Inference Runner Code Sample


This document describes the process of setting up and running the Arm® Ethos™-U55 NPU Inference Runner.

The inference runner is intended for quickly checking profiling results for any wanted network, providing it has been processed by the Vela compiler.

A simple model is provided with the Inference Runner as an example. However, we expect you to replace this model with one that you must profile.

For further details, refer to: Add custom model.

The inference runner populates all input tensors for the provided model with randomly generated data and an inference is then performed. Profiling results are then displayed in the console.

The example use-case code can be found in the following directory: source/use_case/inference_runner.


See Prerequisites

Building the Code Samples application from sources

Build options

In addition to the already specified build option in the main documentation, the Inference Runner use-case adds the following:

  • inference_runner_MODEL_TFLITE_PATH - The path to the NN model file in the TFLite format. The model is then processed and included in the application axf file. The default value points to one of the delivered set of models.

    Note that the parameters TARGET_PLATFORM and ETHOS_U55_ENABLED must be aligned with the chosen model. In other words:

    • If ETHOS_U55_ENABLED is set to On or 1, then the NN model is assumed to be optimized. The model naturally falls back to the Arm® Cortex®-M CPU if an unoptimized model is supplied.
    • if ETHOS_U55_ENABLED is set to Off or 0, the NN model is assumed to be unoptimized. Supplying an optimized model in this case results in a runtime error.
  • inference_runner_ACTIVATION_BUF_SZ: The intermediate, or activation, buffer size reserved for the NN model. By default, it is set to 2MiB and is enough for most models.

To build ONLY the Inference Runner example application, add -DUSE_CASE_BUILD=inferece_runner to the cmake command line, as specified in: Building.

Build process

Note: This section describes the process for configuring the build for the MPS3: SSE-300. To build for a different target platform, please refer to: Building.

Create a build directory and navigate inside, like so:

mkdir build_inference_runner && cd build_inference_runner

On Linux, when providing only the mandatory arguments for the CMake configuration, execute the following command to build only Image Classification application to run on the Ethos-U55 Fast Model:

cmake ../ -DUSE_CASE_BUILD=inference_runner

To configure a build that can be debugged using Arm DS specify the build type as Debug and then use the Arm Compiler toolchain file:

cmake .. \
    -DCMAKE_TOOLCHAIN_FILE=scripts/cmake/toolchains/bare-metal-armclang.cmake \

For further information, please refer to:

Note: If re-building with changed parameters values, we recommend that you clean the build directory and re-run the CMake command.

If the CMake command succeeds, build the application as follows:

make -j4

To see compilation and link details, add VERBOSE=1.

Results of the build are placed under the build/bin folder, like so:

 ├── ethos-u-inference_runner.axf
 ├── ethos-u-inference_runner.htm
 └── sectors
      └── inference_runner
        ├── ddr.bin
        └── itcm.bin

The bin folder contains the following files:

  • ethos-u-inference_runner.axf: The built application binary for the Inference Runner use-case.

  • Information from building the application. For example: The libraries used, what was optimized, and the location of objects.

  • ethos-u-inference_runner.htm: Human readable file containing the call graph of application functions.

  • sectors/inference_runner: Folder containing the built application. It is split into files for loading into different FPGA memory regions.

  • sectors/images.txt: Tells the FPGA which memory regions to use for loading the binaries in the sectors/.. folder.

Add custom model

The application performs inference using the model pointed to by the CMake parameter inference_runner_MODEL_TFLITE_PATH.

Note: If you want to run the model using an Ethos-U55, ensure that your custom model has been successfully run through the Vela compiler before continuing.

For further information: Optimize model with Vela compiler.

Then, you must set inference_runner_MODEL_TFLITE_PATH to the location of the Vela processed model file.

An example:

cmake .. \
  -Dinference_runner_MODEL_TFLITE_PATH=<path/to/custom_model_after_vela.tflite> \

Note: Clean the build directory before re-running the CMake command.

The .tflite model file pointed to by inference_runner_MODEL_TFLITE_PATH is converted to C++ files during the CMake configuration stage. It is then compiled into the application for performing inference with.

The log from the configuration stage tells you what model path and labels file have been used, for example:

-- User option inference_runner_MODEL_TFLITE_PATH is set to <path/to/custom_model_after_vela.tflite>
-- Using <path/to/custom_model_after_vela.tflite>
++ Converting custom_model_after_vela.tflite to\

After compiling, your custom model has now replaced the default one in the application.

Setting up and running Ethos-U55 code sample

Setting up the Ethos-U55 Fast Model

The FVP is available publicly from Arm Ecosystem FVP downloads.

For the Ethos-U55 evaluation, please download the MPS3 version of the Arm® Corstone™-300 model that contains both the Ethos-U55 and Cortex-M55. The model is currently only supported on Linux-based machines.

To install the FVP:

  • Unpack the archive.

  • Run the install script in the extracted package:

  • Follow the instructions to install the FVP to the required location.

Starting Fast Model simulation

Once completed the building step, the application binary ethos-u-infernce_runner.axf can be found in the build/bin folder.

Assuming that the install location of the FVP was set to ~/FVP_install_location, then the simulation can be started by using:


A log output appears on the terminal:

telnetterminal0: Listening for serial connection on port 5000
telnetterminal1: Listening for serial connection on port 5001
telnetterminal2: Listening for serial connection on port 5002
telnetterminal5: Listening for serial connection on port 5003

This also launches a telnet window with the standard output of the sample application. It also includes error log entries containing information about the pre-built application version, TensorFlow Lite Micro library version used, and data types. The log also includes the input and output tensor sizes of the model compiled into the executable binary.

Running Inference Runner

After the application has started the inference starts immediately and it outputs the results on the telnet terminal.

The following example illustrates application output:

INFO - Final results:
INFO - Profile for Inference :
INFO - NPU ACTIVE cycles: 33145
INFO - NPU IDLE cycles: 1033
INFO - NPU TOTAL cycles: 34178

After running an inference on randomly generated data, the output of the log shows the profiling results that for this inference. For example:

  • Ethos-U55 PMU report:

    • 34,178 total cycle: The number of NPU cycles.

    • 33,145 active cycles: The number of NPU cycles that were used for computation.

    • 1,033 idle cycles: The number of cycles for which the NPU was idle.

    • 9,332 AXI0 read beats: The number of AXI beats with read transactions from AXI0 bus. AXI0 is the bus where the Ethos-U55 NPU reads and writes to the computation buffers, activation buf, or tensor arenas.

    • 3,248 AXI0 write beats: The number of AXI beats with write transactions to AXI0 bus.

    • 2,219 AXI1 read beats: The number of AXI beats with read transactions from AXI1 bus. AXI1 is the bus where the Ethos-U55 NPU reads the model. So, read-only.

  • For FPGA platforms, a CPU cycle count can also be enabled. However, do not use cycle counters for FVP, as the CPU model is not cycle-approximate or cycle-accurate.