1. Department of Computer Science and Technology, Tsinghua University, Beijing, China
2. Department of Computer Science, University of Illinois Urbana-Champaign, Champaign, IL, USA
3. Key Laboratory of Data Engineering and Knowledge Engineering (MOE), School of Information, Renmin University of China, Beijing, China
4. Ming Hsieh Department of Electrical Engineering, University of Southern California, Los Angeles, CA, USA
5. School of Computing, National University of Singapore, Singapore