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Galen: Hardware-specific Automatic Compression of Neural Networks

Galen is a framework for automatic compression of neural networks by applying layer-specific pruning and quantization. The layer-wise compression is determined by a RL algorithm which uses the sensitivity but also hardware inference latency as features.

We presented the details of the algorithm at the Practical-DL workshop collocated with AAAI-23: Towards Hardware-Specific Automatic Compression of Neural Networks (AAAI-23: 2nd International Workshop on Practical Deep Learning in the Wild)

Happy to announce that we received the best paper award of this workshop!

Abstract:

Compressing neural network architectures is important to allow the deployment of models to embedded or mobile devices, and pruning and quantization are the major approaches to compress neural networks nowadays. Both methods benefit when compression parameters are selected specifically for each layer. Finding good combinations of compression parameters, so-called compression policies, is hard as the problem spans an exponentially large search space. Effective compression policies consider the influence of the specific hardware architecture on the used compression methods. We propose an algorithmic framework called Galen to search such policies using reinforcement learning utilizing pruning and quantization, thus providing automatic compression for neural networks. Contrary to other approaches we use inference latency measured on the target hardware device as an optimization goal. With that, the framework supports the compression of models specific to a given hardware target. We validate our approach using three different reinforcement learning agents for pruning, quantization and joint pruning and quantization. Besides proving the functionality of our approach we were able to compress a ResNet18 for CIFAR-10, on an embedded ARM processor, to 20% of the original inference latency without significant loss of accuracy. Moreover, we can demonstrate that a joint search and compression using pruning and quantization is superior to an individual search for policies using a single compression method.

Algorithmic Schema

Dependencies

Most important dependencies of the project are PyTorch and Apache TVM. To install all dependencies first create a new conda environment using the included environment.yml by:

conda env create --name galen --file=environments.yml

# activate created environment
conda activate galen

Apache TVM

It is recommended to build Apache TVM from source when you want to test with hardware feedback. For tests without hardware feedback you could install using pip:

# CPU build only
pip install apache-tvm

Manual installation

To build from source using the created conda environment:

conda activate galen
cd ..
git clone --recursive https://github.com/apache/tvm tvm
cd tvm
mkdir build

cp ../cmake_build_config.cmake ./build/config.cmake
# or use the original tvm cmake config / configure by yourself (details: https://tvm.apache.org/docs/install/from_source.html#build-the-shared-library):
# cp cmake/config.cmake build

cd build
cmake ..

# -j specifies the number of compile threads
make -j4

To make the TVM python library usable on your system, add the following to your .bashrc (or .zshrc (...)):

export TVM_HOME=/path/to/tvm # replace with tvm repository location on your system
export PYTHONPATH=$TVM_HOME/python:${PYTHONPATH}

Run a Search: No Hardware Feedback

Pre-Train the ResNet18 on CIFAR-10

To search a compression policy for a ResNet18 model you first have to train the model on CIFAR-10. The model itself and the used data is downloaded automatically. To train for 100 epochs, use the provided script:

bash ./scripts/train_resnet18.sh

Search Policies

To run a compression search using the joint pruning & quantization agent without measuring the latency:

bash ./scripts/search_pq_no_hw.sh

Per episode a result dict will be stored as pickle file in the directory ./logs. The result dict holds the search accuracy per episode but also the found policy and additional metrics.

Retrain & Test a Compressed Model

After searching for policies the compressed model has to be retrained first, before testing the achieved accuracy. To retrain and test, select any episode result stored in the ./logs directory. Adapt the path in scripts/retrain.sh for the selected pickle file. Afterwards run by:

bash ./scripts/retrain.sh

This automatically compresses the model with the selected policy, retrains the model for 30 epochs and validates the test accuracy using the CIFAR-10 test dataset.

Run a Search: Measure Hardware Latency

To run a search with measured hardware latency some extra effort is required. First you have to set-up a TVM remote device. Currently, the implementation supports ARM devices only. For setting up a TVM remote device please refer to the TVM documentation [1] (the section regarding cross-compilation is not relevant, as this is done by the provided implementation).

Adapt following parameters in subsequent scripts accordingly:

  • tvm_target
  • tvm_device_key
  • tvm_rpc_ip
  • tvm_rpc_port

The pre-training and retraining steps are the same as for the search without latency measurement. To run a search using the pruning agent:

bash ./scripts/search_p.sh

For a search using the quantization agent:

bash ./scripts/search_q.sh

Finally, using the joint agent:

bash ./scripts/search_pq.sh

[1] https://tvm.apache.org/docs/tutorial/cross_compilation_and_rpc.html

Apache-TVM: Missing Property kernel.data.shape

If you run into an issue that the kernel object has no property or attribute data or data.shape while running the search (during the hardware latency measurement phase) you have to change a single source file of the Apache TVM python package.

  • navigate to the above cloned TVM repository on your machine
  • open the file python/tvm/topi/arm_cpu/bitserial_conv2d.py
    • comment out the if statement in line 455 (if len(kernel.data.shape) == 4:)
    • fix indention for line 456 to 467