Model Tracing#

The easiest way to generate TorchScript for your model is to use PyTorch’s JIT tracer. Tracing runs an example input tensor through your model, and captures the operations that are invoked as that input makes its way through the model’s layers.

Tracing Limitations

Tracing the model captures only the operations that are performed for a specific input. If your model uses a data-dependent control flow, such as a loop or conditional, the traced model won’t generalize to other inputs. In such cases you can experiment with applying PyTorch’s JIT scripter to your model as described in Model Scripting. Use the JIT scripter only on control flow sections, and trace all other sections of the graph. You should keep the control flow section as small as possible.

The following example builds a simple model from scratch and traces it to generate the TorchScript object needed by the converter. Follow these steps:

  1. Define a simple layer module to reuse:

    import torch
    import torch.nn as nn
    import torch.nn.functional as F
    # Define a simple layer module we'll reuse in our network.
    class Layer(nn.Module):
    	def __init__(self, dims):
    		super(Layer, self).__init__()
    		self.conv1 = nn.Conv2d(*dims)
    	def forward(self, x):
    		x = F.relu(self.conv1(x))
    		x = F.max_pool2d(x, (2, 2))
    		return x
  2. Define a simple network consisting of several base layers:

    # A simple network consisting of several base layers.
    class SimpleNet(nn.Module):
    	def __init__(self):
    		super(SimpleNet, self).__init__()
    		self.layer1 = Layer((3, 6, 3))
    		self.layer2 = Layer((6, 16, 1))
    	def forward(self, x):
    		x = self.layer1(x)
    		x = self.layer2(x)
    		return x
  3. Instantiate the network:

    model = SimpleNet()  # Instantiate the network.
  4. Define the input, which is needed by jit tracer, and trace the model.

    Run torch.jit.trace on your model with an example input, and save the resulting traced object. For an example input, you can use one sample of training or validation data, or even use randomly-generated data as shown in the following code snippet:

    example = torch.rand(1, 3, 224, 224)  # Example input, needed by jit tracer.
    traced = torch.jit.trace(model, example)  # Generate TorchScript by tracing.


Don’t worry if your model is fully convolutional or otherwise has variable-sized inputs. You can fully describe your model’s input shape when you convert the TorchScript model to Core ML.

  1. Optionally pass the traced model directly to the converter:“”) # Optional, can pass traced model directly to converter.