Authors’ addresses: G. Chen, X. Song (corresponding author), Department of Electrical and Computer Engineering, Portland State University, PO Box 751, Portland, OR 97207-0751; email: song@ee.pdx.edu; F. Liu, Q. Tan, National Lab of Parallel Distributed Processing, Changsha, Hunan, China; F. He, School of Software, Tsinghua University, Beijing, 100084, P.R. China. Permission to make digital or hard copies of part or all of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or direct commercial advantage and that copies show this notice on the first page or initial screen of a display along with the full citation. Copyrights for components of this work owned by others than ACM must be honored. Abstracting with credit is permitted. To copy otherwise, to republish, to post on servers, to redistribute to lists, or to use any component of this work in other works requires prior specific permission and/or a fee. Permissions may be requested from Publications Dept., ACM, Inc., 2 Penn Plaza, Suite 701, New York, NY 10121-0701, fax +1 (212) 869-0481, or permissions@acm.org. C © 2007 ACM 1084-4309/2007/09-ART52 $5.00 DOI 10.1145/1278349.1278365 http://doi.acm.org/ 10.1145/1278349.1278365