Comment 7 for bug 1786055

Reviewed: https://review.openstack.org/590388
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=9ea340eb0d3bdb103bd64ca40b999bd2b10b80aa
Submitter: Zuul
Branch: master

commit 9ea340eb0d3bdb103bd64ca40b999bd2b10b80aa
Author: Jay Pipes <email address hidden>
Date: Thu Aug 9 10:46:20 2018 -0400

    placement: use simple code paths when possible

    Somewhere in the past release, we started using extremely complex code
    paths involving sharing providers, anchor providers, and nested resource
    provider calculations when we absolutely don't need to do so.

    There was a _has_provider_trees() function in the
    nova/api/openstack/placement/objects/resource_provider.py file that used
    to be used for top-level switching between a faster, simpler approach to
    finding allocation candidates for a simple search of resources and
    traits when no sharing providers and no nesting was used. That was
    removed at some point and all code paths -- even for simple "get me
    these amounts of these resources" when no trees or sharing providers are
    present (which is the vast majority of OpenStack deployments) -- were
    going through the complex tree-search-and-match queries and algorithms.

    This patch changes that so that when there's a request for some
    resources and there's no trees or sharing providers, we do the simple
    code path. Hopefully this gets our performance for the simple, common
    cases back to where we were pre-Rocky.

    This change is a prerequisite for the following change which adds
    debugging output to help diagnose which resource classes are running
    out of inventory when GET /allocation_candidates returns 0 results.
    That code is not possible without the changes here as they only
    work if we can identify when a "simpler approach" is possible and
    call that simpler code.

    Related-Bug: #1786055
    Partial-Bug: #1786519
    Change-Id: I1fdbcdb7a1dd51e738924c8a30238237d7ac74e1