openstack/nova
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doc: Add documentation for cpu_realtime, cpu_realtime_mask

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This wasn't documented anywhere but the spec [1]. Fix this.

We may want to provide a more in-depth overview of using RT features of
OpenStack, but that's a future work item.

[1] https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/libvirt-real-time.html

Change-Id: Id30bc8447a6b482ad114ec6ebd3d5dab20ca0e3a
Closes-Bug: #1715437
This commit is contained in:
Stephen Finucane 2017-09-08 14:05:38 +01:00
parent 7ab5ee982d
commit bd3a4d242f
1 changed files with 53 additions and 1 deletions
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54
doc/source/user/flavors.rst
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@ -474,11 +474,63 @@ NUMA topology
greater than the available number of CPUs or memory respectively, an
exception is raised.
.. _extra-specs-realtime-policy:
CPU real-time policy
For the libvirt driver, you can state that one or more of your instance
virtual CPUs (vCPUs), though not all of them, run with a real-time policy.
When used on a correctly configured host, this provides stronger guarantees
for worst case scheduler latency for vCPUs and is a requirement for certain
applications.
.. todo::
Document the required steps to configure hosts and guests. There are a lot
of things necessary, from isolating hosts and configuring the
``vcpu_pin_set`` nova configuration option on the host, to choosing a
correctly configured guest image.
.. important::
While most of your instance vCPUs can run with a real-time policy, you must
mark at least one vCPU as non-real-time, to be used for both non-real-time
guest processes and emulator overhead (housekeeping) processes.
.. important::
To use this extra spec, you must enable pinned CPUs. Refer to
:ref:`CPU policy <extra-specs-cpu-policy>` for more information.
.. code:: console
$ openstack flavor set FLAVOR-NAME \
--property hw:cpu_realtime=CPU-REALTIME-POLICY \
--property hw:cpu_realtime_mask=CPU-REALTIME-MASK
Where:
CPU-REALTIME-POLICY (enum):
One of:
- ``no``: (default) The guest vCPUs will not have a real-time policy
- ``yes``: The guest vCPUs will have a real-time policy
CPU-REALTIME-MASK (coremask):
A coremask indicating which vCPUs **will not** have a real-time policy. This
should start with a ``^``. For example, a value of ``^0-1`` indicates that
all vCPUs *except* vCPUs ``0`` and ``1`` will have a real-time policy.
.. note::
The ``hw:cpu_realtime_mask`` option is only valid if ``hw:cpu_realtime``
is set to ``yes``.
Emulator threads policy
For the libvirt driver, you can assign a separate pCPU to an instance that
will be used for emulator threads, which are emulator processes not directly
related to the guest OS. This pCPU will used in addition to the pCPUs used
for the guest. This is generally required for use with a real-time OS.
for the guest. This is generally required for use with a :ref:`real-time
workload <extra-specs-realtime-policy>`.
.. important::