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Red Hat Enterprise Linux 6

Resource Management Guide

Managing system resources on Red Hat Enterprise Linux 6

Edition 1.0

Rüdiger Landmann

Red Hat Engineering Content Services

Douglas Silas

Red Hat Engineering Content Services

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Abstract
Managing system resources on Red Hat Enterprise Linux 6
Preface
1. Document Conventions
1.1. Typographic Conventions
1.2. Pull-quote Conventions
1.3. Notes and Warnings
2. Getting Help and Giving Feedback
2.1. Do You Need Help?
2.2. We Need Feedback!
1. Introduction to Control Groups (Cgroups)
1.1. How Control Groups Are Organized
1.2. Relationships Between Subsystems, Hierarchies, Control Groups and Tasks
1.3. Implications for Resource Management
2. Using Control Groups
2.1. The cgconfig Service
2.1.1. The cgconfig.conf File
2.2. Creating a Hierarchy and Attaching Subsystems
2.3. Attaching Subsystems to, and Detaching Them From, an Existing Hierarchy
2.4. Unmounting a Hierarchy
2.5. Creating Cgroups
2.6. Removing Cgroups
2.7. Setting Parameters
2.8. Moving a Process to a Control Group
2.8.1. The cgred Daemon
2.9. Starting a Process in a Control Group
2.9.1. Starting a Service in a Control Group
2.10. Obtaining Information About Control Groups
2.10.1. Finding a Process
2.10.2. Finding a Subsystem
2.10.3. Finding Hierarchies
2.10.4. Finding Control Groups
2.10.5. Displaying Parameters of Control Groups
2.11. Unloading Groups
2.12. Additional Resources
3. Subsystems and Tunable Parameters
3.1. blkio
3.2. cpu
3.3. cpuacct
3.4. cpuset
3.5. devices
3.6. freezer
3.7. memory
3.8. net_cls
3.9. ns
3.10. Additional Resources
A. Revision History

Preface

1. Document Conventions

This manual uses several conventions to highlight certain words and phrases and draw attention to specific pieces of information.
In PDF and paper editions, this manual uses typefaces drawn from the Liberation Fonts set. The Liberation Fonts set is also used in HTML editions if the set is installed on your system. If not, alternative but equivalent typefaces are displayed. Note: Red Hat Enterprise Linux 5 and later includes the Liberation Fonts set by default.

1.1. Typographic Conventions

Four typographic conventions are used to call attention to specific words and phrases. These conventions, and the circumstances they apply to, are as follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file names and paths. Also used to highlight keycaps and key combinations. For example:
To see the contents of the file my_next_bestselling_novel in your current working directory, enter the cat my_next_bestselling_novel command at the shell prompt and press Enter to execute the command.
The above includes a file name, a shell command and a keycap, all presented in mono-spaced bold and all distinguishable thanks to context.
Key combinations can be distinguished from keycaps by the hyphen connecting each part of a key combination. For example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to the first virtual terminal. Press Ctrl+Alt+F1 to return to your X-Windows session.
The first paragraph highlights the particular keycap to press. The second highlights two key combinations (each a set of three keycaps with each set pressed simultaneously).
If source code is discussed, class names, methods, functions, variable names and returned values mentioned within a paragraph will be presented as above, in mono-spaced bold. For example:
File-related classes include filesystem for file systems, file for files, and dir for directories. Each class has its own associated set of permissions.
Proportional Bold
This denotes words or phrases encountered on a system, including application names; dialog box text; labeled buttons; check-box and radio button labels; menu titles and sub-menu titles. For example:
Choose System → Preferences → Mouse from the main menu bar to launch Mouse Preferences. In the Buttons tab, click the Left-handed mouse check box and click Close to switch the primary mouse button from the left to the right (making the mouse suitable for use in the left hand).
To insert a special character into a gedit file, choose Applications → Accessories → Character Map from the main menu bar. Next, choose Search → Find… from the Character Map menu bar, type the name of the character in the Search field and click Next. The character you sought will be highlighted in the Character Table. Double-click this highlighted character to place it in the Text to copy field and then click the Copy button. Now switch back to your document and choose Edit → Paste from the gedit menu bar.
The above text includes application names; system-wide menu names and items; application-specific menu names; and buttons and text found within a GUI interface, all presented in proportional bold and all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of italics indicates replaceable or variable text. Italics denotes text you do not input literally or displayed text that changes depending on circumstance. For example:
To connect to a remote machine using ssh, type ssh username@domain.name at a shell prompt. If the remote machine is example.com and your username on that machine is john, type ssh [email protected].
The mount -o remount file-system command remounts the named file system. For example, to remount the /home file system, the command is mount -o remount /home.
To see the version of a currently installed package, use the rpm -q package command. It will return a result as follows: package-version-release.
Note the words in bold italics above — username, domain.name, file-system, package, version and release. Each word is a placeholder, either for text you enter when issuing a command or for text displayed by the system.
Aside from standard usage for presenting the title of a work, italics denotes the first use of a new and important term. For example:
Publican is a DocBook publishing system.

1.2. Pull-quote Conventions

Terminal output and source code listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and presented thus: 
books        Desktop   documentation  drafts  mss    photos   stuff  svn
books_tests  Desktop1  downloads      images  notes  scripts  svgs
Source-code listings are also set in mono-spaced roman but add syntax highlighting as follows: 
package org.jboss.book.jca.ex1;

import javax.naming.InitialContext;

public class ExClient
{
   public static void main(String args[]) 
       throws Exception
   {
      InitialContext iniCtx = new InitialContext();
      Object         ref    = iniCtx.lookup("EchoBean");
      EchoHome       home   = (EchoHome) ref;
      Echo           echo   = home.create();

      System.out.println("Created Echo");

      System.out.println("Echo.echo('Hello') = " + echo.echo("Hello"));
   }
}

1.3. Notes and Warnings

Finally, we use three visual styles to draw attention to information that might otherwise be overlooked.

Note

Notes are tips, shortcuts or alternative approaches to the task at hand. Ignoring a note should have no negative consequences, but you might miss out on a trick that makes your life easier.

Important

Important boxes detail things that are easily missed: configuration changes that only apply to the current session, or services that need restarting before an update will apply. Ignoring a box labeled 'Important' will not cause data loss but may cause irritation and frustration.

Warning

Warnings should not be ignored. Ignoring warnings will most likely cause data loss.

2. Getting Help and Giving Feedback

2.1. Do You Need Help?

If you experience difficulty with a procedure described in this documentation, visit the Red Hat Customer Portal at http://access.redhat.com. Through the customer portal, you can:
  • search or browse through a knowledgebase of technical support articles about Red Hat products.
  • submit a support case to Red Hat Global Support Services (GSS).
  • access other product documentation.
Red Hat also hosts a large number of electronic mailing lists for discussion of Red Hat software and technology. You can find a list of publicly available mailing lists at https://www.redhat.com/mailman/listinfo. Click on the name of any mailing list to subscribe to that list or to access the list archives.

2.2. We Need Feedback!

If you find a typographical error in this manual, or if you have thought of a way to make this manual better, we would love to hear from you! Please submit a report in Bugzilla: http://bugzilla.redhat.com/ against the product Red Hat Enterprise Linux 6.
When submitting a bug report, be sure to mention the manual's identifier: doc-Resource_Management_Guide
If you have a suggestion for improving the documentation, try to be as specific as possible when describing it. If you have found an error, please include the section number and some of the surrounding text so we can find it easily.

Chapter 1. Introduction to Control Groups (Cgroups)

1.1. How Control Groups Are Organized
1.2. Relationships Between Subsystems, Hierarchies, Control Groups and Tasks
1.3. Implications for Resource Management
Red Hat Enterprise Linux 6 provides a new kernel feature: control groups, which are called by their shorter name cgroups in this guide. Cgroups allow you to allocate resources—such as CPU time, system memory, network bandwidth, or combinations of these resources—among user-defined groups of tasks (processes) running on a system. You can monitor the cgroups you configure, deny cgroups access to certain resources, and even reconfigure your cgroups dynamically on a running system. The cgconfig (“control group config ”) service can be configured to start up at boot time and reestablish your predefined cgroups, thus making them persistent across reboots.
By using cgroups, system administrators gain fine-grained control over allocating, prioritizing, denying, managing, and monitoring system resources. Hardware resources can be smartly divided up amongst tasks and users, increasing overall efficiency.

1.1. How Control Groups Are Organized

Cgroups are organized hierarchically, like processes, and child cgroups inherit some of the attributes of their parents. However, there are differences between the two models.

The Linux Process Model

All processes on a Linux system are child processes of a common parent: the init process, which is executed by the kernel at boot time and starts other processes (which may in turn start child processes of their own). Because all processes descend from a single parent, the Linux process model is a single hierarchy, or tree.
Additionally, every Linux process except init inherits the environment (such as the PATH variable)[1] and certain other attributes (such as open file descriptors) of its parent process.

The Cgroup Model

Cgroups are similar to processes in that:
  • they are hierarchical, and
  • child cgroups inherit certain attributes from their parent cgroup.
The fundamental difference is that many different hierarchies of cgroups can exist simultaneously on a system. If the Linux process model is a single tree of processes, then the cgroup model is one or more separate, unconnected trees of tasks (i.e. processes).
Multiple separate hierarchies of cgroups are necessary because each hierarchy is attached to one or more subsystems. A subsystem[2] represents a single resource, such as CPU time or memory. Red Hat Enterprise Linux 6 provides nine control group subsystems, listed below by name and function.
Available Subsystems in Red Hat Enterprise Linux
  • blkio — this subsystem sets limits on input/output access to and from block devices such as physical drives (disk, solid state, USB, etc.).
  • cpu — this subsystem uses the scheduler to provide cgroup tasks access to the CPU.
  • cpuacct — this subsystem generates automatic reports on CPU resources used by tasks in a cgroup.
  • cpuset — this subsystem assigns individual CPUs (on a multicore system) and memory nodes to tasks in a cgroup.
  • devices — this subsystem allows or denies access to devices by tasks in a cgroup.
  • freezer — this subsystem suspends or resumes tasks in a cgroup.
  • memory — this subsystem sets limits on memory use by tasks in a cgroup, and generates automatic reports on memory resources used by those tasks.
  • net_cls — this subsystem tags network packets with a class identifier (classid) that allows the Linux traffic controller (tc) to identify packets originating from a particular cgroup task.
  • ns — the namespace subsystem

Subsystems are also known as resource controllers

You may come across the term resource controller or simply controller in control group literature such as the man pages or kernel documentation. Both of these terms are synonymous with “subsystem”, and arise from the fact that a subsystem typically schedules a resource or applies a limit to the cgroups in the hierarchy it is attached to.
The definition of a subsystem (resource controller) is quite general: it is something that acts upon a group of tasks, i.e. processes.

1.2. Relationships Between Subsystems, Hierarchies, Control Groups and Tasks

Remember that system processes are called tasks in cgroup terminology.
Here are a few simple rules governing the relationships between subsystems, hierarchies of cgroups, and tasks, along with explanatory consequences of those rules.
Rule 1
Any single subsystem (such as cpu) can be attached to at most one hierarchy.
As a consequence, the cpu subsystem can never be attached to two different hierarchies.
Rule 2
A single hierarchy can have one or more subsystems attached to it.
As a consequence, the cpu and memory subsystems (or any number of subsystems) can be attached to a single hierarchy, as long as each one is not attached to any other hierarchy.
Rule 3
Each time a new hierarchy is created on the systems, all tasks on the system are initially members of the default cgroup of that hierarchy, which is known as the root cgroup. For any single hierarchy you create, each task on the system can be a member of exactly one cgroup in that hierarchy. A single task may be in multiple cgroups, as long as each of those cgroups is in a different hierarchy. As soon as a task is made a member of a second cgroup in the same hierarchy, it is removed from the first cgroup in that hierarchy. At no time is a task ever in two different cgroups in the same hierarchy.
As a consequence, if the cpu and memory subsystems are attached to a hierarchy named cpu_and_mem, and the net_cls subsystem is attached to a hierarchy named net, then a running httpd process could be a member of any one cgroup in cpu_and_mem, and any one cgroup in net.
The cgroup in cpu_and_mem that the http process is a member of might restrict its CPU time to half of that allotted to other processes, and limit its memory usage to a maximum of 1024 MB. Additionally, the cgroup in net that it is a member of might limit its transmission rate to 30 megabytes per second.
When the first hierarchy is created, every task on the system is a member of at least one cgroup: the root cgroup. When using control groups, therefore, every system task is always in at least one cgroup.
Rule 4
Any process (task) on the system which forks itself creates a child process (task). The child task automatically becomes members of all of the cgroups its parent is members of. The child task can then be moved to different cgroups as needed, but initially, it always inherits the cgroups (the "environment" in process terminology) of its parent task.
As a consequence, consider the httpd task that is a member of the cgroup named half_cpu_1gb_max in the cpu_and_mem hierarchy, and a member of the cgroup trans_rate_30 in the net hierarchy. When that httpd process forks itself, its child process automatically becomes a member of the half_cpu_1gb_max cgroup, and the trans_rate_30 cgroup. It inherits the exact same cgroups its parent task belongs to.
From that point forward, the parent and child tasks are completely independent of each other: changing the cgroups that one task belongs to does not affect the other. Neither will changing cgroups of a parent task affect any of its grandchildren in any way. To summarize: any child task always initially inherit memberships to the exact same cgroups as their parent task, but those memberships can be changed or removed later.

1.3. Implications for Resource Management

  • Because a task can belong to only a single cgroup in any one hierarchy, there is only one way that a task can be limited or affected by any single subsystem. This is logical: a feature, not a limitation.
  • You can group several subsystems together so that they affect all tasks in a single hierarchy. Because cgroups in that hierarchy have different parameters set, those tasks will be affected differently.
  • It may sometimes be necessary to refactor a hierarchy. An example would be removing a subsystem from a hierarchy that has several subsystems attached, and attaching it to a new, separate hierarchy.
  • Conversely, if the need for splitting subsystems among separate hierarchies is reduced, you can remove a hierarchy and attach its subsystems to an existing one.
  • The design allows for simple control group usage, such as setting a few parameters for specific tasks in a single hierarchy, such as one with just the cpu and memory subsystems attached.
  • The design also allows for highly specific configuration: each task (process) on a system could be a member of each hierarchy, each of which has a single attached subsystem. Such a configuration would give the system administrator absolute control over all parameters for every single task.

[1] The parent process is able to alter the environment before passing it to a child process.

[2] You should be aware that subsystems are also called resource controllers, or simply controllers, in the libcgroup man pages and other documentation.

Chapter 2. Using Control Groups

2.1. The cgconfig Service
2.1.1. The cgconfig.conf File
2.2. Creating a Hierarchy and Attaching Subsystems
2.3. Attaching Subsystems to, and Detaching Them From, an Existing Hierarchy
2.4. Unmounting a Hierarchy
2.5. Creating Cgroups
2.6. Removing Cgroups
2.7. Setting Parameters
2.8. Moving a Process to a Control Group
2.8.1. The cgred Daemon
2.9. Starting a Process in a Control Group
2.9.1. Starting a Service in a Control Group
2.10. Obtaining Information About Control Groups
2.10.1. Finding a Process
2.10.2. Finding a Subsystem
2.10.3. Finding Hierarchies
2.10.4. Finding Control Groups
2.10.5. Displaying Parameters of Control Groups
2.11. Unloading Groups
2.12. Additional Resources
The easiest way to work with control groups is to install the libcgroup package, which contains a number of cgroup-related command line utilities and their associated man pages. It is possible to mount hierarchies and set cgroup parameters (non-persistently) using shell commands and utilities available on any system. However, using the libcgroup-provided utilities simplifies the process and extends your capabilities. Therefore, this guide focuses on libcgroup commands throughout. In most cases, we have included the equivalent shell commands to help describe the underlying mechanism. However, we recommend that you use the libcgroup commands wherever practical.

Note: Installing the libcgroup package

In order to use cgroups, first ensure the libcgroup package is installed on your system by running, as root: 
~]# yum install libcgroup

2.1. The cgconfig Service

The cgconfig service installed with the libcgroup package provides a convenient way to create hierarchies, attach subsystems to hierarchies, and manage cgroups within those hierarchies. We recommend that you use cgconfig to manage hierarchies and cgroups on your system.
The cgconfig service is not started by default on Red Hat Enterprise Linux 6. When you start the service with chkconfig, it reads the control group configuration file — /etc/cgconfig.conf. Control groups are therefore recreated from session to session and become persistent. Depending on the contents of the configuration file, cgconfig can create hierarchies, mount necessary file systems, create control groups, and set subsystem parameters for each group.
The default cgconfig.conf file installed with the libcgroup package creates and mounts an individual hierarchy for each subsystem, and attaches the subsystems to these hierarchies.
If you stop the cgconfig service (with service cgconfig stop), it unmounts all the hierarchies that it mounted.

2.1.1. The cgconfig.conf File

The cgconfig.conf file contains two major types of entry — mount and group. Mount entries create and mount hierarchies as virtual filesystems, and attach subsystems to those hierarchies. For example: 
mount {
    cpuset = /cgroup/cpuset;
}
creates a hierarchy for the cpuset subsystem, the equivalent of the shell commands: 
        mkdir /cgroup/cpuset
        mount -t cgroup -o cpuset cpuset /cgroup/cpuset
Group entries create control groups and set subsystem parameters. For example: 
group daemons/sql {
    perm {
        task {
            uid = root;
            gid = sqladmin;
        } admin {
            uid = root;
            gid = root;
        }
    } cpuset {
        cpuset.cpus = 0-3;
    }
}
creates a control group for sql daemons, with permissions for users in the sqladmin group to add tasks to the control group and the root user to modify subsystem parameters. When combined with the example of the mount entry above, the equivalent shell commands are: 
mkdir -p /cgroup/cpu/daemons/sql
chown root:root /cgroup/cpu/daemons/sql/*
chown root:sqladmin /cgroup/cpu/daemons/sql/tasks
echo 0-3 > /cgroup/cpu/daemons/sql/cpuset.cpus
When you install cgroups, a sample config file is written to /etc/cgconfig.conf. The # symbols at the start of each line comment that line out and make it invisible to the cgconfig service.

2.2. Creating a Hierarchy and Attaching Subsystems

Warning — Effects on running systems

The following instructions, which cover creating a new hierarchy and attaching subsystems to it, assume that control groups are not already configured on your system. In this case, these instructions will not affect the operation of the system. Changing the tunable parameters in a cgroup with tasks, however, may immediately affect those tasks. This guide alerts you the first time it illustrates changing a tunable cgroup parameter that may affect one or more tasks.
On a system on which control groups are already configured (either manually, or by the cgconfig service) these commands will fail unless you first unmount existing hierarchies, which will affect the operation of the system. Do not experiment with these instructions on production systems.
To create a hierarchy and attach subsystems to it, edit the mount section of the /etc/cgconfig.conf file as root. Entries in the mount section have the following format: 
subsystem = /cgroup/hierarchy;
When cgconfig next starts, it will create the hierarchy and attach the subsystems to it.
The following example creates a hierarchy called cpu_and_mem and attaches the cpu, cpuset, cpuacct, and memory subsystems to it. 
mount {
    cpuset  = /cgroup/cpu_and_mem;
    cpu     = /cgroup/cpu_and_mem;
    cpuacct = /cgroup/cpu_and_mem;
    memory  = /cgroup/cpu_and_mem;
}

Alternative method

You can also use shell commands and utilities to create hierarchies and attach subsystems to them.
Create a mount point for the hierarchy as root. Include the name of the control group in the mount point: 
~]# mkdir /cgroup/name
For example: 
~]# mkdir /cgroup/cpu_and_mem
Next, use the mount command to mount the hierarchy and simultaneously attach one or more subsystems. For example: 
mount -t cgroup -o subsystems name /cgroup/name
Where subsystems is a comma-separated list of subsystems and name is the name of the hierarchy. Brief descriptions of all available subsystems are listed in Available Subsystems in Red Hat Enterprise Linux, and Chapter 3, Subsystems and Tunable Parameters provides a detailed reference.
Example 2.1. Using the mount command to attach subsystems
In this example, a directory named /cgroup/cpu_and_mem already exists, which will serve as the mount point for the hierarchy that we create. We will attach the cpu, cpuset and memory subsystems to a hierarchy we name cpu_and_mem, and mount the cpu_and_mem hierarchy on /cgroup/cpu_and_mem: 
~]# mount -t cgroup -o cpu,cpuset,memory cpu_and_mem /cgroup/cpu_and_mem
You can list all available subsystems along with their current mount points (i.e. where the hierarchy they are attached to is mounted) with the lssubsys [3] command: 
~]# lssubsys -am
cpu,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
cpu
cpuacct
devices
freezer
blkio
This output indicates that:
  • the cpu, cpuset and memory subsystems are attached to a hierarchy mounted on /cgroup/cpu_and_mem, and
  • the net_cls, ns, cpu, cpuacct, devices, freezer and blkio subsystems are as yet unattached to any hierarchy, as illustrated by the lack of a corresponding mount point.

2.3. Attaching Subsystems to, and Detaching Them From, an Existing Hierarchy

To add a subsystem to an existing hierarchy, detach it from an existing hierarchy, or move it to a different hierarchy, edit the mount section of the /etc/cgconfig.conf file as root, using the same syntax described in Section 2.2, “Creating a Hierarchy and Attaching Subsystems”. When cgconfig next starts, it will reorganize the subsystems according to the hierarchies that you specify.

Alternative method

To add an unattached subsystem to an existing hierarchy, remount the hierarchy. Include the extra subsystem in the mount command, together with the remount option.
Example 2.2. Remounting a hierarchy to add a subsystem
The lssubsys command shows cpu, cpuset, and memory subsystems attached to the cpu_and_mem hierarchy: 
~]# lssubsys -am
cpu,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
cpu
cpuacct
devices
freezer
blkio
We remount the cpu_and_mem hierarchy, using the remount option, and including cpuacct in the list of subsystems: 
~]# mount -t cgroup -o remount,cpu,cpuset,cpuacct,memory cpu_and_mem /cgroup/cpu_and_mem
The lssubsys command now shows cpuacct attached to the cpu_and_mem hierarchy: 
~]# lssubsys -am
cpu,cpuacct,cpuset,memory /cgroup/cpu_and_mem
net_cls
ns
devices
freezer
blkio

Analogously, you can detach a subsystem from an existing hierarchy by remounting the hierarchy and omitting the subsystem name from the -o options. For example, to then detach the cpuacct subsystem, simply remount and omit it: 
~]# mount -t cgroup -o remount,cpu,cpuset,memory cpu_and_mem /cgroup/cpu_and_mem

2.4. Unmounting a Hierarchy

You can unmount a hierarchy of cgroups with the umount command: 
~]# umount /cgroup/name
For example: 
~]# umount /cgroup/cpu_and_mem
If the hierarchy is currently empty (that is, it contains only the root cgroup) the hierarchy is deactivated when it is unmounted. If the hierarchy contains any other cgroups, the hierarchy remains active in the kernel even though it is no longer mounted.
To remove a hierarchy, ensure that all child cgroups are removed before you unmount the hierarchy, or use the cgclear command which can deactivate a hierarchy even when it is not empty — refer to Section 2.11, “Unloading Groups”.

2.5. Creating Cgroups

Use the cgcreate command to create cgroups. The syntax for cgcreate is: cgcreate -t uid:gid -a uid:gid -g subsystems:path , where:
  • -t (optional) — specifies a user (by user ID, uid) and a group (by group ID, gid) to own the tasks pseudofile for this control group. This user can add tasks to the control group.

    Note — Removing tasks

    Note that the only way to remove a task from a control group is to move it to a different control group. To move a task, the user must have write access to the destination control group; write access to the source control group is unimportant.
  • -a (optional) — specifies a user (by user ID, uid) and a group (by group ID, gid) to own all pseudofiles other than tasks for this control group. This user can modify the access that the tasks in this control group have to system resources.
  • -g — specifies the hierarchy in which the cgroup should be created, as a comma-separated list of the subsystems associated with those hierarchies. If the subsystems in this list are in different hierarchies, the group is created in each of these hierarchies. The list of hierarchies is followed by a colon and the path to the child group relative to the hierarchy. Do not include the hierarchy mount point in the path.
    For example, the control group located in the directory /cgroup/cpu_and_mem/lab1/ is called just lab1 — its path is already uniquely determined because there is at most one hierarchy for a given subsystem. Note also that the group is controlled by all the subsystems that exist in the hierarchies in which the cgroup is created, even though these subsystems have not been specified in the cgcreate command — refer to Example 2.3, “cgcreate usage”.
Because all control groups in the same hierarchy have the same controllers, the child group has the same controllers as its parent.
Example 2.3. cgcreate usage
Consider a system where the cpu and memory subsystems are mounted together in the cpu_and_mem hierarchy, and the net_cls controller is mounted in a separate hierarchy called net. We now run: 
cgcreate -g cpu,net_cls:/test-subgroup
The cgcreate command creates two groups named test-subgroup, one in the cpu_and_mem hierarchy and one in the net hierarchy. The test-subgroup group in the cpu_and_mem hierarchy is controlled by the memory subsystem, even though we did not specify it in the cgcreate command.

Alternative method

To create a child of the control group directly, use the mkdir command: 
mkdir /cgroup/hierarchy/name/child_name
For example: 
mkdir /cgroup/cpuset/lab1/group1

2.6. Removing Cgroups

Remove cgroups with the cgdelete, which has a syntax similar to that of cgcreate. Run: cgdelete subsystems:path, where:
  • subsystems is a comma-separated list of subsystems.
  • path is the path to the cgroup relative to the root of the hierarchy.
For example: 
cgdelete cpu,net_cls:/test-subgroup
cgdelete can also recursively remove all subgroups with the option -r.
When you delete a control group, all its tasks move to its parent group.

2.7. Setting Parameters

Set subsystem parameters by running the cgset command from a user account with permission to modify the relevant control group. For example, if /cgroup/cpuset/group1 exists, specify the CPUs to which this group has access with the following command: 
cgset -r cpuset.cpus=0-1 group1
The syntax for cgset is: cgset -r parameter=value path_to_cgroup , where:
  • parameter is the parameter to be set, which corresponds to the file in the directory of the given cgroup
  • value is the value for the parameter
  • path_to_cgroup is the path to the control group relative to the root of the hierarchy. For example, to set the parameter of the root group, run: 
    $ cgset -r cpuset.cpus=1 /
    Alternatively, because . is relative to the root group (that is, the root group itself) you could also run: 
    $ cgset -r cpuset.cpus=1 .
    Note, however, that / is the preferred syntax.
    To set the parameter of group1, which is a subgroup of the root group, run: 
    $ cgset -r cpuset.cpus=1 group1
    A trailing slash on the name of the group (for example, cpuset.cpus=1 group1/) is optional.
The values that you can set with cgset might depend on values set higher in a particular hierarchy. For example, if group1 is limited to use only CPU 0 on a system, you cannot set group1/subgroup1 to use CPUs 0 and 1, or to use only CPU 1.
You can also use cgset to copy the parameters of one cgroup into another, existing cgroup. For example: 
cgset --copy-from group1/ group2/
The syntax to copy parameters with cgset is: cgset --copy-from path_to_source_cgroup path_to_target_cgroup , where:
  • path_to_source_cgroup is the path to the control group whose parameters are to be copied, relative to the root group of the hierarchy
  • path_to_target_cgroup is the path to the destination control group, relative to the root group of the hierarchy
Ensure that any mandatory parameters for the various subsystems are set before you copy parameters from one group to another, or the command will fail.

Alternative method

To set parameters in a control group directly, insert values into the relevant subsystem pseudofile using the echo command. For example, this command inserts the value 0-1 into the cpuset.cpus pseudofile of the control group group1: 
echo 0-1 > /cgroup/cpuset/group1/cpuset.cpus
With this value in place, the tasks in this control group are restricted to CPUs 0 and 1 on the system.

2.8. Moving a Process to a Control Group

Move a process into a control group by running the cgclassify command: 
cgclassify -g cpu,memory:group1 1701
The syntax for cgclassify is: cgclassify -g subsystems:path_to_cgroup pidlist, where:
  • subsystems is a comma-separated list of subsystems, or * to launch the process in the hierarchies associated with all available subsystems. Note that if control groups of the same name exist in multiple hierarchies, the -g option moves the processes in each of those groups. Ensure that the cgroup exists within each of the hierarchies whose subsystems you specify here.
  • path_to_cgroup is the path to the control group within its hierarchies
  • pidlist is a space-separated list of process identifier (PIDs)
You can also add the --sticky option before the pid to keep any child processes in the same control group. If you do not set this option and the cgred daemon is running, child processes will be allocated to control groups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the control group in which you started it.
Using cgclassify, you can move several processes simultaneously. For example, this command moves the processes with PIDs 1701 and 1138 into control group group1/: 
cgclassify -g cpu,memory:group1 1701 1138
Note that the PIDs to be moved are separated by spaces and that the groups specified should be in different hierarchies.

Alternative method

To move a process into a control group directly, write its PID to the tasks file of the control group. For example, to move a process with the PID 1701 into a control group at /cgroup/lab1/group1/: 
echo 1701 > /cgroup/lab1/group1/tasks

2.8.1. The cgred Daemon

Cgred is a daemon that moves tasks into control groups according to parameters set in the /etc/cgrules.conf file. Entries in the /etc/cgrules.conf file can take one of the two forms:
  • user hierarchies control_group
  • user:command hierarchies control_group
For example: 
maria			devices		/usergroup/staff
This entry specifies that any processes that belong to the user named maria access the devices subsystem according to the parameters specified in the /usergroup/staff control group. To associate particular commands with particular control groups, add the command parameter, as follows: 
maria:ftp		devices		/usergroup/staff/ftp
The entry now specifies that when the user named maria uses the ftp command, the process is automatically moved to the /usergroup/staff/ftp control group in the hierarchy that contains the devices subsystem. Note, however, that the daemon moves the process to the control group only after the appropriate condition is fulfilled. Therefore, the ftp process might run for a short time in the wrong group. Furthermore, if the process quickly spawns children while in the wrong group, these children might not be moved.
Entries in the /etc/cgrules.conf file can include the following extra notation:
  • @ — when prefixed to user, indicates a group instead of an individual user. For example, @admins are all users in the admins group.
  • * — represents "all". For example, * in the subsystem field represents all subsystems.
  • % — represents an item the same as the item in the line above. For example: 
    @adminstaff		devices		/admingroup
@labstaff		%		%

2.9. Starting a Process in a Control Group

Important — Mandatory parameters

Some controllers have mandatory parameters that you must set before you run a task in a hierarchy that includes those controllers. For example, before you use the cpuset controller, cpuset.cpus and cpuset.mems must be defined.
The examples in this section illustrate the correct syntax for the command, but only work on systems on which the relevant mandatory parameters have been set for any controllers used in the examples. If you have not already configured the relevant controllers, you cannot copy example commands directly from this section and expect them to work on your system.
Refer to Section 3.10, “Additional Resources” for a description of which parameters are mandatory for given subsystems.
Launch processes in a control group by running the cgexec command. For example, this command launches the lynx web browser within the group1 control group, subject to the limitations imposed on that group by the cpu subsystem: 
cgexec -g cpu:group1 lynx http://www.redhat.com
The syntax for cgexec is: cgexec -g subsystems:path_to_cgroup command arguements , where:
  • subsystems is a comma-separated list of subsystems, or * to launch the process in the hierarchies associated with all available subsystems. Note that, as with cgset described in Section 2.7, “Setting Parameters”, if control groups of the same name exist in multiple hierarchies, the -g option creates processes in each of those groups. Ensure that the cgroup exists within each of the hierarchies whose subsystems you specify here.
  • path_to_cgroup is the path to the control group relative to the hierarchy.
  • command is the command to run
  • arguements are any arguements for the command
You can also add the --sticky option before the command to keep any child processes in the same control group. If you do not set this option and the cgred daemon is running, child processes will be allocated to control groups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the control group in which you started it.

Alternative method

When you start a new process, it inherits the group of its parent process. Therefore, an alternative method for starting a process in a particular control group is to move your shell process to that group (refer to Section 2.8, “Moving a Process to a Control Group”), and then launch the process from that shell. For example: 
echo $$ > /cgroup/lab1/group1/tasks
lynx
Note that after exiting lynx, your existing shell is still in the group1 control group. Therefore, an even better way would be: 
sh -c "echo \$$ > /cgroup/lab1/group1/tasks && lynx"

2.9.1. Starting a Service in a Control Group

You can start some services in a control group. Services that can be started in control groups must:
  • use a /etc/sysconfig/servicename file
  • use the daemon() function from /etc/init.d/functions to start the service
To make an eligible service start in a control group, edit its file in the /etc/sysconfig directory to include an entry in the form CGROUP_DAEMON="subsystem:control_group" where subsystem is a subsystem associated with a particular hierarchy, and control_group is a control group in that hierarchy. For example: 
CGROUP_DAEMON="cpuset:daemons/sql"

2.10. Obtaining Information About Control Groups

2.10.1. Finding a Process

To find the control group to which a process belongs, run: 
ps -O cgroup
Or, if you know the PID for the process, run: 
cat /proc/PID/cgroup

2.10.2. Finding a Subsystem

To find the subsystems that are availabe in your kernel and how are they mounted together to hierarchies, run: 
cat /proc/cgroups
Or, to find the mount points of particular subsystems, run: 
lssubsys -m subsystems
where subsystems is a list of the subsystems in which you are interested.

2.10.3. Finding Hierarchies

We recommend that you mount hierarchies under /cgroup. Assuming this is the case on your system, list or browse the contents of that directory to obtain a list of hierarchies. If tree is installed on your system, run it to obtain an overview of all hierarchies and the control groups within them: 
tree /cgroup

2.10.4. Finding Control Groups

To list the control groups on a system, run: 
lscgroup
You can restrict the output to a specific hierarchy by specifying a controller and path in the format controller:path. For example: 
lscgroup cpuset:adminusers
lists only subgroups of the adminusers control group in the hierarchy to which the cpuset subsystem is attached.

2.10.5. Displaying Parameters of Control Groups

To display the parameters of specific control groups, run: 
cgget -r parameter list_of_cgroups
where parameter is a pseudofile that contains values for a subsystem, and list_of_cgroups is a list of control groups separated with spaces. For example: 
cgget -r cpuset.cpus -r memory.limit_in_bytes lab1 lab2
displays the values of cpuset.cpus and memory.limit_in_bytes for control groups lab1 and lab2.
If you do not know the names of the parameters themselves, use a command like: 
cgget -g cpuset /

2.11. Unloading Groups

Warning — This Command Destroys all Control Groups

The cgclear command destroys all control groups in all hierarchies. If you do not have these hierarchies stored in a configuration file, you will not be able to readily reconstruct them.
To clear an entire control group file system, use the cgclear command.
All tasks in the control group are reallocated to the root node of the hierarchies, all control groups are removed, and the filesystem itself is unmounted from the system, thus destroying all previously mounted hierarchies. Finally, the directory where the cgroup filesystem was mounted is actually deleted.

2.12. Additional Resources

The definitive documentation for control group commands are the manual pages provided with the libcgroup package. The section numbers are specified in the list of man pages below.
The libcgroup Man Pages
  • man 1 cgclassify — the cgclassify command is used to move running tasks to one or more cgroups.
    man 1 cgclear — the cgclear command is used to delete all cgroups in a hierarchy.
    man 5 cgconfig.conf — cgroups are defined in the cgconfig.conf file.
    man 8 cgconfigparser — the cgconfigparser command parses the cgconfig.conf file and mounts hierarchies.
    man 1 cgcreate — the cgcreate command creates new cgroups in hierarchies.
    man 1 cgdelete — the cgdelete command removes specified cgroups.
    man 1 cgexec — the cgexec command runs tasks in specified cgroups.
    man 1 cgget — the cgget command displays cgroup parameters.
    man 5 cgred.conf — cgred.conf is the configuration file for the cgred service.
    man 5 cgrules.conf — cgrules.conf contains the rules used for determining when tasks belong to certain cgroups.
    man 8 cgrulesengd — the cgrulesengd service distributes tasks to cgroups.
    man 1 cgset — the cgset command sets parameters for a cgroup.
    man 1 lscgroup — the lscgroup command lists the cgroups in a hierarchy.
    man 1 lssubsys — the lssubsys command lists the hierarchies containing the specified subsystems.

[3] The lssubsys command is one of the utiilties provided by the libcgroup package. You must install libcgroup to use it: refer to Chapter 2, Using Control Groups if you are unable to run lssubsys.

Chapter 3. Subsystems and Tunable Parameters

3.1. blkio
3.2. cpu
3.3. cpuacct
3.4. cpuset
3.5. devices
3.6. freezer
3.7. memory
3.8. net_cls
3.9. ns
3.10. Additional Resources
Subsystems are kernel modules that are aware of control groups. Typically, they are resource controllers that allocate varying levels of system resources to different control groups. However, subsystems could be programmed for any other interaction with the kernel where the need exists to treat different groups of processes differently. The application programming interface (API) to develop new subsystems is documented in cgroups.txt in the kernel documentation, installed on your system at /usr/share/doc/kernel-doc-kernel-version/Documentation/cgroups/. The latest version of the cgroups documentation is also available on line at http://www.kernel.org/doc/Documentation/cgroups/cgroups.txt. Note, however, that the features in the latest documentation might not match those available in the kernel installed on your system.
State objects that contain the subsystem parameters for a control group are represented as pseudofiles within the control group's virtual file system. These pseudofiles can be manipulated by shell commands or their equivalent system calls. For example, cpuset.cpus is a pseudofile that specifies which CPUs a control group is permitted to access. If /cgroup/cpuset/webserver is a control group for the web server that runs on a system, and we run the following command: 
~]# echo 0,2 > /cgroup/cpuset/webserver/cpuset.cpus
The value 0,2 is written to the cpuset.cpus pseudofile and therefore limits any tasks whose PIDs are listed in /cgroup/cpuset/webserver/tasks to use only CPU 0 and CPU 2 on the system.

3.1. blkio

The blkio subsystem controls and monitors access to I/O on block devices by tasks in control groups. Writing values to some of these pseudofiles limits access or bandwidth, and reading values from some of these pseudofiles provides information on I/O operations.
blkio.weight
specifies the relative proportion (weight) of block I/O access available by default to a control group, in the range 100 to 1000. This value is overriden for specific devices by the blkio.weight_device parameter.For example, to assign a default weight of 500 to a control group for access to block devices, run: 
echo 500 > blkio.weight
blkio.weight_device
specifies the relative proportion (weight) of I/O access on specific devices available to a control group, in the range 100 to 1000. The value of this parameter overrides the value of blkio.weight for the devices specified. Values take the format major:minor weight, where major and minor are device types and node numbers specified in Linux Allocated Devices, otherwise known as the Linux Devices List and available from http://www.kernel.org/doc/Documentation/devices.txt. For example, to assign a weight of 500 to a control group for access to /dev/sda, run: 
echo 8:0 500 > blkio.weight_device
In the Linux Allocated Devices notation, 8:0 represents /dev/sda.
blkio.time
reports the time that a control group had I/O access to specific devices. Entries have three fields: major, minor, and time. Major and minor are device types and node numbers specified in Linux Allocated Devices, and time is the length of time in milliseconds (ms).
blkio.sectors
reports the number of sectors transferred to or from specific devices by a control group. Entries have three fields: major, minor, and sectors. Major and minor are device types and node numbers specified in Linux Allocated Devices, and sectors is the number of disk sectors.
blkio.io_service_bytes
reports the number of bytes transferred to or from specific devices by a control group. Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and bytes is the number of bytes transferred.
blkio.io_serviced
reports the number of I/O operations performed on specific devices by a control group. Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and number represents the number of operations.
blkio.io_service_time
reports the total time between request dispatch and request completion for I/O operations on specific devices by a control group. Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and time is the length of time in nanoseconds (ns). The time is reported in nanoseconds rather than a larger unit so that this report is meaningful even for solid-state devices.
blkio.io_wait_time
reports the total time I/O operations on specific devices by a control group spent waiting for service in the scheduler queues. When you interpret this report, note:
  • the time reported can be greated than the total time elapsed, because the time reported is the cumulative total of all I/O operations for the control group rather than the time that the control group itself spent waiting for I/O operations. To find the time that the group as a whole has spent waiting, use blkio.group_wait_time.
  • if the device has a queue_depth > 1, the time reported only includes the time until the request is dispatched to the device, not any time spent waiting for service while the device re-orders requests.
Entries have four fields: major, minor, operation, and bytes. Major and minor are device types and node numbers specified in Linux Allocated Devices, operation represents the type of operation (read, write, sync, or async) and time is the length of time in nanoseconds (ns). The time is reported in nanoseconds rather than a larger unit so that this report is meaningful even for solid-state devices.
blkio.io_merged
reports the number of BIOS requests merged into requests for I/O operations by a control group. Entries have two fields: number and operation. Number is the number of requests, and operation represents the type of operation (read, write, sync, or async).
blkio.io_queued
reports the number of requests queued for I/O operations by a control group. Entries have two fields: number and operation. Number is the number of requests, and operation represents the type of operation (read, write, sync, or async).
blkio.avg_queue_size
reports the average queue size for I/O operations by a control group, over the entire length of time of the group's existence. The queue size is sampled every time a queue for this control group gets a timeslice. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.group_wait_time
reports the total time (in nanoseconds — ns) a control group spent waiting for a timeslice for one of its queues. The report is updated every time a queue for this control group gets a timeslice, so if you read this pseudofile while the control group is waiting for a timeslice, the report will not contain time spent waiting for the operation currently queued. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.empty_time
reports the total time (in nanoseconds — ns) a control group spent without any pending requests. The report is updated every time a queue for this control group has a pending request, so if you read this pseudofile while the control group has no pending requests, the report will not contain time spent in the current empty state. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.idle_time
reports the total time (in nanoseconds — ns) the scheduler spent idling for a control group in anticipation of a better request than those requests already in other queues or from other groups. The report is updated every time the group is no longer idling, so if you read this pseudofile while the control group is idling, the report will not contain time spent in the current idling state. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.dequeue
reports the number of times requests for I/O operations by a control group were dequeued by specific devices. Entries have three fields: major, minor, and number. Major and minor are device types and node numbers specified in Linux Allocated Devices, and number is the number of requests the group was dequeued. Note that this report is available only if CONFIG_DEBUG_BLK_CGROUP=y is set on the system.
blkio.reset_stats
resets the statistics recorded in the other pseudofiles. Write an integer to this file to reset the statistics for this cgroup.

3.2. cpu

The cpu subsystem schedules CPU access to control groups. Access to CPU resources can be scheduled according to the following parameters, each one in a separate pseudofile within the control group virtual file system:
cpu.shares
contains an integer value that specifies a relative share of CPU time available to the tasks in a control group. For example, tasks in two control groups that have cpu.shares set to 1 will receive equal CPU time, but tasks in a control group that has cpu.shares set to 2 receive twice the CPU time of tasks in a control group where cpu.shares is set to 1.
cpu.rt_runtime_us
specifies a period of time in microseconds (µs, represented here as "us") for the longest continuous period in which the tasks in a control group have access to CPU resources. Establishing this limit prevents tasks in one control group from monopolizing CPU time. If the tasks in a control group should be able to access CPU resources for 4 seconds out of every 5 seconds, set cpu.rt_runtime_us to 4000000 and cpu.rt_period_us to 5000000.
cpu.rt_period_us
specifies a period of time in microseconds (µs, represented here as "us") for how regularly a control group's access to CPU resource should be reallocated. If the tasks in a control group should be able to access CPU resources for 4 seconds out of every 5 seconds, set cpu.rt_runtime_us to 4000000 and cpu.rt_period_us to 5000000.

3.3. cpuacct

The cpuacct subsystem generates automatic reports on CPU resources used by the tasks in a control group, including tasks in child groups. Three reports are available:
cpuacct.stat
reports the number of CPU cycles (in the units defined by USER_HZ on the system) consumed by tasks in this control group and its children in both user mode and system (kernel) mode.
cpuacct.usage
reports the total CPU time (in nanoseconds) consumed by all tasks in this control group (including tasks lower in the hierarchy).
cpuacct.usage_percpu
reports the CPU time (in nanoseconds) consumed on each CPU by all tasks in this control group (including tasks lower in the hierarchy).

3.4. cpuset

The cpuset subsystem assigns individual CPUs and memory nodes to control groups. Each cpuset can be specified according to the following parameters, each one in a separate pseudofile within the control group virtual file system:
cpuset.cpus (mandatory)
specifies the CPUs that tasks in this control group are permitted to access. This is a comma-separated list in ASCII format, with dashes ("-") to represent ranges. For example, 
0-2,16
represents CPUs 0, 1, 2, and 16.
cpuset.mems (mandatory)
specifies the memory nodes that tasks in this control group are permitted to access. This is a comma-separated list in ASCII format, with dashes ("-") to represent ranges. For example, 
0-2,16
represents memory nodes 0, 1, 2, and 16.
cpuset.memory_migrate
contains a flag (0 or 1) that specifies whether a page in memory should migrate to a new node if the values in cpuset.mems change. By default, memory migration is disabled (0) and pages stay on the node to which they were originally allocated, even if this node is no longer one of the nodes now specified in cpuset.mems. If enabled (1), the system will migrate pages to memory nodes within the new parameters specified by cpuset.mems, maintaining their relative placement if possible — for example, pages on the second node on the list originally specified by cpuset.mems will be allocated to the second node on the list now specified by cpuset.mems, if this place is available.
cpuset.cpu_exclusive
contains a flag (0 or 1) that specifies whether cpusets other than this one and its parents and children can share the CPUs specified for this cpuset. By default (0), CPUs are not allocated exclusively to one cpuset.
cpuset.mem_exclusive
contains a flag (0 or 1) that specifies whether other cpusets can share the memory nodes specified for this cpuset. By default (0), memory nodes are not allocated exclusively to one cpuset. Reserving memory nodes for the exclusive use of a cpuset (1) is functionally the same as enabling a memory hardwall with cpuset.mem_hardwall.
cpuset.mem_hardwall
contains a flag (0 or 1) that specifies whether kernel allocations of memory page and buffer data should be restricted to the memory nodes specified for this cpuset. By default (0), page and buffer data is shared across processes belonging to multiple users. With a hardwall enabled (1), each tasks's user allocation can be kept separate.
cpuset.memory_pressure
a read-only file that contains a running average of the memory pressure created by the processes in this cpuset. The value in this pseudofile is automatically updated when cpuset.memory_pressure_enabled is enabled, otherwise, the pseudofile contains the value 0.
cpuset.memory_pressure_enabled
contains a flag (0 or 1) that specifies whether the system should compute the memory pressure created by the processes in this control group. Computed values are output to cpuset.memory_pressure and represent the rate at which processes attempt to free in-use memory, reported as an integer value of attempts to reclaim memory per second, multiplied by 1000.
cpuset.memory_spread_page
contains a flag (0 or 1) that specifies whether file system buffers should be spread evenly across the memory nodes allocated to this cpuset. By default (0), no attempt is made to spread memory pages for these buffers evenly, and buffers are placed on the same node on which the process that created them is running.
cpuset.memory_spread_slab
contains a flag (0 or 1) that specifies whether kernel slab caches for file input/output operations should be spread evenly across the cpuset. By default (0), no attempt is made to spread kernel slab caches evenly, and slab caches are placed on the same node on which the process that created them is running.
cpuset.sched_load_balance
contains a flag (0 or 1) that specifies whether the kernel will balance loads across the CPUs in this cpuset. By default (1), the kernel balances loads by moving processes from overloaded CPUs to less heavily used CPUs.
Note, however, that setting this flag in a control group has no effect if load balancing is enabled in any parent control group, as load balancing is already being carried out at a higher level. Therefore, to disable load balancing in a control group, disable load balancing also in each of its parents in the hierarchy. In this case, you should also consider whether load balancing should be enabled for any siblings of the control group in question.
cpuset.sched_relax_domain_level
contains an integer between -1 and a small positive value, which represents the width of the range of CPUs across which the kernel should attempt to balance loads. This value is meaningless if cpuset.sched_load_balance is disabled.
The precise effect of this value varies according to system architecture, but the following values are typical:
Values of cpuset.sched_relax_domain_level
ValueEffect
-1 Use the system default value for load balancing
0 Do not perform immediate load balancing; balance loads only periodically
1 Immediately balance loads across threads on the same core
2 Immediately balance loads across cores in the same package
3 Immediately balance loads across CPUs on the same node or blade
4 Immediately balance loads across several CPUs on architectures with non-uniform memory access (NUMA)
5 Immediately balance loads across all CPUs on architectures with NUMA

3.5. devices

The devices subsystem allows or denies access to devices by tasks in a control group.

Technology Preview

The devices subsystem is considered to be a Technology Preview in Red Hat Enterprise Linux 6.
Technology preview features are currently not supported under Red Hat Enterprise Linux 6 subscription services, might not be functionally complete, and are generally not suitable for production use. However, Red Hat includes these features in the operating system as a customer convenience and to provide the feature with wider exposure. You might find these features useful in a non-production environment and are also free to provide feedback and functionality suggestions for a technology preview feature before it becomes fully supported.
devices.allow
specifies devices to which tasks in a control group have access. Each entry has four fields: type, major, minor, and access. The values used in the type, major, and minor fields correspond to device types and node numbers specified in Linux Allocated Devices, otherwise known as the Linux Devices List and available from http://www.kernel.org/doc/Documentation/devices.txt.
type
type can have one of the following three values:
  • a — applies to all devices, both character devices and block devices
  • b — specifies a block device
  • c — specifies a character device
major, minor
major and minor are device node numbers specified by Linux Allocated Devices. The major and minor numbers are separated by a colon. For example, 8 is the major number that specifies SCSI disk drives, and the minor number 1 specifies the first partition on the first SCSI disk drive; therefore 8:1 fully specifies this partition, corresponding to a file system location of /dev/sda1.
* can stand for all major or all minor device nodes, for example 9:* (all RAID devices) or *:* (all devices).
access
access is a sequence of one or more of the following letters:
  • r — allows tasks to read from the specified device
  • w — allows tasks to write to the specified device
  • m — allows tasks to create device files that do not yet exist
For example, when access is specified as r, tasks can only read from the specified device, but when access is specified as rw, tasks can read from and write to the device.
devices.deny
specifies devices that tasks in a control group cannot access. The syntax of entries is identical with devices.allow.
devices.list
reports the devices for which access controls have been set for tasks in this control group.

3.6. freezer

The freezer subsystem suspends or resumes tasks in a control group.
freezer.state
freezer.state has three possible values:
  • FROZEN — tasks in the control group are suspended.
  • FREEZING — the system is in the process of suspending tasks in the control group.
  • THAWED — tasks in the control group have resumed.
Note that while the FROZEN and THAWED values can be written to freezer.state, FREEZING cannot be written, only read.

3.7. memory

The memory subsystem generates automatic reports on memory resources used by the tasks in a control group, and sets limits on memory use by those tasks:
memory.stat
reports a wide range of memory statistics, as described in the following table:
Table 3.1. Values reported by memory.stat
Statistic Description
cache page cache, including tmpfs (shmem), in bytes
rss anonymous and swap cache, not including tmpfs (shmem), in bytes
mapped_file size of memory-mapped mapped files, including tmpfs (shmem), in bytes
pgpgin number of pages paged into memory
pgpgout number of pages paged out of memory
swap swap usage, in bytes
active_anon anonymous and swap cache on active least-recently-used (LRU) list, including tmpfs (shmem), in bytes
inactive_anon anonymous and swap cache on inactive LRU list, including tmpfs (shmem), in bytes
active_file file-backed memory on active LRU list, in bytes
inactive_file file-backed memory on inactive LRU list, in bytes
unevictable memory that cannot be reclaimed, in bytes
hierarchical_memory_limit memory limit for the hierarchy that contains the memory cgroup, in bytes
hierarchical_memsw_limit memory plus swap limit for the hierarchy that contains the memory cgroup, in bytes

Additionally, each of these files other than hierarchical_memory_limit and hierarchical_memsw_limit has a counterpart prefixed total_ that reports not only on the control group, but on all its children as well. For example, swap reports the swap usage by a control group and total_swap reports the total swap usage by the control group and all its child groups.
When you interpret the values reported by memory.stat, note how the various statistics inter-relate:
  • active_anon + inactive_anon = anonymous memory + file cache for tmpfs + swap cache
    Therefore, active_anon + inactive_anon ≠ rss, because rss does not include tmpfs.
  • active_file + inactive_file = cache - size of tmpfs
memory.usage_in_bytes
reports the total current memory usage by processes in the control group (in bytes).
memory.memsw.usage_in_bytes
reports the sum of current memory usage plus swap space used by processes in the control group (in bytes).
memory.max_usage_in_bytes
reports the maximum memory used by processes in the control group (in bytes).
memory.memsw.max_usage_in_bytes
reports the maximum amount of memory and swap space used by processes in the control group (in bytes).
memory.limit_in_bytes
sets the maximum amount of user memory (including file cache). If no units are specified, the value is interpreted as bytes. However, it is possible to use suffixes to represent larger units — k or K for kilobytes, m or M for Megabytes, and g or G for Gigabytes.
You cannot use memory.limit_in_bytes to limit the root control group; you can only apply values to groups lower in the hierarchy.
Write -1 to memory.limit_in_bytes to remove any existing limits.
memory.memsw.limit_in_bytes
sets the maximum amount for the sum of memory and swap usage. If no units are specified, the value is interpreted as bytes. However, it is possible to use suffixes to represent larger units — k or K for kilobytes, m or M for Megabytes, and g or G for Gigabytes.
You cannot use memory.memsw.limit_in_bytes to limit the root control group; you can only apply values to groups lower in the hierarchy.
Write -1 to memory.memsw.limit_in_bytes to remove any existing limits.
memory.failcnt
reports the number of times that the memory limit has reached the value set in memory.limit_in_bytes.
memory.memsw.failcnt
reports the number of times that the memory plus swap space limit has reached the value set in memory.memsw.limit_in_bytes.
memory.force_empty
when set to 0, empties memory of all pages used by tasks in this control group. This interface can only be used when the control group has no tasks. If memory cannot be freed, it is moved to a parent control group if possible. Use memory.force_empty before removing a control group to avoid moving out-of-use page caches to its parent control group.
memory.swappiness
sets the tendency of the kernel to swap out process memory used by tasks in this control group instead of reclaiming pages from the page cache. This is the same tendency, calculated the same way, as set in /proc/sys/vm/swappiness for the system as a whole. The default value is 60. Values lower than 60 decrease the kernel's tendency to swap out process memory, values greater than 60 increase the kernel's tendency to swap out process memory, and values greater than 100 permit the kernel to swap out pages that are part of the address space of the processes in this control group.
Note that a value of 0 does not prevent process memory being swapped out; swap out might still happen when there is a shortage of system memory because the global virtual memory management logic does not read the cgroup value. To lock pages completely, use mlock() instead of cgroups.
You cannot change the swappiness of the following groups:
  • the root control group, which uses the swappiness set in /proc/sys/vm/swappiness.
  • a control group that has child groups below it.
memory.use_hierarchy
contains a flag (0 or 1) that specifies whether memory usage should be accounted for throughout a hierarchy of control groups. If enabled (1), the memory controller reclaims memory from the children of and process that exceeds its memory limit. By default (0), the controller does not reclaim memory from a task's children.

3.8. net_cls

The net_cls subsystem tags network packets with a class identifier (classid) that allows the Linux traffic controller (tc) to identify packets originating from a particular control group. The traffic controller can be configured to assign different priorities to packets from different control groups.
net_cls.classid
net_cls.classid contains a single value in hexadecimal format that indicates a traffic control handle. For example, 0x100001 represents the handle conventionally written as 10:1 in the format used by iproute2.
The format for these handles is: 0xAAAABBBB, where AAAA is the major number in hexadecimal and BBBB is the minor number in hexadecimal. You can omit any leading zeroes; 0x10001 is the same as 0x00010001, and represents 1:1.
Refer to the man page for tc to learn how to configure the traffic controller to use the handles that the net_cls adds to network packets.

3.9. ns

The ns subsystem provides a way to group processes into separate namespaces. Within a particular namespace, processes can interact with each other but are isolated from processes running in other namespaces. These separate namespaces are sometimes referred to as containers when used for operating-system-level virtualization.

3.10. Additional Resources

Subsystem-Specific Kernel Documentation
All of the following files are located under the /usr/share/doc/kernel-doc-<kernel_version>/Documentation/cgroups/ directory.
  • blkio subsystem — blkio-controller.txt
  • cpuacct subsystem — cpuacct.txt
  • cpuset subsystem — cpusets.txt
  • devices subsystem — devices.txt
  • freezer subsystem — freezer-subsystem.txt
  • memory subsystem — memory.txt

Revision History

Revision History
Revision 1.0-3Wed Nov 17 2010Rüdiger Landmann
Correct remount example — BZ#612805
Revision 1.0-2Thu Nov 11 2010Rüdiger Landmann
Remove pre-release feedback instructions
Revision 1.0-1Wed Nov 10 2010Rüdiger Landmann
Corrections from QE — BZ#581702 and BZ#612805
Revision 1.0-0Tue Nov 9 2010Rüdiger Landmann
Feature-complete version for GA