The Scalable Coherent Interface (SCI) is an IEEE standard originally designed to provide an interconnect for cache-coherent shared memory systems. One of the first major deployments of SCI was on the Convex Exemplar SPP-1000 in 1994. SCI has not been able to gain ground in traditional networking markets, despite its ability to serve as a general purpose interconnect. The main reason Beowulf designers choose to use SCI is for its low latency of less than 10 microseconds. Current PC motherboard chip sets do not support the coherency mechanisms required to construct an SCI-based shared memory Beowulf. But if that functionality is ever added to commodity motherboards, we may see an increase in the popularity of SCI as researchers experiment with shared-memory Beowulf systems. Five years ago SCI delivered many clear advantages, but today commodity network technology has caught up, although SCI still delivers significantly lower latency.
5.2.4 Myrinet
Myrinet is a system area network designed by Myricom, Inc. On November 2, 1998, it was approved as American National Standard ANSI/VITA 26-1998. It is designed around simple low-latency blocking switches. The path through these switches is implemented with "header-stripping" source routing, where the sending node prepends the route through the network, and each switch remove the first byte of the message and uses it as the output port. Packets can be of arbitrary length.
The bandwidth of the adapter and switch is hidden from the application, and has regularly increased over time from the original 640 MB/s to the current 2.4 GB/s. A limitation of Myrinet is that the switches are incrementally blocking. If a destination port is busy in a multistage network, the packet is stalled and that stalled packet potentially blocks other packets traveling the network, even with unrelated source and destination nodes. This problem is mitigated by the network's high speed and the ability to construct topologies with rich interconnects. Blocking is minimized by higher density switches that reduce the number of a stages traversed by a typical message in a network of fixed size.
While Myrinet is the strongest provider of high-bandwidth SANs, it has the limitation of being provided by a single vendor. The price of the network adapters has remained high, typically exceeding the price of the entire computing node. Where Myrinet has a big advantage is customized protocols. It effectively provides a second processor that can do much of the protocol work and avoid interrupting the primary processor. The former advantage could be obtained for less money by adding
