Nova supports configuring real-time
policies for instances. This builds upon the improved performance
offered by CPU pinning <cpu-topologies> by providing
stronger guarantees for worst case scheduler latency for vCPUs.

Enabling Real-Time

Currently the creation of real-time instances is only supported when
using the libvirt compute driver with a libvirt.virt_type of kvm
or qemu. It requires extensive configuration of the host
and this document provides but a rough overview of the changes required.
Configuration will vary depending on your hardware, BIOS configuration,
host and guest OS’ and application.

BIOS configuration

Configure your host BIOS as recommended in the rt-wiki
page. The most important steps are:

• Disable power management, including CPU sleep states
• Disable SMT (hyper-threading) or any option related to logical
  processors

These are standard steps used in benchmarking as both sets of
features can result in non-deterministic behavior.

OS configuration

This is inherently specific to the distro used, however, there are
some common steps:

• Install the real-time (preemptible) kernel
  (PREEMPT_RT_FULL) and real-time KVM modules
• Configure hugepages
• Isolate host cores to be used for instances from the kernel
• Disable features like CPU frequency scaling (e.g. P-States on Intel
  processors)

RHEL and RHEL-derived distros like CentOS provide packages in their
repositories to accomplish. The kernel-rt and
kernel-rt-kvm packages will provide the real-time kernel
and real-time KVM module, respectively, while the
tuned-profiles-realtime package will provide tuned profiles to configure the
host for real-time workloads. You should refer to your distro
documentation for more information.

Validation

Once your BIOS and the host OS have been configured, you can validate
“real-time readiness” using the hwlatdetect and
rteval utilities. On RHEL and RHEL-derived hosts, you can
install these using the rt-tests package. More information
about the rteval tool can be found here.

Configuring a flavor or
image

Compared to configuring the host, configuring the guest is relatively
trivial and merely requires a combination of flavor extra specs and
image metadata properties, along with a suitable real-time guest OS.

Enable real-time by setting the hw:cpu_realtime flavor extra spec to
yes or a truthy value. When this is configured, it is
necessary to specify where guest overhead processes should be scheduled
to. This can be accomplished in one of three ways. Firstly, the hw:cpu_realtime_mask extra spec or
equivalent image metadata property can be used to indicate which guest
cores should be scheduled as real-time cores, leaving the remainder to
be scheduled as non-real-time cores and to handle overhead processes.
For example, to allocate the first two cores of an 8 core instance as
the non-real-time cores:

$ openstack flavor set $FLAVOR \
    --property hw:cpu_realtime=yes \
    --property hw:cpu_realtime_mask=2-7  # so 0,1 are non-real-time

In this configuration, any non-real-time cores configured will have
an implicit dedicated CPU pinning policy <cpu-pinning-policies>
applied. It is possible to apply a shared policy for these
non-real-time cores by specifying the mixed CPU pinning policy <cpu-pinning-policies> via
the hw:cpu_policy extra spec. This can be
useful to increase resource utilization of the host. For example:

$ openstack flavor set $FLAVOR \
    --property hw:cpu_policy=mixed \
    --property hw:cpu_realtime=yes \
    --property hw:cpu_realtime_mask=2-7  # so 0,1 are non-real-time and unpinned

Finally, you can explicitly offload guest overhead processes to another
host core <emulator-thread-pinning-policies> using the
hw:emulator_threads_policy extra spec. For
example:

$ openstack flavor set $FLAVOR \
    --property hw:cpu_realtime=yes \
    --property hw:emulator_thread_policy=share

In addition to configuring the instance CPUs, it is also likely that
you will need to configure guest huge pages. For information on how to
configure these, refer to the documentation <huge-pages>

References

• Libvirt
  real time instances (spec)
• The Real
  Time Linux collaborative project
• Deploying
  Real Time OpenStack