The MIT Alewife Machine

Abstract

A variety of models for parallel architectures, such as shared memory, message passing, and data flow, have converged in the recent past to a hybrid architecture form called distributed shared memory (DSM). By using a combination of hardware and software mechanisms, DSM combines the nice features of all the above models and is able to achieve both the scalability of messagepassing machines and the programmability of shared memory systems. Alewife, an early prototype of such DSM architectures, uses a hybrid of software and hardware mechanisms to support coherent shared memory, efficient user-level messaging, fine-grain synchronization, and latency tolerance. Alewife supports up to 512 processing nodes connected over a scalable and cost-effective mesh network at a constant cost per node. Four mechanisms combine to achieve Alewife's goals of scalability and programmability: software-extended coherent shared memory provides a global, linear address space; integrated message passing allows compiler and operating system designers to provide efficient communication and synchronization; support for fine-grain computation allows many processors to cooperate on small problem sizes; and latency-tolerance mechanisms-including block multithreading and prefetching-mask unavoidable delays due to communication.