Ah, I think the part that I didn't consider was that an "idle" VM is not zero CPU cost, unlike a container, so indeed from a hypervisor owner perspective you'd like other active VMs to be able to use that CPU time. But again, doesn't that presuppose oversubscription? If a node is fully reserved, it doesn't matter if all of the running VMs are idle, you're still not going to be able to schedule another job on that node, so your costs remain the same unless you oversubscribe the host and count on the fact that there will be unused capacity available most of the time (similar to AWS Flex instances).

Yes definitely as an operator, you want to oversubscribe hosts. What I was mentioning is that there are still small benefits when an host is not full: the CPU gains _and_, for users, the fact that they're not paying/paying less (even though the operator is still paying for the full underutilized hypervisor, but hey, that's the game)