In recent years, the huge resource consumption problem of data centers is being widely concerned. Virtual machine monitor (VMM) can consolidate virtual machines (VMs) onto fewer servers via VM migration to improve the energy efficiency of data centers. This paper surveys the recent works on energy-efficient VM consolidation, and summarizes three research challenges. Among them, this work considers the resource consumption overhead caused by the waiting of virtual machines for server resource scheduling. The study theoretically and experimentally proves that under realistic constraints, this overhead remains steady as the number of consolidating VMs grows. Experiments based on a representative benchmark show that, on average, 11.7% of the server's CPU resource is occupied by the overhead. In addition, in order to fill in the gap on existing approaches, this paper proposes margin reserved consolidation (MRC) algorithm. Simulation results show that MRC outperforms the state of the art baseline in terms of server resource violation probability.