SHMT: An SRAM and HBM Hybrid Computing-in-Memory Architecture With Optimized KV Cache for Multimodal Transformer

Multimodal Transformer (MMT) algorithms have become the state-of-the-art for multimodal tasks such as image captioning. The Encoder-Decoder (E-D) structure, consisting of Encoder, Decoder-causal, and Decoder-cross components, provides a flexible and effective framework for multimodal tasks. However, previous accelerators mainly focus on the dataflow and hardware optimization of the Encoder, which fails to accelerate the entire E-D structure efficiently. There remain three challenges: 1) the lack of pipeline and multicore optimization at the module, layer, and E-D level; 2) the Decoder-causal and Decoder-cross computations have lower arithmetic intensity compared to the Encoder, requiring a better solution for the varying arithmetic intensities; and 3) the autoregressive algorithm in Decoder-causal leads to redundant KV Cache accesses and considerable idle power. In this paper, SHMT, an SRAM and HBM hybrid computing-in-memory (CIM) architecture, is designed to efficiently support multimodal Transformers with three key contributions: 1) a multi-level pipelined multicore scheme, including pipeline optimization across E-D layer-head-module levels and a multicore network-on-chip (NoC) architecture, to reduce inference latency and off-chip accesses; 2) a heterogeneous SRAM-HBM architecture, utilizing high-density HBM-CIM for low-arithmetic-intensity (LAI) parts and high-performance SRAM-CIM for high-arithmetic-intensity (HAI) parts; and 3) by integrating KV Cache with zero-padding in SRAM-CIM, SHMT eliminates redundant read-write operations in KV Cache, reducing idle power consumption. Experiment results show that SHMT achieves $212\times $ speedup, reduces energy consumption by $208\times \sim 2000\times $ per token, and achieves $13.3\times $ higher energy efficiency compared to NVIDIA A100 GPU.