[libvirt] [PATCH] Add docs about cgroups layout and usage
Daniel P. Berrange
berrange at redhat.com
Fri May 3 16:49:26 UTC 2013
From: "Daniel P. Berrange" <berrange at redhat.com>
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 at redhat.com>
---
docs/cgroups.html.in | 285 +++++++++++++++++++++++++++++++++++++++++++++++++++
docs/sitemap.html.in | 4 +
2 files changed, 289 insertions(+)
create mode 100644 docs/cgroups.html.in
diff --git a/docs/cgroups.html.in b/docs/cgroups.html.in
new file mode 100644
index 0000000..3be0672
--- /dev/null
+++ b/docs/cgroups.html.in
@@ -0,0 +1,285 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+ <body>
+ <h1>Control Groups Resource Management</h1>
+
+ <ul id="toc"></ul>
+
+ <p>
+ The QEMU and LXC drivers make use of the Linux "Control Groups" facility
+ for applying resource management to their virtual machines & containers.
+ </p>
+
+ <h2><a name="requiredControllers">Required controllers</a></h2>
+
+ <p>
+ The control groups filesystem supports multiple "controllers". By default
+ the init system (such as systemd) should mount all controllers compiled
+ into the kernel at <code>/sys/fs/cgroup/$CONTROLLER-NAME</code>. Libvirt
+ will never attempt to mount any controllers itself, merely detect where
+ they are mounted.
+ </p>
+
+ <p>
+ The QEMU driver is capable of using the <code>cpuset</code>,
+ <code>cpu</code>, <code>memory</code>, <code>blkio</code> and
+ <code>devices</code> controllers. None of them are compulsory.
+ If any controller is not mounted, the resource management APIs
+ which use it will cease to operate. It is possible to explicitly
+ turn off use of a controller, even when mounted, via the
+ <code>/etc/libvirt/qemu.conf</code> configuration file.
+ </p>
+
+ <p>
+ The LXC driver is capable of using the <code>cpuset</code>,
+ <code>cpu</code>, <code>cpuset</code>, <code>freezer</code>,
+ <code>memory</code>, <code>blkio</code> and <code>devices</code>
+ controllers. The <code>cpuset</code>, <code>devices</code>
+ and <code>memory</code> controllers are compulsory. Without
+ them mounted, no containers can be started. If any of the
+ other controllers are not mounted, the resource management APIs
+ which use them will cease to operate.
+ </p>
+
+ <h2><a name="currentLayout">Current cgroups layout</a></h2>
+
+ <p>
+ As of libvirt 1.0.5 or later, the cgroups layout created by libvirt has been
+ simplified, in order to facilitate the setup of resource control policies by
+ administrators / management applications. The layout is based on the concepts of
+ "partitions" and "consumers". Each virtual machine or container is a consumer,
+ and has a corresponding cgroup named <code>$VMNAME.libvirt-{qemu,lxc}</code>.
+ Each consumer is associated with exactly one partition, which also have a
+ corresponding cgroup usually named <code>$PARTNAME.partition</code>. The
+ exceptions to this naming rule are the three top level default partitions,
+ named <code>/system</code> (for system services), <code>/user</code> (for
+ user login sessions) and <code>/machine</code> (for virtual machines and
+ containers). By default every consumer will of course be associated with
+ the <code>/machine</code> partition. This leads to a hierarchy that looks
+ like
+ </p>
+
+ <pre>
+$ROOT
+ |
+ +- system
+ | |
+ | +- libvirtd.service
+ |
+ +- machine
+ |
+ +- vm1.libvirt-qemu
+ | |
+ | +- emulator
+ | +- vcpu0
+ | +- vcpu1
+ |
+ +- vm2.libvirt-qemu
+ | |
+ | +- emulator
+ | +- vcpu0
+ | +- vcpu1
+ |
+ +- vm3.libvirt-qemu
+ | |
+ | +- emulator
+ | +- vcpu0
+ | +- vcpu1
+ |
+ +- container1.libvirt-lxc
+ |
+ +- container2.libvirt-lxc
+ |
+ +- container3.libvirt-lxc
+ </pre>
+
+ <p>
+ The default cgroups layout ensures that, when there is contention for
+ CPU time, it is shared equally between system services, user sessions
+ and virtual machines / containers. This prevents virtual machines from
+ locking the administrator out of the host, or impacting execution of
+ system services. ConverselyWhen there is no contention from
+ system services / user sessions, it is possible for virtual machines
+ to fully utilize the host CPUs.
+ </p>
+
+ <h2><a name="customPartiton">Using custom partitions</a></h2>
+
+ <p>
+ If there is a need to apply resource constraints to groups of
+ virtual machines or containers, then the single default
+ partition <code>/machine</code> may not be sufficiently
+ flexible. The administrator may wish to sub-divide the
+ default partition, for example into "testing" and "production"
+ partitions, and then assign each guest to a specific
+ sub-partition. This is achieved via a small element addition
+ to the guest domain XML config, just below the main <code>domain</code>
+ element
+ </p>
+
+ <pre>
+ ...
+ <resource>
+ <partition>/machine/production</partition>
+ </resource>
+ ...
+ </pre>
+
+ <p>
+ Libvirt will not auto-create the cgroups directory to back
+ this partition. In the future, libvirt / virsh will provide
+ APIs / commands to create custom partitions, but currently
+ this is left as an exercise for the administrator. For
+ example, given the XML config above, the admin would need
+ to create a cgroup named '/machine/production.partition'
+ </p>
+
+ <pre>
+# cd /sys/fs/cgroup
+# for i in blkio cpu,cpuacct cpuset devices freezer memory net_cls perf_event
+ do
+ mkdir $i/machine/production.partition
+ done
+# for i in cpuset.cpus cpuset.mems
+ do
+ cat cpuset/machine/$i > cpuset/machine/production.partition/$i
+ done
+</pre>
+
+ <p>
+ <strong>Note:</strong> the cgroups directory created as a ".partition"
+ suffix, but the XML config does not require this suffix.
+ </p>
+
+ <p>
+ <strong>Note:</strong> the ability to place guests in custom
+ partitions is only available with libvirt >= 1.0.5, using
+ the new cgroup layout. The legacy cgroups layout described
+ later did not support customization per guest.
+ </p>
+
+ <h2><a name="resourceAPIs">Resource management APIs/commands</a></h2>
+
+ <p>
+ Since libvirt aims to provide an API which is portable across
+ hypervisors, the concept of cgroups is not exposed directly
+ in the API or XML configuration. It is considered to be an
+ internal implementation detail. Instead libvirt provides a
+ set of APIs for applying resource controls, which are then
+ mapped to corresponding cgroup tunables
+ </p>
+
+ <h3>Scheduler tuning</h3>
+
+ <p>
+ Parameters from the "cpu" controller are exposed via the
+ <code>schedinfo</code> command in virsh.
+ </p>
+
+ <pre>
+# virsh schedinfo demo
+Scheduler : posix
+cpu_shares : 1024
+vcpu_period : 100000
+vcpu_quota : -1
+emulator_period: 100000
+emulator_quota : -1</pre>
+
+
+ <h3>Block I/O tuning</h3>
+
+ <p>
+ Parameters from the "blkio" controller are exposed via the
+ <code>bkliotune</code> command in virsh.
+ </p>
+
+
+ <pre>
+# virsh blkiotune demo
+weight : 500
+device_weight : </pre>
+
+ <h3>Memory tuning</h3>
+
+ <p>
+ Parameters from the "memory" controller are exposed via the
+ <code>memtune</code> command in virsh.
+ </p>
+
+ <pre>
+# virsh memtune demo
+hard_limit : 580192
+soft_limit : unlimited
+swap_hard_limit: unlimited
+ </pre>
+
+ <h3>Network tuning</h3>
+
+ <p>
+ The <code>net_cls</code> is not currently used. Instead traffic
+ filter policies are set directly against individual virtual
+ network interfaces.
+ </p>
+
+ <h2><a name="legacyLayout">Legacy cgroups layout</a></h2>
+
+ <p>
+ Prior to libvirt 1.0.5, the cgroups layout created by libvirt was different
+ from that described above, and did not allow for administrator customization.
+ Libvirt used a fixed, 3-level hiearchy <code>libvirt/{qemu,lxc}/$VMNAME</code>
+ which was rooted at the point in the hiearchy where libvirtd itself was
+ located. So if libvirtd was placed at <code>/system/libvirtd.service</code>
+ by systemd, the groups for each virtual machine / container would be located
+ at <code>/system/libvirtd.service/libvirt/{qemu,lxc}/$VMNAME</code>. In addition
+ to this, the QEMU drivers further child groups for each vCPU thread and the
+ emulator thread(s). This leads to a hiearchy that looked like
+ </p>
+
+
+ <pre>
+$ROOT
+ |
+ +- system
+ |
+ +- libvirtd.service
+ |
+ +- libvirt
+ |
+ +- qemu
+ | |
+ | +- vm1
+ | | |
+ | | +- emulator
+ | | +- vcpu0
+ | | +- vcpu1
+ | |
+ | +- vm2
+ | | |
+ | | +- emulator
+ | | +- vcpu0
+ | | +- vcpu1
+ | |
+ | +- vm3
+ | |
+ | +- emulator
+ | +- vcpu0
+ | +- vcpu1
+ |
+ +- lxc
+ |
+ +- container1
+ |
+ +- container2
+ |
+ +- container3
+ </pre>
+
+ <p>
+ Although current releases are much improved, historically the use of deep
+ hiearchies has had a significant negative impact on the kernel scalability.
+ The legacy libvirt cgroups layout highlighted these problems, to the detriment
+ of the performance of virtual machines and containers.
+ </p>
+ </body>
+</html>
diff --git a/docs/sitemap.html.in b/docs/sitemap.html.in
index 619e4a1..cb7cc5b 100644
--- a/docs/sitemap.html.in
+++ b/docs/sitemap.html.in
@@ -89,6 +89,10 @@
<span>Ensuring exclusive guest access to disks</span>
</li>
<li>
+ <a href="cgroups.html">CGroups</a>
+ <span>Control groups integration</span>
+ </li>
+ <li>
<a href="hooks.html">Hooks</a>
<span>Hooks for system specific management</span>
</li>
--
1.8.1.4
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