** Also affects: nova/train
Importance: Undecided
Status: New
** Also affects: nova/pike
Importance: Undecided
Status: New
** Also affects: nova/stein
Importance: Undecided
Status: New
** Also affects: nova/rocky
Importance: Undecided
Status: New
** Also affects: nova/queens
Importance: Undecided
Status: New
** Changed in: nova
Importance: Undecided => Medium
** Tags added: scheduler
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Engineering Team, which is subscribed to OpenStack Compute (nova).
https://bugs.launchpad.net/bugs/1870096
Title:
soft-affinity weight not normalized base on server group's maximum
Status in OpenStack Compute (nova):
In Progress
Status in OpenStack Compute (nova) pike series:
New
Status in OpenStack Compute (nova) queens series:
New
Status in OpenStack Compute (nova) rocky series:
New
Status in OpenStack Compute (nova) stein series:
New
Status in OpenStack Compute (nova) train series:
New
Bug description:
Description
===========
When using soft-affinity to schedule instances on the same host, the
weight is unexpectedly low if a server was previously scheduled to any
server-group with more members on a host. This low weight can then be
easily outweighed by differences in resources (e.g. RAM/CPU).
Steps to reproduce
==================
Do not restart nova-scheduler in the process or the bug doesn't
appear. You need to change the ServerGroupSoftAffinityWeigher to
actually log the weights it computes to see the problem.
* Create a server-group with soft-affinity (let's call it A)
* Create 6 servers in server-group A, one after the other so they end up on
the same host.
* Create another server-group with soft-affinity (B)
* Create 1 server in server-group B
* Create 1 server in server-group B and look at the scheduler's weights
assigned to the hosts by the ServerGroupSoftAffinityWeigher.
Expected result
===============
The weight assigned to the host by the ServerGroupSoftAffinityWeigher
should be 1, as the maximum number of instances for server-group B is
on that host (the one we created there before).
Actual result
=============
The weight assigned to the host by the ServerGroupSoftAffinityWeigher is 0.2,
as the maximum number of instances ever encountered on a host is 5.
Environment
===========
We noticed this on a queens version of nova a year ago. Can't give the
exact commit anymore, but the code still looks broken in current
master.
I've opened a review-request for fixing this bug here:
https://review.opendev.org/#/c/713863/
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