Comment 13 for bug 1737465

Reviewed: https://review.openstack.org/569247
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=91f5af7ee7f7140eafb7237875f6cd6ea1abcd38
Submitter: Zuul
Branch: master

commit 91f5af7ee7f7140eafb7237875f6cd6ea1abcd38
Author: Matt Riedemann <email address hidden>
Date: Thu May 17 16:06:39 2018 -0400

    Trim the fat on HostState.instances

    The only in-tree filters that rely on HostState.instances
    are the affinity filters (and one of the weighers). And they
    don't even look at the values in the HostState.instances dict,
    just the keys (which are the instance uuids for the instances
    on the host).

    So rather than pull the full instance objects, we can just get
    the list of instance uuids off the host and fake out the object.

    Custom filters/weighers will still be able to lazy-load fields
    on the Instance objects in HostState.instances if needed, but
    it will mean a performance penalty due to the round trip to the
    database per instance, per host. Out of tree filters/weighers
    are encouraged to be contributed upstream.

    Related to blueprint put-host-manager-instance-info-on-a-diet

    Related-Bug: #1737465

    Change-Id: I766bb5645e3b598468d092fb9e4f18e720617c52