Features/ControlGroups

Summary

Control Groups consists of two parts:

1. an upstream kernel feature that allows system resources to be partitioned/divided up amongst different processes, or a group of processes.
2. user-space tools which handle kernel control groups mechanism. We want to improve them where necessary and feasible and/or to create new ones e.g. to create or modify cgroups configuration or display control groups data (using libcgroups package).

Owners

• Linda Wang
  • email: lwang@redhat.com
• Nils Philippsen
  • email: nphilipp@redhat.com
• Ivana Varekova
  • email: varekova@redhat.com
• Jan Šafránek
  • email: jsafrane@redhat.com

Current status

• Targeted release: Fedora 11
• Last updated: 2009-03-02
• Percentage of completion: 90%
  • kernel part: done
  • tools part: missing possibility to start services in control groups

Detailed Description

Kernel Part

Control Groups provide a mechanism for aggregating/partitioning sets of tasks, and all their future children, into hierarchical groups with specialized behaviour.

Definitions:

A *cgroup* associates a set of tasks with a set of parameters for one or more subsystems.

A *subsystem* is a module that makes use of the task grouping facilities provided by cgroups to treat groups of tasks in particular ways. A subsystem is typically a "resource controller" that schedules a resource or applies per-cgroup limits, but it may be anything that wants to act on a group of processes, e.g. a virtualization subsystem.

A *hierarchy* is a set of cgroups arranged in a tree, such that every task in the system is in exactly one of the cgroups in the hierarchy, and a set of subsystems; each subsystem has system-specific state attached to each cgroup in the hierarchy. Each hierarchy has an instance of the cgroup virtual filesystem associated with it.

At any one time there may be multiple active hierachies of task cgroups. Each hierarchy is a partition of all tasks in the system.

User level code may create and destroy cgroups by name in an instance of the cgroup virtual file system, specify and query to which cgroup a task is assigned, and list the task pids assigned to a cgroup. Those creations and assignments only affect the hierarchy associated with that instance of the cgroup file system.

On their own, the only use for cgroups is for simple job tracking. The intention is that other subsystems hook into the generic cgroup support to provide new attributes for cgroups, such as accounting/limiting the resources which processes in a cgroup can access. For example, cpusets (see Documentation/cpusets.txt) allows you to associate a set of CPUs and a set of memory nodes with the tasks in each cgroup.

User space tools

Libcgroups makes that functionality available to programmers and contains two tools, cgexec and cgclassify, to start processes in a control group or move existing processes from one control group to another. In Fedora libcgroups package is already incorporated, but the overall quality is very poor. There is almost no documentation, no man pages, no configuration file samples, there should be done code review and created other necessary tools and improve installations:

The goal for Fedora 11 is to improve this package where necessary, i.e.:

• bugfixing
• add/fix documentation and man-pages
• add examples
• fix error handling
• rework logging
• create displaying tool (to see, in which control group is given process)
• prepare a way, how to start a service daemon in given context group

The long term goal is to create new tools to e.g. create or modify persistent cgroups configuration and display control groups data. At the beginning the focus will be on command line tools, but we'll keep in mind that in the long term we'll likely want to have graphical tools. These would offer similar functionality and we should try to make sure that any non-UI code written is usable from both kinds of frontends.

Benefit to Fedora

The implementation of of "control groups" schema and its improvement should enable users to partitioned/divided resources up amongst different processes, or a group of processes. Libcgroups should helps them to create persistent configuration of partitioning devices and handle cgroups from user point of view. This project should help the user to make the best of control groups kernel feature.

Scope

• Kernel Part:

There are several sub-features under control group:

* CGROUPS (grouping infrastructure mechanism)
* CPUSET (cpuset controller, in F10)
* CPUACCT (cpu account controller, in F10)
* SCHED (schedule controller, in F10)
* MEMCTL (memory controller, in F10)
* DEVICES
* NETCTL (network controller, new in Rawhide/F11)

• tools part:

Required extended testing and fixing of libcgroups package and in time when libcgroups will be stable enough try to add start to write another parts - based on existing ones.

How To Test

To help test, and use the control group features in Fedora; there are multiple way to test, depends on the feature set that you are interested in.

From now to other tests it is necessary to have a kernel with cgroups support and the libcgroup package.

1. yum install libcgroup

User space tools

• Creating cgroups:

1. Configure /etc/cgconfig.conf file - there should be nice example and man page packaged.
2. Start/stop cgconfig service and test whether the created groups are as expected.

• Moving task to groups:

1. Prepare some cgroups, i.e. prepare /etc/cgconfig.conf and start cgconfig service.
2. Start/stop new proces using cgexec and check that it's in appropriate cgroup.
3. Prepare cgrules.conf file - there should be some sample and man page available.
4. Test cgrulesengd daemon (it should automatically move processes as written in cgrules.conf).
5. Configure cgroup pam module and test that works if a user logs in (again, driven by cgrules.conf).

• Looking in which cgroup the task is

ps -o cgroup

Kernel features

Read kernel docs (see below). Each controller should have a documentation there

CPUSET

• Create a group controlled by cpuset controller, e.g. use following cgconfig.conf:

mount {
    cpuset = /mnt/cgroup;
}
group test {
    perm {
        task {
            uid = root;
            gid = root;
        }
        admin {
            uid = root;
            gid = root;
        }
    }

# following section is cpuset specific,
# replace with appropriate content when testing other controllers

# allow only the first cpu and the first memory region
    cpuset {
        cpuset.cpus = 0;
        cpuset.mems = 0;
    }
}

• Start the cgconfig service
• Execute a task in this group

$ cgexec -g cpuset:test /bin/bash

• Check the started bash (and all its children) are in the right group

$ cat /proc/self/cgroup
12:cpuset:/test
$ ps -o cgroup
...

• Check, that all children of the bash can use only first cpu (e.g. compile kernel with -j3 or so).

CPUACCT

• Same as before, use following cgconfig.conf snippet instead of cpuset { }:

cpuacct {
}

• Start a process in the group as before, check, that /mnt/cgroup/test/cpuacct.usage counts CPU cycles of the process and all its future children

Memory Controller

• Use following cgconfig.conf snippet:

memory {
    memory.limit_in_bytes = 40M;
}

• Again, start something in the group. The process there can use 40 megabytes of memory.
• Look at /mnt/cgroup/tests/usage_in_bytes,there should be current memory usage of all processes in the group.

Test other controllers, as described in kernel documentation.

User Experience

End-user who will use this feature will hopefully find it useful to help partition their server/machine resources into different functional units that they can dedicate these resources to.

The control group user interfaces are very straight forward, and are a set of common easy to use command-line operations. The concept of allocating different system resources such as number of CPUs, amount of memories, and network bandwidth should be easy.

libcgroups package should help the user to create persistent configuration and would help to reduce the barrier of entry to using control groups on Linux significantly.

Dependencies

Majority of the implementation is done inside of the kernel. Tools part is implemented in package libcgroups

Contingency Plan

The contingency plan for under develop sub-feature is to simply not enable the kernel option during development freeze. Hence it will not expose the incomplete sub-feature to the fedora community. Currently, nothing depends on libcgroup or the tools which would use it. If things go really wrong, we can always go back to the last working version of libcgroup.

Documentation

• kernel documentation:
  • Documentation/cgroups - control group's directory
  • Documentation/cgroups/cgroups.txt - overall top level description of the feature
  • Documentation/cgroups/cpusets.txt - doc describing CPU/memory nodes to a set of tasks
  • Documentation/cgroups/cpuacct.txt - doc describing CPU acct ctrl to cal. usage of cpu time
  • Documentation/cgroups/devices.txt - doc describing device file
  • Documentation/cgroups/memory.txt
  • Documentation/cgroups/resource_counter.txt

• libcgroup:
  • upstream site
  • LWN.net article: libcg: design and plans
  • documentation from source tarball (directories doc and samples)
  • libcgroup man pages

Release Notes

Fedora 11 includes a new feature called Control Group where it allows system administrator to partition the system resources into different sub groups, and dedicate these sub groups resources to different applications' need. It can be use to dedicate specific applications such as interactive applications; cpu, memory, or network bandwidth intensive application; or database application to a set of pre-allocated system resources.

There is also libcgroups tool which helps to manipulate, control and administrate control groups and the associated controllers. Using this tool it is possible to aggregate/partition set of tasks and their future children into hierarchical groups with specialized access to resources.

Comments and Discussion

• See Talk:Features/ControlGroups