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OpenVINO TensorFlow Frontend

The TensorFlow Frontend (TF FE) is a C++ based OpenVINO Frontend component that is responsible for reading and converting a TensorFlow model to an ov::Model object that further can be serialized into the Intermediate Representation (IR) format. This is an internal API for OpenVINO that is used to implement user-facing API such as MO tool, Model Conversion API, and OpenVINO Runtime read_model function for reading TensorFlow models of the original format in run-time. Also, OpenVINO Model Server uses the frontend for serving models. Regular users should not use the frontend directly.

flowchart BT
    model[(model1.pb)]
    style model fill:#427cb0
    model2[(model2.pb)]
    style model2 fill:#427cb0
    model3[(model3.pb)]
    style model3 fill:#427cb0
    ov_model[(ov::Model)]
    style ov_model fill:#427cb0
    ovms(OpenVINO Model Server)
    style ovms fill:#ffffc2
    tf_fe(TensorFlow Frontend)
    style tf_fe fill:#ee9a4d
    fem(Frontend Manager)
    mo(Model Optimizer)
    ov_runtime(OpenVINO Runtime)
    model --> mo --> fem --> tf_fe
    model2 --> ov_runtime --> fem
    model3 --> ovms --> ov_runtime
    tf_fe --> ov_model
    click ovms "https://github.com/openvinotoolkit/model_server"
Loading

The MO tool and model conversion API now use the TensorFlow Frontend as the default path for conversion to IR. Known limitations of TF FE are described here.

Key contacts

People from the openvino-tf-frontend-maintainers have the rights to approve and merge PRs to the TensorFlow Frontend component. They can assist with any questions about the component.

Components

The structure of OpenVINO TensorFlow Frontend sources includes the following directories:

  • include is a public frontend API.
  • src folder contains the sources of the component.
  • tests cover internal transformations.

Additionally, there is a shared TensorFlow Common directory with same structure and purposes. Its content depend only on common FrontEnd APIs thus is free to use in other FrontEnds.

Architecture

OpenVINO TensorFlow Frontend uses TensorFlow Protobuf files to read and parse different TensorFlow model formats. The whole workflow can be split into two steps: model loading and conversion.

During loading, the FrontEnd::load() method creates InputModel that encapsulates the GraphIterator object. GraphIterator is a reader that iterates through the graph nodes in the topological order. GraphIterator::get_decoder() provides a decoder for the current graph node to read its attributes. Each TensorFlow model format has its implementation of GraphIterator. Currently, the frontend supports SavedModel, MetaGraph (.meta), and frozen protobuf (.pb and .pbtxt) formats. The base class GraphIteratorProto is used for reading and parsing these formats. The architecture of the loading step is shown in the picture below:

classDiagram
    direction BT
    class TensorFrontend {
        +load()
    }
    TensorFrontend --|> InputModel
    GraphIterator "1" --o "1" InputModel
    Place --o "1..*" InputModel
    DecoderBase "1" --o "1" Place
    GraphIteratorProto ..|> GraphIterator
    GraphIteratorProtoTxt ..|> GraphIterator
    GraphIteratorMeta ..|> GraphIterator
    GraphIteratorSavedModel ..|> GraphIterator
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After the loading step, InputModel includes a container of topologically sorted operation Place objects. During conversion, each Place provides a DecoderBase object to retrieve attributes of the current operation to be transformed into the OpenVINO opset. Frontend converts operations in topological order and requires NodeContext for the current operation node, which includes Decoder and OutputVector inputs from already converted nodes.

The workflow of the conversion step is presented in the diagram below:

flowchart LR
    subgraph tf_fe["Frontend::convert()"]
    first_pass["1st transform pass (Loaders)"]
    NodeContext --> first_pass
    end
    ov::InputModel --> tf_fe
    tf_fe --> ov::Model
Loading

OpenVINO TensorFlow Frontend supports extensions. To add an extension, use ov::frontend::tensorflow::Frontend::add_extension() API. The next extension types are supported:

  • ov::frontend::tensorflow::ConversionExtension or ov::frontend::ConversionExtension - add new Loader into the conversion pipeline
  • ov::TelemetryExtension - enable telemetry for the frontend
  • ov::BaseOpExtension - enable support of a custom operation
  • ov::detail::SOExtension - allow to support ov::BaseOpExtension extensions loaded from the external library.

How to implement support of a new TensorFlow operation

TensorFlow conversion into the OpenVINO opset operation requires one pass or two passes:

  • One pass using Loaders directly transforms TF operation into a sub-graph of OpenVINO opset.
  • Two passes consist of Loaders and Internal Transformations, where the first pass transforms a TF operation into a sub-graph with Internal Operations, and the second pass avoids internal operations. Two transformation passes are used when a TensorFlow operation cannot be mapped into a sub-graph of the OpenVINO opset, and the conversion depends on the succeeding operations in the graph.

In most cases, it is sufficient to use just one pass for TensorFlow operation conversion.

One transformation pass using Loader

Most TensorFlow operations can be converted by one transformation pass using Loader. The dictionary of Loaders is placed in the op_table.cpp file and loaders are in the op directory:

https://github.com/openvinotoolkit/openvino/blob/7f3c95c161bc78ab2aefa6eab8b008142fb945bc/src/frontends/tensorflow/src/op_table.cpp#L129-L134

Here is an example of Loader for TensorFlow Einsum operation:

https://github.com/openvinotoolkit/openvino/blob/7f3c95c161bc78ab2aefa6eab8b008142fb945bc/src/frontends/tensorflow/src/op/einsum.cpp#L15-L28

In this example, the loader checks the consistency of the operation by using default_op_checks and retrieves an attribute of the equation by using the NodeContext::get_attribute() method. The loader uses OpenVINO Core API for building the OpenVINO sub-graph to replace the TensorFlow operation.

The support of a new TensorFlow operation requires implementing a new Loader in a separate file in the op directory and registering it into the dictionary of Loaders.

The main rules for loaders implementation:

  1. Support dynamic shapes and ranks, undefined types, including for the future support of new types, such as strings and complex numbers.
  2. Try to save the same algorithmic complexity of the decomposition.
  3. Use information about operation types. For example, input data with an undefined rank to Conv2D must be of rank equal to 4.
  4. Use the latest OpenVINO opset version for the transformation.
  5. Preserve output tensor names.
  6. Use helpers routines for operation check and construction of a graph from util.hpp.

Two transformation passes using Loader and Internal Transformation

In rare cases, TensorFlow operation conversion requires two transformations (Loader and Internal Transformation). In the first step, Loader must convert a TF operation into Internal Operation that is used temporarily by the conversion pipeline. The internal operation implementation must also contain the validate_and_infer_types() method as similar to OpenVINO Core operations.

Here is an example of an implementation for the internal operation SparseFillEmptyRows used to convert Wide and Deep models.

https://github.com/openvinotoolkit/openvino/blob/7f3c95c161bc78ab2aefa6eab8b008142fb945bc/src/frontends/tensorflow/src/helper_ops/sparse_fill_empty_rows.hpp#L17-L55

In the second step, Internal Transformation based on ov::pass::MatcherPass must convert sub-graphs with internal operations into sub-graphs consisting only of the OpenVINO opset. For more information about ov::pass::MatcherPass based transformations and their development, read Overview of Transformations API and OpenVINO Matcher Pass documentation. The internal transformation must be called in the ov::frontend::tensorflow::FrontEnd::normalize() method. It is important to check the order of applying internal transformations to avoid situations when some internal operation breaks a graph pattern with an internal operation for another internal transformation.

OpenVINO TensorFlow Frontend tests

There are two types of tests for the TensorFlow Frontend (TF FE): layer tests and unit tests.

The layer tests are used to validate support of TensorFlow operation by the frontend.

The unit tests cover TensorFlow format reading functionality, conversion pipeline, and internal transformations for Transpose Sinking and conversion of sub-graphs with TF FE internal operations into the OpenVINO opset.

For operation conversion that requires just Loader, implement layers tests:

In case of two transformation passes using Loader and Internal Transformation, implement them in addition to the layer tests:

How to build and run unit tests

For building the TF FE unit tests, use the CMake target ov_tensorflow_frontend_tests. CMake automatically runs generation scripts to create TensorFlow models used in the testing.

Once the build is complete, launch the ov_tensorflow_frontend_tests (ov_tensorflow_frontend_tests.exe for Windows) executable file to run all tests for the TensorFlow Frontend. The unit tests use the GoogleTest framework for execution.

To get a tests coverage report for the TensorFlow Frontend, read the page on measuring coverage.

How to run TensorFlow Frontend layer tests

The layer tests are Python-based and check that a TensorFlow operation is supported by TF FE. The testing pipeline of the layer tests consists of four steps:

  1. Create a single-layer model with tested operation using TensorFlow.
  2. Convert this model into IR with TF FE.
  3. Infer the original model using TensorFlow and infer the IR model using OpenVINO.
  4. Compare the inference results from both frameworks.

The layer tests include two suites for TensorFlow 1 and TensorFlow 2 Keras operation set support.

To set up environment for running the layer tests, follow these instructions.

To test the whole suite of the TensorFlow 1 operation set support, run the following command:

py.test tensorflow_tests --use_new_frontend

The command line for one operation:

py.test tensorflow_tests/test_tf_Unique.py --use_new_frontend

See also