mirror of
https://gitlab.com/libvirt/libvirt.git
synced 2024-12-22 21:55:25 +00:00
184245f53b
When running on host with systemd we register VMs with machined.
In this case systemd creates the root VM cgroup for us. This has some
implications where one of them is that systemd owns all files inside
the root VM cgroup and we should not touch them.
We already use DBus calls for some of the APIs but for the remaining
ones we will continue accessing the files directly. Systemd doesn't
support threaded cgroups so we need to do this.
The reason why we don't use DBus for most of the APIs is that we already
have a code that works with files and we would have to check if systemd
supports each API.
This change introduces new topology on systemd hosts:
$ROOT
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+- machine.slice
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+- machine-qemu\x2d1\x2dvm1.scope
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+- libvirt
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+- emulator
+- vcpu0
+- vcpu0
compared to the previous topology:
$ROOT
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+- machine.slice
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+- machine-qemu\x2d1\x2dvm1.scope
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+- emulator
+- vcpu0
+- vcpu0
Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
425 lines
14 KiB
XML
425 lines
14 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE html>
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<html xmlns="http://www.w3.org/1999/xhtml">
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<body>
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<h1>Control Groups Resource Management</h1>
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<ul id="toc"></ul>
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<p>
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The QEMU and LXC drivers make use of the Linux "Control Groups" facility
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for applying resource management to their virtual machines and containers.
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</p>
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<h2><a id="requiredControllers">Required controllers</a></h2>
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<p>
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The control groups filesystem supports multiple "controllers". By default
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the init system (such as systemd) should mount all controllers compiled
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into the kernel at <code>/sys/fs/cgroup/$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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| +- libvirt
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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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| +- libvirt
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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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| +- libvirt
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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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| | +- 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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<p>
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Prior libvirt 7.1.0 the topology doesn't have extra
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<code>libvirt</code> directory.
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</p>
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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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is the top level default partition for virtual machines and containers
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<code>/machine</code>.
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</p>
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<p>
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Given this, a possible non-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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+- 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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