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# Deploying Torch-TensorRT Programs

After compiling and saving Torch-TensorRT programs there is no longer a strict dependency on the full Torch-TensorRT library. All that is required to run a compiled program is the runtime. There are therefore a couple options to deploy your programs other than shipping the full Torch-TensorRT compiler with your applications.

## Torch-TensorRT package / libtorchtrt.so

Once a program is compiled, you run it using the standard PyTorch APIs. All that is required is that the package must be imported in python or linked in C++.

## Runtime Library

Distributed with the C++ distribution is `libtorchtrt_runtime.so`. This library only contains the components necessary to run Torch-TensorRT programs. Instead of linking `libtorchtrt.so` or importing `torch_tensorrt` you can link `libtorchtrt_runtime.so` in your deployment programs or use `DL_OPEN` or `LD_PRELOAD`. For python you can load the runtime with `torch.ops.load_library("libtorchtrt_runtime.so")`. You can then continue to use programs just as you would otherwise via PyTorch API.

Note

If you are using the standard distribution of PyTorch in Python on x86, likely you will need the pre-cxx11-abi variant of `libtorchtrt_runtime.so`, check Installation documentation for more details.

Note

If you are linking `libtorchtrt_runtime.so`, likely using the following flags will help `-Wl,--no-as-needed -ltorchtrt -Wl,--as-needed` as there’s no direct symbol dependency to anything in the Torch-TensorRT runtime for most Torch-TensorRT runtime applications

An example of how to use `libtorchtrt_runtime.so` can be found here: https://github.com/pytorch/TensorRT/tree/master/examples/torchtrt_runtime_example

## Plugin Library

In the case you use Torch-TensorRT as a converter to a TensorRT engine and your engine uses plugins provided by Torch-TensorRT, Torch-TensorRT ships the library `libtorchtrt_plugins.so` which contains the implementation of the TensorRT plugins used by Torch-TensorRT during compilation. This library can be `DL_OPEN` or `LD_PRELOAD` similarly to other TensorRT plugin libraries.

## Multi Device Safe Mode

Multi-device safe mode is a setting in Torch-TensorRT which allows the user to determine whether the runtime checks for device consistency prior to every inference call.

There is a non-negligible, fixed cost per-inference call when multi-device safe mode is enabled, which is why it is now disabled by default. It can be controlled via the following convenience function which doubles as a context manager.

    # Enables Multi Device Safe Mode
    torch_tensorrt.runtime.set_multi_device_safe_mode(True)

    # Disables Multi Device Safe Mode [Default Behavior]
    torch_tensorrt.runtime.set_multi_device_safe_mode(False)

    # Enables Multi Device Safe Mode, then resets the safe mode to its prior setting
    with torch_tensorrt.runtime.set_multi_device_safe_mode(True):
        ...

TensorRT requires that each engine be associated with the CUDA context in the active thread from which it is invoked. Therefore, if the device were to change in the active thread, which may be the case when invoking engines on multiple GPUs from the same Python process, safe mode will cause Torch-TensorRT to display an alert and switch GPUs accordingly. If safe mode is not enabled, there could be a mismatch in the engine device and CUDA context device, which could lead the program to crash.

One technique for managing multiple TRT engines on different GPUs while not sacrificing performance for multi-device safe mode is to use Python threads. Each thread is responsible for all of the TRT engines on a single GPU, and the default CUDA device on each thread corresponds to the GPU for which it is responsible (can be set via `torch.cuda.set_device(...)`). In this way, multiple threads can be used in the same Python script without needing to switch CUDA contexts and incur performance overhead.

## Cudagraphs Mode

Cudagraphs mode is a setting in Torch-TensorRT which allows the user to determine whether the runtime uses cudagraphs to accelerate inference in certain cases.

Cudagraphs can accelerate certain models by reducing kernel overheads, as documented further [here](https://pytorch.org/blog/accelerating-pytorch-with-cuda-graphs/).

    # Enables Cudagraphs Mode
    torch_tensorrt.runtime.set_cudagraphs_mode(True)

    # Disables Cudagraphs Mode [Default Behavior]
    torch_tensorrt.runtime.set_cudagraphs_mode(False)

    # Enables Cudagraphs Mode, then resets the mode to its prior setting
    with torch_tensorrt.runtime.enable_cudagraphs(trt_module):
        ...

In the current implementation, use of a new input shape (for instance in dynamic shape cases), will cause the cudagraph to be re-recorded. Cudagraph recording is generally not latency intensive, and future improvements include caching cudagraphs for multiple input shapes.

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