Self over allocating VCPUs is not a good idea. If your machine only has
4 CPUs and you expose that as 5 CPUs to a guest, you'll get
pathologically bad performance as the guest tries to optimize workloads
across those, which are causing cache flushes in the CPUs below.
Definitely in the Won't Fix category
** Changed in: nova
Status: New => Won't Fix
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https://bugs.launchpad.net/bugs/1707085
Title:
Max_unit should account for allocation_ratio
Status in OpenStack Compute (nova):
Won't Fix
Bug description:
It's great that the Placement API v1.4 provides a way to query for
hosts that can fulfill a set of allocations. However, because the
max_unit doesn't account for allocation_ratio, the result is not as
expected.
For example, my host has a total of 4 vCPU and 2 vCPU have been
allocated. cpu_allocation_ratio is 3. I expect that this host is
considered a qualified host if I make a request to the <placement
api>/resource_providers?resources=VCPU:5 because Capacity = (total-
reserved) x allocation_ratio - used which is 10. However, because the
max_unit is set as the total which is 4, my host is not in the
response although it can fulfill the requested allocation.
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