Adaptive Coherence Granularity for Multi-Socket Systems

The emerging large-scale multi-socket systems make the need for more sophisticated large-scale coherence management of necessity. Directory-based coherence has been an ad hoc solution and a clear candidate for large-scale shared-memory systems. A vanilla directory design, however, suffers from inefficient use of storage to keep coherence metadata, resulting in a high storage overhead for large-scale systems. In this paper, we propose a dynamic multi-grain directory for large multi-socket systems. The idea is to track coherence of regions of different sizes which requires storing much less information in the directory than having a directory entry per each data block. It dynamically refines granularity according to the application phase and therefore tracks coherence information for regions of varying sizes. The results show that the proposal allows to reduce the directory storage by an order of magnitude, while the loss of precision does not cause performance penalty. The paper demonstrates that different applications and different application phases have different requirements for the region size. Performance results are compared against two state-of-the-art multi-grain directories and it is the one that obtains the better results.