diff --git a/doc/design/kernel_hint_design.md b/doc/design/kernel_hint_design.md new file mode 100644 index 0000000000000000000000000000000000000000..a54b7da045e1a362626ef066f9ebb56af2c3181a --- /dev/null +++ b/doc/design/kernel_hint_design.md @@ -0,0 +1,57 @@ +## Problem +In PaddlePaddle's [Design](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md), one Operator may have multiple kernels. Users may have some personal preference to choose a certain type of kernel for an operator, such as `force_cpu` to choose a CPU kernel, `use_cudnn` to choose a CUDNN kernel, we need to provide a way for users to do this. + +In the current design, we use KernelType to describe one kernel. + +```cpp +struct KernelType { + Place place_; + DataType data_type_; + LayoutType layout_; +}; +``` + `place_` `data_type_` and `layout_` can be got from the input tensors of the operator, `GetActualKernelType(inputs)` use inputs to infer the proper kernel key that fit the incoming data, but users can not directly configure it. + +The [design](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md) also provides a virtual method `GetExpectedKernelType` that user can overload and use to choose the KernelType they want to use. + +So we should send the information user defined in proto to `GetExpectedKernelType` for choosing a kernel. + +The problem is, how should we define and send the information for `GetExpectedKernelType` to use? + +## Solution + +### Potential choice +1. Do nothing, let the user add the information they want to operator‘s attribute and get them inside `GetExpectedKernelType`, this can work properly. But there is a little problem that users may define many kinds of hints for the same purpose, such as `force_cpu`, `use_cpu`, `cpu_kernel` to choose CPU kernel, and `use_cudnn`, `force_cudnn`, `cudnn_kernel` to choose CUDNN kernel. + +2. Pre-define all the needed option and use a single attr key such as `kernel_hint` for the user, this is not so flexible if the user wants to define some more kind of hint. + +### Final choice +To provide enough flexibility while avoiding confusion definition, we can define some global constants for these attribute names, such as `force_cpu`, `use_cudnn`, `use_mkldnn` for a user to choose. + +In C++ + +```cpp +const std::string kForceCPU = "force_cpu"; +const std::string kUseCUDNN = "use_cudnn"; +const std::string kUseMKLDNN = "use_mkldnn"; + +KernelType GetExpectedKernelType() { + if (Attr(kForceCPU)) { + return KernelType(CPUPlace, ...) + } else { + ... + } +} +``` + +In Python code + +```python +FORCE_CPU = core.kForceCPU() + +def xx_layer(..., force_cpu=false): + layer_helper = LayerHelper(...) + layer_helper.append_op( + type="xx", + attr={FORCE_CPU: force_cpu}) +```