Scalability of SCI workstation clusters, a preliminary study

SCI is based on unidirectional point-to-point links connected in rings. Separate rings can be connected using switches to allow for larger configurations than are feasible with a single ring. The SCI standard is implemented in the LinkController (LC-1) chip from Dolphin Interconnect Solutions. Experiments are conducted on a set of Sun UltraSparc workstations connected by the new Sbus/SCI adapters based on the LC-1. The practical limit on the number of nodes on a ring depends on the host interface and the link capacity and tuning, as well as the expected interconnect load. Single ring configurations of up to 10 nodes on a ring and 4 and 8 node systems using multiple smaller rings connected through a 4-way switch are analysed with respect to latency and network throughput. Results demonstrate a peak aggregate ring bandwidth of 187 Mbytes/s in a 10 node ring configuration, and a peak bandwidth of 153 Mbytes/s for the 4-way switched 8 node system. Results also show that the interconnect is sensitive to certain traffic patterns, and that different topologies have different weak spots in this sense.