Scheduling UET task systems with concurrency on two parallel identical processors

Problems with unit execution time (UET) tasks and two identical parallel processors have received a great deal of attention in scheduling theory. In contrast to the conventional models, where each task requires only one processor, we consider a situation when a task may require both processors simultaneously. Following Lloyd, we term such task systems systems with concurrency. For problems without precedence constraints we present several polynomial time algorithms which complement recent results of Lee and Cai. We also show that the introduction of precedence constraints leads to NP-hardness results for maximum lateness and mean ow time objective functions. For the maximum lateness problem, a family of algorithms, based upon the idea of modiied due dates, is considered. The worst case behaviour of these algorithms is analysed, and it is shown that the same upper bound is tight for each algorithm of this family.