4.5 System Configuration

		5 Network Hardware and Software

		Networking converts a shelf full of PCs into a single system. Networking also allows a system to be accessed remotely, and to provide services to remote clients. The incredible growth of both the Internet and enterprise-specific intranets has resulted in the availability of high performance, low cost networking hardware which Beowulf systems use to create a single system from a collection of nodes. After we review networking hardware, with a particular emphasis on Fast Ethernet because of its superb price/performance ratio, we turn to the networking software options available to the Beowulf programmer, administrator and user. Networking software is usually described as a stack, made up of different protocol layers that interoperate with one another. We survey a few of the layers in the networking stack, focusing on those services and tools that are used extensively on Beowulf systems.

		For Beowulf systems, the most demanding communication requirements are not with the external environment but with the other nodes on the system area network. In a Beowulf system, every Beowulf node may need to interact with every other node, independently or together, to move a wide range of data types between processors. Such data may be large blocks of contiguous information representing subpartitions of very large global data sets, small packets containing single values, or synchronization signals in support of collective operation. In the former case, a high bandwidth communication path may be required. In the latter case, low latency communication is required to expedite execution. Requirements in both cases are highly sensitive to the characteristics of the parallel program being executed. In any case, communications capability will determine the generality of the Beowulf-class system and the degree of difficulty in constructing efficient programs. The choice of network hardware and software dictate the nature of this capability.

		5.1 Fast Ethernet

		Beowulf was enabled by the availability of a low cost, moderate bandwidth networking technology. Ethernet, initially 10 Megabit per second (Mbps) and shortly thereafter 100 Mbps peak bandwidth, provided a cost effective means of interconnecting PCs to form an integrated cluster. Used primarily for commercial local area network (LAN) technology, Ethernet supplied the means of implementing a system area network (SAN) at about 25% the cost of the total system, even when employing low cost personal computers. While other networking was and continues to be available (and used in some Beowulfs), Ethernet has been a mainstay of many Beowulf implementations. With the very low cost of 100 Mbps Fast Ethernet and