e3ada6332f
Currently we uses cpu_model in nova.conf (when cpu_mode is set to ‘custom’) to specify the CPU model the instance should use on this host. This could have implications on availability of compute nodes for live migration if you ended up with an "advanced" CPU model when all you really cared about was an older feature flag. To mitigate the problem , we use cpu_models to replace cpu_model, where cpu_models is an ordered list of CPU models the host supports. The list is ordered so that the more common and less advanced CPU models are listed earlier. End users specify CPU features they required through traits, libvirt driver then select the first one in the list that can provide the required feature traits. This would make it more likely that the instance could be live-migrated later on. blueprint: cpu-model-selection Change-Id: I546230ac115eb3de14eac25b9dd0c74aafdaccd5 Signed-off-by: Jack Ding <jack.ding@windriver.com>
4.1 KiB
Select cpu model from a list of cpu models
https://blueprints.launchpad.net/nova/+spec/cpu-model-selection
Problem description
In the libvirt virt driver, currently we use cpu_model in nova.conf
(when cpu_mode is set to custom) to specify the CPU model
the instance should use on this host. This could have implications on
availability of compute nodes for live migration if you ended up with an
advanced CPU model when all you really cared about was an
older feature flag.
Use Cases
As a user, I would like to boot an instance on a host supporting specific CPU features and for the instance to be live-migratable to as many other hosts as possible that also support the instance.
Proposed change
Replace cpu_model with cpu_models which is
an ordered list of CPU models the host supports. It is expected that the
list is ordered so that the more common and less advanced CPU models are
listed earlier. The reported cpu feature traits will be the union of
features of all the cpu models.
End users specify CPU features they required through traits1. If the cpu_mode is set
to custom, libvirt driver will select the first CPU model in
the cpu_models list (combined with
cpu_model_extra_flags if it is specified) that can provide
the required feature traits. This would make it more likely that the
instance could be live-migrated later on. If no CPU feature traits are
specified then the instance will be configured with the first CPU model
in the list.
For example, if the end user specifies CPU features avx and avx2 as following:
openstack flavor set 1 --property trait:HW_CPU_X86_AVX=required --property trait:HW_CPU_X86_AVX2=requiredand cpu_models is configured like this:
[libvirt]
cpu_mode = custom
cpu_models = Conroe,Penryn,Nehalem,Westmere,SandyBridge,IvyBridge,Haswell,Broadwell,Skylake-Client,Skylake-Serverthen Haswell, the first cpu model supporting both avx
and avx2 will be chosen by libvirt.
If cpu_model_extra_flags is specified, it should be
checked against each cpu model in the cpu_models list using
host.compare_cpu() to make sure it is compatible with all
models in the cpu_models list. Any incompatibility should
prevent compute host from starting and user needs to correct the
configuration.
If both cpu_models and cpu_model are set,
cpu_model will be ignored.
Alternatives
None
Data model impact
None
REST API impact
None
Security impact
None
Notifications impact
None
Other end user impact
None
Performance Impact
Negligible.
Other deployer impact
Add some information in the config option help text indicating that the operator should be careful to only specify models which can be fully supported in hardware. If they specify models with CPU features that are emulated by qemu it could result in performance degredation.
Developer impact
None
Upgrade impact
The operator needs to set the config option appropriately after an upgrade. If cpu_models is not set it should default to the value of cpu_model.
Implementation
Assignee(s)
TBD
Work Items
- Conf: define
[libvirt]cpu_models.[libvirt]cpu_modelis deprecated. - Virt driver changes.
- Add/modify unit tests.
Dependencies
None
Testing
Will add unit tests.
Documentation Impact
Update release note for introducing [libvirt]cpu_models.
References
History
| Release Name | Description |
|---|---|
| Stein | Introduced |