Quantization-DNNs

simulate TensorFlow Lite with PyTorch API
reference: https://www.zhihu.com/people/xu-yong-zhe-65/posts
Since I just use Pytorch API to implement INT8 quantization, I cannot obtain the speed-up effect proposed in the paper, but just simulate the influence brought be the quantization operation.

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Usage

• train full-precision model

  python ./train.py

• post training quantization

  python ./PTQ.py

• quantization aware training

  python ./QAT.py

Experiment Results

• full-precision model

  FashionMNIST dataset

  model type	accuracy
  CNNs	90%
  CNNs_BN	90%

  CIFAR10 dataset

  model type	accuracy
  CNNs	69%
  CNNs_BN	73%

• quant model

  FashionMNIST dataset

  num_bits	4	6	8
  cnn_PTQ	71%	88%	90%
  cnn_QAT	81%	90%	91%
  cnnBN_PTQ	52%	63%	59%
  cnnBN_QAT	62%	71%	84%

  CIFAR10 dataset

  num_bits	4	6	8
  cnn_PTQ	47%	67%	69%
  cnn_QAT	47%	67%	69%
  cnnBN_PTQ	42%	45%	57%
  cnnBN_QAT	43%	53%	59%

Observation

• The quantization precision of bias vectors has huge influence to the final results

  As proposed in the paper, the use of higher precision(int32) of bias vectors meets the real need since they are added to many activations. The following experiments verify the explanation:

• Learning rate sometimes may determine QAT's final performance

  During the experiment, I found that improper choice of learning rate of QAT would make the performance of it worse than simple PTQ. After many adjustments to the learning rate, QAT's final performance can surpass PTQ. The empirical law is: When we fine-tune a full precision model, use lower learning rate for lower bits quantized model and higher learning rate for higher bits quantized model(anyway, the learning rate for QAT is much lower than full precision model training setting).

• Batch-Normalization layers lower the accuracy of quantized model

  From the above experiment results we can find that the quantized models with folded Conv-BN-RELU layers actually have worse performance. One possible explanation is: We fix the folded Conv-BN-RELU layers' weights and bias using exponential average running mean and running std obtained during quantization aware training, however, the actual folded weights and bias corresponds to the actual inputs during inference , and this operation brings estimated error. Since the mean and std of inputs may be more sensitive than the max and min value of them(used to calculate S and Z), the final results of quantized models with Conv-BN-RELU layers are worse.

• Optimizer affects QAT's performance

  At first, I choose to use Adam for quantization aware training. However, its performance is worse than use SGD with momentum. Currently, I cannot figure out the reason behind this phenomenon.

TODO list

• Train a model from scratch with quantization aware training
• Read more paper like LSQ, LSQ+
• Try to read the source code of TensorFlow Lite and reproduce it from the underlying code