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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 john@example.com.
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:
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)
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 among 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 moresubsystems. A subsystem[2] represents a single resource, such as CPU time or memory. Red Hat Enterprise Linux 6 provides nine cgroup 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 cgroup 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
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 which has any other subsystems attached to it already (see Rule 2).
Attaching one or more subsystems to a single hierarchy.
Figure 1.1. Rule 1
Rule 2
Any single subsystem (such as cpu) cannot be attached to more than one hierarchy if one of those hierarchies has a different subsystem attached to it already.
As a consequence, the cpu subsystem can never be attached to two different hierarchies if one of those hierarchies already has the memory subsystem attached to it. However, a single subsystem can be attached to two hierarchies if both of those hierarchies have only that subsystem attached.
Attaching a single subsystem to multiple hierarchies.
Figure 1.2. Rule 2—The numbered bullets represent a time sequence in which the subsystems are attached.
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 becomes 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_mem_cg, 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_mem_cg that the httpd 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 cgroups, therefore, every system task is always in at least one cgroup.
A task cannot be a member of two different cgroups in the same hierarchy.
Figure 1.3. Rule 3
Rule 4
Any process (task) on the system which forks itself creates a child process (task). A child task automatically inherits the cgroup membership of its parent but can be moved to different cgroups as needed. Once forked, the parent and child processes are completely independent.
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.
A forked task inherits cgroup properties of its parent.
Figure 1.4. Rule 4—The numbered bullets represent a time sequence in which the task forks.
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 cgroup 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.
The easiest way to work with cgroups 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.
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 cgroup configuration file — /etc/cgconfig.conf. Cgroups 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 cgroups, and set subsystem parameters for each group.
The default /etc/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 the service cgconfig stop command), it unmounts all the hierarchies that it mounted.
2.1.1. The /etc/cgconfig.conf File
The /etc/cgconfig.conf file contains two major types of entry — mount and group. Mount entries create and mount hierarchies as virtual file systems, and attach subsystems to those hierarchies. Mount entries are defined using the following syntax:
The following example creates a cgroup for SQL daemons, with permissions for users in the sqladmin group to add tasks to the cgroup and the root user to modify subsystem parameters:
Restart the cgconfig service for the changes to take effect
You must restart the cgconfig service for the changes in the /etc/cgconfig.conf to take effect:
~]# service cgconfig restart
When you install the libcgroup package, a sample configuration file is written to /etc/cgconfig.conf. The hash 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
Effects on running systems
The following instructions, which cover creating a new hierarchy and attaching subsystems to it, assume that cgroups 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 cgroups 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 cgroup 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 subsystemsname /cgroup/name
Example 2.3. 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:
the cpu, cpuset and memory subsystems are attached to a hierarchy mounted on /cgroup/cpu_and_mem, and
the net_cls, ns, 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 2.4. Remounting a hierarchy to add a subsystem
The lssubsys command shows cpu, cpuset, and memory subsystems attached to the cpu_and_mem hierarchy:
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 2.12, “Unloading Control Groups”.
2.5. Creating Control Groups
Use the cgcreate command to create cgroups. The syntax for cgcreate is:
cgcreate -tuid:gid-auid: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 pseudo-file for this cgroup. This user can add tasks to the cgroup.
Removing tasks
Note that the only way to remove a task from a cgroup is to move it to a different cgroup. To move a task, the user must have write access to the destination cgroup; write access to the source cgroup is unimportant.
-a (optional) — specifies a user (by user ID, uid) and a group (by group ID, gid) to own all pseudo-files other than tasks for this cgroup. This user can modify the access that the tasks in this cgroup 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 cgroup 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 2.5, “cgcreate usage”.
Because all cgroups in the same hierarchy have the same controllers, the child group has the same controllers as its parent.
Example 2.5. 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 cgroup directly, use the mkdir command:
~]# mkdir /cgroup/hierarchy/name/child_name
For example:
~]# mkdir /cgroup/cpuset/lab1/group1
2.6. Removing Control Groups
Remove cgroups with the cgdelete, which has a syntax similar to that of cgcreate. Run the following command:
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 cgroup, 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 cgroup. For example, if /cgroup/cpuset/group1 exists, specify the CPUs to which this group has access with the following command:
cpuset]# cgset -r cpuset.cpus=0-1 group1
The syntax for cgset is:
cgset -rparameter=valuepath_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 cgroup relative to the root of the hierarchy. For example, to set the parameter of the root group (if /cgroup/cpuacct/ exists), run:
cpuacct]# cgset -r cpuacct.usage=0 /
Alternatively, because . is relative to the root group (that is, the root group itself) you could also run:
cpuacct]# cgset -r cpuacct.usage=0 .
Note, however, that / is the preferred syntax.
Setting parameters for the root group
Only a small number of parameters can be set for the root group (such as the cpuacct.usage parameter shown in the examples above). This is because a root group owns all of the existing resources, therefore, it would make no sense to limit all existing processes by defining certain parameters, for example the cpuset.cpu parameter.
To set the parameter of group1, which is a subgroup of the root group, run:
cpuacct]# cgset -r cpuacct.usage=0 group1
A trailing slash on the name of the group (for example, cpuacct.usage=0 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:
path_to_source_cgroup is the path to the cgroup whose parameters are to be copied, relative to the root group of the hierarchy
path_to_target_cgroup is the path to the destination cgroup, 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. For more information on mandatory parameters, refer to Mandatory parameters.
Alternative method
To set parameters in a cgroup directly, insert values into the relevant subsystem pseudo-file using the echo command. For example, this command inserts the value 0-1 into the cpuset.cpus pseudo-file of the cgroup group1:
~]# echo 0-1 > /cgroup/cpuset/group1/cpuset.cpus
With this value in place, the tasks in this cgroup are restricted to CPUs 0 and 1 on the system.
2.8. Moving a Process to a Control Group
Move a process into a cgroup by running the cgclassify command:
~]# cgclassify -g cpu,memory:group1 1701
The syntax for cgclassify is:
cgclassify -gsubsystems:path_to_cgrouppidlist
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 cgroups 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 cgroup 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 cgroup. If you do not set this option and the cgred daemon is running, child processes will be allocated to cgroups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the cgroup 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 cgroup 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 cgroup directly, write its PID to the tasks file of the cgroup. For example, to move a process with the PID 1701 into a cgroup at /cgroup/lab1/group1/:
~]# echo 1701 > /cgroup/lab1/group1/tasks
2.8.1. The cgred Daemon
Cgred is a daemon that moves tasks into cgroups according to parameters set in the /etc/cgrules.conf file. Entries in the /etc/cgrules.conf file can take one of the two forms:
userhierarchiescontrol_group
user:commandhierarchiescontrol_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 cgroup. To associate particular commands with particular cgroups, 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 cgroup in the hierarchy that contains the devices subsystem. Note, however, that the daemon moves the process to the cgroup 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
Mandatory parameters
Some subsystems have mandatory parameters that must be set before you can move a task into a cgroup which uses any of those subsystems. For example, before you move a task into a cgroup which uses the cpuset subsystem, the cpuset.cpus and cpuset.mems parameters must be defined for that cgroup.
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.
Launch processes in a cgroup by running the cgexec command. For example, this command launches the lynx web browser within the group1 cgroup, subject to the limitations imposed on that group by the cpu subsystem:
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 cgroups 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 cgroup relative to the hierarchy.
command is the command to run.
arguments are any arguments for the command.
You can also add the --sticky option before the command to keep any child processes in the same cgroup. If you do not set this option and the cgred daemon is running, child processes will be allocated to cgroups based on the settings found in /etc/cgrules.conf. The process itself, however, will remain in the cgroup 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 cgroup 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/taskslynx
Note that after exiting lynx, your existing shell is still in the group1 cgroup. 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 certain services in a cgroup. Services that can be started in cgroups 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 cgroup, 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 cgroup in that hierarchy. For example:
CGROUP_DAEMON="cpuset:daemons/sql"
2.9.2. Process Behavior in the Root Control Group
Certain blkio and cpu configuration options affect processes (tasks) running in the root cgroup in a different way than those in a subgroup. Consider the following example:
Create two subgroups under one root group: /rootgroup/red/ and /rootgroup/blue/
In each subgroup and in the root group, define the cpu.shares configuration option and set it to 1.
In the scenario configured above, one process placed in each group (that is, one task in /rootgroup/tasks, /rootgroup/red/tasks and /rootgroup/blue/tasks) ends up consuming 33.33% of the CPU:
Any other processes placed in subgroups blue and red result in the 33.33% percent of the CPU assigned to that specific subgroup to be split among the multiple processes in that subgroup.
However, multiple processes placed in the root group cause the CPU resource to be split per process, rather than per group. For example, if /rootgroup/ contains three processes, /rootgroup/red/ contains one process and /rootgroup/blue/ contains one process, and the cpu.shares option is set to 1 in all groups, the CPU resource is divided as follows:
Therefore, it is advisable to move all processes from the root group to a specific subgroup when using the blkio and cpu configuration options which divide an available resource based on a weight or a share (for example, cpu.shares or blkio.weight). To move all tasks from the root group into a specific subgroup, you can use the following command:
rootgroup]# for i in `cat tasks`; do echo $i > red/tasks; done
2.10. Generating the /etc/cgconfig.conf File
Configuration for the /etc/cgconfig.conf file can be generated from the current cgroup configuration using the cgsnapshot utility. This utility takes a snapshot of the current state of all subsystems and their cgroups and returns their configuration as it would appear in the /etc/cgconfig.conf file. Example 2.6, “Using the cgsnapshot utility” shows an example usage of the cgsnapshot utility.
Example 2.6. Using the cgsnapshot utility
Assume we configured cgroups on our system using the following commands:
The above commands mounted two subsystems and created two cgroups, for the cpu subsystem, with specific values for some of their parameters. Executing the cgsnapshot command (with the -s option and an empty /etc/cgsnapshot_blacklist.conf file[4]) then produces the following output:
~]$ cgsnapshot -s
# Configuration file generated by cgsnapshot
mount {
cpu = /cgroup/cpu;
cpuacct = /cgroup/cpuacct;
}
group lab2 {
cpu {
cpu.rt_period_us="1000000";
cpu.rt_runtime_us="0";
cpu.shares="3";
}
}
group lab1 {
cpu {
cpu.rt_period_us="5000000";
cpu.rt_runtime_us="4000000";
cpu.shares="2";
}
}
The -s option used in the example above tells cgsnapshot to ignore all warnings in the output file caused by parameters not being defined in the blacklist or whitelist of the cgsnapshot utility. For more information on parameter blacklisting, refer to Section 2.10.1, “Blacklisting Parameters”. For more information on parameter whitelisting, refer to Section 2.10.2, “Whitelisting Parameters”.
When not specifying any options, the output generated by cgsnapshot is returned on the standard output. Use the -f to specify a file to which the output should be redirected. For example:
~]$ cgsnapshot -f ~/test/cgconfig_test.conf
The -f option overwrites the specified file
When using the -f option, note that it overwrites any content in the file you specify. Therefore, it is advisable not to direct the output straight to the /etc/cgconfig.conf file.
The cgsnapshot utility can also create configuration files per subsystem. By specifying the name of a subsystem, the output will consist of the corresponding configuration for that subsystem:
~]$ cgsnapshot cpuacct
# Configuration file generated by cgsnapshot
mount {
cpuacct = /cgroup/cpuacct;
}
2.10.1. Blacklisting Parameters
The cgsnapshot utility allows parameter blacklisting. If a parameter is blacklisted, it does not appear in the output generated by cgsnapshot. By default, the /etc/cgsnapshot_blacklist.conf file is checked for blacklisted parameters. If a parameter is not present in the blacklist, the whitelist is checked. To specify a different blacklist, use the -b option. For example:
~]$ cgsnapshot -b ~/test/my_blacklist.conf
2.10.2. Whitelisting Parameters
The cgsnapshot utility also allows parameter whitelisting. If a parameter is whitelisted, it appears in the output generated by cgsnapshot. If a parameter is neither blacklisted or whitelisted, a warning appears informing of this:
~]$ cgsnapshot -f ~/test/cgconfig_test.conf
WARNING: variable cpu.rt_period_us is neither blacklisted nor whitelisted
WARNING: variable cpu.rt_runtime_us is neither blacklisted nor whitelisted
By default, there is no whitelist configuration file. To specify which file to use as a whitelist, use the -w option. For example:
~]$ cgsnapshot -w ~/test/my_whitelist.conf
Specifying the -t option tells cgsnapshot to generate a configuration with parameters from the whitelist only.
2.11. Obtaining Information About Control Groups
2.11.1. Finding a Process
To find the cgroup to which a process belongs, run:
~]$ ps -O cgroup
Or, if you know the PID for the process, run:
~]$ cat /proc/PID/cgroup
2.11.2. Finding a Subsystem
To find the subsystems that are available 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. Note that the lssubsys -m command returns only the top-level mount point per each hierarchy.
2.11.3. 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 cgroups within them:
~]$ tree /cgroup/
2.11.4. Finding Control Groups
To list the cgroups 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 cgroup in the hierarchy to which the cpuset subsystem is attached.
2.11.5. Displaying Parameters of Control Groups
To display the parameters of specific cgroups, run:
~]$ cgget -r parameterlist_of_cgroups
where parameter is a pseudo-file that contains values for a subsystem, and list_of_cgroups is a list of cgroups separated with spaces. For example:
displays the values of cpuset.cpus and memory.limit_in_bytes for cgroups lab1 and lab2.
If you do not know the names of the parameters themselves, use a command like:
~]$ cgget -g cpuset /
2.12. Unloading Control Groups
This command destroys all control groups
The cgclear command destroys all cgroups 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 cgroup file system, use the cgclear command.
All tasks in the cgroup are reallocated to the root node of the hierarchies, all cgroups are removed, and the file system itself is unmounted from the system, thus destroying all previously mounted hierarchies. Finally, the directory where the cgroup file system was mounted is actually deleted.
Accurate listing of all mounted cgroups
Using the mount command to create cgroups (as opposed to creating them using the cgconfig service) results in the creation of an entry in the /etc/mtab file (the mounted file systems table). This change is also reflected into the /proc/mounts file. However, the unloading of cgroups with the cgclear command, along with other cgconfig commands, uses a direct kernel interface which does not reflect its changes into the /etc/mtab file and only writes the new information into the /proc/mounts file. Thus, after unloading cgroups with the cgclear command, the unmounted cgroups may still be visible in the /etc/mtab file, and, consequently, displayed when the mount command is executed. It is advisable to refer to the /proc/mounts file for an accurate listing of all mounted cgroups.
2.13. Additional Resources
The definitive documentation for cgroup 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 1 cgsnapshot — the cgsnapshot command generates a configuration file from existing subsystems.
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 utilities 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.
[4]
The cpu.shares parameter is specified in the /etc/cgsnapshot_blacklist.conf file by default, which would cause it to be omitted in the generated output in Example 2.6, “Using the cgsnapshot utility”. Thus, for the purposes of the example, an empty /etc/cgsnapshot_blacklist.conf file is used.
Subsystems are kernel modules that are aware of cgroups. Typically, they are resource controllers that allocate varying levels of system resources to different cgroups. 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/ (provided by the kernel-doc package). 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 cgroup are represented as pseudofiles within the cgroup virtual file system. These pseudo-files can be manipulated by shell commands or their equivalent system calls. For example, cpuset.cpus is a pseudo-file that specifies which CPUs a cgroup is permitted to access. If /cgroup/cpuset/webserver is a cgroup for the web server that runs on a system, and we run the following command:
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 Block I/O (blkio) subsystem controls and monitors access to I/O on block devices by tasks in cgroups. 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.
The blkio subsystem offers two policies for controlling access to I/O:
Proportional weight division — implemented in the Completely Fair Queuing I/O scheduler, this policy allows you to set weights to specific cgroups. This means that each cgroup has a set percentage (depending on the weight of the cgroup) of all I/O operations reserved. For more information, refer to Section 3.1.1, “Proportional Weight Division Configuration Options”
I/O throttling (Upper limit) — used to set an upper limit for the number of I/O operations performed by a specific device. This means that a device can have a limited rate of read or write operations. For more information, refer to Section 3.1.2, “I/O Throttling Configuration Options”
Buffered write operations
Currently, the Block I/O subsystem does not work for buffered write operations. It is primarily targeted at direct I/O, although it works for buffered read operations.
specifies the relative proportion (weight) of block I/O access available by default to a cgroup, in the range 100 to 1000. This value is overridden for specific devices by the blkio.weight_device parameter. For example, to assign a default weight of 500 to a cgroup 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 cgroup, in the range 100 to 1000. The value of this parameter overrides the value of the blkio.weight parameter for the devices specified. Values take the format major:minorweight, 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 cgroup 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 cgroup 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 cgroup. 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_serviced
reports the number of I/O operations performed on specific devices by a cgroup as seen by the CFQ scheduler. Entries have four fields: major, minor, operation, and number. 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_bytes
reports the number of bytes transferred to or from specific devices by a cgroup as seen by the CFQ scheduler. 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_service_time
reports the total time between request dispatch and request completion for I/O operations on specific devices by a cgroup as seen by the CFQ scheduler. Entries have four fields: major, minor, operation, and time. 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 cgroup spent waiting for service in the scheduler queues. When you interpret this report, note:
the time reported can be greater than the total time elapsed, because the time reported is the cumulative total of all I/O operations for the cgroup rather than the time that the cgroup itself spent waiting for I/O operations. To find the time that the group as a whole has spent waiting, use the blkio.group_wait_time parameter.
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 time. 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 cgroup. 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 cgroup. 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 cgroup, over the entire length of time of the group's existence. The queue size is sampled every time a queue for this cgroup 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 cgroup spent waiting for a timeslice for one of its queues. The report is updated every time a queue for this cgroup gets a timeslice, so if you read this pseudofile while the cgroup 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 cgroup spent without any pending requests. The report is updated every time a queue for this cgroup has a pending request, so if you read this pseudofile while the cgroup 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 cgroup 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 cgroup 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 cgroup 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.
3.1.2. I/O Throttling Configuration Options
blkio.throttle.read_bps_device
specifies the upper limit on the number of read operations a device can perform. The rate of the read operations is specified in bytes per second. Entries have three fields: major, minor, and bytes_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and bytes_per_second is the upper limit rate at which read operations can be performed. For example, to allow the /dev/sda device to perform read operations at a maximum of 10 MBps, run:
specifies the upper limit on the number of read operations a device can perform. The rate of the read operations is specified in operations per second. Entries have three fields: major, minor, and operations_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and operations_per_second is the upper limit rate at which read operations can be performed. For example, to allow the /dev/sda device to perform a maximum of 10 read operations per second, run:
specifies the upper limit on the number of write operations a device can perform. The rate of the write operations is specified in bytes per second. Entries have three fields: major, minor, and bytes_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and bytes_per_second is the upper limit rate at which write operations can be performed. For example, to allow the /dev/sda device to perform write operations at a maximum of 10 MBps, run:
specifies the upper limit on the number of write operations a device can perform. The rate of the write operations is specified in operations per second. Entries have three fields: major, minor, and operations_per_second. Major and minor are device types and node numbers specified in Linux Allocated Devices, and operations_per_second is the upper limit rate at which read operations can be performed. For example, to allow the /dev/sda device to perform a maximum of 10 write operations per second, run:
reports the number of I/O operations performed on specific devices by a cgroup as seen by the throttling policy. Entries have four fields: major, minor, operation, and number. 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.throttle.io_service_bytes
reports the number of bytes transferred to or from specific devices by a cgroup. The only difference between blkio.io_service_bytes and blkio.throttle.io_service_bytes is that the former is not updated when the CFQ scheduler is operating on a request queue. 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.
3.1.3. Common Configuration Option
The following configuration option may be used for either of the policies listed in Section 3.1, “blkio”.
blkio.reset_stats
resets the statistics recorded in the other pseudofiles. Write an integer to this file to reset the statistics for this cgroup.
The above commands create two files (file_1 and file_2) of size 4 GB.
For each of the test cgroups, execute a dd command (which reads the contents of a file and outputs it to the null device) on one of the large files:
~]# cgexec -g blkio:test1 time dd if=file_1 of=/dev/null
~]# cgexec -g blkio:test2 time dd if=file_2 of=/dev/null
Both commands will output their completion time once they have finished.
Simultaneously with the two running dd threads, you can monitor the performance in real time by using the iotop utility. To install the iotop utility, execute, as root, the yum install iotop command. The following is an example of the output as seen in the iotop utility while running the previously-started dd threads:
Total DISK READ: 83.16 M/s | Total DISK WRITE: 0.00 B/s
TIME TID PRIO USER DISK READ DISK WRITE SWAPIN IO COMMAND
15:18:04 15071 be/4 root 27.64 M/s 0.00 B/s 0.00 % 92.30 % dd if=file_2 of=/dev/null
15:18:04 15069 be/4 root 55.52 M/s 0.00 B/s 0.00 % 88.48 % dd if=file_1 of=/dev/null
In order to get the most accurate result in Example 3.1, “blkio proportional weight division”, prior to the execution of the dd commands, flush all file system buffers and free pagecache, dentries and inodes using the following commands:
~]# sync
~]# echo 3 > /proc/sys/vm/drop_caches
Additionally, you can enable group isolation which provides stronger isolation between groups at the expense of throughput. When group isolation is disabled, fairness can be expected only for a sequential workload. By default, group isolation is enabled and fairness can be expected for random I/O workloads as well. To enable group isolation, use the following command:
where <disk_device> stands for the name of the desired device, for example sda.
3.2. cpu
The cpu subsystem schedules CPU access to cgroups. Access to CPU resources can be scheduled according to the following parameters, each one in a separate pseudofile within the cgroup virtual file system:
cpu.shares
contains an integer value that specifies a relative share of CPU time available to the tasks in a cgroup. For example, tasks in two cgroups that have cpu.shares set to 1 will receive equal CPU time, but tasks in a cgroup that has cpu.shares set to 2 receive twice the CPU time of tasks in a cgroup where cpu.shares is set to 1.
cpu.rt_runtime_us
applicable to realtime scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "us") for the longest continuous period in which the tasks in a cgroup have access to CPU resources. Establishing this limit prevents tasks in one cgroup from monopolizing CPU time. If the tasks in a cgroup 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
applicable to realtime scheduling tasks only, this parameter specifies a period of time in microseconds (µs, represented here as "us") for how regularly a cgroup's access to CPU resource should be reallocated. If the tasks in a cgroup 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 CPU Accounting (cpuacct) subsystem generates automatic reports on CPU resources used by the tasks in a cgroup, including tasks in child groups. Three reports are available:
cpuacct.usage
reports the total CPU time (in nanoseconds) consumed by all tasks in this cgroup (including tasks lower in the hierarchy).
Resetting cpuacct.usage
To reset the value in cpuacct.usage, execute the following command:
~]# echo 0 > /cgroups/cpuacct/cpuacct.usage
The above command also resets values in cpuacct.usage_percpu.
cpuacct.stat
reports the user and system CPU time consumed by all tasks in this cgroup (including tasks lower in the hierarchy) in the following way:
user — CPU time consumed by tasks in user mode.
system — CPU time consumed by tasks in system (kernel) mode.
CPU time is reported in the units defined by the USER_HZ variable.
cpuacct.usage_percpu
reports the CPU time (in nanoseconds) consumed on each CPU by all tasks in this cgroup (including tasks lower in the hierarchy).
3.4. cpuset
The cpuset subsystem assigns individual CPUs and memory nodes to cgroups. Each cpuset can be specified according to the following parameters, each one in a separate pseudofile within the cgroup virtual file system:
Mandatory parameters
Some subsystems have mandatory parameters that must be set before you can move a task into a cgroup which uses any of those subsystems. For example, before you move a task into a cgroup which uses the cpuset subsystem, the cpuset.cpus and cpuset.mems parameters must be defined for that cgroup.
cpuset.cpus (mandatory)
specifies the CPUs that tasks in this cgroup 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 cgroup 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 the cpuset.mem_hardwall parameter.
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' 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 cgroup. 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 cgroup has no effect if load balancing is enabled in any parent cgroup, as load balancing is already being carried out at a higher level. Therefore, to disable load balancing in a cgroup, 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 cgroup 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
Value
Effect
-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 cgroup.
Technology preview
The Device Whitelist (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 cgroup 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 cgroup 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 cgroup.
3.6. freezer
The freezer subsystem suspends or resumes tasks in a cgroup.
freezer.state
freezer.state has three possible values:
FROZEN — tasks in the cgroup are suspended.
FREEZING — the system is in the process of suspending tasks in the cgroup.
THAWED — tasks in the cgroup have resumed.
To suspend a specific process:
Move that process to a cgroup in a hierarchy which has the freezer subsystem attached to it.
Freeze that particular cgroup to suspend the process contained in it.
It is not possible to move a process into a suspended (frozen) cgroup.
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 cgroup, 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 cgroup, but on all its children as well. For example, swap reports the swap usage by a cgroup and total_swap reports the total swap usage by the cgroup and all its child groups.
When you interpret the values reported by memory.stat, note how the various statistics inter-relate:
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 cgroup (in bytes).
memory.memsw.usage_in_bytes
reports the sum of current memory usage plus swap space used by processes in the cgroup (in bytes).
memory.max_usage_in_bytes
reports the maximum memory used by processes in the cgroup (in bytes).
memory.memsw.max_usage_in_bytes
reports the maximum amount of memory and swap space used by processes in the cgroup (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 cgroup; 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 cgroup; you can only apply values to groups lower in the hierarchy.
Write -1 to memory.memsw.limit_in_bytes to remove any existing limits.
Setting the memory.memsw.limit_in_bytes and memory.limit_in_bytes parameters
It is important to set the memory.limit_in_bytes parameter before setting the memory.memsw.limit_in_bytes parameter; attempting to do so in the reverse order results in an error. This is because memory.memsw.limit_in_bytes becomes available only after all memory limitations (previously set in memory.limit_in_bytes) are exhausted.
Consider the following example: setting memory.limit_in_bytes = 2G and memory.memsw.limit_in_bytes = 4G for a certain cgroup will allow processes in that cgroup to allocate 2 GB of memory and, once exhausted, allocate another 2 GB of swap only; the memory.memsw.limit_in_bytes parameter represents a sum of memory and swap. Processes in a cgroup that does not have the memory.memsw.limit_in_bytes parameter set can potentially use up all the available swap (after exhausting the set memory limitation) and trigger an Out Of Memory situation caused by the lack of available swap.
The order in which the memory.limit_in_bytes and memory.memsw.limit_in_bytes parameters are set in the /etc/cgconfig.conf file is important as well. The following is a correct example of such a configuration:
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 cgroup. This interface can only be used when the cgroup has no tasks. If memory cannot be freed, it is moved to a parent cgroup if possible. Use the memory.force_empty parameter before removing a cgroup to avoid moving out-of-use page caches to its parent cgroup.
memory.swappiness
sets the tendency of the kernel to swap out process memory used by tasks in this cgroup 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 cgroup.
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 cgroup, which uses the swappiness set in /proc/sys/vm/swappiness.
a cgroup 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 cgroups. If enabled (1), the memory subsystem reclaims memory from the children of and process that exceeds its memory limit. By default (0), the subsystem 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 cgroup. The traffic controller can be configured to assign different priorities to packets from different cgroups.
net_cls.classid
net_cls.classid contains a single value that indicates a traffic control handle. The value of classid read from the net_cls.classid file is presented in the decimal format while the value to be written to the file is expected in the hexadecimal format. For example, 0x100001 represents the handle conventionally written as 10:1 in the format used by iproute2. In the net_cls.classid file, it would be represented by the number 1048577.
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. The following is an example of setting a 10:1 handle in the net_cls.classid file:
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 (provided by the kernel-doc package).
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 1.0-6
Tue Dec 6 2011
MartinPrpič
Red Hat Enterprise Linux 6.2 GA release of the Resource Management Guide.
Revision 1.0-5
Thu May 19 2011
MartinPrpič
Red Hat Enterprise Linux 6.1 GA release of the Resource Management Guide.
