Comment 3 for bug 1787977

Reviewed: https://review.openstack.org/593131
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=c3a77f80b1863e114109af9c32ea01b205c1a735
Submitter: Zuul
Branch: master

commit c3a77f80b1863e114109af9c32ea01b205c1a735
Author: Dan Smith <email address hidden>
Date: Fri Aug 17 07:56:05 2018 -0700

    Make instance_list perform per-cell batching

    This makes the instance_list module support batching across cells
    with a couple of different strategies, and with room to add more
    in the future.

    Before this change, an instance list with limit 1000 to a
    deployment with 10 cells would generate a query to each cell
    database with the same limit. Thus, that API request could end
    up processing up to 10,000 instance records despite only
    returning 1000 to the user (because of the limit).

    This uses the batch functionality in the base code added in
    Iaa4759822e70b39bd735104d03d4deec988d35a1
    by providing a couple of strategies by which the batch size
    per cell can be determined. These should provide a lot of gain
    in the short term, and we can extend them with other strategies
    as we identify some with additional benefits.

    Closes-Bug: #1787977
    Change-Id: Ie3a5f5dc49f8d9a4b96f1e97f8a6ea0b5738b768