Recognizing images with Azure Machine Learning and the ONNX ResNet50v2 model

Featured image from: https://medium.com/comet-app/review-of-deep-learning-algorithms-for-object-detection-c1f3d437b852

In a previous post, I discussed the creation of a container image that uses the ResNet50v2 model for image classification. If you want to perform tasks such as localization or segmentation, there are other models that serve that purpose. The image was built with GPU support. Adding GPU support was pretty easy:

  • Use the enable_gpu flag in the Azure Machine Learning SDK or check the GPU box in the Azure Portal; the service will build an image that supports NVIDIA cuda
  • Add GPU support in your score.py file and/or conda dependencies file (scoring script uses the ONNX runtime, so we added the onnxruntime-gpu package)

In this post, we will deploy the image to a Kubernetes cluster with GPU nodes. We will use Azure Kubernetes Service (AKS) for this purpose. Check my previous post if you want to use NVIDIA V100 GPUs. In this post, I use hosts with one V100 GPU.

To get started, make sure you have the Kubernetes cluster deployed and that you followed the steps in my previous post to create the GPU container image. Make sure you attached the cluster to the workspace’s compute.

Deploy image to Kubernetes

Click the container image you created from the previous post and deploy it to the Kubernetes cluster you attached to the workspace by clicking + Create Deployment:

Starting the deployment from the image in the workspace

The Create Deployment screen is shown. Select AKS as deployment target and select the Kubernetes cluster you attached. Then press Create.

Azure Machine Learning now deploys the containers to Kubernetes. Note that I said containers in plural. In addition to the scoring container, another frontend container is added as well. You send your requests to the front-end container using HTTP POST. The front-end container talks to the scoring container over TCP port 5001 and passes the result back. The front-end container can be configured with certificates to support SSL.

Check the deployment and wait until it is healthy. We did not specify advanced settings during deployment so the default settings were chosen. Click the deployment to see the settings:

Deployment settings including authentication keys and scoring URI

As you can see, the deployment has authentication enabled. When you send your HTTP POST request to the scoring URI, make sure you pass an authentication header like so: bearer primary-or-secondary-key. The primary and secondary key are in the settings above. You can regenerate those keys at any time.

Checking the deployment

From the Azure Cloud Shell, issue the following commands in order to list the pods deployed to your Kubernetes cluster:

  • az aks list -o table
  • az aks get-credentials -g RESOURCEGROUP -n CLUSTERNAME
  • kubectl get pods
Listing the deployed pods

Azure Machine Learning has deployed three front-ends (default; can be changed via Advanced Settings during deployment) and one scoring container. Let’s check the container with: kubectl get pod onnxgpu-5d6c65789b-rnc56 -o yaml. Replace the container name with yours. In the output, you should find the following:

resources:
limits:
nvidia.com/gpu: "1"
requests:
cpu: 100m
memory: 500m
nvidia.com/gpu: "1"

The above allows the pod to use the GPU on the host. The nvidia drivers on the host are mapped to the pod with a volume:

volumeMounts:
- mountPath: /usr/local/nvidia
name: nvidia

Great! We did not have to bother with doing this ourselves. Let’s now try to recognize an image by sending requests to the front-end pods.

Recognizing images

To recognize an image, we need to POST a JSON payload to the scoring URI. The scoring URI can be found in the deployment properties in the workspace. In my case, the URI is:

http://23.97.218.34/api/v1/service/onnxgpu/score

The JSON payload needs to be in the below format:

{"data": [[[[143.06100463867188, 130.22100830078125, 122.31999969482422, ... ]]]]} 

The data field is a multi-dimensional array, serialized to JSON. The shape of the array is (1,3,224,224). The dimensions correspond to the batch size, channels (RGB), height and width.

You only have to read an image and put the pixel values in the array! Easy right? Well, as usual the answer is: “it depends”! The easiest way to do it, according to me, is with Python and a collection of helper packages. The code is in the following GitHub gist: https://gist.github.com/gbaeke/b25849f3813e9eb984ee691659d1d05a. You need to run the code on a machine with Python 3 installed. Make sure you also install Keras and NumPy (pip3 install keras / pip3 install numpy). The code uses two images, cat.jpg and car.jpg but you can use your own. When I run the code, I get the following result:

Using TensorFlow backend.
channels_last
Loading and preprocessing image… cat.jpg
Array shape (224, 224, 3)
Array shape afer moveaxis: (3, 224, 224)
Array shape after expand_dims (1, 3, 224, 224)
prediction time (as measured by the scoring container) 0.025304794311523438
Probably a: Egyptian_cat 0.9460222125053406
Loading and preprocessing image… car.jpg
Array shape (224, 224, 3)
Array shape afer moveaxis: (3, 224, 224)
Array shape after expand_dims (1, 3, 224, 224)
prediction time (as measured by the scoring container) 0.02526378631591797
Probably a: sports_car 0.948998749256134

It takes about 25 milliseconds to classify an image, or 40 images/second. By increasing the number of GPUs and scoring containers (we only deployed one), we can easily scale out the solution.

With a bit of help from Keras and NumPy, the code does the following:

  • check the image format reported by the keras back-end: it reports channels_last which means that, by default, the RGB channels are the last dimensions of the image array
  • load the image; the resulting array has a (224,224,3) shape
  • our container expects the channels_first format; we use moveaxis to move the last axis to the front; the array now has a (3,224,224) shape
  • our container expects a first dimension with a batch size; we use expand_dims to end up with a (1,3,224,224) shape
  • we convert the 4D array to a list and construct the JSON payload
  • we send the payload to the scoring URI and pass an authorization header
  • we get a JSON response with two fields: result and time; we print the inference time as reported by the container
  • from keras.applications.resnet50, we use the decode_predictions class to process the result field; result contains the 1000 values computed by the softmax function in the container; decode_predictions knows the categories and returns the first five
  • we print the name and probability of the category with the highest probability (item 0)

What happens when you use a scoring container that uses the CPU? In that case, you could run the container in Azure Container Instances (ACI). Using ACI is much less costly! In ACI with the default setting of 0.1 CPU, it will take around 2 seconds to score an image. Ouch! With a full CPU (in ACI), the scoring time goes down to around 180-220ms per image. To achieve better results, simply increase the number of CPUs. On the Standard_NC6s_v3 Kubernetes node with 6 cores, scoring time with CPU hovers around 60ms.

Conclusion

In this post, you have seen how Azure Machine Learning makes it straightforward to deploy GPU scoring images to a Kubernetes cluster with GPU nodes. The service automatically configures the resource requests for the GPU and maps the NVIDIA drivers to the scoring container. The only thing left to do is to start scoring images with the service. We have seen how easy that is with a bit of help from Keras and NumPy. In practice, always start with CPU scoring and scale out that solution to match your requirements. But if you do need GPUs for scoring, Azure Machine Learning makes it pretty easy to do so!

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