Comment 4 for bug 1767303

OK I understand the issue now. The problem is that when we get results from placement, all of the allocation candidates (compute nodes) might be in a single cell, because maybe the request is tied to an aggregate which represents that cell. But when the HostManager queries the cell databases for the compute nodes, it iterates overall enabled cells, which could be ~70 in CERNs case. So we're doing a lot of extra DB queries that won't yield results, and might be on older slower cell DBs which take longer to return a response.

If we could filter the cells up front based on the computes (via host_mappings maybe) like we do for filtering instances by project mapped to cells in the API using config:

https://docs.openstack.org/nova/latest/configuration/config.html#api.instance_list_per_project_cells

Then that might make scheduling faster, assuming the compute nodes are in fact restricted to a small subset of cells.