Superconducting processors for HTMT: issues and challenges

The Hybrid Technology Multi-Threading project is a long-term study of the feasibility of combining several emerging technologies to reach 1 petaFLOPS within ten years. HTMT will combine high-speed superconductor processors, semiconductor memories with built-in processors, high-speed optical interconnects, and high-density holographic storage. While there are major challenges in all aspects of this project, those in processor architecture are the focus of this paper. Fundamental differences between RSFQ circuits and conventional semiconductor circuits, including a radical jump in clock speed, make today's processor design approaches inappropriate for HTMT. Sequential instruction dispatching, even within the lowest programming unit (a strand), will lead to unacceptably high latencies, hence poor performance. We propose alternative processor designs which use fine-grain synchronizations between individual instructions in order to avoid these bottlenecks.