The Scalable Coherent Interface (SCI)

There is rapidly increasing demand for very-high-performance networked communication for workstation clusters, distributed databases, multiprocessors, industrial data acquisition and control systems, shared access to distributed data, and so on. Higher-bandwidth hardware using the traditional protocols is not sufficient. Even at 100 Mb/s, and certainly at 250 Mb/s, throughput for many applications is so limited by delays due to architecturally induced inefficiencies, such as software overheads (often hundreds of microseconds), that higher bandwidth generally raises cost without improving performance. A new approach to communication is required, one that can eliminate the delay due to software overheads, if we are to reap the full benefit of the far higher bandwidths that modern hardware can provide. The SCI solves this problem by using the distributed-shared-memory paradigm, typically offering submicrosecond delays and bandwidths currently in the range of 1250 to 8000 Mb/s per network node. The article first reviews the general properties that an appropriate system architecture should have, and introduces an architectural model, the local area multiprocessor, distinguished by its shared-memory performance and its ability to handle LAN-style distances. These desired properties are then considered in more detail, and practical design decisions are made, illustrated by the evolution of the ISO/ANSI/IEEE standard Scalable Coherent Interface (SCI) as it addressed these issues. Finally, the current status of the various SCI follow-on and support projects is reported.