pytorch经onnx转tensorrt初体验上、下中学习了tensorrt如何调用onnx模型，但其中遇到的问题是tensorrt7没有办法直接输入动态batchsize的数据，当batchsize>1时只有第一个sample的结果是正确的，而其后的samples的输出都为0. 本文主要是探索如何进行批量化的处理。

1. 添加辅助引擎。

这是TensorRT/samples/sampleDynamicReshape/sampleDynamicReshape.cpp 中给出的一个解决方案，其主要思路是在原有的INetwork 之前再创建一个用于input resize的Network， 该Network的主要功能是对可变的输入进行resize，以及设置配置文件和参数绑定。

其中，最主要的部分如下：

    // Finally, configure and build the preprocessor engine.
    auto preprocessorConfig = makeUnique(builder->createBuilderConfig());

    // Create an optimization profile so that we can specify a range of input dimensions.
    auto profile = builder->createOptimizationProfile();

    // This profile will be valid for all images whose size falls in the range of [(1, 1, 1, 1), (1, 1, 56, 56)]
    // but TensorRT will optimize for (1, 1, 28, 28)
    profile->setDimensions(input->getName(), OptProfileSelector::kMIN, Dims4{1, 1, 1, 1});
    profile->setDimensions(input->getName(), OptProfileSelector::kOPT, Dims4{1, 1, 28, 28});
    profile->setDimensions(input->getName(), OptProfileSelector::kMAX, Dims4{1, 1, 56, 56});
    preprocessorConfig->addOptimizationProfile(profile);
    mPreprocessorEngine = makeUnique(builder->buildEngineWithConfig(*preprocessorNetwork, *preprocessorConfig));

其中配置器profile指定了输入的最小尺寸、最优尺寸和最大尺寸。那么真实输入时，处在最小和最大尺寸中都行。

2. 直接给context 配置profile文件。

参考：# TensorRT 7 ONNX models with variable batch size

image.png

• 安装文档，首先我们需要生成一个onnx模型，我们这里生成两个模型，分别对应batchsize固定 resnet18.onnx 和batchsize可变resnet18_dynamic.onnx

#--*-- coding:utf-8 --*--
import onnx 
import torch
import torchvision 
import netron

net = torchvision.models.resnet18(pretrained=True).cuda()
net.eval()

export_onnx_file = "./resnet18.onnx"
x=torch.onnx.export(net,  # 待转换的网络模型和参数
                torch.randn(1, 3, 224, 224, device='cuda'), # 虚拟的输入，用于确定输入尺寸和推理计算图每个节点的尺寸
                export_onnx_file,  # 输出文件的名称
                verbose=False,      # 是否以字符串的形式显示计算图
                input_names=["input"],# + ["params_%d"%i for i in range(120)],  # 输入节点的名称，这里也可以给一个list，list中名称分别对应每一层可学习的参数，便于后续查询
                output_names=["output"], # 输出节点的名称
                opset_version=10,   # onnx 支持采用的operator set, 应该和pytorch版本相关，目前我这里最高支持10
                do_constant_folding=True, # 是否压缩常量
                )

export_onnx_file = "./resnet18_dynamic.onnx"
x=torch.onnx.export(net,  # 待转换的网络模型和参数
                torch.randn(1, 3, 224, 224, device='cuda'), # 虚拟的输入，用于确定输入尺寸和推理计算图每个节点的尺寸
                export_onnx_file,  # 输出文件的名称
                verbose=False,      # 是否以字符串的形式显示计算图
                input_names=["input"],# + ["params_%d"%i for i in range(120)],  # 输入节点的名称，这里也可以给一个list，list中名称分别对应每一层可学习的参数，便于后续查询
                output_names=["output"], # 输出节点的名称
                opset_version=10,   # onnx 支持采用的operator set, 应该和pytorch版本相关，目前我这里最高支持10
                do_constant_folding=True, # 是否压缩常量
                dynamic_axes={"input":{0: "batch_size"}, "output":{0: "batch_size"},} #设置动态维度，此处指明input节点的第0维度可变，命名为batch_size
                )

• 然后我们使用 TensorRT 提供的 trtexec 工具由onnx模型直接生成并保存cuda引擎。
  trtexec指令的位置： <path-to-TensorRT>/bin, 所以把该路径添加到PATH环境变量中
  export PATH=/home/zwzhou/packages/TensorRT-7.0.0.11/bin:$PATH

测试 trtexec -h 发现

image.png

image.png

export LD_LIBRARY_PATH=/home/zwzhou/cuda-9.0/bin:${LD_LIBRARY_PATH}

再次 trtexec -h 即正确显示帮助信息。其中给出了 model options、build options、 inference options和system options等。
a. 从固定尺寸的onnx转cudaEngine
···
trtexec --explicitBatch --onnx=./resnet18.onnx --saveEngine=resnet18.engine
···
b.从可变尺寸的onnx转cudaEngine，需要指定profile。

trtexec --onnx=./resnet18_dynamic.onnx --explicitBatch \
            --minShapes="input":1x3x224x224\
            --optShapes="input":16x3x224x224\
            --maxShapes="input":32x3x224x224\
            --shapes="input":1x3x224x224\
            --saveEngine=resnet18_dynamic.engine

c. 接下来看一下python API的调用

import argparse
from typing import Tuple, List

import numpy as np
import pycuda.driver as cuda
import pycuda.autoinit
import tensorrt as trt

TRT_LOGGER = trt.Logger(trt.Logger.WARNING)
BatchSize = 32
// 判断 shape是否是动态的或者固定的
def is_fixed(shape: Tuple[int]):
    return not is_dynamic(shape)
def is_dynamic(shape: Tuple[int]):
    return any(dim is None or dim < 0 for dim in shape)

def load_engine(filename: str):
    # Load serialized engine file into memory 加载序列化的cuda引擎并进行反序列化
    with open(filename, "rb") as f, trt.Runtime(TRT_LOGGER) as runtime:
        return runtime.deserialize_cuda_engine(f.read())

def get_binding_idxs(engine: trt.ICudaEngine, profile_index: int):
    # Calculate start/end binding indices for current context's profile
    num_bindings_per_profile = engine.num_bindings // engine.num_optimization_profiles
    start_binding = profile_index * num_bindings_per_profile
    end_binding = start_binding + num_bindings_per_profile
    print("Engine/Binding Metadata")
    print("\tNumber of optimization profiles: {}".format(engine.num_optimization_profiles))
    print("\tNumber of bindings per profile: {}".format(num_bindings_per_profile))
    print("\tFirst binding for profile {}: {}".format(profile_index, start_binding))
    print("\tLast binding for profile {}: {}".format(profile_index, end_binding-1))

    # Separate input and output binding indices for convenience
    input_binding_idxs = []
    output_binding_idxs = []
    for binding_index in range(start_binding, end_binding):
        if engine.binding_is_input(binding_index):
            input_binding_idxs.append(binding_index)
        else:
            output_binding_idxs.append(binding_index)

    return input_binding_idxs, output_binding_idxs
# 指定输入的shape，同时根据输入的shape指定输出的shape，并未输出赋予cuda空间
def setup_binding_shapes(
    engine: trt.ICudaEngine,
    context: trt.IExecutionContext,
    host_inputs: List[np.ndarray],
    input_binding_idxs: List[int],
    output_binding_idxs: List[int],
):
    # Explicitly set the dynamic input shapes, so the dynamic output
    # shapes can be computed internally
    for host_input, binding_index in zip(host_inputs, input_binding_idxs):
        context.set_binding_shape(binding_index, host_input.shape)

    assert context.all_binding_shapes_specified

    host_outputs = []
    device_outputs = []
    for binding_index in output_binding_idxs:
        output_shape = context.get_binding_shape(binding_index)
        # Allocate buffers to hold output results after copying back to host
        buffer = np.empty(output_shape, dtype=np.float32)
        host_outputs.append(buffer)
        # Allocate output buffers on device
        device_outputs.append(cuda.mem_alloc(buffer.nbytes))

    return host_outputs, device_outputs

def get_random_inputs(
    engine: trt.ICudaEngine,
    context: trt.IExecutionContext,
    input_binding_idxs: List[int],
    seed: int = 42,
):
    # Input data for inference
    host_inputs = []
    print("Generating Random Inputs")
    print("\tUsing random seed: {}".format(seed))
    np.random.seed(seed)
    for binding_index in input_binding_idxs:
        # If input shape is fixed, we'll just use it
        input_shape = context.get_binding_shape(binding_index)
        input_name = engine.get_binding_name(binding_index)
        print("\tInput [{}] shape: {}".format(input_name, input_shape))
        # If input shape is dynamic, we'll arbitrarily select one of the
        # the min/opt/max shapes from our optimization profile
        if is_dynamic(input_shape):
            profile_index = context.active_optimization_profile
            profile_shapes = engine.get_profile_shape(profile_index, binding_index)
            print("\tProfile Shapes for [{}]: [kMIN {} | kOPT {} | kMAX {}]".format(input_name, *profile_shapes))
            # 0=min, 1=opt, 2=max, or choose any shape, (min <= shape <= max)
            input_shape = (BatchSize, 3, 224, 224)#profile_shapes[1]
            print("\tInput [{}] shape was dynamic, setting inference shape to {}".format(input_name, input_shape))

        host_inputs.append(np.random.random(input_shape).astype(np.float32))

    return host_inputs

主函数：

def main():
    parser = argparse.ArgumentParser()
    parser.add_argument("-e", "--engine", required=True, type=str,
                        help="Path to TensorRT engine file.")
    parser.add_argument("-s", "--seed", type=int, default=42,
                        help="Random seed for reproducibility.")
    args = parser.parse_args()

    # Load a serialized engine into memory
    engine = load_engine(args.engine)  // 加载序列化的cuda引擎
    print("Loaded engine: {}".format(args.engine))

    # Create context, this can be re-used  创建 执行环境
    context = engine.create_execution_context()
    # Profile 0 (first profile) is used by default  context可以设置多个profile， 这里选择第一个，也是默认的profile，其中规定了输入尺寸的变化区间
    context.active_optimization_profile = 0 
    print("Active Optimization Profile: {}".format(context.active_optimization_profile))

    # These binding_idxs can change if either the context or the
    # active_optimization_profile are changed  获得输入输出变量名对应profile的idx
    input_binding_idxs, output_binding_idxs = get_binding_idxs(
        engine, context.active_optimization_profile
    )
    # 获得输入变量的变量名 
    input_names = [engine.get_binding_name(binding_idx) for binding_idx in input_binding_idxs]
    
    # Generate random inputs based on profile shapes， 随机产生输入变量
    host_inputs = get_random_inputs(engine, context, input_binding_idxs, seed=args.seed)

    # Allocate device memory for inputs. This can be easily re-used if the
    # input shapes don't change  为输入变量赋予host空间，该空间可复用
    device_inputs = [cuda.mem_alloc(h_input.nbytes) for h_input in host_inputs]
    # Copy host inputs to device, this needs to be done for each new input， 由host拷贝到device
    for h_input, d_input in zip(host_inputs, device_inputs):
        cuda.memcpy_htod(d_input, h_input)

    print("Input Metadata")
    print("\tNumber of Inputs: {}".format(len(input_binding_idxs)))
    print("\tInput Bindings for Profile {}: {}".format(context.active_optimization_profile, input_binding_idxs))
    print("\tInput names: {}".format(input_names))
    print("\tInput shapes: {}".format([inp.shape for inp in host_inputs]))

    # This needs to be called everytime your input shapes change
    # If your inputs are always the same shape (same batch size, etc.),
    # then you will only need to call this once 重新指定网络输入输出的大小。
    host_outputs, device_outputs = setup_binding_shapes(
        engine, context, host_inputs, input_binding_idxs, output_binding_idxs,
    ) # 返回的是输出的idx和device buffer
    output_names = [engine.get_binding_name(binding_idx) for binding_idx in output_binding_idxs]

    print("Output Metadata")
    print("\tNumber of Outputs: {}".format(len(output_binding_idxs)))
    print("\tOutput names: {}".format(output_names))
    print("\tOutput shapes: {}".format([out.shape for out in host_outputs]))
    print("\tOutput Bindings for Profile {}: {}".format(context.active_optimization_profile, output_binding_idxs))
    
    # Bindings are a list of device pointers for inputs and outputs
    bindings = device_inputs + device_outputs  # list的合并

    # Inference
    t1 = time.time()
    for i in range(1000):
        context.execute_v2(bindings)  // 执行1000次， 该处和execute_async_v2函数不大一样
    t2 = time.time()
    print("Inference iterations: {}".format(((t2-t1))))
    print("Inference iterations per sample: {}".format(((t2-t1)/BatchSize)))

    # Copy outputs back to host to view results 将输出由gpu拷贝到cpu。
    for h_output, d_output in zip(host_outputs, device_outputs):
        cuda.memcpy_dtoh(h_output, d_output)

    # View outputs
    # print("Inference Outputs:", host_outputs)

    # Cleanup (Can also use context managers instead)
    del context
    del engine

下表给出的是V100上resnet18的前向推断时间(ms)

总结一下，dynamic batchsize的处理流程：

• 生成 batch可变化的onnx，当然这一步非必须，可以后面tensorrt中修改
• 将onnx模型保存成 engine文件，可以使用trtexec工具
• 输入在profile限定尺寸范围内的数据，并分配host和device空间
• 根据输入的尺寸，推到输出变量的尺寸，并分配host和device空间
• execute_v2进行推理
• 将输出由cuda拷贝到cpu进行处理