HyGain: High-performance, Energy-efficient Hybrid Gain Cell-based Cache Hierarchy

In this article, we propose a "full-stack"solution to designing high-Apacity and low-latency on-chip cache hierarchies by starting at the circuit level of the hardware design stack. We propose a novel half VDD precharge 2T Gain Cell (GC) design for the cache hierarchy. The GC has several desirable characteristics, including ∼50% higher storage density and ∼50% lower dynamic energy as compared to the traditional 6T SRAM, even after accounting for peripheral circuit overheads. We also demonstrate data retention time of 350 us (∼17.5× of eDRAM) at 28 nm technology with VDD = 0.9V and temperature = 27°C that, combined with optimizations like staggered refresh, makes it an ideal candidate to architect all levels of on-chip caches. We show that compared to 6T SRAM, for a given area budget, GC-based caches, on average, provide 30% and 36% increase in IPC for single-and multi-programmed workloads, respectively, on contemporary workloads, including SPEC CPU 2017. We also observe dynamic energy savings of 42% and 34% for single-and multi-programmed workloads, respectively. Finally, in a quest to utilize the best of all worlds, we combine GC with STT-RAM to create hybrid hierarchies. We show that a hybrid hierarchy with GC caches at L1 and L2 and an LLC split between GC and STT-RAM is able to provide a 46% benefit in energy-delay product (EDP) as compared to an all-SRAM design, and 13% as compared to an all-GC cache hierarchy, averaged across multi-programmed workloads.