Serving a Torch-TensorRT model with Triton

Optimization and deployment go hand in hand in a discussion about Machine Learning infrastructure. Once network level optimization are done to get the maximum performance, the next step would be to deploy it.

However, serving this optimized model comes with its own set of considerations and challenges like: building an infrastructure to support concurrent model executions, supporting clients over HTTP or gRPC and more.

The Triton Inference Server solves the aforementioned and more. Let’s discuss step-by-step, the process of optimizing a model with Torch-TensorRT, deploying it on Triton Inference Server, and building a client to query the model.

Step 1: Optimize your model with Torch-TensorRT

Most Torch-TensorRT users will be familiar with this step. For the purpose of this demonstration, we will be using a ResNet50 model from Torchhub.

Let’s first pull the NGC PyTorch Docker container. You may need to create an account and get the API key from here. Sign up and login with your key (follow the instructions here after signing up).

# <xx.xx> is the yy:mm for the publishing tag for NVIDIA's Pytorch
# container; eg. 22.04

docker run -it --gpus all -v ${PWD}:/scratch_space nvcr.io/nvidia/pytorch:<xx.xx>-py3
cd /scratch_space

Once inside the container, we can proceed to download a ResNet model from Torchhub and optimize it with Torch-TensorRT.

import torch
import torch_tensorrt
torch.hub._validate_not_a_forked_repo=lambda a,b,c: True

# load model
model = torch.hub.load('pytorch/vision:v0.10.0', 'resnet50', pretrained=True).eval().to("cuda")

# Compile with Torch TensorRT;
trt_model = torch_tensorrt.compile(model,
    inputs= [torch_tensorrt.Input((1, 3, 224, 224))],
    enabled_precisions= { torch.half} # Run with FP32
)

# Save the model
torch.jit.save(trt_model, "model.pt")

After copying the model, exit the container. The next step in the process is to set up a Triton Inference Server.

Step 2: Set Up Triton Inference Server

If you are new to the Triton Inference Server and want to learn more, we highly recommend to checking our Github Repository.

To use Triton, we need to make a model repository. A model repository, as the name suggests, is a repository of the models the Inference server hosts. While Triton can serve models from multiple repositories, in this example, we will discuss the simplest possible form of the model repository.

The structure of this repository should look something like this:

model_repository
|
+-- resnet50
    |
    +-- config.pbtxt
    +-- 1
        |
        +-- model.pt

There are two files that Triton requires to serve the model: the model itself and a model configuration file which is typically provided in config.pbtxt. For the model we prepared in step 1, the following configuration can be used:

name: "resnet50"
platform: "pytorch_libtorch"
max_batch_size : 0
input [
  {
    name: "input__0"
    data_type: TYPE_FP32
    dims: [ 3, 224, 224 ]
    reshape { shape: [ 1, 3, 224, 224 ] }
  }
]
output [
  {
    name: "output__0"
    data_type: TYPE_FP32
    dims: [ 1, 1000 ,1, 1]
    reshape { shape: [ 1, 1000 ] }
  }
]

The config.pbtxt file is used to describe the exact model configuration with details like the names and shapes of the input and output layer(s), datatypes, scheduling and batching details and more. If you are new to Triton, we highly encourage you to check out this section of our documentation for more details.

With the model repository setup, we can proceed to launch the Triton server with the docker command below. Refer this page for the pull tag for the container.

# Make sure that the TensorRT version in the Triton container
# and TensorRT version in the environment used to optimize the model
# are the same.

docker run --gpus all --rm -p 8000:8000 -p 8001:8001 -p 8002:8002 -v /full/path/to/the_model_repository/model_repository:/models nvcr.io/nvidia/tritonserver:<xx.yy>-py3 tritonserver --model-repository=/models

This should spin up a Triton Inference server. Next step, building a simple http client to query the server.

Step 3: Building a Triton Client to Query the Server

Before proceeding, make sure to have a sample image on hand. If you don’t have one, download an example image to test inference. In this section, we will be going over a very basic client. For a variety of more fleshed out examples, refer to the Triton Client Repository

wget  -O img1.jpg "https://www.hakaimagazine.com/wp-content/uploads/header-gulf-birds.jpg"

We then need to install dependencies for building a python client. These will change from client to client. For a full list of all languages supported by Triton, please refer to Triton’s client repository.

pip install torchvision
pip install attrdict
pip install nvidia-pyindex
pip install tritonclient[all]

Let’s jump into the client. Firstly, we write a small preprocessing function to resize and normalize the query image.

import numpy as np
from torchvision import transforms
from PIL import Image
import tritonclient.http as httpclient
from tritonclient.utils import triton_to_np_dtype

# preprocessing function
def rn50_preprocess(img_path="img1.jpg"):
    img = Image.open(img_path)
    preprocess = transforms.Compose([
        transforms.Resize(256),
        transforms.CenterCrop(224),
        transforms.ToTensor(),
        transforms.Normalize(mean=[0.485, 0.456, 0.406], std=[0.229, 0.224, 0.225]),
    ])
    return preprocess(img).numpy()

transformed_img = rn50_preprocess()

Building a client requires three basic points. Firstly, we setup a connection with the Triton Inference Server.

# Setting up client
client = httpclient.InferenceServerClient(url="localhost:8000")

Secondly, we specify the names of the input and output layer(s) of our model.

inputs = httpclient.InferInput("input__0", transformed_img.shape, datatype="FP32")
inputs.set_data_from_numpy(transformed_img, binary_data=True)

outputs = httpclient.InferRequestedOutput("output__0", binary_data=True, class_count=1000)

Lastly, we send an inference request to the Triton Inference Server.

# Querying the server
results = client.infer(model_name="resnet50", inputs=[inputs], outputs=[outputs])
inference_output = results.as_numpy('output__0')
print(inference_output[:5])

The output should look like below:

[b'12.468750:90' b'11.523438:92' b'9.664062:14' b'8.429688:136'
 b'8.234375:11']

The output format here is <confidence_score>:<classification_index>. To learn how to map these to the label names and more, refer to Triton Inference Server’s documentation.