1. Department of Computer Science, Chongqing University, Chongqing 400044, P. R. China
2. Department of Computer Science, School of Systems Engineering, University of Reading, Reading, Berkshire, RG6 6AY, UK
3. Parallelism, Algorithms and Architectures Research Centre, Department of Computer Science, Loughborough University, Loughborough, Leicestershire, LE11 3TU, UK