A multi-cluster system having processor cores of different energy efficiency characteristics is configured to operate with high efficiency such that performance and power requirements can be satisfied. The system includes multiple processor cores in a hierarchy of groups. The hierarchy of groups includes: multiple level-1 groups, each level-1 group including one or more of processor cores having identical energy efficiency characteristics, and each level-1 group configured to be assigned tasks by a level-1 scheduler; one or more level-2 groups, each level-2 group including respective level-1 groups, the processor cores in different level-1 groups of the same level-2 group having different energy efficiency characteristics, and each level-2 group configured to be assigned 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 tasks by a level-3 scheduler.