Toward Large Scale Shared Memory Multiprocessing

We are currently investigating two di erent approaches to scalable shared memory Munin a distributed shared memory DSM system implemented entirely in software and Willow a true shared memory multiprocessor with extensive hardware support for scalability Munin allows parallel programs written for shared memory multiprocessors to be executed e ciently on dis tributed memory multiprocessors Unlike existing DSM systems which only provide a single mechanism for memory consistency Munin provides multiple consistency protocols matching protocol to data object based on the expected pattern of accesses to that object We call this approach type speci c coher ence Munin also employs a relaxed consistency model to mask network latency and to minimize the number of messages required for keeping memory consis tent Willow is intended to be a true shared memory multiprocessor providing memory capacity and performance capable of supporting over a thousand com mercial microprocessors These processors are arranged in cluster fashion with a multi level cache I O synchronization and memory hierarchy Willow is distinguished from other shared memory multiprocessors by a layered memory organization that signi cantly reduces the impact of inclusion on the cache hierarchy and that exploits locality gradients Willow also provides support for adaptive cache coherence an approach similar to Munin s type speci c co herence whereby the consistency protocol used to manage each cache line is selected based on the expected or observed access behavior for the data stored in that line Implementation of Munin is in progress we are still designing Willow