quantization_quick_start_mnist.rst

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Quantization Quickstart
=======================

Quantization reduces model size and speeds up inference time by reducing the number of bits required to represent weights or activations.

In NNI, both post-training quantization algorithms and quantization-aware training algorithms are supported.
Here we use `QAT_Quantizer` as an example to show the usage of quantization in NNI.

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Preparation
-----------

In this tutorial, we use a simple model and pre-train on MNIST dataset.
If you are familiar with defining a model and training in pytorch, you can skip directly to `Quantizing Model`_.

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.. code-block:: default


    import torch
    import torch.nn.functional as F
    from torch.optim import SGD

    from scripts.compression_mnist_model import TorchModel, trainer, evaluator, device, test_trt

    # define the model
    model = TorchModel().to(device)

    # define the optimizer and criterion for pre-training

    optimizer = SGD(model.parameters(), 1e-2)
    criterion = F.nll_loss

    # pre-train and evaluate the model on MNIST dataset
    for epoch in range(3):
        trainer(model, optimizer, criterion)
        evaluator(model)





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    Average test loss: 0.7073, Accuracy: 7624/10000 (76%)
    Average test loss: 0.2776, Accuracy: 9122/10000 (91%)
    Average test loss: 0.1907, Accuracy: 9412/10000 (94%)




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Quantizing Model
----------------

Initialize a `config_list`.
Detailed about how to write ``config_list`` please refer :doc:`compression config specification <../compression/compression_config_list>`.

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    config_list = [{
        'quant_types': ['input', 'weight'],
        'quant_bits': {'input': 8, 'weight': 8},
        'op_types': ['Conv2d']
    }, {
        'quant_types': ['output'],
        'quant_bits': {'output': 8},
        'op_types': ['ReLU']
    }, {
        'quant_types': ['input', 'weight'],
        'quant_bits': {'input': 8, 'weight': 8},
        'op_names': ['fc1', 'fc2']
    }]








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finetuning the model by using QAT

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.. code-block:: default

    from nni.algorithms.compression.pytorch.quantization import QAT_Quantizer
    dummy_input = torch.rand(32, 1, 28, 28).to(device)
    quantizer = QAT_Quantizer(model, config_list, optimizer, dummy_input)
    quantizer.compress()





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    TorchModel(
      (conv1): QuantizerModuleWrapper(
        (module): Conv2d(1, 6, kernel_size=(5, 5), stride=(1, 1))
      )
      (conv2): QuantizerModuleWrapper(
        (module): Conv2d(6, 16, kernel_size=(5, 5), stride=(1, 1))
      )
      (fc1): QuantizerModuleWrapper(
        (module): Linear(in_features=256, out_features=120, bias=True)
      )
      (fc2): QuantizerModuleWrapper(
        (module): Linear(in_features=120, out_features=84, bias=True)
      )
      (fc3): Linear(in_features=84, out_features=10, bias=True)
      (relu1): QuantizerModuleWrapper(
        (module): ReLU()
      )
      (relu2): QuantizerModuleWrapper(
        (module): ReLU()
      )
      (relu3): QuantizerModuleWrapper(
        (module): ReLU()
      )
      (relu4): QuantizerModuleWrapper(
        (module): ReLU()
      )
      (pool1): MaxPool2d(kernel_size=(2, 2), stride=(2, 2), padding=0, dilation=1, ceil_mode=False)
      (pool2): MaxPool2d(kernel_size=(2, 2), stride=(2, 2), padding=0, dilation=1, ceil_mode=False)
    )



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The model has now been wrapped, and quantization targets ('quant_types' setting in `config_list`)
will be quantized & dequantized for simulated quantization in the wrapped layers.
QAT is a training-aware quantizer, it will update scale and zero point during training.

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    for epoch in range(3):
        trainer(model, optimizer, criterion)
        evaluator(model)





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    Average test loss: 0.1542, Accuracy: 9529/10000 (95%)
    Average test loss: 0.1133, Accuracy: 9664/10000 (97%)
    Average test loss: 0.0919, Accuracy: 9726/10000 (97%)




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export model and get calibration_config

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.. code-block:: default

    model_path = "./log/mnist_model.pth"
    calibration_path = "./log/mnist_calibration.pth"
    calibration_config = quantizer.export_model(model_path, calibration_path)

    print("calibration_config: ", calibration_config)





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    calibration_config:  {'conv1': {'weight_bits': 8, 'weight_scale': tensor([0.0031], device='cuda:0'), 'weight_zero_point': tensor([76.], device='cuda:0'), 'input_bits': 8, 'tracked_min_input': -0.4242129623889923, 'tracked_max_input': 2.821486711502075}, 'conv2': {'weight_bits': 8, 'weight_scale': tensor([0.0018], device='cuda:0'), 'weight_zero_point': tensor([113.], device='cuda:0'), 'input_bits': 8, 'tracked_min_input': 0.0, 'tracked_max_input': 12.42452621459961}, 'fc1': {'weight_bits': 8, 'weight_scale': tensor([0.0011], device='cuda:0'), 'weight_zero_point': tensor([124.], device='cuda:0'), 'input_bits': 8, 'tracked_min_input': 0.0, 'tracked_max_input': 31.650196075439453}, 'fc2': {'weight_bits': 8, 'weight_scale': tensor([0.0013], device='cuda:0'), 'weight_zero_point': tensor([122.], device='cuda:0'), 'input_bits': 8, 'tracked_min_input': 0.0, 'tracked_max_input': 25.805370330810547}, 'relu1': {'output_bits': 8, 'tracked_min_output': 0.0, 'tracked_max_output': 12.499907493591309}, 'relu2': {'output_bits': 8, 'tracked_min_output': 0.0, 'tracked_max_output': 32.0243034362793}, 'relu3': {'output_bits': 8, 'tracked_min_output': 0.0, 'tracked_max_output': 26.491384506225586}, 'relu4': {'output_bits': 8, 'tracked_min_output': 0.0, 'tracked_max_output': 17.662996292114258}}




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build tensorRT engine to make a real speedup, for more information about speedup, please refer :doc:`quantization_speedup`.

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.. code-block:: default


    from nni.compression.pytorch.quantization_speedup import ModelSpeedupTensorRT
    input_shape = (32, 1, 28, 28)
    engine = ModelSpeedupTensorRT(model, input_shape, config=calibration_config, batchsize=32)
    engine.compress()
    test_trt(engine)




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    Loss: 0.09358334274291992  Accuracy: 97.21%
    Inference elapsed_time (whole dataset): 0.04445981979370117s





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