Funny... in the VMware whitepaper linked to from the article, even VMware wasn't able to get more than 110% memory over-consolidation from page sharing. I wonder what's so different about KVM's page sharing approach?
wait until memory pressure builds and paging hits performance more than you'd like, then auto-migrate machines off the host
That should happen before the host runs out of memory, or very soon after. The other thing is why wouldn't you do this waaay in advance if you have a node who's memory is not overcommitted, and a node that is. All nodes are overcommitted? Then you are basically screwed if you need to evacuate all VM's from a node at once due to failure.
Memory overcommit is just a dangerous game man. I'd only use it in the case where you provisioned resources for X physical node failures, and you have X+ failures. No other choice, and temporary.
But the demo is showing over-provisioning of desktop machines. Seriously, what enterprise sysadmin cares if it takes an extra two seconds for PowerPoint to load on a user's VDI VM?