Push Multicast: A Speculative and Coherent Interconnect for Mitigating Manycore CPU Communication Bottleneck
Jiayi Huang, Yanhua Chen, Zhe Wang, Christopher J. Hughes, Yufei Ding, Yuan Xie · 2025
As CPUs scale up to many cores, the bandwidth of the network-on-chip (NoC) and cache can soon become the performance bottleneck. In modern processors, the cache hierarchy plays a reactive role to supply data upon request. In parallel programs, shared data accesses from different cores at different times can consume large cache and NoC bandwidth for the same data. These same-data accesses inherently have redundancy and lead to inefficient cache and NoC bandwidth utilization. In this work, we propose Push Multicast, a speculative and coherent interconnect. We transform the last-level cache into a proactive agent to push data to other sharers upon replying to the demand requester. Pushing enables effective multicasting to reduce LLC and NoC bandwidth consumption. A coherent innetwork filter is proposed to prune the outstanding requests in the routers along the way of the pushed data delivery. Moreover, a dynamic mechanism is designed to pause and resume pushing adaptively. Compared with a system with an L1 Bingo data prefetcher and an L2 Stride prefetcher, Push Multicast achieves an average of $\mathbf{3 3 \%}$ NoC bandwidth saving, a geomean of $1.02 \times$ and a maximum of $1.56 \times$ speedup in a 16 -core system. In a 64-core system, it further achieves an average of $\mathbf{4 3 \%}$ NoC bandwidth saving, along with a geomean of $1.11 \times$ and a maximum of $2.08 \times$ speedup.