1. A method for managing mainframe computer system usage comprising: monitoring processing capacity usage of first and second logical partitions in executing a plurality of workload tasks, the first and second logical partitions both defined as being within a first group having a first group policy defined therefor, the first group policy utilizing at least two different time criticality levels applicable to both the first and second logical partitions, the first group also utilizing first and second capacity usage limits applicable, respectively, to the first and second logical partitions; determining, for each of the first and second logical partitions, based on the processing capacity usage monitoring, 4 hour rolling average processing capacity usages at the least two different time criticality levels; determining, based on the processing capacity usage monitoring and the first group policy, if either of the 4 hour rolling average processing capacity usages of the first and second logical partitions will exceed its respective capacity usage limit; if one of the first and second logical partitions is determined to exceed its respective capacity usage limit, automatically adjusting the first and second partition processing capacity limits to allocate processing capacity therebetween, based on the time criticality level processing capacity usage determination, to ensure that processing capacity is made available to workload tasks at a higher of the at least two time criticality levels before workload tasks at a lower of the at least two time criticality levels.