libvirt: Control Groups Resource Management
Description: libvirt, virtualization, virtualization API

# Control Groups Resource Management

- Required controllers
- Current cgroups layout
- Using custom partitions
- Resource management APIs/commands
- Legacy cgroups layout

The QEMU and LXC drivers make use of the Linux "Control Groups" facility for applying resource management to their virtual machines and containers.

## Required controllers

The control groups filesystem supports multiple "controllers". By default the init system (such as systemd) should mount all controllers compiled into the kernel at `/sys/fs/cgroup/$CONTROLLER-NAME`. Libvirt will never attempt to mount any controllers itself, merely detect where they are mounted.

The QEMU driver is capable of using the `cpuset`, `cpu`, `memory`, `blkio` and `devices` controllers. None of them are compulsory. If any controller is not mounted, the resource management APIs which use it will cease to operate. It is possible to explicitly turn off use of a controller, even when mounted, via the `/etc/libvirt/qemu.conf` configuration file.

The LXC driver is capable of using the `cpuset`, `cpu`, `cpuset`, `freezer`, `memory`, `blkio` and `devices` controllers. The `cpuset`, `devices` and `memory` controllers are compulsory. Without them mounted, no containers can be started. If any of the other controllers are not mounted, the resource management APIs which use them will cease to operate.

## Current cgroups layout

As of libvirt 1.0.5 or later, the cgroups layout created by libvirt has been simplified, in order to facilitate the setup of resource control policies by administrators / management applications. The layout is based on the concepts of "partitions" and "consumers". Each virtual machine or container is a consumer, and has a corresponding cgroup named `$VMNAME.libvirt-{qemu,lxc}`. Each consumer is associated with exactly one partition, which also have a corresponding cgroup usually named `$PARTNAME.partition`. The exceptions to this naming rule are the three top level default partitions, named `/system` (for system services), `/user` (for user login sessions) and `/machine` (for virtual machines and containers). By default every consumer will of course be associated with the `/machine` partition. This leads to a hierarchy that looks like

    $ROOT
      |
      +- system
      |   |
      |   +- libvirtd.service
      |
      +- machine
          |
          +- vm1.libvirt-qemu
          |   |
          |   +- emulator
          |   +- vcpu0
          |   +- vcpu1
          |
          +- vm2.libvirt-qemu
          |   |
          |   +- emulator
          |   +- vcpu0
          |   +- vcpu1
          |
          +- vm3.libvirt-qemu
          |   |
          |   +- emulator
          |   +- vcpu0
          |   +- vcpu1
          |
          +- container1.libvirt-lxc
          |
          +- container2.libvirt-lxc
          |
          +- container3.libvirt-lxc

The default cgroups layout ensures that, when there is contention for CPU time, it is shared equally between system services, user sessions and virtual machines / containers. This prevents virtual machines from locking the administrator out of the host, or impacting execution of system services. Conversely, when there is no contention from system services / user sessions, it is possible for virtual machines to fully utilize the host CPUs.

## Using custom partitions

If there is a need to apply resource constraints to groups of virtual machines or containers, then the single default partition `/machine` may not be sufficiently flexible. The administrator may wish to sub-divide the default partition, for example into "testing" and "production" partitions, and then assign each guest to a specific sub-partition. This is achieved via a small element addition to the guest domain XML config, just below the main `domain` element

      ...
      <resource>
        <partition>/machine/production</partition>
      </resource>
      ...

Libvirt will not auto-create the cgroups directory to back this partition. In the future, libvirt / virsh will provide APIs / commands to create custom partitions, but currently this is left as an exercise for the administrator. For example, given the XML config above, the admin would need to create a cgroup named '/machine/production.partition'

    # cd /sys/fs/cgroup
    # for i in blkio cpu,cpuacct cpuset devices freezer memory net_cls perf_event
      do
        mkdir $i/machine/production.partition
      done
    # for i in cpuset.cpus  cpuset.mems
      do
        cat cpuset/machine/$i > cpuset/machine/production.partition/$i
      done

Note: the cgroups directory created as a ".partition" suffix, but the XML config does not require this suffix.

Note: the ability to place guests in custom partitions is only available with libvirt >= 1.0.5, using the new cgroup layout. The legacy cgroups layout described later did not support customization per guest.

## Resource management APIs/commands

Since libvirt aims to provide an API which is portable across hypervisors, the concept of cgroups is not exposed directly in the API or XML configuration. It is considered to be an internal implementation detail. Instead libvirt provides a set of APIs for applying resource controls, which are then mapped to corresponding cgroup tunables

### Scheduler tuning

Parameters from the "cpu" controller are exposed via the `schedinfo` command in virsh.

    # virsh schedinfo demo
    Scheduler      : posix
    cpu_shares     : 1024
    vcpu_period    : 100000
    vcpu_quota     : -1
    emulator_period: 100000
    emulator_quota : -1

### Block I/O tuning

Parameters from the "blkio" controller are exposed via the `bkliotune` command in virsh.

    # virsh blkiotune demo
    weight         : 500
    device_weight  :

### Memory tuning

Parameters from the "memory" controller are exposed via the `memtune` command in virsh.

    # virsh memtune demo
    hard_limit     : 580192
    soft_limit     : unlimited
    swap_hard_limit: unlimited

### Network tuning

The `net_cls` is not currently used. Instead traffic filter policies are set directly against individual virtual network interfaces.

## Legacy cgroups layout

Prior to libvirt 1.0.5, the cgroups layout created by libvirt was different from that described above, and did not allow for administrator customization. Libvirt used a fixed, 3-level hierarchy `libvirt/{qemu,lxc}/$VMNAME` which was rooted at the point in the hierarchy where libvirtd itself was located. So if libvirtd was placed at `/system/libvirtd.service` by systemd, the groups for each virtual machine / container would be located at `/system/libvirtd.service/libvirt/{qemu,lxc}/$VMNAME`. In addition to this, the QEMU drivers further child groups for each vCPU thread and the emulator thread(s). This leads to a hierarchy that looked like

    $ROOT
      |
      +- system
          |
          +- libvirtd.service
               |
               +- libvirt
                   |
                   +- qemu
                   |   |
                   |   +- vm1
                   |   |   |
                   |   |   +- emulator
                   |   |   +- vcpu0
                   |   |   +- vcpu1
                   |   |
                   |   +- vm2
                   |   |   |
                   |   |   +- emulator
                   |   |   +- vcpu0
                   |   |   +- vcpu1
                   |   |
                   |   +- vm3
                   |       |
                   |       +- emulator
                   |       +- vcpu0
                   |       +- vcpu1
                   |
                   +- lxc
                       |
                       +- container1
                       |
                       +- container2
                       |
                       +- container3

Although current releases are much improved, historically the use of deep hierarchies has had a significant negative impact on the kernel scalability. The legacy libvirt cgroups layout highlighted these problems, to the detriment of the performance of virtual machines and containers.