Coherency Hub Design for Multisocket Sun Servers with CoolThreads Technology

To bring the benefits of CMT to larger workloads, these systems had to scale beyond a single socket. Because CMT requires massive memory bandwidth to achieve adequate throughput performance, the challenge was to develop a coherency link and fabric that would allow performance to scale along with thread count in a multinode (that is, multisocket) system. In this article CoHub´s coherency scheme, ASIC design, and transtransaction flows, and discussion of the engineering challenges created by 800-MHz operation and a six-stage pipeline budget is presented. The basic principles embodied in the multinode coherency protocol and CoHub design will be important building blocks for future multinode CMT systems with higher node counts.