4.6 Tools for Program Development

the rapid emergence of Gigabit Ethernet, it is likely to continue to play a critical role in Beowulf-class computing for some time to come. Ethernet, first developed at Xerox PARC in the early 1970s and standardized by the IEEE in the early 1980s, is the most widely used technology for local area networks. Ethernet continues to be an evolving technology. This section provides a brief overview of Ethernet technology and its operation.
Ethernet was originally developed as a packet-based, serial multi-drop network requiring no centralized control. All network access and arbitration control is performed by distributed mechanisms. Variable length message packets are made up of a sequence of bits including a header, data, and error detecting nodes. A fixed topology (no switched line routing) network passes packets from the source to destination through intermediate elements known as hubs or switches. The next step through the network is determined by addressing information in the packet header. The topology can be a shared multi-drop passive cable to which many Ethernet controllers are attached, a tree structure of hubs or switches, or some more complicated switching technology for high bandwidths and low latency under heavy loads.
5.1.1 Ethernet Arbitration
Distributed network control and access arbitration is accomplished by a technique referred to as the Carrier Sense Multiple Access with Collision Detect access protocol or CSMA/CD. Very simply, independent controllers on each Beowulf node monitor the activity of the SAN to determine when there is message traffic or when the network is available. A node that is ready to send a message packet waits for the network to be available and then initiates transmission sometime afterward. More than one node may attempt message transmission simultaneously. However, because of propagation delay time across the network, they would not see each other's packets until some short time thereafter and would be unable to avoid a conflict. The resulting collision of packets on the single shared network produces garbled data and none of the intended message traffic gets through to its destination correctly. Ethernet provides the means by which network nodes can detect the collision of packets



How to Build a Beowulf
How to Build a Beowulf: A Guide to the Implementation and Application of PC Clusters (Scientific and Engineering Computation)
ISBN: 026269218X
EAN: 2147483647
Year: 1999
Pages: 134

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