The Complexity of Mean Flow Time Scheduling Problems with Release Times
  We study the problem of preemptive scheduling n jobs with given release times on m identical parallel machines. The objective is to minimize the average flow time. We show that when all jobs have equal processing times then the problem can be solved in polynomial time using linear programming. Our algorithm can also be applied to the open-shop problem with release times and unit processing times. For the general case (when processing times are arbitrary), we show that the problem is unary NP-hard. 
Philippe Baptiste, Peter Brucker, Marek Chrobak, Christoph Durr,   Svetlana A. Kravchenko, Francis Sourd