How does OpenStack choose which compute node to use when starting an instance?

Solution In Progress - Updated 2016-05-20T18:00:42+00:00 -

Issue

With respect to a server being chosen for a virtual machine, what is stacking and spreading?
Does OpenStack spread or stack by default?
What are the benefits of stacking?
What are the benefits of spreading?
How do I tune to have stacking or spreading?
How do I empirically verify if OpenStack is stacking or spreading and debug if necessary?

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