Wan Jianxiong is Ph.D. candidates majored in computer science in University of Science and Technology, Beijing. His research interests include optimal control and performance evaluation of computer systems, especially avionics systems and distributed service computing systems. Xiang Xudong is Ph.D. candidates majored in computer science in University of Science and Technology, Beijing. His research interests include optimal control and performance evaluation of computer systems, especially avionics systems and distributed service computing systems. Bai Xiaoying is an associate professor in the department of computer science and technology, Tsinghua University. Her research interests include software engineering and system reliability. Corresponding author at: Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China. Tel.: + 86 10 62772487. E-mail address: baixy@tsinghua.edu.cn (X. Bai).