The cache coherence protocol of the Data Diffusion Machine

Abstract

The Data Diffusion Machine (DDM) is a scalable shared virtual memory multiprocessor in which the location of a datum in the machine is completely decoupled from its virtual address. In particular, there is no distinguished home location where a datum must normally reside. Instead data migrates automatically to where it is needed, reducing access times and traffic.

The hardware organisation consists of a hierarchy of buses and data controllers linking an arbitrary number of processors each having a large set-associative memory. Each data controller has a set-associative directory containing status bits for data under its control. The controller supports remote data access by “snooping” on the buses above it and below it. The data access protocol it uses provides for the automatic migration, duplication and replacement of data while maintaining data coherency.

The machine is scalable in that there may be any number of levels in the hierarchy. Only a few levels are necessary in practice for a very large number of processors. Most memory requests are satisfied locally. Requests requiring remote access generally cause only a limited amount of traffic over a limited part of the machine, and are satisfied within a small time that is logarithmic to the number of processors. Although designed particularly to provide good support for the parallel execution of logic programs, the architecture is very general in that it does not assume any particular processor, language or class of application.