Myrinet Clos Network

Figure 7: A 128-host Clos network using 16-port Myrinet switch (upper) and a 64-host Clos network using 16-port Myrinet switch (each line represents two links).

• Myrinet is a high-performance, packet-communication and switching technology. It was produced by Myricom as a high-performance alternative to conventional Ethernet networks.
• Myrinet switches are multiple-port components that route a packet entering on an input channel of a port to the output channel of the port selected by the packet. Myrinet switches have 4, 8, 12, 16 ports. For an $n$-port switch, the ports are addressed $0, 1, 2,\ldots, n-1$. For any switching permutation, there may be as many packets traversing a switch concurrently as the switch has ports. These switches are implemented using two types of chips: crossbar-switch chips and the Myrinet-interface chip.
• The basic building block of the Myrinet-2000 network is a 16-port Myrinet crossbar switch, implemented on a single chip designated as Xbar16. It can be interconnected to build various topologies of varying sizes. The most common topology is the Clos network.
• Figure 7upper shows a 128-host Clos network, which includes 24 Xbar16s. Each Xbar16 switch is represented using a circle. The eight switches forming the upper row is the Clos network spine, which is connected through a Clos spreader network to the 16 leaf switches forming the lower row.
• The Clos network provides routes from any host to any other host. There is a unique shortest route between hosts connected to the same Xbar16. Routes between hosts connected to different Xbar16s traverse three Xbar16 switches.
• Figure 7lower shows a 64-host Clos network. A network of 64 hosts or fewer would require eight-port switches for the spine. In the figure, an Xbar16 switch can serve the purpose of two 8-port switches. The thick line connecting a spine switch to a leaf switch represents two links.
• The routing of Myrinet packets is based on the source routing approach. Each Myrinet packet has a variable length header with complete routing information. When a packet enters a switch, the leading byte of the header determines the outgoing port before being stripped off the packet header. At the host interface, a control program is executed to perform source-route translation.