Pushing the Limit of Post-Training Quantization

Recently, post-training quantization (PTQ) has become the de facto way to produce efficient low-precision neural networks without long-time retraining. Despite its low cost, current PTQ works fail to succeed under the extremely low-bit setting. In this work, we delve into extremely low-bit quantization and construct a unified theoretical analysis, which provides an in-depth understanding of the reason for the failure of low-bit quantization. According to the theoretical study, we argue that the existing methods fail in low-bit schemes due to significant perturbation on weights and lack of consideration of activation quantization. To this end, we propose Brecq and QDrop to respectively solve these two challenges, based on which a Q-Limit framework is constructed. Then the Q-Limit framework is further extended to support a mixed precision quantization scheme. To the best of our knowledge, this is the first work that can push the limit of PTQ down to INT2. Extensive experiments on various handcrafted and searched neural architectures are conducted for both visual recognition/detection tasks and language processing tasks. Without bells and whistles, our PTQ framework can attain low-bit ResNet and MobileNetV2 comparable with quantization-aware training (QAT), establishing a new state-of-the-art for PTQ.