1. A method comprising: calculating, by use of a processor, an expected energy efficiency for a prior mapping of process threads for a plurality of cores; calculating current reliability levels for the plurality of cores from degradation rates for the plurality of cores; calculating desired reliability levels for the plurality of cores from the current reliability levels and initial reliability levels for the plurality of cores; calculating an error desired level from the current reliability levels and the desired reliability levels; calculating a workload acceptance capacity (WAC) from the error desired level; and mapping the process threads to the plurality of cores, wherein a process thread is mapped to a core if a mapping policy is satisfied by a utilization for the core not exceeding an energy delay and the WAC for the core not reaching a specified WAC threshold and a specified number of the plurality of cores are not powered and are not mapped to any of the process threads.