Efficient Cache Coherence on Manycore Optical Networks

Ever since industry has turned to parallelism instead of frequency scaling to improve processor performance, multicore processors have continued to scale to larger and larger numbers of cores. Some believe that multicores will have 1000 cores or more by the middle of the next decade. However, their promise of increased performance will only be reached if their inherent scaling challenges are overcome. One such major scaling challenge is the viability of efficient cache coherence with large numbers of cores. Meanwhile, recent advances in nanophotonic device manufacturing are making CMOS-integrated optics a reality—interconnect technology which can provide significantly more bandwidth at lower power than conventional electrical analogs. The contributions of this paper are two-fold. (1) It presents ATAC, a new manycore architecture that augments an electrical mesh network with an optical network that performs highly efficient broadcasts. (2) It introduces ACKwise, a novel directorybased cache coherence protocol that provides high performance and scalability on any large-scale manycore interconnection network with broadcast capability. Performance evaluation studies using analytical models show that (i) a 1024-core ATAC chip using ACKwise achieves a speedup of 3.9× compared to a similarly-sized pure electrical mesh manycore with a conventional limited directory protocol; (ii) the ATAC chip with ACKwise achieves a speedup of 1.35× compared to the electrical mesh chip with ACKwise; and (iii) a pure electrical mesh chip with ACKwise achieves a speedup of 2.9× over the same chip using a conventional limited directory protocol.