1383 lines
51 KiB
Plaintext
1383 lines
51 KiB
Plaintext
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Control Group v2
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October, 2015 Tejun Heo <tj@kernel.org>
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This is the authoritative documentation on the design, interface and
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conventions of cgroup v2. It describes all userland-visible aspects
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of cgroup including core and specific controller behaviors. All
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future changes must be reflected in this document. Documentation for
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v1 is available under Documentation/cgroup-legacy/.
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CONTENTS
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1. Introduction
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1-1. Terminology
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1-2. What is cgroup?
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2. Basic Operations
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2-1. Mounting
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2-2. Organizing Processes
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2-3. [Un]populated Notification
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2-4. Controlling Controllers
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2-4-1. Enabling and Disabling
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2-4-2. Top-down Constraint
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2-4-3. No Internal Process Constraint
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2-5. Delegation
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2-5-1. Model of Delegation
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2-5-2. Delegation Containment
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2-6. Guidelines
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2-6-1. Organize Once and Control
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2-6-2. Avoid Name Collisions
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3. Resource Distribution Models
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3-1. Weights
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3-2. Limits
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3-3. Protections
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3-4. Allocations
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4. Interface Files
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4-1. Format
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4-2. Conventions
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4-3. Core Interface Files
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5. Controllers
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5-1. CPU
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5-1-1. CPU Interface Files
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5-2. Memory
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5-2-1. Memory Interface Files
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5-2-2. Usage Guidelines
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5-2-3. Memory Ownership
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5-3. IO
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5-3-1. IO Interface Files
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5-3-2. Writeback
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P. Information on Kernel Programming
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P-1. Filesystem Support for Writeback
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D. Deprecated v1 Core Features
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R. Issues with v1 and Rationales for v2
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R-1. Multiple Hierarchies
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R-2. Thread Granularity
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R-3. Competition Between Inner Nodes and Threads
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R-4. Other Interface Issues
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R-5. Controller Issues and Remedies
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R-5-1. Memory
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1. Introduction
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1-1. Terminology
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"cgroup" stands for "control group" and is never capitalized. The
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singular form is used to designate the whole feature and also as a
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qualifier as in "cgroup controllers". When explicitly referring to
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multiple individual control groups, the plural form "cgroups" is used.
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1-2. What is cgroup?
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cgroup is a mechanism to organize processes hierarchically and
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distribute system resources along the hierarchy in a controlled and
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configurable manner.
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cgroup is largely composed of two parts - the core and controllers.
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cgroup core is primarily responsible for hierarchically organizing
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processes. A cgroup controller is usually responsible for
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distributing a specific type of system resource along the hierarchy
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although there are utility controllers which serve purposes other than
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resource distribution.
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cgroups form a tree structure and every process in the system belongs
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to one and only one cgroup. All threads of a process belong to the
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same cgroup. On creation, all processes are put in the cgroup that
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the parent process belongs to at the time. A process can be migrated
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to another cgroup. Migration of a process doesn't affect already
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existing descendant processes.
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Following certain structural constraints, controllers may be enabled or
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disabled selectively on a cgroup. All controller behaviors are
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hierarchical - if a controller is enabled on a cgroup, it affects all
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processes which belong to the cgroups consisting the inclusive
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sub-hierarchy of the cgroup. When a controller is enabled on a nested
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cgroup, it always restricts the resource distribution further. The
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restrictions set closer to the root in the hierarchy can not be
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overridden from further away.
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2. Basic Operations
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2-1. Mounting
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Unlike v1, cgroup v2 has only single hierarchy. The cgroup v2
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hierarchy can be mounted with the following mount command.
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# mount -t cgroup2 none $MOUNT_POINT
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cgroup2 filesystem has the magic number 0x63677270 ("cgrp"). All
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controllers which support v2 and are not bound to a v1 hierarchy are
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automatically bound to the v2 hierarchy and show up at the root.
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Controllers which are not in active use in the v2 hierarchy can be
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bound to other hierarchies. This allows mixing v2 hierarchy with the
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legacy v1 multiple hierarchies in a fully backward compatible way.
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A controller can be moved across hierarchies only after the controller
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is no longer referenced in its current hierarchy. Because per-cgroup
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controller states are destroyed asynchronously and controllers may
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have lingering references, a controller may not show up immediately on
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the v2 hierarchy after the final umount of the previous hierarchy.
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Similarly, a controller should be fully disabled to be moved out of
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the unified hierarchy and it may take some time for the disabled
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controller to become available for other hierarchies; furthermore, due
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to inter-controller dependencies, other controllers may need to be
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disabled too.
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While useful for development and manual configurations, moving
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controllers dynamically between the v2 and other hierarchies is
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strongly discouraged for production use. It is recommended to decide
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the hierarchies and controller associations before starting using the
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controllers after system boot.
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2-2. Organizing Processes
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Initially, only the root cgroup exists to which all processes belong.
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A child cgroup can be created by creating a sub-directory.
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# mkdir $CGROUP_NAME
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A given cgroup may have multiple child cgroups forming a tree
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structure. Each cgroup has a read-writable interface file
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"cgroup.procs". When read, it lists the PIDs of all processes which
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belong to the cgroup one-per-line. The PIDs are not ordered and the
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same PID may show up more than once if the process got moved to
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another cgroup and then back or the PID got recycled while reading.
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A process can be migrated into a cgroup by writing its PID to the
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target cgroup's "cgroup.procs" file. Only one process can be migrated
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on a single write(2) call. If a process is composed of multiple
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threads, writing the PID of any thread migrates all threads of the
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process.
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When a process forks a child process, the new process is born into the
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cgroup that the forking process belongs to at the time of the
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operation. After exit, a process stays associated with the cgroup
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that it belonged to at the time of exit until it's reaped; however, a
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zombie process does not appear in "cgroup.procs" and thus can't be
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moved to another cgroup.
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A cgroup which doesn't have any children or live processes can be
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destroyed by removing the directory. Note that a cgroup which doesn't
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have any children and is associated only with zombie processes is
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considered empty and can be removed.
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# rmdir $CGROUP_NAME
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"/proc/$PID/cgroup" lists a process's cgroup membership. If legacy
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cgroup is in use in the system, this file may contain multiple lines,
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one for each hierarchy. The entry for cgroup v2 is always in the
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format "0::$PATH".
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# cat /proc/842/cgroup
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...
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0::/test-cgroup/test-cgroup-nested
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If the process becomes a zombie and the cgroup it was associated with
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is removed subsequently, " (deleted)" is appended to the path.
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# cat /proc/842/cgroup
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...
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0::/test-cgroup/test-cgroup-nested (deleted)
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2-3. [Un]populated Notification
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Each non-root cgroup has a "cgroup.events" file which contains
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"populated" field indicating whether the cgroup's sub-hierarchy has
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live processes in it. Its value is 0 if there is no live process in
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the cgroup and its descendants; otherwise, 1. poll and [id]notify
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events are triggered when the value changes. This can be used, for
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example, to start a clean-up operation after all processes of a given
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sub-hierarchy have exited. The populated state updates and
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notifications are recursive. Consider the following sub-hierarchy
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where the numbers in the parentheses represent the numbers of processes
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in each cgroup.
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A(4) - B(0) - C(1)
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\ D(0)
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A, B and C's "populated" fields would be 1 while D's 0. After the one
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process in C exits, B and C's "populated" fields would flip to "0" and
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file modified events will be generated on the "cgroup.events" files of
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both cgroups.
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2-4. Controlling Controllers
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2-4-1. Enabling and Disabling
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Each cgroup has a "cgroup.controllers" file which lists all
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controllers available for the cgroup to enable.
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# cat cgroup.controllers
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cpu io memory
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No controller is enabled by default. Controllers can be enabled and
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disabled by writing to the "cgroup.subtree_control" file.
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# echo "+cpu +memory -io" > cgroup.subtree_control
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Only controllers which are listed in "cgroup.controllers" can be
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enabled. When multiple operations are specified as above, either they
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all succeed or fail. If multiple operations on the same controller
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are specified, the last one is effective.
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Enabling a controller in a cgroup indicates that the distribution of
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the target resource across its immediate children will be controlled.
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Consider the following sub-hierarchy. The enabled controllers are
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listed in parentheses.
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A(cpu,memory) - B(memory) - C()
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\ D()
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As A has "cpu" and "memory" enabled, A will control the distribution
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of CPU cycles and memory to its children, in this case, B. As B has
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"memory" enabled but not "CPU", C and D will compete freely on CPU
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cycles but their division of memory available to B will be controlled.
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As a controller regulates the distribution of the target resource to
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the cgroup's children, enabling it creates the controller's interface
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files in the child cgroups. In the above example, enabling "cpu" on B
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would create the "cpu." prefixed controller interface files in C and
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D. Likewise, disabling "memory" from B would remove the "memory."
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prefixed controller interface files from C and D. This means that the
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controller interface files - anything which doesn't start with
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"cgroup." are owned by the parent rather than the cgroup itself.
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2-4-2. Top-down Constraint
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Resources are distributed top-down and a cgroup can further distribute
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a resource only if the resource has been distributed to it from the
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parent. This means that all non-root "cgroup.subtree_control" files
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can only contain controllers which are enabled in the parent's
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"cgroup.subtree_control" file. A controller can be enabled only if
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the parent has the controller enabled and a controller can't be
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disabled if one or more children have it enabled.
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2-4-3. No Internal Process Constraint
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Non-root cgroups can only distribute resources to their children when
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they don't have any processes of their own. In other words, only
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cgroups which don't contain any processes can have controllers enabled
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in their "cgroup.subtree_control" files.
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This guarantees that, when a controller is looking at the part of the
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hierarchy which has it enabled, processes are always only on the
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leaves. This rules out situations where child cgroups compete against
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internal processes of the parent.
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The root cgroup is exempt from this restriction. Root contains
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processes and anonymous resource consumption which can't be associated
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with any other cgroups and requires special treatment from most
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controllers. How resource consumption in the root cgroup is governed
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is up to each controller.
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Note that the restriction doesn't get in the way if there is no
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enabled controller in the cgroup's "cgroup.subtree_control". This is
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important as otherwise it wouldn't be possible to create children of a
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populated cgroup. To control resource distribution of a cgroup, the
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cgroup must create children and transfer all its processes to the
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children before enabling controllers in its "cgroup.subtree_control"
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file.
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2-5. Delegation
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2-5-1. Model of Delegation
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A cgroup can be delegated to a less privileged user by granting write
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access of the directory and its "cgroup.procs" file to the user. Note
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that resource control interface files in a given directory control the
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distribution of the parent's resources and thus must not be delegated
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along with the directory.
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Once delegated, the user can build sub-hierarchy under the directory,
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organize processes as it sees fit and further distribute the resources
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it received from the parent. The limits and other settings of all
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resource controllers are hierarchical and regardless of what happens
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in the delegated sub-hierarchy, nothing can escape the resource
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restrictions imposed by the parent.
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Currently, cgroup doesn't impose any restrictions on the number of
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cgroups in or nesting depth of a delegated sub-hierarchy; however,
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this may be limited explicitly in the future.
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2-5-2. Delegation Containment
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A delegated sub-hierarchy is contained in the sense that processes
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can't be moved into or out of the sub-hierarchy by the delegatee. For
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a process with a non-root euid to migrate a target process into a
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cgroup by writing its PID to the "cgroup.procs" file, the following
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conditions must be met.
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- The writer's euid must match either uid or suid of the target process.
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- The writer must have write access to the "cgroup.procs" file.
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- The writer must have write access to the "cgroup.procs" file of the
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common ancestor of the source and destination cgroups.
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The above three constraints ensure that while a delegatee may migrate
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processes around freely in the delegated sub-hierarchy it can't pull
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in from or push out to outside the sub-hierarchy.
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For an example, let's assume cgroups C0 and C1 have been delegated to
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user U0 who created C00, C01 under C0 and C10 under C1 as follows and
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all processes under C0 and C1 belong to U0.
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~~~~~~~~~~~~~ - C0 - C00
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~ cgroup ~ \ C01
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~ hierarchy ~
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~~~~~~~~~~~~~ - C1 - C10
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Let's also say U0 wants to write the PID of a process which is
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currently in C10 into "C00/cgroup.procs". U0 has write access to the
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file and uid match on the process; however, the common ancestor of the
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source cgroup C10 and the destination cgroup C00 is above the points
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of delegation and U0 would not have write access to its "cgroup.procs"
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files and thus the write will be denied with -EACCES.
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2-6. Guidelines
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2-6-1. Organize Once and Control
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Migrating a process across cgroups is a relatively expensive operation
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and stateful resources such as memory are not moved together with the
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process. This is an explicit design decision as there often exist
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inherent trade-offs between migration and various hot paths in terms
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of synchronization cost.
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As such, migrating processes across cgroups frequently as a means to
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apply different resource restrictions is discouraged. A workload
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should be assigned to a cgroup according to the system's logical and
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resource structure once on start-up. Dynamic adjustments to resource
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distribution can be made by changing controller configuration through
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the interface files.
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2-6-2. Avoid Name Collisions
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Interface files for a cgroup and its children cgroups occupy the same
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directory and it is possible to create children cgroups which collide
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with interface files.
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All cgroup core interface files are prefixed with "cgroup." and each
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controller's interface files are prefixed with the controller name and
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a dot. A controller's name is composed of lower case alphabets and
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'_'s but never begins with an '_' so it can be used as the prefix
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character for collision avoidance. Also, interface file names won't
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start or end with terms which are often used in categorizing workloads
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such as job, service, slice, unit or workload.
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cgroup doesn't do anything to prevent name collisions and it's the
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user's responsibility to avoid them.
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3. Resource Distribution Models
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cgroup controllers implement several resource distribution schemes
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depending on the resource type and expected use cases. This section
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describes major schemes in use along with their expected behaviors.
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3-1. Weights
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A parent's resource is distributed by adding up the weights of all
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active children and giving each the fraction matching the ratio of its
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weight against the sum. As only children which can make use of the
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resource at the moment participate in the distribution, this is
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work-conserving. Due to the dynamic nature, this model is usually
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used for stateless resources.
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All weights are in the range [1, 10000] with the default at 100. This
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allows symmetric multiplicative biases in both directions at fine
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enough granularity while staying in the intuitive range.
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As long as the weight is in range, all configuration combinations are
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valid and there is no reason to reject configuration changes or
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process migrations.
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"cpu.weight" proportionally distributes CPU cycles to active children
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and is an example of this type.
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3-2. Limits
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A child can only consume upto the configured amount of the resource.
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Limits can be over-committed - the sum of the limits of children can
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exceed the amount of resource available to the parent.
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Limits are in the range [0, max] and defaults to "max", which is noop.
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As limits can be over-committed, all configuration combinations are
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valid and there is no reason to reject configuration changes or
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process migrations.
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"io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
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on an IO device and is an example of this type.
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3-3. Protections
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A cgroup is protected to be allocated upto the configured amount of
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the resource if the usages of all its ancestors are under their
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protected levels. Protections can be hard guarantees or best effort
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soft boundaries. Protections can also be over-committed in which case
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only upto the amount available to the parent is protected among
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children.
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Protections are in the range [0, max] and defaults to 0, which is
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noop.
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As protections can be over-committed, all configuration combinations
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are valid and there is no reason to reject configuration changes or
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process migrations.
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"memory.low" implements best-effort memory protection and is an
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example of this type.
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3-4. Allocations
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A cgroup is exclusively allocated a certain amount of a finite
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resource. Allocations can't be over-committed - the sum of the
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allocations of children can not exceed the amount of resource
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available to the parent.
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Allocations are in the range [0, max] and defaults to 0, which is no
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resource.
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As allocations can't be over-committed, some configuration
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combinations are invalid and should be rejected. Also, if the
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resource is mandatory for execution of processes, process migrations
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may be rejected.
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"cpu.rt.max" hard-allocates realtime slices and is an example of this
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type.
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4. Interface Files
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4-1. Format
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All interface files should be in one of the following formats whenever
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possible.
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New-line separated values
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(when only one value can be written at once)
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VAL0\n
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VAL1\n
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...
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Space separated values
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(when read-only or multiple values can be written at once)
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VAL0 VAL1 ...\n
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Flat keyed
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KEY0 VAL0\n
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KEY1 VAL1\n
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...
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Nested keyed
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KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
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KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
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...
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For a writable file, the format for writing should generally match
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reading; however, controllers may allow omitting later fields or
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implement restricted shortcuts for most common use cases.
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For both flat and nested keyed files, only the values for a single key
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can be written at a time. For nested keyed files, the sub key pairs
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may be specified in any order and not all pairs have to be specified.
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4-2. Conventions
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- Settings for a single feature should be contained in a single file.
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- The root cgroup should be exempt from resource control and thus
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shouldn't have resource control interface files. Also,
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informational files on the root cgroup which end up showing global
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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.
|
|
|
|
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
|
|
|
|
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. General Usage
|
|
|
|
"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.
|
|
|
|
|
|
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.
|
|
|
|
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.
|