PaperReading

Network Slimming: Learning Efficient Convolutional Networks through Network Slimming

May 2020

tl;dr: Channel pruning by learning with L1 sparse constraint on batch norm.

Overall impression

This paper proposes a simple idea of gamma (channel scaling factor) decay. It adds a L1 sparse constraint on BN scale parameter gamma. During inference time, when gamma is smaller than a global threshold, set the entire channel to zero.

The work is concurrent with batchnorm pruning which has a very similar idea.

Key ideas

• Advantages of network slimming:
  • No need to change architecture. Recycles the parameters in BN.
  • No need for special lib for inference.
• The pruning can happen multiple pass.
• Sparsity constraints can actually help performance, even before pruning.
• During training, the channel scaling factor can actually go up or down.

Technical details

• Un-structured pruning can only save model size by storing it with a sparse format.
• In Batch Norm, there are 4*C parameters (2 trainable and 2 un-trainable parameters per channel).
• The additional L1 regularization term rarely hurt model performance
• ResNets are harder to prune. Only 10% or so can be pruned away without hurting performance after a model is trained and without further finetuning. (per pruning filters). The paper also reported that for ResNet-164, the FLOPs saving ratios is around 50%.

Notes

• code on github
• How is the channel selection layer implemented for architectures with cross layer connection such as ResNets?