Anatomy of a Message in the Alewife Multiprocessor

Shared-memory provides a uniform and attractive mechanism for communication. For efficiency, it is often implemented with a layer of interpretive hardware on top of a message-passing communications network. This interpretive layer is responsible for data location, data movement, and cache coherence. It uses patterns of communication that benefit common programming styles, but which are only heuristics. This suggests that certain styles of communication may benefit from direct access to the underlying communications substrate. The Alewife machine, a shared-memory multiprocessor being built at MIT, provides such an interface. The interface is an integral part of the shared memory implementation and affords direct, user-level access to the network queues, supports an efficient DMA mechanism, and includes fast trap handling for message reception. This paper discusses the design and implementation of the Alewife message-passing interface and addresses the issues and advantages of using such an interface to complement hardware-synthesized shared memory.