The poor energy proportionality of server is seen as the principal source for low energy efficiency of mod data centers. We find that different resource configurations of an application lead to similar performance, but have distinct energy consumption. We call this phenomenon as performance-equivalent resource configurations (PERC), and its performance range is called equivalent region (ER). Based on PERC, one basic idea for improving energy efficiency is to select the most efficient configuration from PERC for each application. However, it cannot support every application to obtain optimal solution when thousands of applications are run simultaneously on resource-bounded servers. Here we propose a heuristic scheme, CPicker, based on genetic programming to improve energy efficiency of servers. To speed up convergence, CPicker initializes a high quality population by first choosing configurations from regions that have high energy variation. Experiments show that CPicker obtains above 17%energy efficiency improvement compared with the greedy approach, and less than 4%efficiency loss compared with the oracle case.