Shiva Chaitanya Bhuvan Urgaonkar Anand Sivasubramaniam QDSL: a queuing model for systems with differential service levels. 289-300 2008 SIGMETRICS http://doi.acm.org/10.1145/1375457.1375490 conf/sigmetrics/2008 db/conf/sigmetrics/sigmetrics2008.html#ChaitanyaUS08