What limits prevented Cray from building even “bigger” machines ?
By bigger in supercomputing terms we mean more CPU horsepower in one system. Looking at the T3e range of machines, we see that that the T3e is the most scaleable architecture of any of the Cray machines. The systems can be grown buy adding boards of 4 or 8 processors during a maintenance slot. However adding boards does require some rewiring of the interconnect torus and takes a few hours. There would be various reasons why a limit is set on the size of a T3E. I am not sure which of the following played a part in the upper size limit of a T3e but here are the ideas. Physical size limits: 2176 CPUs, the largest known configuration, would fill 8 system cabinets and beyond that the distance across the frame could have adverse timing influences on the clock circuits. The clock was carried from cab to cab using Fibre optic circuits. The physically bigger a system gets the longer it takes to co-ordinate the parts. Which is why computers get denser by preference rather than larger. Seymour Cr
