Briefly, according to an embodiment of the invention, a computing system comprises: a plurality of tightly coupled processing nodes; a plurality of circuit switched networks using a circuit switching mode, interconnecting the processing nodes, and for handling data transfers that meet one or more criteria; and a plurality of electronic packet switched networks, also interconnecting the processing nodes, for handling data transfers that do meet the at least one criteria. The circuit switched networks and the electronic packet switched networks operate simultaneously. The system additionally comprises a plurality of clusters which comprise the processing nodes, and a plurality of intra-cluster communication links. The electronic packet switched networks are for handling collectives and short-lived data transfers among the processing nodes and comprises one-tenth of the bandwidth of the circuit switched networks.