Emerging chips with hundreds and thousands of cores require networks with unprecedented energy/area efficiency and scalability. To address this, we propose Slim NoC (SN): a new on-chip network design that delivers significant improvements in efficiency and scalability compared to the state-of-the-art. The key idea is to use two concepts from graph and number theory, degree-diameter graphs combined with non-prime finite fields, to enable the smallest number of ports for a given core count. SN is inspired by state-of-the-art off-chip topologies; it identifies and distills their advantages for NoC settings while solving several key issues that lead to significant overheads on-chip. SN provides NoC-specific layouts, which further enhance area/energy efficiency. We show how to augment SN with state-of-the-art router microarchitecture schemes such as Elastic Links, to make the network even more scalable and efficient. Our extensive experimental evaluations show that SN outperforms both traditional low-radix topologies (e.g., meshes and tori) and modern high-radix networks (e.g., various Flattened Butterflies) in area, latency, throughput, and static/dynamic power consumption for both synthetic and real workloads. SN provides a promising direction in scalable and energy-efficient NoC topologies.

Edge buffers along with multiple virtual channels have traditionally been used to provide deadlock freedom guarantees in on-chip networks. The problem with such schemes is their high buffer space requirement which consumes significant power and area. In this work, we propose bubble sharing flow control to provide deadlock freedom with small, shared central buffers, eliminating edge buffers, improving buffer utilization, and decreasing router buffer requirements. The key insight involves sharing of the flit-size bubbles (free buffers) among cyclic network paths via central buffers in the router, reducing the overall router buffering space requirement. This technique effectively reconciles the trade-off between high radix and buffer space, encouraging the use of low hop count, high-radix topologies, with both deterministic and adaptive routing. Comparisons show improvement in average packet latency by 31% as compared to traditional 2VC edge buffer routers with 33% reduction in area for an 8×8 generalized hypercube topology.

While router buffers have been used as performance multipliers, they are also major consumers of area and power in on-chip networks. In this paper, we propose centralized elastic bubble router - a router micro-architecture based on the use of centralized buffers (CB) with elastic buffered (EB) links. At low loads, the CB is power gated, bypassed, and optimized to produce single cycle operation. A novel extension to bubble flow control enables routing deadlock and message dependent deadlock to be avoided with the same mechanism having constant buffer size per router independent of the number of message types. This solution enables end-to-end latency reduction via high radix switches with low overall buffer requirements. Comparisons made with other low latency routers across different topologies show consistent performance improvement, for example 26% improvement in no load latency of a 2D Mesh and 4X improvement in saturation throughput in a 2D-Generalized Hypercube.

The presence of multiple MCs and their integration into the on-chip network fabric creates a highly concurrent system that can support significant levels of memory level parallelism (MLP) across cores. This work exposes the trade-off between DRAM parameters, bank level parallelism (BLP), and row buffer hit rate that exposes the amount of effective BLP that is necessary to approximate a 100% hit rate. We further study how this trade-off can be controlled and propose a class of global (system) and local (within an MC) address mappings that can be tuned to optimize the performance across a set of multiprogrammed benchmarks.