Willow: a scalable shared memory multiprocessor

The authors are currently developing Willow, a shared memory multiprocessor whose design provides system capacity and performance capable of supporting over 1000 commercial microprocessors. Recently, they have focused their attention on the design of a 64-processor prototype that tests most of their ideas about scalability. In this paper, they describe the factors that traditionally limited the scalability of shared memory systems. These include enforcing sequential consistency, inefficient synchronization, memory latency and bandwidth limitations, bus and memory contention, the necessity to enforce inclusion on lower-level caches, and limited I/O (input/output) bandwidth. They then describe how the Willow architecture addresses each of these issues. Finally, they present data in order to evaluate the effect of the major architectural innovations in Willow on the performance of several parallel applications