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gpu_plugin_ops_enabling.md

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GPU plugin operations enabling flow

Terminology

  • NGraph operation: Building block of neural networks, such as convolution or pooling.
  • (clDNN) Primitive: Basic NN operation that was defined in clDNN. One primitive is usually mapped to one ngraph operation, but graph compilation may cause the mapping not to be 1-to-1.
  • Kernel: Actual body of execution in GPU. It also refers to specific implementations of Primitive for GPU, such as convolution_gpu_winograd_2x3_s1.cl. Usually, single kernel fulfills the operation of single primitive, but several kernels may be used to support one primitive.
  • Unittest: Single-layer test within cldnn.
  • Functional test: Single-layer test in IE.

Adding new primitive

  1. Understand the new operation.

  2. Try to find existing primitive that fully or partially covers this operation.

    • It is also possible to transform the network so that the missing primitive is covered from existing primitive.
    • e.g. Replace reduce with pooling

  3. Add new / extend existing cldnn primitive according to the operation spec.

    1. This phase is to enable primitive within cldnn library, without exposing it to IE.

    2. Implement reference parallel kernel that supports all parameters of the operation and all input/output data types and layouts

      File Description
      scatter_elements_update_ref.cl OpenCL Kernel body. For more detail, please see How to write OCL kernel section
      scatter_elements_update_kernel_ref.(cpp,h) Counterpart of kernel body for host
      scatter_elements_update_kernel_selector.(cpp,h) Kernel selector for a primitive
      register_gpu.(cpp,hpp) Primitive registration
      scatter_elements_update_gpu.cpp Primitive registration, input spec
      scatter_elements_update_inst.h Node type declaration for cldnn program
      clDNN/src/scatter_elements_update.cpp Code for scatter_elements_update_inst.h
      clDNN/api/cldnn/primitives/scatter_elements_update.hpp clDNN primitive definition
      common_types.h Enum declaration for KernelType and arguments

    3. Add unit tests for the new operation

      File Description
      scatter_elements_update_gpu_test.cpp Unittest for layer

      • Need to add reference code or expected result for checking the result.

      • You can also specify the kernel with force_implementations in case the primitive contains multiple kernels.

        ...
build_options options;
implementation_desc conv_impl = { format::fs_b_yx_fsv32, "" };
options.set_option(build_option::force_implementations({ {"conv_fsv", conv_impl} }));
network network(engine, topology, options);
...

      • This unit test is built into clDNN_unit_tests. It is a gtest application.

        # Show list of test cases
openvino/bin/intel64/Debug$ ./clDNN_unit_tests64 --gtest_list_tests
# Run test
openvino/bin/intel64/Debug$ ./clDNN_unit_tests64 --gtest_filter=scatter_elements_update_gpu_fp16.*

      • Test scope needs to be comprehensive, but not wasteful. These tests run for every PRs in CI. Let's save the planet.

    4. Support layer fusion, if applicable

      • It is usually easy to fuse some layers, such as scale, activation, quantize and eltwise, into previous layer. This fusing rule can be added to prepare_primitive_fusing::fuse_simple_primitives.
      • fuse_simple_primitives is called during graph compilation phase
      • You can see general description of layer fusion here
      • Unit tests for layer fusion are placed in a single file: fusings_gpu_test.cpp. It is also compiled into clDNN_unit_tests.
      • Code for fused layers are generated with jitter. It is created as FUSED_OPS.. macro in OCL code. This generation logic is in KernelBase::MakeFusedOpsJitConstants.

  4. Add / update factory for this operation in the GPU plugin to use new primitive in inference-engine

    File Description
    cldnn_engine/ops/scatter_elements_update.cpp Instantiation from cldnn plugin for IE
    cldnn_primitives_list.hpp Registration for primitives

  5. Add functional single layer tests for the operation and try to cover most of the difference use cases of this operation

    File Description
    single_layer_tests/scatter_elements_update.cpp Single layer test

  6. [Optional] If there are existing IRs with this operation, try to run the full model(s) to be sure that it's correctly processed within the context

  7. [Optional] If there are existing IRs with this operation, try to run the full model(s) and estimate performance impact from this operation on total model execution time

  8. Create PR with your changes


Adding new kernel for an existing primitive

  • The process is quite similar to previous one. You can skip already existing steps.
  • Main work is adding new kernel and registering it from kernel selector.
  • You may need to add unit test for that new kernel. Specific kernel can be chosen with build_option::force_implementations.
  • It is not possible to specify kernel from functional test(IE).

Writing OCL kernel

Jitter

In GPU OCL kernels, many conditional statements are processed with #ifdef so that it can be handled during compile-time. The definitions are created with jitter.cpp. It is set during graph compilation. You can see generated macros following the steps in source dumps. Jitter also contains run-time parameters such as input and output size. Additional macros can be defined from host-code of kernel itself. For example, see below code snippet. It passes SUB_GROUP_SIZE through macro definition through jitter.

  // GetJitConstants method of the kernel
  const size_t sub_group_size = 16;
  JitConstants jit = MakeBaseParamsJitConstants(params);
  jit.AddConstant(MakeJitConstant("SUB_GROUP_SIZE", sub_group_size ));

Accessing input and output tensor

Jitter generates macros for index calculations. With these macros, you can program ocl kernel in a layout-agnostic way. If you use the macro ${TENSOR_NAME}_GET_INDEX, you can get 1d-index from tensor coordinate whether the format is planar(such as bfyx or byxf) or blocked.(such as b_fs_yx_fsv16). You can check source code for GET_INDEX macro.

Layout support

If a kernel is not performance-critical, you can support bfyx, bfzyx and bfwzyx only for layout. Those are default layouts. As an optimized format, b_fs_yx_fsv16, b_fs_yx_fsv4 or byxf can be used as well. General description of layout can be found here and header file is here

Layer fusion

When layers are fused, jitter will create macros to generate code for fused layers. It is realized into FUSED_OPS.. in OCL kernel. You can understand the usage from other kernels. There is a comment that describes layer fusion.

See also