torch::deploy
=============

``torch::deploy`` is a system that allows you to run multiple embedded Python
interpreters in a C++ process without a shared global interpreter lock. For more
information on how ``torch::deploy`` works internally, please see the related
`arXiv paper <https://arxiv.org/pdf/2104.00254.pdf>`_.


.. warning::

    This is a prototype feature. Only Linux x86 is supported, and the API may
    change without warning.


Getting Started
---------------

Installing ``torch::deploy``
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``torch::deploy`` is not yet built by default in our binary releases, so to get
a copy of libtorch with ``torch::deploy`` enabled, follow the instructions for
`building PyTorch from source <https://github.com/pytorch/pytorch/#from-source>`_.

When running ``setup.py``, you will need to specify ``USE_DEPLOY=1``, like:

.. code-block:: bash

    export CMAKE_PREFIX_PATH=${CONDA_PREFIX:-"$(dirname $(which conda))/../"}
    export USE_DEPLOY=1
    python setup.py bdist_wheel
    python -mpip install dist/*.whl


Creating a model package in Python
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

``torch::deploy`` can load and run Python models that are packaged with
``torch.package``. You can learn more about ``torch.package`` in the
``torch.package`` `documentation <https://pytorch.org/docs/stable/package.html#tutorials>`_.

For now, let's create a simple model that we can load and run in ``torch::deploy``.

.. code-block:: py

    from torch.package import PackageExporter
    import torchvision

    # Instantiate some model
    model = torchvision.models.resnet.resnet18()

    # Package and export it.
    with PackageExporter("my_package.pt") as e:
        e.intern("torchvision.**")
        e.extern("sys")
        e.save_pickle("model", "model.pkl", model)

Now, there should be a file named ``my_package.pt`` in your working directory.

.. note::

    Currently, ``torch::deploy`` supports only the Python standard library and
    ``torch`` as ``extern`` modules in ``torch.package``. In the future we plan
    to transparently support any Conda environment you point us to.



Loading and running the model in C++
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Let's create a minimal C++ program to that loads the model.

.. code-block:: cpp

    #include <torch/deploy.h>
    #include <torch/script.h>

    #include <iostream>
    #include <memory>

    int main(int argc, const char* argv[]) {
        if (argc != 2) {
            std::cerr << "usage: example-app <path-to-exported-script-module>\n";
            return -1;
        }

        // Start an interpreter manager governing 4 embedded interpreters.
        torch::deploy::InterpreterManager manager(4);

        try {
            // Load the model from the torch.package.
            torch::deploy::Package package = manager.loadPackage(argv[1]);
            torch::deploy::ReplicatedObj model = package.loadPickle("model", "model.pkl");
        } catch (const c10::Error& e) {
            std::cerr << "error loading the model\n";
            return -1;
        }

        std::cout << "ok\n";
    }

This small program introduces many of the core concepts of ``torch::deploy``.

An ``InterpreterManager`` abstracts over a collection of independent Python
interpreters, allowing you to load balance across them when running your code.

Using the ``InterpreterManager::loadPackage`` method, you can load a
``torch.package`` from disk and make it available to all interpreters.

``Package::loadPickle`` allows you to retrieve specific Python objects
from the package, like the ResNet model we saved earlier.

Finally, the model itself is a ``ReplicatedObj``. This is an abstract handle to
an object that is replicated across multiple interpreters. When you interact
with a ``ReplicatedObj`` (for example, by calling ``forward``), it will select
an free interpreter to execute that interaction.


Building and running the application
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Assuming the above C++ program was stored in a file called, `example-app.cpp`, a
minimal CMakeLists.txt file would look like:

.. code-block:: cmake

    cmake_minimum_required(VERSION 3.0 FATAL_ERROR)
    project(deploy_tutorial)

    find_package(Torch REQUIRED)

    add_executable(example-app example-app.cpp)
    target_link_libraries(example-app "${TORCH_LIBRARIES}")
    set_property(TARGET example-app PROPERTY CXX_STANDARD 14)


The last step is configuring and building the project. Assuming that our code
directory is laid out like this:

.. code-block:: none

    example-app/
        CMakeLists.txt
        example-app.cpp

We can now run the following commands to build the application from within the
``example-app/`` folder:

.. code-block:: bash

    mkdir build
    cd build
    # Point CMake at the built version of PyTorch we just installed.
    SITE_PACKAGES="$(python -c 'from distutils.sysconfig import get_python_lib; print(get_python_lib())')"
    cmake -DCMAKE_PREFIX_PATH="$SITE_PACKAGES/torch" ..
    cmake --build . --config Release

Now we can run our app:

.. code-block:: bash

        ./example-app /path/to/my_package.pt


Executing ``forward`` in C++
~~~~~~~~~~~~~~~~~~~~~~~~~~~~

One you have your model loaded in C++, it is easy to execute it:

.. code-block:: cpp

    // Create a vector of inputs.
    std::vector<torch::jit::IValue> inputs;
    inputs.push_back(torch::ones({1, 3, 224, 224}));

    // Execute the model and turn its output into a tensor.
    at::Tensor output = model(inputs).toTensor();
    std::cout << output.slice(/*dim=*/1, /*start=*/0, /*end=*/5) << '\n';

Notably, the model's forward function is executing in Python, in an embedded
CPython interpreter. Note that the model is a ``ReplicatedObj``, which means
that you can call ``model()`` from multiple threads and the forward method will
be executed on multiple independent interpreters, with no global interpreter
lock.