On-chip cache coherency evaluation for space applications: Snoop with AHB bus vs. directory with CHI NoC

Abstract

With the end of frequency scaling as a solution for more efficient processors, bigger multi-core systems are getting increasingly popular, including in the space domain. To achieve bigger high-performing multi-core systems, more efficient on-chip data transfer systems are required which in turn are dependent on systems scaling. Increasing the size of a multi-core system introduces more complexity in upholding coherency. Different protocols and hardware mechanisms have been used for supporting coherent systems, such as snooping and directory-based, which can be supported by different interconnects. Although NoC-based interconnects and directory-based mechanisms have been shown to successfully scale multi-core systems, past research lacks a comparison of the coherence overhead introduced in NoC versus bus interconnects. This thesis project has performed an evaluation and comparison of two protocols for upholding coherency, AHB and CHI, used by Frontgrade Gaisler in their current and future state-of-the-art radiation hardened microprocessors. Two evaluation frameworks have been developed, one for each protocol, with configurable simulation stimuli targeted for cache coherency. The limited experiments we managed to perform in this project identified scenarios and topologies where the more expensive CHI system will not deliver a higher and more scalable performance compared to the more simple AHB system. This provides a valuable foundation for the next step, that is performing quantitative analysis of the topologies where CHI delivers superior performance as the system scales to a larger number of cores. However, due to technical challenges we leave that part for future work. Improvements such as increasing parallelism by using non-blocking components and sliced and distributed last-level-cache, as well as using more advanced NoC routers with reduced latency should be part of the future work that demonstrates the advantages of CHI over AHB system. Performance gains in the evaluated CHI system could still be seen when using a coherency protocol that allows for a write-back policy instead of writethrough.