1. An apparatus comprising: at least one processor; a memory coupled to the at least one processor; a first physical host with a virtual machine and a second physical host coupled to the apparatus via a network; an optimizer for optimizing the loading of virtual machines on host computers; an aggressiveness policy mechanism residing in the memory and executed by the at least one processor that provides dynamically changing the aggressiveness settings for loading the virtual machines based on aggressiveness policies that indicate to change aggressiveness settings depending on system conditions comprising: number of migration failures in the past, number of network connections for a physical host, the total number of virtual machines in the system, wherein the aggressiveness settings comprise: a maximum number of migrations per host per hour, and a maximum number of concurrent migrations per host; and wherein the optimizer dynamically changes the aggressiveness of optimization as indicated by the dynamically changing aggressiveness settings.