Simple COMA Node Implementations

Shared memory architectures often have caches to reduce the number of slow remote memory accesses. The largest possible caches exist in shared memory architectures called Cache-Only Memory Architectures (COMAs). In a COMA all the memory resources are used to implement large caches. Unfortunately, these large caches also have their price. Due to its lack of physically shared memory, COMA may su er from a longer remote access latency than alternatives. Large COMA caches might also introduce an extra latency for local memory accesses, unless the node architecture is designed with care. We examine the implementation of COMAs, and consider how to move much of the complex functionality into software. We introduce the idea of a simple COMA architecture a hybrid with hardware support only for the functionality frequently used. Such a system is expected to have good performance, and because of its simplicity it should be quick and cheap to develop and engineer.