** Changed in: nova
Status: Invalid => In Progress
** Changed in: nova
Assignee: Stephen Finucane (sfinucan) => Nikola Đipanov (ndipanov)
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https://bugs.launchpad.net/bugs/1467927
Title:
Odd number of vCPUs breaks 'prefer' threads policy
Status in OpenStack Compute (nova):
In Progress
Bug description:
Using a CPU policy of dedicated ('hw:cpu_policy=dedicated') results in
vCPUs being pinned to pCPUs, per the original blueprint:
http://specs.openstack.org/openstack/nova-
specs/specs/kilo/implemented/virt-driver-cpu-pinning.html
When scheduling instance with this extra spec there appears to be an
implicit use of the 'prefer' threads policy, i.e. where possible vCPUs
are pinned to thread siblings first. This is "implicit" because the
threads policy aspect of this spec has not yet been implemented.
However, this implicit 'prefer' policy breaks when a VM with an odd
number of vCPUs is booted. This has been seen on a Hyper-Threading-
enabled host where "sibling sets" are two long, but it would
presumably happen on any host where the number of siblings (or any
number between this value and one) is not an factor of the number of
vCPUs (i.e. vCPUs % n != 0, for siblings <= n > 0).
It is reasonable to assume that a three vCPU VM, for example, should
try best effort and use siblings for at the first two vCPUs of the VM
(assuming you're on a host system with HyperThreading and sibling sets
are of length two). This would give us a true best effort
implementation.
---
# Testing Configuration
Testing was conducted on a single-node, Fedora 21-based
(3.17.8-300.fc21.x86_64) OpenStack instance (built with devstack). The
system is a dual-socket, 10 core, HT-enabled system (2 sockets * 10
cores * 2 threads = 40 "pCPUs". 0-9,20-29 = node0, 10-19,30-39 =
node1). Two flavors were used:
openstack flavor create --ram 4096 --disk 20 --vcpus 3 demo.odd
nova flavor-key demo.odd set hw:cpu_policy=dedicated
openstack flavor create --ram 4096 --disk 20 --vcpus 4 demo.even
nova flavor-key demo.even set hw:cpu_policy=dedicated
# Results
Correct case ("even" number of vCPUs)
=====================================
The output from 'virsh dumpxml [ID]' for the four vCPU VM is given
below. Similar results can be seen for varying "even" numbers of vCPUs
(2, 4, 10 tested):
<cputune>
<shares>4096</shares>
<vcpupin vcpu='0' cpuset='3'/>
<vcpupin vcpu='1' cpuset='23'/>
<vcpupin vcpu='2' cpuset='26'/>
<vcpupin vcpu='3' cpuset='6'/>
<emulatorpin cpuset='3,6,23,26'/>
</cputune>
Incorrect case ("odd" number of vCPUs)
======================================
The output from 'virsh dumpxml [ID]' for the three vCPU VM is given
below. Similar results can be seen for varying "odd" numbers of vCPUs
(3, 5 tested):
<cputune>
<shares>3072</shares>
<vcpupin vcpu='0' cpuset='1'/>
<vcpupin vcpu='1' cpuset='0'/>
<vcpupin vcpu='2' cpuset='25'/>
<emulatorpin cpuset='0-1,25'/>
</cputune>
This isn't correct. We would expect something closer to this:
<cputune>
<shares>3072</shares>
<vcpupin vcpu='0' cpuset='0'/>
<vcpupin vcpu='1' cpuset='20'/>
<vcpupin vcpu='2' cpuset='1'/>
<emulatorpin cpuset='0-1,20'/>
</cputune>
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