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7966414194
Commit <c3bd0019c0> changed the way how cgroup directory names are constructed but the documentation was not updated. Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
421 lines
14 KiB
XML
421 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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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 id="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>cpuacct</code>, <code>memory</code>,
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<code>blkio</code> and <code>devices</code> controllers.
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None of them are compulsory. If any controller is not mounted,
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the resource management APIs which use it will cease to operate.
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It is possible to explicitly turn off use of a controller,
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even when mounted, via the <code>/etc/libvirt/qemu.conf</code>
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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>cpuacct</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>cpuacct</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 id="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 new layout is based on the concepts
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of "partitions" and "consumers". A "consumer" is a cgroup which holds the
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processes for a single virtual machine or container. A "partition" is a cgroup
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which does not contain any processes, but can have resource controls applied.
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A "partition" will have zero or more child directories which may be either
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"consumer" or "partition".
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</p>
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<p>
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As of libvirt 1.1.1 or later, the cgroups layout will have some slight
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differences when running on a host with systemd 205 or later. The overall
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tree structure is the same, but there are some differences in the naming
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conventions for the cgroup directories. Thus the following docs split
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in two, one describing systemd hosts and the other non-systemd hosts.
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</p>
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<h3><a id="currentLayoutSystemd">Systemd cgroups integration</a></h3>
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<p>
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On hosts which use systemd, each consumer maps to a systemd scope unit,
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while partitions map to a system slice unit.
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</p>
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<h4><a id="systemdScope">Systemd scope naming</a></h4>
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<p>
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The systemd convention is for the scope name of virtual machines / containers
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to be of the general format <code>machine-$NAME.scope</code>. Libvirt forms the
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<code>$NAME</code> part of this by concatenating the driver type with the id
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and truncated name of the guest, and then escaping any systemd reserved
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characters.
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So for a guest <code>demo</code> running under the <code>lxc</code> driver,
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we get a <code>$NAME</code> of <code>lxc-12345-demo</code> which when escaped
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is <code>lxc\x2d12345\x2ddemo</code>. So the complete scope name is
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<code>machine-lxc\x2d12345\x2ddemo.scope</code>.
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The scope names map directly to the cgroup directory names.
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</p>
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<h4><a id="systemdSlice">Systemd slice naming</a></h4>
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<p>
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The systemd convention for slice naming is that a slice should include the
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name of all of its parents prepended on its own name. So for a libvirt
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partition <code>/machine/engineering/testing</code>, the slice name will
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be <code>machine-engineering-testing.slice</code>. Again the slice names
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map directly to the cgroup directory names. Systemd creates three top level
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slices by default, <code>system.slice</code> <code>user.slice</code> and
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<code>machine.slice</code>. All virtual machines or containers created
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by libvirt will be associated with <code>machine.slice</code> by default.
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</p>
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<h4><a id="systemdLayout">Systemd cgroup layout</a></h4>
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<p>
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Given this, a possible systemd cgroups layout involving 3 qemu guests,
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3 lxc containers and 3 custom child slices, would be:
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</p>
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<pre>
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$ROOT
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+- system.slice
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| +- libvirtd.service
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+- machine.slice
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+- machine-qemu\x2d1\x2dvm1.scope
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| |
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- machine-qemu\x2d2\x2dvm2.scope
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| |
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- machine-qemu\x2d3\x2dvm3.scope
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- machine-engineering.slice
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| +- machine-engineering-testing.slice
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| | |
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| | +- machine-lxc\x2d11111\x2dcontainer1.scope
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| +- machine-engineering-production.slice
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| +- machine-lxc\x2d22222\x2dcontainer2.scope
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+- machine-marketing.slice
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+- machine-lxc\x2d33333\x2dcontainer3.scope
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</pre>
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<h3><a id="currentLayoutGeneric">Non-systemd cgroups layout</a></h3>
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<p>
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On hosts which do not use systemd, each consumer has a corresponding cgroup
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named <code>$VMNAME.libvirt-{qemu,lxc}</code>. Each consumer is associated
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with exactly one partition, which also have a corresponding cgroup usually
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named <code>$PARTNAME.partition</code>. The exceptions to this naming rule
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are the three top level default partitions, named <code>/system</code> (for
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system services), <code>/user</code> (for user login sessions) and
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<code>/machine</code> (for virtual machines and containers). By default
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every consumer will of course be associated with the <code>/machine</code>
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partition.
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</p>
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<p>
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Given this, a possible systemd cgroups layout involving 3 qemu guests,
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3 lxc containers and 2 custom child slices, would be:
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</p>
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<pre>
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$ROOT
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+- system
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| +- libvirtd.service
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+- machine
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+- qemu-1-vm1.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- qeme-2-vm2.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- qemu-3-vm3.libvirt-qemu
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| +- emulator
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| +- vcpu0
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| +- vcpu1
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+- engineering.partition
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| +- testing.partition
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| | +- lxc-11111-container1.libvirt-lxc
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| +- production.partition
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| +- lxc-22222-container2.libvirt-lxc
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+- marketing.partition
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+- lxc-33333-container3.libvirt-lxc
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</pre>
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<h2><a id="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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Note that the partition names in the guest XML are using a
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generic naming format, not the low level naming convention
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required by the underlying host OS. That is, you should not include
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any of the <code>.partition</code> or <code>.slice</code>
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suffixes in the XML config. Given a partition name
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<code>/machine/production</code>, libvirt will automatically
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apply the platform specific translation required to get
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<code>/machine/production.partition</code> (non-systemd)
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or <code>/machine.slice/machine-production.slice</code>
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(systemd) as the underlying cgroup name
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</p>
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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.
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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 in this document did not support customization per guest.
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</p>
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<h3><a id="createSystemd">Creating custom partitions (systemd)</a></h3>
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<p>
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Given the XML config above, the admin on a systemd based host would
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need to create a unit file <code>/etc/systemd/system/machine-production.slice</code>
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</p>
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<pre>
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# cat > /etc/systemd/system/machine-testing.slice <<EOF
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[Unit]
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Description=VM testing slice
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Before=slices.target
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Wants=machine.slice
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EOF
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# systemctl start machine-testing.slice
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</pre>
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<h3><a id="createNonSystemd">Creating custom partitions (non-systemd)</a></h3>
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<p>
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Given the XML config above, the admin on a non-systemd based host
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would need 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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<h2><a id="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 id="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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