Architectural support for efficient multicasting in irregular networks

Abstract

Parallel computing on networks of workstations is fast becoming a cost-effective high-performance computing alternative to MPPs. Such a computing environment typically consists of processing nodes interconnected through a switch-based irregular network. Many of the problems that were solved for regular networks have to be solved anew for these systems. One such problem is that of efficient multicast communication. In this paper, we propose two broad categories of schemes for efficient multicasting in such irregular networks: network interface-based (NI-based) and switch-based. The NI-based multicasting schemes use the network interface of intermediate destinations for absorbing and retransmitting messages to other destinations in the multicast tree. In contrast, the switch-based multicasting schemes use hardware support for packet replication at the switches of the network and a concept known as multidestination routing to convey a multicast message from one source to multiple destinations. We first present alternative schemes for efficient multipacket forwarding at the NI and derive an optimal k-binomial multicast tree for multipacket NI-based multicast. We then propose two switch-based multicasting schemes that differ in the power of the encoding scheme and the complexity of the decoding logic at the switches. These multicasting schemes use path-based multidestination worms that can cover all nodes connected to switches along a valid unicast path and tree-based multidestination worms that can cover entire destination sets in a single phase using one worm, respectively. For each scheme, we describe the associated header encoding and decoding operation, the method for deriving multidestination worms that cover arbitrary multicast destination sets, and the multicasting scheme using the derived multidestination worms. We then compare the NI-based multicasting scheme to the switch-based multicasting schemes with path-based and tree-based multidestination worms using simulation to determine the system parameters that affect each of the schemes and the range of system parameters for which each scheme performs best. Our results show that the switch-based multicasting scheme using a single tree-based multidestination worm performs the best among the three schemes. However, the NI-based multicasting scheme is capable of delivering high performance compared to the switch-based multicast using pathbased worms, especially when the software overhead at the network interface is less than half of the overhead at the host. We therefore conclude that support for multicast at the NI is an important first step to improving multicast performance. However, there is still considerable gain that can be achieved by supporting hardware multicast in switches. Finally, while supporting such hardware multicast, it is better to support schemes that can achieve multicast in one phase.