established SLA policy between the service provider and consumer will be violated. The service provider will need to pay a penalty for violating the SLA and the penalty is calculated by multiplying the violated time frame of the SLA violation with the cost per unit. To solve the oversubscription problem, a single VM should be migrated to another host. This migration process will decrease the CPU’s utilization and will help to maintain the utilization threshold. However, during the migration, if another host that is used for migration in that specific time, the energy cost will be doubled. Defining when a migration should be initiated is quite challenging, especially when the minimization of energy cost and SLA violation costs take place.\

![image](Sin_VM_mig){width="4.5in"}

Fig. 1 shows how a single VM migration helps to protect a possible system failure before the host is heavily overloaded using the CPU utilization threshold. The analysis of cost function, optimal offline and online algorithm has been performed by Beloglazov and Buyya \[18\]. The cost of consumed energy by the server is paid by the resource provider and it is calculated as $C_pt_p$ ($C_p$ is the power cost and $t_p$ is time period). The resource capacity of a 