Inference

A Holoscan application that needs to run inference will use an inference operator. The built-in Inference operator (InferenceOp) can be used, and several related use cases are documented in the Inference operator section below. The use cases are created using the parameter set that must be defined in the configuration file of the holoscan application. If the built-in InferenceOp doesn’t cover a specific use case, users can create their own custom inference operator as documented in Creating an Inference operator section.

The core inference functionality in the Holoscan SDK is provided by the Inference Module which is a framework that facilitates designing and executing inference and processing applications through its APIs. It is used by the built-in InferenceOp which supports the same parameters as the Inference Module. All parameters required by the Holoscan Inference Module are passed through a parameter set in the configuration file of an application.

Required parameters and related features available with the Holoscan Inference Module are listed below.

• Data Buffer Parameters: Parameters are provided in the inference settings to enable data buffer locations at several stages of the inference. As shown in the figure below, three parameters input_on_cuda, output_on_cuda and transmit_on_cuda can be set by the user.

  • input_on_cuda refers to the location of the data going into the inference.

    • If value is true, it means the input data is on the device

    • If value is false, it means the input data is on the host

    • Default value: true

  • output_on_cuda refers to the data location of the inferred data.

    • If value is true, it means the inferred data is on the device

    • If value is false, it means the inferred data is on the host

    • Default value: true

  • transmit_on_cuda refers to the data transmission.

    • If value is true, it means the data transmission from the inference extension will be on Device

    • If value is false, it means the data transmission from the inference extension will be on Host

    • Default value: true

• Inference Parameters

  • backend parameter is set to either trt for TensorRT, onnxrt for Onnx runtime, or torch for libtorch. If there are multiple models in the inference application, all models will use the same backend. If it is desired to use different backends for different models, specify the backend_map parameter instead.

    • TensorRT:

      • CUDA-based inference supported both on x86_64 and aarch64

      • End-to-end CUDA-based data buffer parameters supported. input_on_cuda, output_on_cuda and transmit_on_cuda will all be true for end-to-end CUDA-based data movement.

      • input_on_cuda, output_on_cuda and transmit_on_cuda can be either true or false.

      • TensorRT backend expects input models to be in tensorrt engine file format or onnx format.

        • if models are in tensorrt engine file format, parameter is_engine_path must be set to true.

        • if models are in onnx format, it will be automatically converted into tensorrt engine file by the Holoscan inference module.

    • Torch:

      • CUDA and CPU based inference supported both on x86_64 and aarch64.

      • End-to-end CUDA-based data buffer parameters supported. input_on_cuda, output_on_cuda and transmit_on_cuda will all be true for end-to-end CUDA-based data movement.

      • input_on_cuda, output_on_cuda and transmit_on_cuda can be either true or false.

      • Libtorch and TorchVision are included in the Holoscan NGC container, initially built as part of the PyTorch NGC container. To use the Holoscan SDK torch backend outside of these containers, we recommend you download libtorch and torchvision binaries from Holoscan’s third-party repository.

      • Torch backend expects input models to be in torchscript format.

        • It is recommended to use the same version of torch for torchscript model generation, as used in the HOLOSCAN SDK on the respective architectures.

        • Additionally, it is recommended to generate the torchscript model on the same architecture on which it will be executed. For example, torchscript model must be generated on x86_64 to be executed in an application running on x86_64 only.

    • Onnx runtime:

      • Data flow via host only. input_on_cuda, output_on_cuda and transmit_on_cuda must be false.

      • CUDA based inference (supported on x86_64)

      • CPU based inference (supported on x86_64 and aarch64)

  • infer_on_cpu parameter is set to true if CPU based inference is desired.

    The tables below demonstrate the supported features related to the data buffer and the inference with trt and onnxrt based backend, on x86 and aarch64 system respectively.

    x86	input_on_cuda	output_on_cuda	transmit_on_cuda	infer_on_cpu
    Supported values for trt	true or false	true or false	true or false	false
    Supported values for torch	true or false	true or false	true or false	true or false
    Supported values for onnxrt	false	false	true or false	true or false

    Aarch64	input_on_cuda	output_on_cuda	transmit_on_cuda	infer_on_cpu
    Supported values for trt	true or false	true or false	true or false	false
    Supported values for torch	true or false	true or false	true or false	true or false
    Supported values for onnxrt	false	false	true or false	true

  • model_path_map: User can design single or multi AI inference pipeline by populating model_path_map in the config file.

    • With a single entry it is single inference and with more than one entry, multi AI inference is enabled.

    • Each entry in model_path_map has a unique keyword as key (used as an identifier by the Holoscan Inference Module), and the path to the model as value.

    • All model entries must have the models either in onnx or tensorrt engine file or torchscript format.

  • pre_processor_map: input tensor to the respective model is specified in pre_processor_map in the config file.

    • The Holoscan Inference Module supports same input for multiple models or unique input per model.

    • Each entry in pre_processor_map has a unique keyword representing the model (same as used in model_path_map), and a vector of tensor names as the value.

    • The Holoscan Inference Module supports multiple input tensors per model.

  • inference_map: output tensors per model after inference is specified in inference_map in the config file.

    • Each entry in inference_map has a unique keyword representing the model (same as used in model_path_map and pre_processor_map), and a vector of the output tensor names as the value.

    • The Holoscan Inference Module supports multiple output tensors per model.

  • parallel_inference: Parallel or Sequential execution of inferences.

    • If multiple models are input, then user can execute models in parallel.

    • Parameter parallel_inference can be either true or false. Default value is true.

    • Inferences are launched in parallel without any check of the available GPU resources, user must make sure that there is enough memory and compute available to run all the inferences in parallel.

  • enable_fp16: Generation of the TensorRT engine files with FP16 option

    • If backend is set to trt, and if the input models are in onnx format, then users can generate the engine file with fp16 option to accelerate inferencing.

    • It takes few mintues to generate the engine files for the first time.

    • It can be either true or false. Default value is false.

  • is_engine_path: if the input models are specified in trt engine format in model_path_map, this flag must be set to true. Default value is false.

  • in_tensor_names: Input tensor names to be used by pre_processor_map. This parameter is optional. If absent in the parameter map, values are derived from pre_processor_map.

  • out_tensor_names: Output tensor names to be used by inference_map. This parameter is optional. If absent in the parameter map, values are derived from inference_map.

  • device_map: Multi-GPU inferencing is enabled if device_map is populated in the parameter set.

    • Each entry in device_map has a unique keyword representing the model (same as used in model_path_map and pre_processor_map), and GPU identifier as the value. This GPU ID is used to execute the inference for the specified model.

    • GPUs specified in the device_map must have P2P (peer to peer) access and they must be connected to the same PCIE configuration. If P2P access is not possible among GPUs, the host (CPU memory) will be used to transfer the data.

    • Multi-GPU inferencing is supported for all backends.

  • backend_map: Multiple backends can be used in the same application with this parameter.

    • Each entry in backend_map has a unique keyword representing the model (same as used in model_path_map), and the backend as the value.

    • A sample backend_map is shown below. In the example, model_1 uses the tensorRT backend, and model 2 and model 3 uses the torch backend for inference.

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          backend_map:
              "model_1_unique_identifier": "trt"
              "model_2_unique_identifier": "torch"
              "model_3_unique_identifier": "torch"
              

• Other features: Table below illustrates other features and supported values in the current release.

  Feature	Supported values
  Data type	float32, int32, int8
  Inference Backend	trt, torch, onnxrt
  Inputs per model	Multiple
  Outputs per model	Multiple
  GPU(s) supported	Multi-GPU on same PCIE network
  Tensor data dimension	2, 3, 4
  Model Type	All onnx or all torchscript or all trt engine type or a combination of torch and trt engine

• Multi Receiver and Single Transmitter support

  • The Holoscan Inference Module provides an API to extract the data from multiple receivers.

  • The Holoscan Inference Module provides an API to transmit multiple tensors via a single transmitter.

Parameter Specification

All required inference parameters of the inference application must be specified. Below is a sample parameter set for an application that uses three models for inferencing. User must populate all required fields with appropriate values.

            

            
inference:
    backend: "trt"
    model_path_map:
        "model_1_unique_identifier": "path_to_model_1"
        "model_2_unique_identifier": "path_to_model_2"
        "model_3_unique_identifier": "path_to_model_3"
    pre_processor_map:
        "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
        "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
        "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
    inference_map:
        "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
        "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
        "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
    parallel_inference: true
    infer_on_cpu: false
    enable_fp16: false
    input_on_cuda: true
    output_on_cuda: true
    transmit_on_cuda: true
    is_engine_path: false
        

In Holoscan SDK, the built-in Inference operator (InferenceOp) is designed using the Holoscan Inference Module APIs. The Inference operator ingests the inference parameter set (from the configuration file) and the data receivers (from previous connected operators in the application), executes the inference and transmits the inferred results to the next connected operators in the application.

InferenceOp is a generic operator that serves multiple use cases via the parameter set. Parameter sets for some key use cases are listed below:

Note: Some parameters have default values set for them in the InferenceOp. For any parameters not mentioned in the example parameter sets below, their default is used by the InferenceOp. These parameters are used to enable several use cases.

• Single model inference using TensorRT backend.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          

  Value of backend can be modified for other supported backends, and other parameters related to each backend. User must ensure correct model type and model path is provided into the parameter set, along with supported values of all parameters for the respective backend.

  In this example, path_to_model_1 must be an onnx file, which will be converted to a tensorRT engine file at first execution. During subsequent executions, the Holoscan inference module will automatically find the tensorRT engine file (if path_to_model_1 has not changed). Additionally, if user has a pre-built tensorRT engine file, path_to_model_1 must be path to the engine file and the parameter is_engine_path must be set to true in the parameter set.

• Single model inference using TensorRT backend with multiple outputs.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier",
                                        "output_tensor_2_model_1_unique_identifier",
                                        "output_tensor_3_model_1_unique_identifier"]
          

  As shown in example above, Holoscan Inference module automatically maps the model outputs to the named tensors in the parameter set. Users must ensure to use the named tensors in the same sequence in which the model generates the output. Similar logic holds for multiple inputs.

• Single model inference using fp16 precision.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier",
                                        "output_tensor_2_model_1_unique_identifier",
                                        "output_tensor_3_model_1_unique_identifier"]
      enable_fp16: true
          

  If a tensorRT engine file is not available for fp16 precision, it will be automatically generated by the Holoscan Inference module on the first execution. The file is cached for future executions.

• Single model inference on CPU.

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      backend: "onnxrt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
      infer_on_cpu: true
          

  Note that the backend can only be onnxrt or torch for CPU based inference.

• Single model inference with input/output data on Host.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
      input_on_cuda: false
      output_on_cuda: false
          

  Data in the core inference engine is passed through the host and is received on the host. Inference can happen on the GPU. Parameters input_on_cuda and output_on_cuda define the location of the data before and after inference respectively.

• Single model inference with data transmission via Host.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
      transmit_on_host: true
          

  Data from inference operator to the next connected operator in the application is transmitted via the host.

• Multi model inference with a single backend.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
          "model_2_unique_identifier": "path_to_model_2"
          "model_3_unique_identifier": "path_to_model_3"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
          

  By default multiple model inferences are launched in parallel. The backend specified via parameter backend is used for all models in the application.

• Multi model inference with sequential inference.

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      backend: "trt"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
          "model_2_unique_identifier": "path_to_model_2"
          "model_3_unique_identifier": "path_to_model_3"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
      parallel_inference: false
          

  parallel_inference is set to true by default. To launch model inferences in sequence, parallel_inference must be set to false.

• Multi model inference with multiple backends.

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      backend_map:
          "model_1_unique_identifier": "trt"
          "model_2_unique_identifier": "torch"
          "model_3_unique_identifier": "torch"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
          "model_2_unique_identifier": "path_to_model_2"
          "model_3_unique_identifier": "path_to_model_3"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
          

  In the above sample parameter set, the first model will do inference using the tensorRT backend, and model 2 and 3 will do inference using the torch backend.

  Note: the combination of backends in backend_map must support all other parameters that will be used during the inference. For. e.g. onnxrt and tensorRT combination with CPU based inference will not be supported.

• Multi model inference with a single backend on multi-GPU.

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      backend: "trt"
      device_map:
          "model_1_unique_identifier": "1"
          "model_2_unique_identifier": "0"
          "model_3_unique_identifier": "1"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
          "model_2_unique_identifier": "path_to_model_2"
          "model_3_unique_identifier": "path_to_model_3"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
          

  In the sample above, model 1 and model 3 will do inference on the GPU with ID 1 and model 2 will do inferene on the GPU with ID 0. GPUs must have P2P (peer to peer) access among them. If it is not enabled, the Holoscan inference module enables it by default. If P2P access is not possible between GPUs, then the data transfer will happen via the Host.

• Multi model inference with multiple backends on multiple GPUs.

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      backend_map:
          "model_1_unique_identifier": "trt"
          "model_2_unique_identifier": "torch"
          "model_3_unique_identifier": "torch"
      device_map:
          "model_1_unique_identifier": "1"
          "model_2_unique_identifier": "0"
          "model_3_unique_identifier": "1"
      model_path_map:
          "model_1_unique_identifier": "path_to_model_1"
          "model_2_unique_identifier": "path_to_model_2"
          "model_3_unique_identifier": "path_to_model_3"
      pre_processor_map:
          "model_1_unique_identifier": ["input_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["input_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["input_tensor_1_model_3_unique_identifier"]
      inference_map:
          "model_1_unique_identifier": ["output_tensor_1_model_1_unique_identifier"]
          "model_2_unique_identifier": ["output_tensor_1_model_2_unique_identifier"]
          "model_3_unique_identifier": ["output_tensor_1_model_3_unique_identifier"]
          

  In the sample above, three models are used during the inference. Model 1 uses the trt backend and runs on the GPU with ID 1, model 2 uses the torch backend and runs on the GPU with ID 0, and model 3 uses the torch backend and runs on the GPU with ID 1.

The Inference operator is the core inference unit in an inference application. The built-in Inference operator (InferenceOp) can be used for inference, or users can create their own custom inference operator as explained in this section. In Holoscan SDK, the inference operator can be designed using the Holoscan Inference Module APIs.

Arguments in the code sections below are referred to as ….

• Parameter Validity Check: Input inference parameters via the configuration (from step 1) are verified for correctness.

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  auto status = HoloInfer::inference_validity_check(...);
          

• Inference specification creation: For a single AI, only one entry is passed into the required entries in the parameter set. There is no change in the API calls below. Single AI or multi AI is enabled based on the number of entries in the parameter specifications from the configuration (in step 1).

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  // Declaration of inference specifications
  std::shared_ptr<HoloInfer::InferenceSpecs> inference_specs_;
  
  // Creation of inference specification structure
  inference_specs_ = std::make_shared<HoloInfer::InferenceSpecs>(...);
          

• Inference context creation.

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  // Pointer to inference context.
  std::unique_ptr<HoloInfer::InferContext> holoscan_infer_context_;
  // Create holoscan inference context
  holoscan_infer_context_ = std::make_unique<HoloInfer::InferContext>();
          

• Parameter setup with inference context: All required parameters of the Holoscan Inference Module are transferred in this step, and relevant memory allocations are initiated in the inference specification.

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  // Set and transfer inference specification to inference context
  auto status = holoscan_infer_context_->set_inference_params(inference_specs_);
          

• Data extraction and allocation: The following API is used from the Holoinfer utility to extract and allocate data for the specified tensor.

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  // Extract relevant data from input, and update inference specifications
  gxf_result_t stat = HoloInfer::get_data_per_model(...);
          

• Inference execution

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  // Execute inference and populate output buffer in inference specifications
  auto status = holoscan_infer_context_->execute_inference(inference_specs_->data_per_model_,
                                                           inference_specs_->output_per_model_);
          

• Transmit inferred data:

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  // Transmit output buffers
  auto status = HoloInfer::transmit_data_per_model(...);
          

Figure below demonstrates the Inference operator in the Holoscan SDK. All blocks with blue color are the API calls from the Holoscan Inference Module.