Datensatz

Zusammenfassung

We study the problem of exchanging a set of messages among a group of processors, where messages may consist of different numbers of packets. We consider the model of half-duplex communication. Let $\Lmax$ denote the maximum number of packets that a processor must send and receive. If all the packets need to be delivered directly, at least $\frac{3}{2}\Lmax$ communication steps are needed to solve the problem in the worst case. We show that by allowing forwarding, only $\frac{6}{5}\Lmax + \Oh{1}$ time steps are needed to exchange all the messages, and this is optimal. Our work was motivated by the importance of irregular message exchanges in distributed-memory parallel computers, but it can also be viewed as an answer to an open problem on scheduling file transfers posed by Coffmann, Garey, Johnsson, and LaPaugh in 1985.