1. A computing system comprising: a plurality of processor cores in a hierarchy of groups, the hierarchy of groups comprising: a plurality of level-1 groups, each of the level-1 groups including one or more of the processor cores having identical energy efficiency characteristics, and each of the level-1 groups configured to be assigned first tasks by a respective level-1 scheduler; one or more level-2 groups, each of the one or more level-2 groups including a respective plurality of level-1 groups, the processor cores in different level-1 groups of a same level-2 group having different energy efficiency characteristics, and each of the one or more level-2 groups configured to be assigned second tasks by a respective level-2 scheduler; and a level-3 group including the one or more level-2 groups and configured to be assigned third tasks by a level-3 scheduler.