[openstack-dev] [nova] Queens PTG recap - placement

Matt Riedemann mriedemos at gmail.com
Mon Sep 18 15:28:14 UTC 2017

Placement related items came up a lot at the Queens PTG. Some on Tuesday 
[1], some on Wednesday [2], some on Thursday [3] and some on Friday [4].

Priorities for Queens

The priorities for placement/scheduler related items in Queens are:

1. Migration allocations [5] - we realized late in Pike that the way we 
were tracking allocations across source and dest nodes during a move 
operation (cold migrate, live migrate, resize, evacuate) was confusing 
and error prone, and we had to "double up" allocations for the instance 
during the move. The idea here is to simplify the resource allocation 
modeling during a move operation by having the migration record be a 
consumer of resource allocations during the move, so we can keep the 
source/dest node allocations separate using the instance/migration 
records. This is mostly internal technical debt reduction and to 
simplify our accounting which should mean fewer bugs.

2. Alternate hosts - this is the work to have the scheduler determine a 
set of alternative hosts for reschedules. This is important for cells v2 
where the cell conductor and nova-compute services can't reach the API 
database or scheduler, so reschedules need to happen within the cell 
given a list of pre-determined hosts chosen by the scheduler at the top. 
Ed Leafe has already started on some of this [6].

3. Nested resource providers [7] - this has been around for awhile now 
but hasn't had the proper reviewer focus due to other priorities. We are 
making this a priority in Queens as it enables a lot of other use cases 
like bandwidth-aware scheduling and being able to eventually remove 
major chunks of the claims code in the ResourceTracker in the compute 
service. We agreed that in Queens we want to try and keep the scope of 
this small and focus on being able to model a simple SR-IOV PF/VF 
relationship. Modeling NUMA use cases will be post-Queens. We will need 
quite a bit of work on functional testing done along with this so that 
we have some fixtures and/or fake virt drivers in place to model things 
like CPU pinning, huge pages, NUMA, SR-IOV, etc which also verify 
allocations in Placement to know we are doing things correctly from the 
client perspective, similar to the functional tests added for verifying 
allocations during move operations in Pike.

General device management

This was a more forward looking discussion and the notes are in the 
etherpad [3]. This is not really slated for Queens work except to make 
sure that things we do in Queens don't limit what we can do for 
generically managing devices later, and is tied heavily to the nested 
resource providers work.

Other discussion

Traits - supporting required traits in a flavor is on-going and the spec 
is here [8].

Shared storage providers [9] - we have decided to defer working on this 
from Queens given other priorities. Modeling move allocations with 
migration records should help here though.

Modeling distance for (anti-)affinity use cases - this is being deferred 
from Queens. There are workarounds when running with multiple cells.

Limits and ordering in Placement - Chris Dent has proposed a spec [10] 
so that we can limit the size of a response when getting resource 
providers from Placement during scheduling and also optionally configure 
the behavior of how Placement orders the returned set, so you can pack 
or spread possible build candidates.

OSC plugin - I'm trying to push this work forward. We have the plugin 
installed with devstack now and a functional CI job for the repo but 
need to move some of the patches forward that add the CLI functionality.

There was lots of other random stuff in [2] and [4] but for the most 
part are not prioritized, spec'ed out or have a clear owner, so those 
are not really getting attention for Queens.

[1] https://etherpad.openstack.org/p/placement-nova-neutron-queens-ptg
[2] https://etherpad.openstack.org/p/nova-ptg-queens-placement
[4] https://etherpad.openstack.org/p/nova-ptg-queens
[6] https://review.openstack.org/#/c/498830/
[8] https://review.openstack.org/#/c/468797/
[9] https://bugs.launchpad.net/nova/+bug/1707256
[10] https://review.openstack.org/#/c/504540/




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