Relaxing cache coherence protocol with QOLB synchronizations

Abstract

Cache memories are widely accepted in shared-memory multiprocessor systems because they make possible the reduction of network traffic and memory latencies. However, they impose substantial overheads of cache coherency maintenance and also engender some inefficiencies of coherence misses. The paper considers that constraints of cache coherency can be relaxed in a region where exclusive accesses are guaranteed by synchronization operations, so that the execution time of parallel programs can be decreased. When an acquire operation occurs, the cache coherence protocol could not be applied to the operations following an acquire operation until a release operation is issued. The updated data in the region are transferred to other processors when a release operation is performed. The authors alleviate the overheads of release operations using QOLB synchronization primitives. Using a program-driven simulation, the new cache coherence protocol shows performance improvements in most parallel applications, and execution times can also be reduced effectively as the number of processors is increased.