153 lines
5.9 KiB
ReStructuredText
153 lines
5.9 KiB
ReStructuredText
=========
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Real Time
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=========
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.. versionadded:: 13.0.0 (Mitaka)
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Nova supports configuring `real-time policies`__ for instances. This builds upon
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the improved performance offered by :doc:`CPU pinning <cpu-topologies>` by
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providing stronger guarantees for worst case scheduler latency for vCPUs.
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.. __: https://en.wikipedia.org/wiki/Real-time_computing
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Enabling Real-Time
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------------------
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Currently the creation of real-time instances is only supported when using the
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libvirt compute driver with a :oslo.config:option:`libvirt.virt_type` of
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``kvm`` or ``qemu``. It requires extensive configuration of the host and this
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document provides but a rough overview of the changes required. Configuration
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will vary depending on your hardware, BIOS configuration, host and guest OS'
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and application.
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BIOS configuration
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~~~~~~~~~~~~~~~~~~
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Configure your host BIOS as recommended in the `rt-wiki`__ page.
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The most important steps are:
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- Disable power management, including CPU sleep states
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- Disable SMT (hyper-threading) or any option related to logical processors
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These are standard steps used in benchmarking as both sets of features can
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result in non-deterministic behavior.
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.. __: https://rt.wiki.kernel.org/index.php/HOWTO:_Build_an_RT-application
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OS configuration
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~~~~~~~~~~~~~~~~
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This is inherently specific to the distro used, however, there are some common
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steps:
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- Install the real-time (preemptible) kernel (``PREEMPT_RT_FULL``) and
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real-time KVM modules
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- Configure hugepages
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- Isolate host cores to be used for instances from the kernel
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- Disable features like CPU frequency scaling (e.g. P-States on Intel
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processors)
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RHEL and RHEL-derived distros like CentOS provide packages in their
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repositories to accomplish. The ``kernel-rt`` and ``kernel-rt-kvm``
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packages will provide the real-time kernel and real-time KVM module,
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respectively, while the ``tuned-profiles-realtime`` package will provide
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`tuned`__ profiles to configure the host for real-time workloads. You should
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refer to your distro documentation for more information.
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.. __: https://tuned-project.org/
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Validation
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~~~~~~~~~~
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Once your BIOS and the host OS have been configured, you can validate
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"real-time readiness" using the ``hwlatdetect`` and ``rteval`` utilities. On
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RHEL and RHEL-derived hosts, you can install these using the ``rt-tests``
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package. More information about the ``rteval`` tool can be found `here`__.
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.. __: https://git.kernel.org/pub/scm/utils/rteval/rteval.git/tree/README
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Configuring a flavor or image
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-----------------------------
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.. versionchanged:: 22.0.0 (Victoria)
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Previously, it was necessary to specify
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:nova:extra-spec:`hw:cpu_realtime_mask` when realtime mode was enabled via
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:nova:extra-spec:`hw:cpu_realtime`. Starting in Victoria, it is possible
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to omit this when an emulator thread policy is configured using the
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:nova:extra-spec:`hw:emulator_threads_policy` extra spec, thus allowing all
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guest cores to be be allocated as real-time cores.
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.. versionchanged:: 22.0.0 (Victoria)
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Previously, a leading caret was necessary when specifying the value for
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:nova:extra-spec:`hw:cpu_realtime_mask` and omitting it would be equivalent
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to not setting the mask, resulting in a failure to spawn the instance.
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Compared to configuring the host, configuring the guest is relatively trivial
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and merely requires a combination of flavor extra specs and image metadata
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properties, along with a suitable real-time guest OS.
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Enable real-time by setting the :nova:extra-spec:`hw:cpu_realtime` flavor extra
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spec to ``yes`` or a truthy value. When this is configured, it is necessary to
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specify where guest overhead processes should be scheduled to. This can be
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accomplished in one of three ways. Firstly, the
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:nova:extra-spec:`hw:cpu_realtime_mask` extra spec or equivalent image metadata
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property can be used to indicate which guest cores should be scheduled as
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real-time cores, leaving the remainder to be scheduled as non-real-time cores
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and to handle overhead processes. For example, to allocate the first two cores
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of an 8 core instance as the non-real-time cores:
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.. code-block:: console
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$ openstack flavor set $FLAVOR \
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--property hw:cpu_realtime=yes \
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--property hw:cpu_realtime_mask=2-7 # so 0,1 are non-real-time
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In this configuration, any non-real-time cores configured will have an implicit
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``dedicated`` :ref:`CPU pinning policy <cpu-pinning-policies>` applied. It is
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possible to apply a ``shared`` policy for these non-real-time cores by
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specifying the ``mixed`` :ref:`CPU pinning policy <cpu-pinning-policies>` via
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the :nova:extra-spec:`hw:cpu_policy` extra spec. This can be useful to increase
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resource utilization of the host. For example:
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.. code-block:: console
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$ openstack flavor set $FLAVOR \
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--property hw:cpu_policy=mixed \
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--property hw:cpu_realtime=yes \
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--property hw:cpu_realtime_mask=2-7 # so 0,1 are non-real-time and unpinned
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Finally, you can explicitly :ref:`offload guest overhead processes to another
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host core <emulator-thread-pinning-policies>` using the
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:nova:extra-spec:`hw:emulator_threads_policy` extra spec. For example:
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.. code-block:: console
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$ openstack flavor set $FLAVOR \
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--property hw:cpu_realtime=yes \
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--property hw:emulator_thread_policy=share
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.. note::
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Emulator thread pinning requires additional host configuration.
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Refer to :ref:`the documentation <emulator-thread-pinning-policies>` for
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more information.
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In addition to configuring the instance CPUs, it is also likely that you will
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need to configure guest huge pages. For information on how to configure these,
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refer to :doc:`the documentation <huge-pages>`
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References
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----------
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* `Libvirt real time instances (spec)`__
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* `The Real Time Linux collaborative project`__
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* `Deploying Real Time OpenStack`__
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.. __: https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/libvirt-real-time.html
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.. __: https://wiki.linuxfoundation.org/realtime/start
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.. __: https://that.guru/blog/deploying-real-time-openstack/
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