General scheduling non-approximability results in presence of hierarchical communications

We investigate on the issue of minimizing the makespan (resp. the sum of the completion times) for the multiprocessor scheduling problem in presence of hierarchical communications. We consider a model with two levels of communication: interprocessor and intercluster. The processors are grouped in fully connected clusters. We propose general non-approximability results in the case where all the tasks of the precedence graph have unit execution times, and where the multiprocessor is composed of an unrestricted number of machines with l ⩾ 4 identical processors each.