Variation-Aware Demonstration and Optimization of Block Floating Point and Integer Neural Network Acceleration on RRAM Compute in-Memory Hardware

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In this work, we propose a device-circuit-system co-design approach to achieve the best performance from a given variation-prone resistive random access memory (RRAM) compute-in-memory (CIM) hardware for deep neural network (DNN) acceleration and also demonstrate an optimized hardware implementation of block floating point (BFP) acceleration exploiting FP8 format on RRAM CIM hardware using a 1T-1R array of single-level cell (SLC) and multilevel cell (MLC) devices. Further, bit-level accurate network simulations incorporating device-to-device (D2D) variations on multi-bit SLC / MLC weight mantissa / weight are also conducted and benchmarking is performed with integer (INT4, INT8) formats. In particular, a word line read voltage ($V_{\boldsymbol{WLread}}$) tuning scheme is proposed and experimentally analyzed to (a) optimize D2D variation (tested on 144 1T-1R array bit cells) in SLC-based implementation to reduce partial MAC (PMAC) mapping errors for robust inference and b) obtain linearly separated conductance states for linear accumulation in MLC-based implementation (tested on 200 1T-1R bitcells). An optimized write-verify scheme is also proposed to obtain a tight MLC states distribution. Extensive analysis on hardware obtained FP8 inference results is also performed by measuring 16,384$<$PMACs$<$1,31,072 from hardware to identify sources of error for different word-line parallelism, SLC / MLC mapping and their trade-off with sparsity and energy consumption. For SLC based hardware implementation, the test accuracy of the FP8 trained networks at$\sigma/\mu =$0.5 is 1.2x better than INT4 at only${\boldsymbol\sim}$1.3x the area consumption of the INT4 RRAM array. INT8 network performance on the other hand, is comparable to FP8 but at${\boldsymbol\sim}$2x FP8 RRAM array area consumption. Hardware inference using 2-bit MLC mapped weight mantissas is also demonstrated to analyze the effect of individual state variation dependence and identify critical variability states.

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