Scalable Directory Protocols for 1000s of Cores Dominic DiTomaso EE 6633

Outline Introduction Background ATAC SPATL Cuckoo Directory SCD Conclusions

Directory Protocols Snoopy (broadcast) -> Directory (multicast) Large Directory Overhead Overhead = P*M P - # of processors M - # of memory blocks 64 nodes: 12.7% overhead 256 nodes: 50% overhead 1024 nodes: 200% overhead P M

Directory Protocols Requirements Small area, energy, and latency overheads Accurate sharer information Limited directory-induced invalidations Duplicate Tags Area-efficient High associativity -> high power Sparse Directory Power-efficient Large capacity -> large area Coarse-grain vectors, Hierarchical, etc.

ATAC Optical Broadcast Network

SPATL Tagless Bloom Filters

Cuckoo Directory N-ary Cuckoo Hash Table

SCD Variable directory tags

SCD (cont.)

Conclusions Large directory overhead at 1000s of cores Solutions Optics – ATAC Tagless – SPATL Hash Tables – Cuckoo Variable Tags – SCD

References [1] George Kurian, Jason E. Miller, James Psota, Jonathan Eastep, Jifeng Liu, Jurgen Michel, Lionel C. Kimerling, and Anant Agarwal, “ATAC: a 1000-core cache-coherent processor with on-chip optical network,” In Proceedings of the 19th international conference on Parallel architectures and compilation techniques (PACT '10), [2] Daniel Sanchez and Christos Kozyrakis, “SCD: A scalable coherence directory with flexible sharer set encoding,” In Proceedings of the 2012 IEEE 18th International Symposium on High-Performance Computer Architecture (HPCA '12), [3] H. Zhao, A. Shriraman, S. Dwarkadas, and V. Srinivasan, “SPATL: Honey, I Shrunk the Coherence Directory,” In Proceedings of the 20th international conference on Parallel architectures and compilation techniques (PACT ’11), [4] M. Ferdman, P. Lotfi-Kamran, K. Balet, B. Falsafi, "Cuckoo directory: A scalable directory for many-core systems," 2011 IEEE 17th International Symposium on High Performance Computer Architecture (HPCA), pp , Feb