  
  Each process gets its namespace-specific view of "/proc/$PID/cgroup"
  
  Processes running inside the cgroup namespace will be able to see
  cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
  From within an unshared cgroupns:
  
    # sleep 100000 &
    [1] 7353
    # echo 7353 > sub_cgrp_1/cgroup.procs
    # cat /proc/7353/cgroup
    0::/sub_cgrp_1
  
  From the initial cgroup namespace, the real cgroup path will be
  visible:
  
    $ cat /proc/7353/cgroup
    0::/batchjobs/container_id1/sub_cgrp_1
  
  From a sibling cgroup namespace (that is, a namespace rooted at a
  different cgroup), the cgroup path relative to its own cgroup
  namespace root will be shown.  For instance, if PID 7353's cgroup
  namespace root is at '/batchjobs/container_id2', then it will see
  
    # cat /proc/7353/cgroup
    0::/../container_id2/sub_cgrp_1
  
  Note that the relative path always starts with '/' to indicate that
  its relative to the cgroup namespace root of the caller.
  
  
  6-3. Migration and setns(2)
  
  Processes inside a cgroup namespace can move into and out of the
  namespace root if they have proper access to external cgroups.  For
  example, from inside a namespace with cgroupns root at
  /batchjobs/container_id1, and assuming that the global hierarchy is
  still accessible inside cgroupns:
  
    # cat /proc/7353/cgroup
    0::/sub_cgrp_1
    # echo 7353 > batchjobs/container_id2/cgroup.procs
    # cat /proc/7353/cgroup
    0::/../container_id2
  
  Note that this kind of setup is not encouraged.  A task inside cgroup
  namespace should only be exposed to its own cgroupns hierarchy.
  
  setns(2) to another cgroup namespace is allowed when:
  
  (a) the process has CAP_SYS_ADMIN against its current user namespace
  (b) the process has CAP_SYS_ADMIN against the target cgroup
      namespace's userns
  
  No implicit cgroup changes happen with attaching to another cgroup
  namespace.  It is expected that the someone moves the attaching
  process under the target cgroup namespace root.
  
  
  6-4. Interaction with Other Namespaces
  
  Namespace specific cgroup hierarchy can be mounted by a process
  running inside a non-init cgroup namespace.
  
    # mount -t cgroup2 none $MOUNT_POINT
  
  This will mount the unified cgroup hierarchy with cgroupns root as the
  filesystem root.  The process needs CAP_SYS_ADMIN against its user and
  mount namespaces.
  
  The virtualization of /proc/self/cgroup file combined with restricting
  the view of cgroup hierarchy by namespace-private cgroupfs mount
  provides a properly isolated cgroup view inside the container.

  P. Information on Kernel Programming
  
  This section contains kernel programming information in the areas
  where interacting with cgroup is necessary.  cgroup core and
  controllers are not covered.
  
  
  P-1. Filesystem Support for Writeback
  
  A filesystem can support cgroup writeback by updating
  address_space_operations->writepage[s]() to annotate bio's using the
  following two functions.
  
    wbc_init_bio(@wbc, @bio)
  
  	Should be called for each bio carrying writeback data and
  	associates the bio with the inode's owner cgroup.  Can be
  	called anytime between bio allocation and submission.
  
    wbc_account_io(@wbc, @page, @bytes)
  
  	Should be called for each data segment being written out.
  	While this function doesn't care exactly when it's called
  	during the writeback session, it's the easiest and most
  	natural to call it as data segments are added to a bio.
  
  With writeback bio's annotated, cgroup support can be enabled per
  super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
  selective disabling of cgroup writeback support which is helpful when
  certain filesystem features, e.g. journaled data mode, are
  incompatible.
  
  wbc_init_bio() binds the specified bio to its cgroup.  Depending on
  the configuration, the bio may be executed at a lower priority and if
  the writeback session is holding shared resources, e.g. a journal
  entry, may lead to priority inversion.  There is no one easy solution
  for the problem.  Filesystems can try to work around specific problem
  cases by skipping wbc_init_bio() or using bio_associate_blkcg()
  directly.
  
  
  D. Deprecated v1 Core Features
  
  - Multiple hierarchies including named ones are not supported.
  
  - All mount options and remounting are not supported.
  
  - The "tasks" file is removed and "cgroup.procs" is not sorted.
  
  - "cgroup.clone_children" is removed.
  
  - /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" file
    at the root instead.
  
  
  R. Issues with v1 and Rationales for v2
  
  R-1. Multiple Hierarchies
  
  cgroup v1 allowed an arbitrary number of hierarchies and each
  hierarchy could host any number of controllers.  While this seemed to
  provide a high level of flexibility, it wasn't useful in practice.
  
  For example, as there is only one instance of each controller, utility
  type controllers such as freezer which can be useful in all
  hierarchies could only be used in one.  The issue is exacerbated by
  the fact that controllers couldn't be moved to another hierarchy once
  hierarchies were populated.  Another issue was that all controllers
  bound to a hierarchy were forced to have exactly the same view of the
  hierarchy.  It wasn't possible to vary the granularity depending on
  the specific controller.
  
  In practice, these issues heavily limited which controllers could be
  put on the same hierarchy and most configurations resorted to putting
  each controller on its own hierarchy.  Only closely related ones, such
  as the cpu and cpuacct controllers, made sense to be put on the same
  hierarchy.  This often meant that userland ended up managing multiple
  similar hierarchies repeating the same steps on each hierarchy
  whenever a hierarchy management operation was necessary.
  
  Furthermore, support for multiple hierarchies came at a steep cost.
  It greatly complicated cgroup core implementation but more importantly
  the support for multiple hierarchies restricted how cgroup could be
  used in general and what controllers was able to do.
  
  There was no limit on how many hierarchies there might be, which meant
  that a thread's cgroup membership couldn't be described in finite
  length.  The key might contain any number of entries and was unlimited
  in length, which made it highly awkward to manipulate and led to
  addition of controllers which existed only to identify membership,
  which in turn exacerbated the original problem of proliferating number
  of hierarchies.
  
  Also, as a controller couldn't have any expectation regarding the
  topologies of hierarchies other controllers might be on, each
  controller had to assume that all other controllers were attached to
  completely orthogonal hierarchies.  This made it impossible, or at
  least very cumbersome, for controllers to cooperate with each other.
  
  In most use cases, putting controllers on hierarchies which are
  completely orthogonal to each other isn't necessary.  What usually is
  called for is the ability to have differing levels of granularity
  depending on the specific controller.  In other words, hierarchy may
  be collapsed from leaf towards root when viewed from specific
  controllers.  For example, a given configuration might not care about
  how memory is distributed beyond a certain level while still wanting
  to control how CPU cycles are distributed.
  
  
  R-2. Thread Granularity
  
  cgroup v1 allowed threads of a process to belong to different cgroups.
  This didn't make sense for some controllers and those controllers
  ended up implementing different ways to ignore such situations but
  much more importantly it blurred the line between API exposed to
  individual applications and system management interface.
  
  Generally, in-process knowledge is available only to the process
  itself; thus, unlike service-level organization of processes,
  categorizing threads of a process requires active participation from
  the application which owns the target process.
  
  cgroup v1 had an ambiguously defined delegation model which got abused
  in combination with thread granularity.  cgroups were delegated to
  individual applications so that they can create and manage their own
  sub-hierarchies and control resource distributions along them.  This
  effectively raised cgroup to the status of a syscall-like API exposed
  to lay programs.
  
  First of all, cgroup has a fundamentally inadequate interface to be
  exposed this way.  For a process to access its own knobs, it has to
  extract the path on the target hierarchy from /proc/self/cgroup,
  construct the path by appending the name of the knob to the path, open
  and then read and/or write to it.  This is not only extremely clunky
  and unusual but also inherently racy.  There is no conventional way to
  define transaction across the required steps and nothing can guarantee
  that the process would actually be operating on its own sub-hierarchy.
  
  cgroup controllers implemented a number of knobs which would never be
  accepted as public APIs because they were just adding control knobs to
  system-management pseudo filesystem.  cgroup ended up with interface
  knobs which were not properly abstracted or refined and directly
  revealed kernel internal details.  These knobs got exposed to
  individual applications through the ill-defined delegation mechanism
  effectively abusing cgroup as a shortcut to implementing public APIs
  without going through the required scrutiny.
  
  This was painful for both userland and kernel.  Userland ended up with
  misbehaving and poorly abstracted interfaces and kernel exposing and
  locked into constructs inadvertently.
  
  
  R-3. Competition Between Inner Nodes and Threads
  
  cgroup v1 allowed threads to be in any cgroups which created an
  interesting problem where threads belonging to a parent cgroup and its
  children cgroups competed for resources.  This was nasty as two
  different types of entities competed and there was no obvious way to
  settle it.  Different controllers did different things.
  
  The cpu controller considered threads and cgroups as equivalents and
  mapped nice levels to cgroup weights.  This worked for some cases but
  fell flat when children wanted to be allocated specific ratios of CPU
  cycles and the number of internal threads fluctuated - the ratios
  constantly changed as the number of competing entities fluctuated.
  There also were other issues.  The mapping from nice level to weight
  wasn't obvious or universal, and there were various other knobs which
  simply weren't available for threads.
  
  The io controller implicitly created a hidden leaf node for each
  cgroup to host the threads.  The hidden leaf had its own copies of all
  the knobs with "leaf_" prefixed.  While this allowed equivalent
  control over internal threads, it was with serious drawbacks.  It
  always added an extra layer of nesting which wouldn't be necessary
  otherwise, made the interface messy and significantly complicated the
  implementation.
  
  The memory controller didn't have a way to control what happened
  between internal tasks and child cgroups and the behavior was not
  clearly defined.  There were attempts to add ad-hoc behaviors and
  knobs to tailor the behavior to specific workloads which would have
  led to problems extremely difficult to resolve in the long term.
  
  Multiple controllers struggled with internal tasks and came up with
  different ways to deal with it; unfortunately, all the approaches were
  severely flawed and, furthermore, the widely different behaviors
  made cgroup as a whole highly inconsistent.
  
  This clearly is a problem which needs to be addressed from cgroup core
  in a uniform way.
  
  
  R-4. Other Interface Issues
  
  cgroup v1 grew without oversight and developed a large number of
  idiosyncrasies and inconsistencies.  One issue on the cgroup core side
  was how an empty cgroup was notified - a userland helper binary was
  forked and executed for each event.  The event delivery wasn't
  recursive or delegatable.  The limitations of the mechanism also led
  to in-kernel event delivery filtering mechanism further complicating
  the interface.
  
  Controller interfaces were problematic too.  An extreme example is
  controllers completely ignoring hierarchical organization and treating
  all cgroups as if they were all located directly under the root
  cgroup.  Some controllers exposed a large amount of inconsistent
  implementation details to userland.
  
  There also was no consistency across controllers.  When a new cgroup
  was created, some controllers defaulted to not imposing extra
  restrictions while others disallowed any resource usage until
  explicitly configured.  Configuration knobs for the same type of
  control used widely differing naming schemes and formats.  Statistics
  and information knobs were named arbitrarily and used different
  formats and units even in the same controller.
  
  cgroup v2 establishes common conventions where appropriate and updates
  controllers so that they expose minimal and consistent interfaces.
  
  
  R-5. Controller Issues and Remedies
  
  R-5-1. Memory
  
  The original lower boundary, the soft limit, is defined as a limit
  that is per default unset.  As a result, the set of cgroups that
  global reclaim prefers is opt-in, rather than opt-out.  The costs for
  optimizing these mostly negative lookups are so high that the
  implementation, despite its enormous size, does not even provide the
  basic desirable behavior.  First off, the soft limit has no
  hierarchical meaning.  All configured groups are organized in a global
  rbtree and treated like equal peers, regardless where they are located
  in the hierarchy.  This makes subtree delegation impossible.  Second,
  the soft limit reclaim pass is so aggressive that it not just
  introduces high allocation latencies into the system, but also impacts
  system performance due to overreclaim, to the point where the feature
  becomes self-defeating.
  
  The memory.low boundary on the other hand is a top-down allocated
  reserve.  A cgroup enjoys reclaim protection when it and all its
  ancestors are below their low boundaries, which makes delegation of
  subtrees possible.  Secondly, new cgroups have no reserve per default
  and in the common case most cgroups are eligible for the preferred
  reclaim pass.  This allows the new low boundary to be efficiently
  implemented with just a minor addition to the generic reclaim code,
  without the need for out-of-band data structures and reclaim passes.
  Because the generic reclaim code considers all cgroups except for the
  ones running low in the preferred first reclaim pass, overreclaim of
  individual groups is eliminated as well, resulting in much better
  overall workload performance.
  
  The original high boundary, the hard limit, is defined as a strict
  limit that can not budge, even if the OOM killer has to be called.
  But this generally goes against the goal of making the most out of the
  available memory.  The memory consumption of workloads varies during
  runtime, and that requires users to overcommit.  But doing that with a
  strict upper limit requires either a fairly accurate prediction of the
  working set size or adding slack to the limit.  Since working set size
  estimation is hard and error prone, and getting it wrong results in
  OOM kills, most users tend to err on the side of a looser limit and
  end up wasting precious resources.
  
  The memory.high boundary on the other hand can be set much more
  conservatively.  When hit, it throttles allocations by forcing them
  into direct reclaim to work off the excess, but it never invokes the
  OOM killer.  As a result, a high boundary that is chosen too
  aggressively will not terminate the processes, but instead it will
  lead to gradual performance degradation.  The user can monitor this
  and make corrections until the minimal memory footprint that still
  gives acceptable performance is found.
  
  In extreme cases, with many concurrent allocations and a complete
  breakdown of reclaim progress within the group, the high boundary can
  be exceeded.  But even then it's mostly better to satisfy the
  allocation from the slack available in other groups or the rest of the
  system than killing the group.  Otherwise, memory.max is there to
  limit this type of spillover and ultimately contain buggy or even
  malicious applications.

  Setting the original memory.limit_in_bytes below the current usage was
  subject to a race condition, where concurrent charges could cause the
  limit setting to fail. memory.max on the other hand will first set the
  limit to prevent new charges, and then reclaim and OOM kill until the
  new limit is met - or the task writing to memory.max is killed.

  The combined memory+swap accounting and limiting is replaced by real
  control over swap space.
  
  The main argument for a combined memory+swap facility in the original
  cgroup design was that global or parental pressure would always be
  able to swap all anonymous memory of a child group, regardless of the
  child's own (possibly untrusted) configuration.  However, untrusted
  groups can sabotage swapping by other means - such as referencing its
  anonymous memory in a tight loop - and an admin can not assume full
  swappability when overcommitting untrusted jobs.
  
  For trusted jobs, on the other hand, a combined counter is not an
  intuitive userspace interface, and it flies in the face of the idea
  that cgroup controllers should account and limit specific physical
  resources.  Swap space is a resource like all others in the system,
  and that's why unified hierarchy allows distributing it separately.