1. Department of Computer Science and Technology Tsinghua University, Beijing, China
2. Key Laboratory of Pervasive Computing, Ministry of Education, China
3. Department of Computer Sciences, University of Wisconsin-Madison, Madison, USA
4. Department of Computer Science and Technology, Tsinghua University,, Beijing, China
5. DepartmenDepartment of Computer Science and Technology, Tsinghua University,, Beijingt of Computer science and Technology Tsinghua University, China