Therefore, servers consume more than half their full power even when they are idling [13]. In our case, however, a hypervisor cannot host many VMs effectively due to its limited resources. For example, it takes 20 s for the low-power server we used to boot a new VM, which is 5 times longer than waking up a new physical server. Therefore, we managed the system in units of physical servers, not VMs. This phrase indicates an ideal feature of energy-efficient servers, namely, power consumption is proportional to the load or CPU usage. Since this feature is not available in existing servers as aforementioned, it is often emulated roughly in a server cluster by turning off excess servers depending on the load of the cluster.