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42b5e496a7
Signed-off-by: Peter Krempa <pkrempa@redhat.com> Reviewed-by: Erik Skultety <eskultet@redhat.com>
365 lines
12 KiB
ReStructuredText
365 lines
12 KiB
ReStructuredText
==================================
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Control Groups Resource Management
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==================================
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.. contents::
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The QEMU and LXC drivers make use of the Linux "Control Groups" facility for
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applying resource management to their virtual machines and containers.
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Required controllers
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--------------------
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The control groups filesystem supports multiple "controllers". By default the
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init system (such as systemd) should mount all controllers compiled into the
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kernel at ``/sys/fs/cgroup/$CONTROLLER-NAME``. Libvirt will never attempt to
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mount any controllers itself, merely detect where they are mounted.
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The QEMU driver is capable of using the ``cpuset``, ``cpu``, ``cpuacct``,
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``memory``, ``blkio`` and ``devices`` controllers. None of them are compulsory.
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If any controller is not mounted, the resource management APIs which use it will
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cease to operate. It is possible to explicitly turn off use of a controller,
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even when mounted, via the ``/etc/libvirt/qemu.conf`` configuration file.
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The LXC driver is capable of using the ``cpuset``, ``cpu``, ``cpuacct``,
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``freezer``, ``memory``, ``blkio`` and ``devices`` controllers. The ``cpuacct``,
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``devices`` and ``memory`` controllers are compulsory. Without them mounted, no
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containers can be started. If any of the other controllers are not mounted, the
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resource management APIs which use them will cease to operate.
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Current cgroups layout
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----------------------
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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
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concepts of "partitions" and "consumers". A "consumer" is a cgroup which holds
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the processes for a single virtual machine or container. A "partition" is a
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cgroup which does not contain any processes, but can have resource controls
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applied. A "partition" will have zero or more child directories which may be
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either "consumer" or "partition".
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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 tree
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structure is the same, but there are some differences in the naming conventions
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for the cgroup directories. Thus the following docs split in two, one describing
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systemd hosts and the other non-systemd hosts.
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Systemd cgroups integration
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~~~~~~~~~~~~~~~~~~~~~~~~~~~
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On hosts which use systemd, each consumer maps to a systemd scope unit, while
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partitions map to a system slice unit.
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Systemd scope naming
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^^^^^^^^^^^^^^^^^^^^
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The systemd convention is for the scope name of virtual machines / containers to
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be of the general format ``machine-$NAME.scope``. Libvirt forms the ``$NAME``
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part of this by concatenating the driver type with the id and truncated name of
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the guest, and then escaping any systemd reserved characters. So for a guest
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``demo`` running under the ``lxc`` driver, we get a ``$NAME`` of
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``lxc-12345-demo`` which when escaped is ``lxc\x2d12345\x2ddemo``. So the
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complete scope name is ``machine-lxc\x2d12345\x2ddemo.scope``. The scope names
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map directly to the cgroup directory names.
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Systemd slice naming
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^^^^^^^^^^^^^^^^^^^^
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The systemd convention for slice naming is that a slice should include the name
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of all of its parents prepended on its own name. So for a libvirt partition
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``/machine/engineering/testing``, the slice name will be
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``machine-engineering-testing.slice``. Again the slice names map directly to the
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cgroup directory names. Systemd creates three top level slices by default,
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``system.slice`` ``user.slice`` and ``machine.slice``. All virtual machines or
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containers created by libvirt will be associated with ``machine.slice`` by
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default.
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Systemd cgroup layout
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^^^^^^^^^^^^^^^^^^^^^
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Given this, a possible systemd cgroups layout involving 3 qemu guests, 3 lxc
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containers and 3 custom child slices, would be:
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::
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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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Prior libvirt 7.1.0 the topology doesn't have extra ``libvirt`` directory.
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Non-systemd cgroups layout
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~~~~~~~~~~~~~~~~~~~~~~~~~~
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On hosts which do not use systemd, each consumer has a corresponding cgroup
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named ``$VMNAME.libvirt-{qemu,lxc}``. Each consumer is associated with exactly
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one partition, which also have a corresponding cgroup usually named
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``$PARTNAME.partition``. The exceptions to this naming rule is the top level
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default partition for virtual machines and containers ``/machine``.
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Given this, a possible non-systemd cgroups layout involving 3 qemu guests, 3 lxc
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containers and 2 custom child slices, would be:
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::
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$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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Using custom partitions
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-----------------------
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If there is a need to apply resource constraints to groups of virtual machines
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or containers, then the single default partition ``/machine`` may not be
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sufficiently flexible. The administrator may wish to sub-divide the default
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partition, for example into "testing" and "production" partitions, and then
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assign each guest to a specific sub-partition. This is achieved via a small
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element addition to the guest domain XML config, just below the main ``domain``
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element
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::
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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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Note that the partition names in the guest XML are using a generic naming
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format, not the low level naming convention required by the underlying host OS.
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That is, you should not include any of the ``.partition`` or ``.slice`` suffixes
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in the XML config. Given a partition name ``/machine/production``, libvirt will
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automatically apply the platform specific translation required to get
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``/machine/production.partition`` (non-systemd) or
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``/machine.slice/machine-production.slice`` (systemd) as the underlying cgroup
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name
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Libvirt will not auto-create the cgroups directory to back this partition. In
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the future, libvirt / virsh will provide APIs / commands to create custom
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partitions, but currently this is left as an exercise for the administrator.
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**Note:** the ability to place guests in custom partitions is only available
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with libvirt >= 1.0.5, using the new cgroup layout. The legacy cgroups layout
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described later in this document did not support customization per guest.
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Creating custom partitions (systemd)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Given the XML config above, the admin on a systemd based host would need to
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create a unit file ``/etc/systemd/system/machine-production.slice``
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::
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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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Creating custom partitions (non-systemd)
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Given the XML config above, the admin on a non-systemd based host would need to
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create a cgroup named '/machine/production.partition'
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::
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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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Resource management APIs/commands
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---------------------------------
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Since libvirt aims to provide an API which is portable across hypervisors, the
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concept of cgroups is not exposed directly in the API or XML configuration. It
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is considered to be an internal implementation detail. Instead libvirt provides
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a set of APIs for applying resource controls, which are then mapped to
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corresponding cgroup tunables
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Scheduler tuning
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~~~~~~~~~~~~~~~~
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Parameters from the "cpu" controller are exposed via the ``schedinfo`` command
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in virsh.
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::
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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
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Block I/O tuning
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~~~~~~~~~~~~~~~~
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Parameters from the "blkio" controller are exposed via the ``bkliotune`` command
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in virsh.
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::
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# virsh blkiotune demo
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weight : 500
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device_weight :
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Memory tuning
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~~~~~~~~~~~~~
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Parameters from the "memory" controller are exposed via the ``memtune`` command
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in virsh.
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::
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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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Network tuning
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~~~~~~~~~~~~~~
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The ``net_cls`` is not currently used. Instead traffic filter policies are set
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directly against individual virtual network interfaces.
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Legacy cgroups layout
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---------------------
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Prior to libvirt 1.0.5, the cgroups layout created by libvirt was different from
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that described above, and did not allow for administrator customization. Libvirt
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used a fixed, 3-level hierarchy ``libvirt/{qemu,lxc}/$VMNAME`` which was rooted
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at the point in the hierarchy where libvirtd itself was located. So if libvirtd
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was placed at ``/system/libvirtd.service`` by systemd, the groups for each
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virtual machine / container would be located at
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``/system/libvirtd.service/libvirt/{qemu,lxc}/$VMNAME``. In addition to this,
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the QEMU drivers further child groups for each vCPU thread and the emulator
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thread(s). This leads to a hierarchy that looked like
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::
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$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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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. The
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legacy libvirt cgroups layout highlighted these problems, to the detriment of
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the performance of virtual machines and containers.
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