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#65 in Machine learning

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Latest Version Documentation

RTen (the Rust Tensor engine) † is a runtime for machine learning models converted from ONNX format, which you can export from PyTorch and other frameworks.

The project also provides supporting libraries for common pre-processing and post-processing tasks in various domains. This makes RTen a more complete toolkit for running models in Rust applications.

The name is also a reference to PyTorch's ATen library.


  • Provide a (relatively) small and efficient neural network runtime that makes it easy to take models created in frameworks such as PyTorch and run them in Rust applications.
  • Be easy to compile and run on a variety of platforms, including WebAssembly
  • End-to-end Rust. This project and all of its required dependencies are written in Rust.


This project has a number of limitations to be aware of. Addressing them is planned for the future:

  • Supports CPU inference only. There is currently no support for running models on GPUs or other accelerators.
  • Not all ONNX operators are currently supported. See OperatorType in src/schema.fbs for currently supported operators. For implemented operators, some attributes or input shapes may not be supported.
  • A limited set of data types are supported: float32 and int32 tensors. int64 and boolean tensors are converted to int32.
  • RTen is not as well optimized as more mature runtimes such as ONNX Runtime or TensorFlow Lite. The performance difference depends on the operators used, model structure, CPU architecture and platform.

Getting started

The best way to get started is to clone this repository and try running some of the examples locally. The conversion scripts use popular Python machine learning libraries, so you will need Python >= 3.10 installed.

The examples are located in the rten-examples/ directory. See the README for descriptions of all the examples and steps to run them. As a quick-start, here are the steps to run the image classification example:

git clone https://github.com/robertknight/rten.git
cd rten

# Install model conversion tool
pip install -e rten-convert

# Install dependencies for Python scripts
pip install -r tools/requirements.txt

# Export an ONNX model. We're using resnet-50, a classic image classification model.
python -m tools.export-timm-model timm/resnet50.a1_in1k

# Convert model to this library's format
rten-convert resnet50.a1_in1k.onnx resnet50.rten

# Run image classification example. Replace `image.png` with your own image.
cargo run -p rten-examples --release --bin imagenet mobilenet resnet50.rten image.png

Converting ONNX models

RTen does not load ONNX models directly. ONNX models must be run through a conversion tool which produces an optimized model in a FlatBuffers-based format (.rten) that the engine can load. This is conceptually similar to the .tflite and .ort formats that TensorFlow Lite and ONNX Runtime use.

The conversion tool requires Python >= 3.10. To convert an existing ONNX model, run:

pip install rten-convert
rten-convert your-model.onnx

See the rten-convert README for more information about usage and version compatibility.

Usage in JavaScript

To use this library in a JavaScript application, there are two approaches:

  1. Prepare model inputs in JavaScript and use the rten library's built-in WebAssembly API to run the model and return a tensor which will then need to be post-processed in JS. This approach may be easiest for tasks where the pre-processing is simple.

    The image classification example uses this approach.

  2. Create a Rust library that uses rten and does pre-processing of inputs and post-processing of outputs on the Rust side, exposing a domain-specific WebAssembly API. This approach is more suitable if you have complex and/or computationally intensive pre/post-processing to do.

Before running the examples, you will need to follow the steps under "Building the WebAssembly library" below.

The general steps for using RTen's built-in WebAssembly API to run models in a JavaScript project are:

  1. Develop a model or find a pre-trained one that you want to run. Pre-trained models in ONNX format can be obtained from the ONNX Model Zoo or Hugging Face.
  2. If the model is not already in ONNX format, convert it to ONNX. PyTorch users can use torch.onnx for this.
  3. Use the rten-convert package in this repository to convert the model to the optimized format RTen uses. See the section above on converting models.
  4. In your JavaScript code, fetch the WebAssembly binary and initialize RTen using the init function.
  5. Fetch the prepared .rten model and use it to an instantiate the Model class from this library.
  6. Each time you want to run the model, prepare one or more Float32Arrays containing input data in the format expected by the model, and call Model.run. This will return a TensorList that provides access to the shapes and data of the outputs.

After building the library, API documentation for the Model and TensorList classes is available in dist/rten.d.ts.

Building the WebAssembly library


To build RTen for WebAssembly you will need:

  • A recent stable version of Rust
  • make
  • (Optional) The wasm-opt tool from Binaryen can be used to optimize .wasm binaries for improved performance
  • (Optional) A recent version of Node for running demos

Building rten

git clone https://github.com/robertknight/rten.git
cd rten
make wasm-all

The make wasm-all command will build two versions of the library, one for browsers that support SIMD (Chrome 91, Firefox 89, Safari 16.4) and one for those which do not (primarily older Safari releases). See the WebAssembly Roadmap for a full list of which features different engines support. The SIMD build is significantly faster.

During development, you can speed up the testing cycle by running make wasm to build only the SIMD version, or make wasm-nosimd for the non-SIMD version.

At runtime, you can find out which build is supported by calling the binaryName() function exported by this package.


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