提交 661f03b4 编写于 作者: T tensor-tang

add mkldnn fluid design doc

上级 fc5c432a
# Design Doc: Add MKLDNN Kernel in Fluid Operator
## Principles
First of all, we should follow some basical principles like:
1. [How to write a new operator](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/howto/dev/new_op_en.md). We are trying to add a new kind of kernel into operators, so basically we should follow this doc.
2. [Supporting new Device/Library](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/support_new_device.md). Since MKLDNN is a new library to fluid, we should add `MKLDNNDeviceContext` and maybe `mkldnn_helper.h`, just like [cudnn_helper.h](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/platform/cudnn_helper.h).
3. [Switch Kernel](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/switch_kernel.md). Another important point is that we should ensure the data synchronization among different divices, which is the topic #6549. So basically we should override `GetActualKernelType` and `trans` functions to support switching kernels.
4. [The Keys of Operator Kernel Type](https://github.com/PaddlePaddle/Paddle/blob/develop/doc/design/operator_kernel_type.md). Kernel Type is a pivotal conception which can record the `Place`, `Library`, `DataType` and `Layout`.
## Sulution
In general, there are three part we should follow to run a MKL-DNN primitive.
- create a primitive descriptor that describe this operator
- create a memory that handle all memory buffers needed
- stream the primitive create by first two
We do not want to see the first two would be re-initialized every iteration again and again. \
So we plan to create a map to record all the `primitive` and `memory`, which should not take too much memories as discussed [here](https://github.com/PaddlePaddle/Paddle/issues/6822).
Assuming that three condition would be confirmed:
1. there is a unique key for each operator instance. May be the actual name of `Output Tensor`.
2. the `Input Tensor` inside `Compute` function is the one after converted.
3. we can get the phase(eg. `is_test`) inside `Compute` function, otherwise we need to expose this attribue to user.
### Compute
The algorithm of `Compute` would be described as follow, let's take conv like an example.
```c++
PADDLE_ENFORCE(platform::is_cpu_place(ctx.GetPlace()), "It must use CPUPlace.");
PADDLE_ENFORCE(platform::is_mkldnn_library(ctx.GetLibrary()), "It must use MKLDNN Library.");
auto& ctx = executionContext.template device_context<platform::MKLDNNDeviceContext>();
// find primitive by unique key from mkldnn context
// the op_key should be a unique name of this op instance
auto& p = ctx.findPrimitive(op_key + "_fwd");
// assuming the input tensor inside this compute function is the one after converted
// this point should be guarantee by another mechanism
auto& i = ctx.findMemory(op_key + "_input");
if (p == nullptr || i == nullptr || inputSizeChanged(p, i)) {
auto fwd_primitive_desc = createPrimitiveDesc(ctx);
auto* input = ctx.Input<Tensor>("Input");
auto* filter = ctx.Input<Tensor>("Filter");
auto* output = ctx.Output<Tensor>("Output");
shared_ptr<mkldnn::memory> in(new mkldnn::memory(fwd_primitive_desc->src_primitive_desc(), input->data<T>()));
shared_ptr<mkldnn::memory> wgt(new mkldnn::memory(fwd_primitive_desc->weights_primitive_desc(), filter->data<T>()));
shared_ptr<mkldnn::memory> out(new mkldnn::memory(fwd_primitive_desc->dst_primitive_desc(), output->mutable_data<T>(ctx.GetPlace())));
shared_ptr<mkldnn::conv_fwd> fwd_primitive(new mkldnn::conv_fwd(*fwd_primitive_desc, *in, *wgt, *out)
);
ctx.addMemory(op_key+"_input", in);
ctx.addMemory(op_key+"_output", out);
ctx.addMemory(op_key+"_filer", wgt);
ctx.addPrimitive(op_key+"_fwd", fwd_primitive);
ctx.addPrimitiveDesc(op_key+"_fwd_PD", fwd_primitive_desc);
}
p = ctx.findPrimitive(op_key + "_fwd");
PADDLE_ENFORCE(p, "Should have Forward Primitive");
PADDLE_ENFORCE(ctx.findMemory(op_unique_key+"_input"), "Should have input memory");
PADDLE_ENFORCE(ctx.findMemory(op_unique_key+"_output"), "Should have output memory");
PADDLE_ENFORCE(ctx.findMemory(op_unique_key+"_filter"), "Should have filter memory");
PADDLE_ENFORCE(ctx.findPrimitiveDesc(op_unique_key+"_fwd_PD"), "Should have forward PrimitiveDesc");
ctx.submit(p);
ctx.execute(); // the convert primitive should have already contained.
```
The `createPrimitiveDesc` returns the primitive descripotor of this operator, would be like this:
```c++
auto* input = ctx.Input<Tensor>("Input");
auto* filter = ctx.Input<Tensor>("Filter");
auto* output = ctx.Output<Tensor>("Output");
std::vector<int> strides = ctx.Attr<std::vector<int>>("strides");
std::vector<int> paddings = ctx.Attr<std::vector<int>>("paddings");
std::vector<int> dilations = ctx.Attr<std::vector<int>>("dilations");
int groups = ctx.Attr<int>("groups");
algorithm algo = static_cast<algorithm>(ctx.Attr<int>("convolution_algorithm_option"));
prop_kind pk = ctx.Attr<bool>("is_test") ? prop_kind::forward_inference : prop_kind::forward_training;
auto fwd_desc = mkldnn::conv_fwd::desc(/* all the setting above*/);
shared_ptr<mkldnn::conv_fwd::primitive_desc> fwd_primitive_desc(new mkldnn::conv_fwd::primitive_desc(fwd_desc, ctx.getEngine()));
return fwd_primitive_desc;
}
```
### MKLDNNDeviceContext
`MKLDNNDeviceContext`, which is very straightforward, should contain some base information like: `stream`, `engine` and the map needed.
### mkldnn_helper
Some functions would be put in `paddle/platform/mkldnn_helper.h`.
- create MKLDNN memories
- create MKLDNN primitives
- error check function
- etc
### Kernel Switch
We should `reorder` the different Layout from other device or to other device. `GetExpectedKernelType` and `trans` functions can help us to implement it.
`GetExpectedKernelType` should get the context, and this operator can return the best `KernelType`.
`trans` would be like this:
```c++
void trans(inputs, ctx) override {
if (NoNeedTrasn()) {
return;
}
// find reorder primitive by op_key from context
auto& p = ctx.findPrimitive(op_key + "_reorder_input");
auto& i = ctx.findMemory(op_key + "_src_input");
if (p == nullptr || i == nullptr || changeSized(i, input)) {
auto prim = createPrimitiveDesc(ctx);
auto src = createMemory(memoryDesc(input->dims(), actual_layout), input->data);
auto newbuffer = paddle::memory::Alloc(ctx.GetPlace(), input->size_in_bytes());
auto dst = createMemory(p->expected_desc(), newbuffer->data);
auto reorder_primitive(new mkldnn::reorder(src, dst));
ctx.addMemory(op_key+"_src_input", src);
ctx.addMemory(op_key+"_input", dst);
ctx.addPrimitive(op_key+"_reorder_input", reorder_primitive);
}
p = ctx.findPrimitive(op_key + "_reorder_input");
PADDLE_ENFORCE(p, "Should have Reorder Primitive");
ctx.submit(p);
if (! this->isMKLDNNKernel()) {
// execute immediately only if this is not mkldnn kernel function.
// otherwise, it can be executed with the operator primitive in Compute
ctx.stream();
}
// after submit, the input tensor in ctx should be changes as the converted one
// there should be another mechanism to ensure this
}
```
### Unit Test
All the functions should be tested corresponding。
TBD
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