  In this paper, we present a novel multi-objective ant colony system algorithm
for virtual machine (VM) consolidation in cloud data centers. The proposed
algorithm builds VM migration plans, which are then used to minimize
over-provisioning of physical machines (PMs) by consolidating VMs on
under-utilized PMs. It optimizes two objectives that are ordered by their
importance. The first and foremost objective in the proposed algorithm is to
maximize the number of released PMs. Moreover, since VM migration is a
resource-intensive operation, it also tries to minimize the number of VM
migrations. The proposed algorithm is empirically evaluated in a series of
experiments. The experimental results show that the proposed algorithm provides
an efficient solution for VM consolidation in cloud data centers. Moreover, it
outperforms two existing ant colony optimization based VM consolidation
algorithms in terms of number of released PMs and number of VM migrations.
