AWS isn't so stupid or poor
To waste brain cells on dumbass optimization like allowing over commit, and then trying to manage an self inflicted problem
Every rack in AWS datacenter is committed to exactly once instance type. Each server is sliced into whole numbers of instances that fit the memory size and core count that corresponds to the chosen type. RDS racks are just special cases of ec2 racks. If there are left over cores or gb of ram, nobody gives a shit.
Alibaba apparently isn't so clever, they are stupid for worrying about minutia that is not remotely i.portant.