Ok, given the docs are fixed, lets put this into Opinion (which is a
closed state) for the actual code changes which don't have concensus.
** Changed in: nova
Status: New => Opinion
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https://bugs.launchpad.net/bugs/1687067
Title:
problems with cpu and cpu-thread policy where flavor/image specify
different settings
Status in OpenStack Compute (nova):
Opinion
Bug description:
nova version: Newton (and later)
There are a number of issues related to CPU policy and CPU thread
policy where the flavor extra-spec and image properties do not match
up.
The docs at https://docs.openstack.org/admin-guide/compute-cpu-
topologies.html say the following:
"Image metadata takes precedence over flavor extra specs. Thus,
configuring competing policies causes an exception. By setting a
shared policy through image metadata, administrators can prevent users
configuring CPU policies in flavors and impacting resource
utilization."
For the CPU policy this is exactly backwards based on the code. The
flavor is specified by the admin, and so it generally takes priority
over the image which is specified by the end user. If the flavor
specifies "dedicated" then the result is dedicated regardless of what
the image specifies. If the flavor specifies "shared" then the result
depends on the image--if it specifies "dedicated" then we will raise
an exception, otherwise we use "shared". If the flavor doesn't
specify a CPU policy then the image can specify whatever policy it
wants.
The issue around CPU threading policy is more complicated.
Back in Mitaka, if the flavor specified a CPU threading policy of
either None or "prefer" then we would use the threading policy
specified by the image (if it was set). If the flavor specified a CPU
threading policy of "isolate" or "require" and the image specified a
different CPU threading policy then we raised
exception.ImageCPUThreadPolicyForbidden(), otherwise we used the CPU
threading policy specified by the flavor. This behaviour is described
in the spec at https://specs.openstack.org/openstack/nova-
specs/specs/mitaka/implemented/virt-driver-cpu-thread-pinning.html
In git commit 24997343 (which went into Newton) Nikola Dipanov made a
code change that doesn't match the intent in the git commit message:
if flavor_thread_policy in [None,
fields.CPUThreadAllocationPolicy.PREFER]:
- cpu_thread_policy = image_thread_policy
+ cpu_thread_policy = flavor_thread_policy or image_thread_policy
The effect of this is that if the flavor specifies a CPU threading
policy of "prefer" then we will use a policy of "prefer" regardless of
the policy from the image. If the flavor specifies a CPU threading
policy of None then we will use the policy from the image.
This is a bug, because the original intent was to treat None and
"prefer" identically, since "prefer" was just an explicit way to
specify the default behaviour.
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