Pin or Fuse? Exploiting Scratchpad Memory to Reduce Off-Chip Data Transfer in DNN Accelerators

Growing interests in on-device AI have led to the proliferation of accelerators dedicated to neural network inference. Most ASIC accelerators are equipped with compiler-controlled scratchpad memory (SPM) used as a last-level cache to reduce the number of accesses to off-chip memory. A widely-used strategy for utilizing SPM is fused-layer execution, which divides a DNN model into groups of layers and forwards the intermediate results within each group without eviction to the off-chip memory. However, layer fusion has an inherent limitation that the fusion of consecutive layers increases the amount of computations, leading to sub-optimal performance. This paper introduces a new dimension to SPM usage, which temporarily pins a feature map on SPM. Pinning reduces off-chip transfer without computation increase, but it is not applicable to all feature maps due to limited SPM size. We find that superior performance can be achieved by combination of pinning and fusion in MobileNet. Based on this observation, we propose a model-level optimization method that jointly applies pinning and fusion to minimize inference latency under memory constraints. Scheduling and allocation schemes are presented for automatic generation of optimized codes. Evaluation on the commercial AI accelerator shows that the proposed method reduces off-chip transfer of feature maps by 50% and improves inference latency by 15% on average without additional hardware, compared to the state-of-the-art fusion approach.