14 releases (5 breaking)

0.7.2 Mar 19, 2024
0.7.1 Mar 9, 2024
0.7.0 Feb 20, 2024
0.6.1 Jan 26, 2024
0.2.1 Sep 25, 2023

#145 in Science

Used in 2 crates

Apache-2.0 OR MIT



Crates.io kn-graph Crates.io kn-cuda-sys Crates.io kn-cuda-eval docs.rs CI status


A neural network inference library, written in/for Rust. It can run ONNX files either on the CPU or on Nvidia GPUs using cuda/cudnn/cublas.

It is general enough to run all kinds of networks, it has been tested with:

  • Simple fully connected networks
  • ResNet-based CNNs
  • Large language models like LLaMA
  • Image generation models like Stable Diffusion. For a demo see the stable_diffusion example in the kn-runtime crate.

The framework consists of the following crates:

  • kn-graph: The core crate, containing the intermediate representation and the CPU executor.
  • kn-cuda-sys: The Cuda FFI bindings, generated with rust-bindgen.
  • kn-cuda-eval: The Cuda executor and planner.
  • kn-runtime: A wrapper around the other crates to allow selecting between CPU and GPU execution at runtime.
  • kn-python: An experimental python wrapper around the runtime crate, using PyO3.

Quick demo

// Graph operations (using kn-graph)
// Load on onnx file into a graph
let graph = load_graph_from_onnx_path("test.onnx", false)?;
// Optimize the graph
let graph = optimize_graph(&graph, Default::default());
// Render the graph as an svg file
graph_to_svg("test.svg", &graph, false, false)?;

// Build the inputs
let batch_size = 8;
let inputs = [DTensor::F32(Tensor::zeros(IxDyn(&[batch_size, 16])))];

// CPU: (using kn-graph)
// just evaluate the graph
let outputs: Vec<DTensor> = cpu_eval_graph(&graph, batch_size, &inputs);

// GPU: (using kn-cuda-eval)
// build an executor
let device = CudaDevice::new(0).unwrap();
let mut executor = CudaExecutor::new(device, &graph, batch_size);
// run the executor on the inputs
let outputs: &[DTensor] = executor.evaluate(&inputs);

// Runtime device selection: (using kn-runtime)
let device = Device::best();
let mut prepared = device.prepare(graph, batch_size);
let outputs: Vec<DTensor> = prepared.eval( & inputs);

System requirements

To use the CUDA crates, the appropriate libraries need to be installed on this system; they are not downloaded automatically:

  • CUDA (includes CUDA, cuBLAS, NVRTC): installer, follow the instructions. Ensure that the environment variable CUDA_PATH points to the root directory of the install (i.e., CUDA_PATH/bin/ should exist).
  • cuDNN: archive file, to be extracted to a location of your choosing. If you choose the same location as CUDA_PATH, you don't need to do anything else. Otherwise, set the environment variable CUDNN_PATH to the root directory of the cuDNN installation (i.e., CUDNN_PATH/bin should exist).

The project has been tested with CUDA v12.2 and cuDNN version v8.9.5. Newer versions might work, but this is not guaranteed since CUDA sometimes changes the name of or removes certain functions.


The typical pipeline is shown in the first figure below. The second figure shows the results of running this pipeline on a simple NN architecture.

NN inference diagram


Graph IR

Central is the Graph IR, the intermediate representation for neural network graphs.

The structure is an SSA-style directed acyclic graph, where nodes are values with a shape, data type, and the operation that computes it. These values are abstract; they don't have strides or memory locations yet.

The operations are similar to those of other frameworks but are kept as orthogonal as possible. Some example operations: convolution, matmul, reshape, broadcast, slice, unary, binary, reduce, softmax, ... See the docs for the full list of graph operations.

The graph can be constructed directly in code using the graph builder API, but for convenience, an ONNX loader exists. It can read ONNX files and convert the supported subset of operations into those supported by the IR.

Because the graph IR is much more orthogonal than the ONNX specification, many ONNX operations are decomposed into separate steps, some examples:

  • ONNX binary operations implicitly broadcast their operands, but this step is a separate operation in the IR.
  • ONNX convolution and matmul have a built-in optional bias operand; this also becomes a separate broadcast plus binary addition operation.

To figure out if an ONNX operation is supported, check the branches of the top-level match statement in the visit_node function in load.rs. Many common operations are already implemented, and adding more operations shouldn't be too hard.

For a larger example of a typical graph, see stable_diffusion_piece.svg, a small section taken from the start of the stable diffusion model.


The graph can optionally be optimized by the optimizer. Since the graph is append-only, a new graph is returned.

The optimizations that are currently implemented are:

  • Constant folding
  • Fusing consecutive affine (bias, scale, batchnorm) operations into a single bias+scale operation.
  • Fusing consecutive clamping operations (relu, min, max) into a single min+max operation.
  • Strength reduction: replacing division by a constant with multiplication by the inverse constant.
  • Recognizing the layernorm template (reduce, subtract, power, reduce, divide) and replacing it with the layernorm operator.

CPU executor

Finally, the graph needs to be executed. There is a simple CPU executor that just directly runs each operation. No major optimizations are attempted here, except for using BLAS routines for matmuls and im2col for convolutions. It's important that this executor is as simple as possible because it serves as the baseline for unit tests that check the correctness of the GPU executor.

Cuda Executor

The second (and more useful) way to run these graphs is with the Cuda executor. This involves running the graph through the Cuda Planner, which outputs a predetermined schedule of Cuda operations and allocates the necessary memory buffers. This is split out as a separate step so this expensive planning step only needs to be carried out once per network architecture; the resulting plan can then be reused many times in the executor.

The planner has the following major responsibilities:

  • Determine the memory layout of tensors: the strides and the memory offsets
    • This implicitly handles most reshape, broadcast, stride, ... operations.
    • Buffers are also reused if possible, minimizing total memory usage. There is much room for improvement here; currently, this is just a single pass algorithm.
  • Decide which cuDNN/cuBLAS operations to run for convolutions and matmuls. If possible, operations are fused together. Some examples:
    • cuDNN supports a single "convolution + residual + bias + relu" operation
    • cuBLAS matmuls can include a transpose of either input matrix, and equivalently the output by swapping the inputs.
    • cuDNN and cuBLAS operations sometimes include a "scalar" argument that is multiplied by some of the operands
  • Compile custom kernels for the remaining scalar and compound operations using an autokernel framework based on NVRTC (Runtime Compilation).
    • The operations handled by autokernel are: scalar operations, reduce, softmax, layernorm, gather.
    • Handwritten kernel templates are used, with details such as tensor shapes, strides, scalar operations, ... substituted in before compilation at runtime.
    • More operator fusion happens here
      • Multiple scalar operations get compiled to a single kernel
      • Constant scalars are inlined
      • Some compound kernels support fusing input or output scalar operations

This final operator fusion can be significant and save a lot of redundant transfers to and from main memory. The same performance could be achieved by manually writing kernels for each used combination of operations, but the combinatorial explosion and associated maintenance would be huge.

An example generated scalar kernel with some handwritten clarifying comments is shown below:

Example scalar autokernel for residual + batchnorm + relu6
#include "util.cu"

// constants that got inserted into the template
// this scalar operation happens on a tensor of rank 4, with 7 operands
const int RANK = 4;
const int OPERANDS = 7;
const int STRIDES_DENSE[RANK] = {648, 81, 9, 1};
    // these are full input tensors with normal, dense strides
    {648, 81, 9, 1},
    {648, 81, 9, 1},
    // these values have zero strides for all axes except the channel one,
    //    so these are probably biases and scaling factors
    //    that are broadcast across the other axes
    {0, 1, 0, 0},
    {0, 1, 0, 0},
    {0, 1, 0, 0},
    {0, 1, 0, 0},
    // the output tensor is just another operand
    {648, 81, 9, 1}

// the template function, the body of which is generated at runtime
__device__ void operation(void *pointers[OPERANDS], int offsets[OPERANDS]) {
    // all input operand memory locations are cast to the right type
    float *x0 = &((float *) pointers[0])[offsets[0]];
    float *x1 = &((float *) pointers[1])[offsets[1]];
    float *x2 = &((float *) pointers[2])[offsets[2]];
    float *x3 = &((float *) pointers[3])[offsets[3]];
    float *x4 = &((float *) pointers[4])[offsets[4]];
    float *x5 = &((float *) pointers[5])[offsets[5]];
    float *x6 = &((float *) pointers[6])[offsets[6]];
    // input operands are loaded
    float y0 = *x0;
    float y1 = *x1;
    // this is probably a residual connection
    float y2 = y0 + y1;
    // these 4 steps look like they're implementing a batchnorm layer  
    float y3 = *x2;
    float y4 = y2 - y3;
    float y5 = *x3;
    float y6 = y4 / y5;
    float y7 = *x4;
    float y8 = y6 * y7;
    float y9 = *x5;
    float y10 = y8 + y9;
    // this implements a relu6 activation function
    float y11 = 6;
    float y12 = min(y10, y11);
    float y13 = (0.0);
    float y14 = max(y12, y13);
    // finally the output is stored
    *x6 = y14;

// the kernel main function is the same for all scalar kernels
__global__ void scalar_kernel(
        int batch_size,
        Array<void *, OPERANDS> pointers
) {
    KernelInfo info = kernel_info();
    int size = batch_size * STRIDES_DENSE[0];

    // the main loop, following https://developer.nvidia.com/blog/cuda-pro-tip-write-flexible-kernels-grid-stride-loops/
    for (int flat = info.global_thread_id; flat < size; flat += info.thread_count) {
        Array<int, OPERANDS> offsets = flat_index_to_offsets<RANK, OPERANDS>(flat, STRIDES_DENSE, STRIDES);
        operation(pointers.data, &offsets[0]);

Comparison to other crates

See Are We Learning Yet? for a full list of potential alternatives.

Rust wrappers around existing runtimes


  • extensive support for many neural network operations
  • support for many different backends (CPU, GPU (Nvidia + AMD), TPU, ...)


  • not always great support for loading ONNX files (ort is great at this though, as the name suggests)
  • large and somewhat black-box external dependency
  • less operator fusion in many cases, although this is expected to improve in the future

Performance should be about the same as Kyanite for cases where operator fusion does not matter much; all libraries mostly use the same underlying cuDNN and cuBLAS kernels.

From-scratch Rust projects

  • tract: larger coverage of the ONNX specification but only does CPU inference


While developing this crate, to update the ONNX proto, the prost-build crate is used. This requires that protoc is installed and that the PROTOC environment variable is set to point to the executable. See their installation instructions (or the error message the build script shows if any) for more details.

To actually update the proto definition, replace kn-graph/proto/onnx.proto3 with the newer version and run cargo run --bin proto-to-rust. Then commit both the onnx.proto3 file and the generated onnx.rs file.


~46K SLoC