mlpfile

Simple and fast library for working with small multilayer perceptrons (aka fully-connected neural networks) on the CPU. Python/NumPy bindings and C++/Eigen core.

Features:

• Evaluate the forward pass faster than alternatives.
• Compute derivatives of the network's output with respect to its input or parameters.
• Take a step of online gradient descent in place.
• Generate fast allocation-free C or C++/Eigen code for the forward pass.
• Convert from a torch.nn.Sequential to our file format.
• Binary file I/O (no C++ dependency on Protobuf, etc.)

API docs: https://jpreiss.github.io/mlpfile/api.html

Installation

To use the Python export and/or bindings, install the pip package:

pip install mlpfile

If you only need to load and evaluate networks in C++, the easiest way is to either 1) copy the files from mlpfile/cpp into your project, or 2) include this repo as a submodule.

Example code

Python:

model_torch = <train a torch.nn.Sequential somehow>
mlpfile.torch.write(model_torch, "net.mlp")

model_ours = mlpfile.Model.load("net.mlp")
x = <appropriate input>
y = model.forward(x)

C++:

mlpfile::Model model = mlpfile::Model::load("net.mlp");
Eigen::VectorXf x = <appropriate input>;
Eigen::VectorXf y = model.forward(x);

Performance

mlpfile is faster than popular alternatives for small networks on the CPU. Small networks can appear in time-sensitive realtime applications.

On a 2021 MacBook Pro M1, mlpfile is over 3x faster than ONNX and TorchScript on the forward pass. You can test on your own hardware by running benchmark.py.

$ python benchmark.py

┌─────────────────┐
│ Model structure │
└─────────────────┘
mlpfile::Model with 5 Layers, 40 -> 10
Linear: 40 -> 100
ReLU
Linear: 100 -> 100
ReLU
Linear: 100 -> 10

┌─────────┐
│ Forward │
└─────────┘
        torch:   15.72 usec
  torchscript:    6.97 usec
         onnx:    5.98 usec
         ours:    1.91 usec
    codegen_c:   10.10 usec
codegen_eigen:    1.11 usec

┌──────────┐
│ Jacobian │
└──────────┘
    torch-autodiff:   88.19 usec
      torch-manual:   40.82 usec
torchscript-manual:   16.81 usec
              onnx:   42.00 usec
              ours:   11.97 usec

┌────────────┐
│ OGD-update │
└────────────┘
torch:  129.38 usec
 ours:   10.17 usec

Motivation

The performance shown above is the #1 motivation. Aside from that:

Popular tools for NN deployment from PyTorch to C++ are TorchScript and ONNX. Both are heavyweight because they handle general computation graphs like ResNets and Transformers. Their C++ packages are hard to compile, and some are platform-specific. Their file formats are complicated to parse manually.

To take the NN's Jacobian with respect to its input, PyTorch's torch.func.jacrev generates a computation graph that can't be serialized with TorchScript or PyTorch's own ONNX exporter (circa 2023).

File format

It is a binary file format. All numerical types are little-endian, but the code currently assumes it's running on a little-endian machine.

The file format is not stable!

layer types enum:
    2 - linear
    3 - relu

header:
    number of layers (uint32)
    input dimension (uint32)

    for each layer:
        enum layer type (uint32)
        if linear:
            output dim (uint32)
        if relu:
            nothing

data:
    for each layer:
        if linear:
            weight (float32[], row-major)
            bias (float32[])
        otherwise:
            nothing