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486a86eb18
Describe the new cgroups layout, how to customize placement of guests and what virsh commands are used to access the parameters. Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
286 lines
9.2 KiB
XML
286 lines
9.2 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Control Groups Resource Management</h1>
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<ul id="toc"></ul>
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<p>
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The QEMU and LXC drivers make use of the Linux "Control Groups" facility
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for applying resource management to their virtual machines and containers.
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</p>
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<h2><a name="requiredControllers">Required controllers</a></h2>
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<p>
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The control groups filesystem supports multiple "controllers". By default
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the init system (such as systemd) should mount all controllers compiled
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into the kernel at <code>/sys/fs/cgroup/$CONTROLLER-NAME</code>. Libvirt
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will never attempt to mount any controllers itself, merely detect where
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they are mounted.
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</p>
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<p>
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The QEMU driver is capable of using the <code>cpuset</code>,
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<code>cpu</code>, <code>memory</code>, <code>blkio</code> and
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<code>devices</code> controllers. None of them are compulsory.
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If any controller is not mounted, the resource management APIs
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which use it will cease to operate. It is possible to explicitly
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turn off use of a controller, even when mounted, via the
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<code>/etc/libvirt/qemu.conf</code> configuration file.
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</p>
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<p>
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The LXC driver is capable of using the <code>cpuset</code>,
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<code>cpu</code>, <code>cpuset</code>, <code>freezer</code>,
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<code>memory</code>, <code>blkio</code> and <code>devices</code>
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controllers. The <code>cpuset</code>, <code>devices</code>
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and <code>memory</code> controllers are compulsory. Without
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them mounted, no containers can be started. If any of the
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other controllers are not mounted, the resource management APIs
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which use them will cease to operate.
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</p>
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<h2><a name="currentLayout">Current cgroups layout</a></h2>
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<p>
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As of libvirt 1.0.5 or later, the cgroups layout created by libvirt has been
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simplified, in order to facilitate the setup of resource control policies by
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administrators / management applications. The layout is based on the concepts of
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"partitions" and "consumers". Each virtual machine or container is a consumer,
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and has a corresponding cgroup named <code>$VMNAME.libvirt-{qemu,lxc}</code>.
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Each consumer is associated with exactly one partition, which also have a
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corresponding cgroup usually named <code>$PARTNAME.partition</code>. The
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exceptions to this naming rule are the three top level default partitions,
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named <code>/system</code> (for system services), <code>/user</code> (for
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user login sessions) and <code>/machine</code> (for virtual machines and
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containers). By default every consumer will of course be associated with
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the <code>/machine</code> partition. This leads to a hierarchy that looks
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like
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</p>
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<pre>
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$ROOT
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+- system
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| +- libvirtd.service
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+- machine
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+- vm1.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- vm2.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- vm3.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- container1.libvirt-lxc
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+- container2.libvirt-lxc
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+- container3.libvirt-lxc
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</pre>
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<p>
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The default cgroups layout ensures that, when there is contention for
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CPU time, it is shared equally between system services, user sessions
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and virtual machines / containers. This prevents virtual machines from
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locking the administrator out of the host, or impacting execution of
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system services. Conversely, when there is no contention from
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system services / user sessions, it is possible for virtual machines
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to fully utilize the host CPUs.
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</p>
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<h2><a name="customPartiton">Using custom partitions</a></h2>
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<p>
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If there is a need to apply resource constraints to groups of
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virtual machines or containers, then the single default
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partition <code>/machine</code> may not be sufficiently
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flexible. The administrator may wish to sub-divide the
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default partition, for example into "testing" and "production"
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partitions, and then assign each guest to a specific
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sub-partition. This is achieved via a small element addition
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to the guest domain XML config, just below the main <code>domain</code>
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element
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</p>
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<pre>
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...
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<resource>
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<partition>/machine/production</partition>
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</resource>
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...
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</pre>
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<p>
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Libvirt will not auto-create the cgroups directory to back
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this partition. In the future, libvirt / virsh will provide
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APIs / commands to create custom partitions, but currently
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this is left as an exercise for the administrator. For
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example, given the XML config above, the admin would need
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to create a cgroup named '/machine/production.partition'
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</p>
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<pre>
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# cd /sys/fs/cgroup
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# for i in blkio cpu,cpuacct cpuset devices freezer memory net_cls perf_event
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do
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mkdir $i/machine/production.partition
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done
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# for i in cpuset.cpus cpuset.mems
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do
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cat cpuset/machine/$i > cpuset/machine/production.partition/$i
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done
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</pre>
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<p>
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<strong>Note:</strong> the cgroups directory created as a ".partition"
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suffix, but the XML config does not require this suffix.
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</p>
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<p>
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<strong>Note:</strong> the ability to place guests in custom
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partitions is only available with libvirt >= 1.0.5, using
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the new cgroup layout. The legacy cgroups layout described
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later did not support customization per guest.
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</p>
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<h2><a name="resourceAPIs">Resource management APIs/commands</a></h2>
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<p>
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Since libvirt aims to provide an API which is portable across
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hypervisors, the concept of cgroups is not exposed directly
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in the API or XML configuration. It is considered to be an
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internal implementation detail. Instead libvirt provides a
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set of APIs for applying resource controls, which are then
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mapped to corresponding cgroup tunables
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</p>
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<h3>Scheduler tuning</h3>
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<p>
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Parameters from the "cpu" controller are exposed via the
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<code>schedinfo</code> command in virsh.
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</p>
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<pre>
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# virsh schedinfo demo
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Scheduler : posix
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cpu_shares : 1024
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vcpu_period : 100000
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vcpu_quota : -1
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emulator_period: 100000
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emulator_quota : -1</pre>
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<h3>Block I/O tuning</h3>
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<p>
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Parameters from the "blkio" controller are exposed via the
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<code>bkliotune</code> command in virsh.
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</p>
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<pre>
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# virsh blkiotune demo
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weight : 500
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device_weight : </pre>
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<h3>Memory tuning</h3>
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<p>
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Parameters from the "memory" controller are exposed via the
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<code>memtune</code> command in virsh.
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</p>
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<pre>
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# virsh memtune demo
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hard_limit : 580192
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soft_limit : unlimited
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swap_hard_limit: unlimited
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</pre>
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<h3>Network tuning</h3>
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<p>
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The <code>net_cls</code> is not currently used. Instead traffic
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filter policies are set directly against individual virtual
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network interfaces.
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</p>
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<h2><a name="legacyLayout">Legacy cgroups layout</a></h2>
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<p>
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Prior to libvirt 1.0.5, the cgroups layout created by libvirt was different
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from that described above, and did not allow for administrator customization.
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Libvirt used a fixed, 3-level hierarchy <code>libvirt/{qemu,lxc}/$VMNAME</code>
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which was rooted at the point in the hierarchy where libvirtd itself was
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located. So if libvirtd was placed at <code>/system/libvirtd.service</code>
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by systemd, the groups for each virtual machine / container would be located
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at <code>/system/libvirtd.service/libvirt/{qemu,lxc}/$VMNAME</code>. In addition
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to this, the QEMU drivers further child groups for each vCPU thread and the
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emulator thread(s). This leads to a hierarchy that looked like
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</p>
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<pre>
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$ROOT
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+- system
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+- libvirtd.service
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+- libvirt
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+- qemu
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| +- vm1
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| | +- emulator
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| | +- vcpu0
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| | +- vcpu1
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| +- vm2
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| | +- emulator
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| | +- vcpu0
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| | +- vcpu1
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| +- vm3
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- lxc
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+- container1
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+- container2
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+- container3
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</pre>
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<p>
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Although current releases are much improved, historically the use of deep
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hierarchies has had a significant negative impact on the kernel scalability.
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The legacy libvirt cgroups layout highlighted these problems, to the detriment
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of the performance of virtual machines and containers.
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</p>
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</body>
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</html>
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