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I tried bringing this up on the irc channel, but nobody took the bait. 
Hopefully this will generate some discussion.

I just filed bug 1485631.  Nikola suggested one way of handling it, but there 
are some complications that I thought I should highlight so we're all on the 
same page.

The basic question is, if a host has X CPUs in total for VMs, and a single 
instance wants X+1 vCPUs, should we allow it?  (Regardless of overcommit ratio.) 
  There is also an equivalent question for RAM.

Currently we have two different answers depending on whether numa topology is 
involved or not.  Should we change one of them to make it consistent with the 
other?  If so, a) which one should we change, and b) how would we do that given 
that it results in a user-visible behaviour change?  (Maybe a microversion, even 
though the actual API doesn't change, just whether the request passes the 
scheduler filter or not?)

Chris