magicwyzh / trained-ternary-quantization

Reducing the size of convolutional neural networks

Trained Ternary Quantization

pytorch implementation of Trained Ternary Quantization, a way of replacing full precision weights of a neural network by ternary values. I tested it on Tiny ImageNet dataset. The dataset consists of 64x64 images and has 200 classes.

The quantization roughly proceeds as follows.

1. Train a model of your choice as usual (or take a trained model).
2. Copy all full precision weights that you want to quantize. Then do the initial quantization:
   in the model replace them by ternary values {-1, 0, +1} using some heuristic.
3. Repeat until convergence:
   • Make the forward pass with the quantized model.
   • Compute gradients for the quantized model.
   • Preprocess the gradients and apply them to the copy of full precision weights.
   • Requantize the model using the changed full precision weights.
4. Throw away the copy of full precision weights and use the quantized model.

Results

I believe that this results can be made better by spending more time on hyperparameter optimization.

model	accuracy, %	top5 accuracy, %	number of parameters
DenseNet-121	74	91	7151176
TTQ DenseNet-121	55	79	~7M 2-bit, 89% are zeros
small DenseNet	49	75	440264
TTQ small DenseNet	37	65	~0.4M 2-bit, 65% are zeros
SqueezeNet	52	77	827784
TTQ SqueezeNet	36	63	~0.8M 2-bit, 66% are zeros

Implementation details

• I use pretrained DenseNet-121, but I train SqueezeNet and small DenseNet from scratch.
• I modify the SqueezeNet architecture by adding batch normalizations and skip connections.
• I quantize all layers except the first CONV layer, the last FC layer, and all BATCH_NORM layers.

How to reproduce results

For example, for small DenseNet:

1. Download Tiny ImageNet dataset and extract it to ~/data folder.
2. Run python utils/move_tiny_imagenet_data.py to prepare the data.
3. Go to vanilla_densenet_small/. Run train.ipynb to train the model as usual.
   Or you can skip this step and use model.pytorch_state (the model already trained by me).
4. Go to ttq_densenet_small/.
5. Run train.ipynb to do TTQ.
6. Run test_and_explore.ipynb to explore the quantized model.

To use this on your data you need to edit utils/input_pipeline.py and to change the model architecture in files like densenet.py and get_densenet.py as you like.

Requirements

• pytorch 0.2, Pilllow, torchvision
• numpy, sklearn, tqdm, matplotlib