In one embodiment, a computer-implemented method includes receiving a plurality of tasks to be assigned to a plurality of subgroups of virtual servers. A first plurality of the tasks is assigned to a first subgroup, where the first subgroup includes two or more virtual servers. For each of the first plurality of tasks assigned to the first subgroup, a virtual server is selected within the first subgroup, and the task is assigned to the selected virtual server. A first virtual server is migrated, by a computer processor, from the first subgroup of virtual servers to a second subgroup of virtual servers, if at least one predetermined condition is met, where the migration maintains in the first subgroup at least one of the first plurality of tasks assigned to the first subgroup.