inference engine related design

doc/fluid/design/inference_engine.md

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+# Utilize Engines to Accelerate Inference
+
+The inference phase need to support some special hardware for acceleration, 
+such as GPU, FPGA, and ARM.
+Special softwares power some of these hardwares and the inner states are hidden, for example, the TensorRT is released by NVidia to improve the inference performance on GPUs, it takes a computation graph as input, 
+optimize and execute it, but the users can't directly modify its internal logics. 
+
+In other words, these software acts like a black box, the external logics prepare its inputs, execute it and process its output. 
+In the Paddle inference module, we call such software an engine, and the inference phase will partition sub-blocks(sub graph) and execute them on the engines to improve performance.
+
+## Use Engines to Execute Sub-blocks
+
+Compared to Paddle Fluid, the engines covers limited number of operators and can only power several kinds of models. In other words, the engines can only support a part of Fluid.
+
+The Block in Fluid acts like a computation graph and it is natural to partition the Block into several sub-blocks which are powered by several different engines.
+
+<p align="center">
+
+<img src="./images/inference engine.jpg"/>
+
+</p>
+
+It is easy to parallelize the computation by scheduling several engines on different devices, for example, the CPU and GPU engines can be dispatched in the meantime
+
+<p align="center">
+
+<img src="./images/parallel engine.png"/>
+
+</p>
+
+
+
+## Partition the Sub-blocks supported by a Specific Engine
+
+As mentioned above, one engine can support a partition of Fluid operators, the sub-block dispatched should be composed by the operators this engine fully supports.
+
+The Inference framework needs a mechanism to mark the sub-block and deliver it to an engine.
+
+We use a `with-statement` to mark the sub-block as follows.
+
+```python
+with infer.power_by_engine('tensorrt'):
+    o = some_op()
+    o = some_op()
+    ...
+```
+
+The operators inside the `infer.power_by_tensorrt` code block will combine into a sub-block and transfers to a TensorRT engine. We call this API-powered sub-block marking the way the **manul sub-block marking**, which means the users directly decide the operators runs on some engine.
+
+For large models, it is trivial to mark all the sub-blocks, so an elicitation method is raised to make it automatic, we call this way the **sub-block automatic detection mode**.
+
+```python
+# if min_ops is set, turn on sub-block automatic detection mode
+# if the more than two adjacent operators are supported by some engine, combine them to 
+# a sub-block and transmit to some engine.
+infer.init_subblock_optimizer(min_ops=2)
+
+o = some_op()
+o = some_op()
+
+# one can still set one or a code block to use some specific engine
+with infer.power_by_engine('X'):
+    o = op1()
+
+o = some_op()
+
+# several different engines can be utilized in one model, the elicitation method 
+# will greedily detect more adjacent operators that powered by the specified engine,
+#, and partition to a larger sub-block.
+with infer.power_by_engine('Y'):
+    o = op2()
+
+o = some_op()
+```
+
+## Transmit the sub-blocks to an Engine
+
+The marked code blocks will be written into a `BlockDesc`, to make the inference phase more clear to support the engine execution, we break up the whole architecture into three layers:
+
+- Frontend, the python syntax, generate the basic fluid model description with some inference customized configurations.
+- Optimizer, rewrite the fluid description, such as pruning the unused operators, reuse some variable memory.
+- Backend, simply execute the fluid description.
+
+<p align="center">
+
+<img src="images/inference architecture.png"/>
+
+</p>
+
+To support engines, there are following phases.
+
+1. the user uses the APIs supplied by **Frontend**, generate a Fluid model description with some adjacent operators marked with some engine label
+2. the optimizer found out the operators marked with the engine labels
+   1. Extract these operators and combine them to sub-blocks
+   2. Delete them from the original Fluid model description
+   3. Insert a new `EngineOp` into the original Fluid model description with the sub-block set as an attribute
+3. the **Backend** get the optimized Fluid description
+   - the `EngineOp` is treated as normal operators
+   - the Backend execute each operator one by one if in sync mode
+   - the operators(especially different `EngineOp` on different devices) can execute parallelly in async mode
+
+## How Engine works with the Fluid framework
+
+The `EngineOp` described above is the key, it acts like normal Fluid operators, but embed with an engine. When an `EngineOp` is created, the engine inside will build a network which acts equivalent functions with the Fluid sub-block describes.
+
+When the whole Fluid description is executed by Backend, the engine inside will run its runtime engine.
+
+There is a tradeoff between sub-block size and the number of `EngineOp,` each `EngineOp` need a pair of input and output data format converters, those results in additional latency. 
+
+So bigger sub-block with less `EngineOp` is better, but some Fluid operators without alternative ones in the engine will break up the big block into small subblocks, whether to execute these sub-blocks on engines or just on Fluid, that needs more consideration.
+
+To help convert input/output data format between any Fluid operators and some `EngineOp`, a pair of `EngineInputConvert` and `EngineOutputConvert` interface is proposed
+
+- the converter works between the Fluid operators and `EngineOp`s, both of their data formats might be different and need to specify, for example
+  - `RNNOp -> xEngineOp`, the input is from an `LoDTensor` to `xTensor`
+  - `MulOp -> xEngineOp`, the input is from an `Tensor` to `xTensor`
+  - but `RNNOp -> MulOp -> xEngineOp`, the input is from an `LoDTensor` to `xTensor`
+  - the `EngineOp` can not get the external operators those link to it (the `RNNOp` and `MulOp` above), so it is impossible to deduce the data's format interact with the external Fluid framework.
+- the converter will result in additional overhead, to make it an indenpendent interface is more clear and more flexible for further optimization.
+
+## Engine-related Design
+
+###  EngineOp                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        
+
+`EngineOp` is just a normal Fluid operator, which has an attribute called `subblock` to get the Fluid description about a sub-block.
+
+To unify the behavior of different `EngineOp`s, an underlying interface over all the engines is raised as follows 
+
+```c++
+/*
+EngineBase is used like
+class xEngine : EngineBase { ... }
+
+xEngine engine;
+engine.Build(some_desc);
+
+for (const auto& batch : dataset) {
+    auto& input0 = engine.buffer("input0");
+    if (input0.device == DeviceType::CPU) {
+    std::memcpy(input0.buffer, batch.data(), batch.size());
+    input0.size = batch.size();
+    } else if (input0.device == DeviceType::GPU) {
+    	cudaMemCpy(input0.buffer, batch.data(), batch.size());
+        input0.size = batch.size();
+    }
+    engine.Execute();
+
+    const auto& output0 = engine.buffer("output0");
+    // ...
+}
+*/
+enum class DeviceType {
+    CPU = 0,
+    GPU
+};
+
+struct Buffer {
+    void* buffer;
+    int max_size;
+    int size;
+    DeviceType device;
+};
+
+class EngineBase {
+public:
+    // Build once.
+    virtual Build(const BlockDesc& desc) = 0;
+    virtual Execute() = 0;
+    // Clone an engine instance with weights shared.
+    virtual EngineBase* Clone() const = 0;
+    // Expose the engine's internal buffer to write/read data directly
+    virtual Buffer& buffer(const std::string& tensor) = 0;
+
+    virtual ~EngineBase() {}  
+};
+```
+
+
+
+### Data format convert operators
+
+Both the `EngineInputConvertOp` and `EngineOutputConvertOp` has similar interfaces
+
+The operator has following attributes.
+
+1. `input_op_type` 
+2. `output_op_type`
+
+For a convert op that input from `RNNOp` and outputs to `xEngineOp`, the values of these two attributes are *RNNOp* and *xEngineOp*.
+
+to make the implementation of the input and output combination more extensible, the functor and register should be included:
+
+```c++
+struct EngineInputConveterBase {
+    // the `out` is a cuda memory that has been allocated.
+    virtual void operator()(LoDTensor& in, void* out, size_t max_size) = 0;
+    static void Execute(const std::string& in_op_type, const std::string& out_op_type,
+                      const LoDTensor& in, void* out, size_t max_size) {
+        conveters[in_op_type + "_to_" + out_op_type](in, out, max_size);
+    }
+
+    template<typename T>
+    static Register(const std::string& key) { conveters[key] = T(); }
+
+  	static std::map<std::string, EngineInputConveterBase> conveters;
+};
+
+// some specific implementations
+struct RNN2xEngineConveter : public EngineInputConveterBase {
+    void operator()(const LoDTensor& in, void* out, size_t max_size) override;
+};
+
+#define REGISTER_INPUT_CONVERTER(in_op_type__, out_op_type__, Conveter__) \
+    ... some logics \
+    EngineInputConveterBase::Register<Conveter__>(#in_op_type__ "_to_" #out_op_type__); \
+    ... some logics
+
+REGISTER_INPUT_CONVERTER(RNNOp, xEngineOp, RNN2xEngineConveter);
+```
+
+The `EngineOutputConvertOp` is similar.
+
+### Optimizer for sub-block
+
+```c++
+// The InferenceOptimizers input a program desc and output a block desc.
+// Different implementations will rewrite the original program desc by different logics.
+// There might be many different optimizers, such as
+// - CleanUselessOptimizer
+// - PruneOpOptimizer
+// - SubblockToEngineOptimizer
+struct InferenceOptimizer {
+   virtual operator()(const ProgramDesc& desc, ProgramDesc* out) = 0;
+
+    void RunALL() {
+        for (auto& o : optimizers) { o(); }
+    }
+
+    template<typename T>
+    static void Register(const T& t) { optimizers.append(t); }
+    static std::vector<InfernceOptimizer> optimizers;
+};
+
+// Extract the subblock from the Program Desc, insert a xEngineOp and set its attribute
+// to a sub-block description.
+struct SubblockToEngineOptimizer : public InferenceOptimizer {
+    virtual operator() (const ProgramDesc& desc, ProgramDesc* out) override;
+};
+
+REGISTER_INFERENCE_OPTIMIZER(SubblockToEngineOptimizer);
+
+#define REGISTER_INFERENCE_OPTIMIZER(Optimizer__) \
+   InferenceOptimizer::Register(Optimizer__());
+```