SlimPruner on NNI Compressor
===

## 1. Slim Pruner

SlimPruner is a structured pruning algorithm for pruning channels in the convolutional layers by pruning corresponding scaling factors in the later BN layers.

In ['Learning Efficient Convolutional Networks through Network Slimming'](https://arxiv.org/pdf/1708.06519.pdf), authors Zhuang Liu, Jianguo Li, Zhiqiang Shen, Gao Huang, Shoumeng Yan and Changshui Zhang.

![](../../img/slim_pruner.png)

> Slim Pruner **prunes channels in the convolution layers by masking corresponding scaling factors in the later BN layers**, L1 regularization on the scaling factors should be applied in batch normalization (BN) layers while training, scaling factors of BN layers are **globally ranked** while pruning, so the sparse model can be automatically found given sparsity.

## 2. Usage

PyTorch code

```
from nni.compression.torch import SlimPruner
config_list = [{ 'sparsity': 0.8, 'op_types': ['BatchNorm2d'] }]
pruner = SlimPruner(model, config_list)
pruner.compress()
```

#### User configuration for Filter Pruner

- **sparsity:** This is to specify the sparsity operations to be compressed to
- **op_types:** Only BatchNorm2d is supported in Slim Pruner

## 3. Experiment

We implemented one of the experiments in ['Learning Efficient Convolutional Networks through Network Slimming'](https://arxiv.org/pdf/1708.06519.pdf), we pruned $70\%$ channels in the **VGGNet** for CIFAR-10 in the paper, in which $88.5\%$ parameters are pruned. Our experiments results are as follows:

| Model         | Error(paper/ours) | Parameters | Pruned    |
| ------------- | ----------------- | ---------- | --------- |
| VGGNet        | 6.34/6.40     | 20.04M   |           |
| Pruned-VGGNet | 6.20/6.39     | 2.03M    | 88.5% |

The experiments code can be found at [examples/model_compress]( https://github.com/microsoft/nni/tree/master/examples/model_compress/)
