Tensorflow中float32模型强制转为float16半浮点模型

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在Tensorflow框架训练完成后，部署模型时希望对模型进行压缩。一种方案是前面文字介绍的方法《【Ubuntu】Tensorflow对训练后的模型做8位（uint8）量化转换》。另一种方法是半浮点量化，今天我们主要介绍如何通过修改Tensorflow的pb文件中的计算节点和常量（const），将float32数据类型的模型大小压缩减半为float16数据类型的模型。

1 加载pb模型

封装函数，加载pb模型：

def load_graph(model_path):
    graph = tf.Graph()
    with graph.as_default():
        graph_def = tf.GraphDef()
        if model_path.endswith("pb"):
            with open(model_path, "rb") as f:
                graph_def.ParseFromString(f.read())
        else:
            with open(model_path, "r") as pf:
                text_format.Parse(pf.read(), graph_def)
        tf.import_graph_def(graph_def, name="")
        sess = tf.Session(graph=graph)
        ops=graph.get_operations()
        for op in ops:
            print(op.name)
        return sess

2 重写BatchNorm

由于BatchNorm对精度比较敏感，需要保持float32类型，因此BatchNorm需要特殊处理。

#用FusedBatchNormV2替换FusedBatchNorm，以保证反向梯度下降计算时使用的是float
def rewrite_batch_norm_node_v2(node, graph_def, target_type='fp16'): 
    if target_type == 'fp16':
        dtype = types_pb2.DT_HALF
    elif target_type == 'fp64':
        dtype = types_pb2.DT_DOUBLE
    else:
        dtype = types_pb2.DT_FLOAT
    new_node = graph_def.node.add()
    new_node.op = "FusedBatchNormV2"
    new_node.name = node.name
    new_node.input.extend(node.input)
    new_node.attr["U"].CopyFrom(attr_value_pb2.AttrValue(type=types_pb2.DT_FLOAT))
    for attr in list(node.attr.keys()):
        if attr == "T":
            node.attr[attr].type = dtype
        new_node.attr[attr].CopyFrom(node.attr[attr])
    print("rewrite fused_batch_norm done!")

3 Graph转换

重新构造graph，参数从原始pb的graph中拷贝，并转为float16

def convert_graph_to_fp16(model_path, save_path, name, as_text=False, target_type='fp16', input_name=None, output_names=None):
    #生成新的图数据类型
    if target_type == 'fp16':
        dtype = types_pb2.DT_HALF
    elif target_type == 'fp64':
        dtype = types_pb2.DT_DOUBLE
    else:
        dtype = types_pb2.DT_FLOAT

    #加载需要转换的模型
    source_sess = load_graph(model_path)
    source_graph_def = source_sess.graph.as_graph_def()
    #创建新的模图对象
    target_graph_def = graph_pb2.GraphDef()
    target_graph_def.versions.CopyFrom(source_graph_def.versions)
    #对加载的模型遍历计算节点
    for node in source_graph_def.node:
        # 对FusedBatchNorm计算节点替换为FusedBatchNormV2
        if node.op == "FusedBatchNorm":
            rewrite_batch_norm_node_v2(node, target_graph_def, target_type=target_type)
            continue
        # 复制计算节点
        new_node = target_graph_def.node.add()
        new_node.op = node.op
        new_node.name = node.name
        new_node.input.extend(node.input)

        #对attrs属性进行复制，attrs属性主要关注
        attrs = list(node.attr.keys())
        # BatchNorm属性保持不变
        if ("BatchNorm" in node.name) or ('batch_normalization' in node.name):
            for attr in attrs:
                new_node.attr[attr].CopyFrom(node.attr[attr])
            continue
        # 除了BatchNorm以外其他计算节点的属性单独
        for attr in attrs:
            # 对指定的计算节点保持不变
            if node.name in keep_fp32_node_name:
                new_node.attr[attr].CopyFrom(node.attr[attr])
                continue
            #将Float类型修改为设置的目标类型
            if node.attr[attr].type == types_pb2.DT_FLOAT:
                # modify node dtype
                node.attr[attr].type = dtype
                
            #重点关注value，weights都是保存在value属性中
            if attr == "value":
                tensor = node.attr[attr].tensor
                if tensor.dtype == types_pb2.DT_FLOAT:
                    # if float_val exists
                    if tensor.float_val:
                        float_val = tf.make_ndarray(node.attr[attr].tensor)
                        new_node.attr[attr].tensor.CopyFrom(tf.make_tensor_proto(float_val, dtype=dtype))
                        continue
                    # if tensor content exists
                    if tensor.tensor_content:
                        tensor_shape = [x.size for x in tensor.tensor_shape.dim]
                        tensor_weights = tf.make_ndarray(tensor)
                        # reshape tensor
                        tensor_weights = np.reshape(tensor_weights, tensor_shape)
                        tensor_proto = tf.make_tensor_proto(tensor_weights, dtype=dtype)
                        new_node.attr[attr].tensor.CopyFrom(tensor_proto)
                        continue
            new_node.attr[attr].CopyFrom(node.attr[attr])
    # transform graph
    if output_names:
        if not input_name:
            input_name = []
        transforms = ["strip_unused_nodes"]
        target_graph_def = TransformGraph(target_graph_def, input_name, output_names, transforms)
    # write graph_def to model
    tf.io.write_graph(target_graph_def, logdir=save_path, name=name, as_text=as_text)
    print("Converting done ...")

4 完整的代码

import tensorflow as tf
from tensorflow.core.framework import types_pb2, graph_pb2, attr_value_pb2
from tensorflow.tools.graph_transforms import TransformGraph
from google.protobuf import text_format
import numpy as np

# object detection api input and output nodes
input_name = "input_tf"
output_names = ["output:0"]
keep_fp32_node_name = []

def load_graph(model_path):
    graph = tf.Graph()
    with graph.as_default():
        graph_def = tf.GraphDef()
        if model_path.endswith("pb"):
            with open(model_path, "rb") as f:
                graph_def.ParseFromString(f.read())
        else:
            with open(model_path, "r") as pf:
                text_format.Parse(pf.read(), graph_def)
        tf.import_graph_def(graph_def, name="")
        sess = tf.Session(graph=graph)
        ops=graph.get_operations()
        for op in ops:
            print(op.name)
        return sess

#用FusedBatchNormV2替换FusedBatchNorm，以保证反向梯度下降计算时使用的是float
def rewrite_batch_norm_node_v2(node, graph_def, target_type='fp16'): 
    if target_type == 'fp16':
        dtype = types_pb2.DT_HALF
    elif target_type == 'fp64':
        dtype = types_pb2.DT_DOUBLE
    else:
        dtype = types_pb2.DT_FLOAT
    new_node = graph_def.node.add()
    new_node.op = "FusedBatchNormV2"
    new_node.name = node.name
    new_node.input.extend(node.input)
    new_node.attr["U"].CopyFrom(attr_value_pb2.AttrValue(type=types_pb2.DT_FLOAT))
    for attr in list(node.attr.keys()):
        if attr == "T":
            node.attr[attr].type = dtype
        new_node.attr[attr].CopyFrom(node.attr[attr])
    print("rewrite fused_batch_norm done!")

def convert_graph_to_fp16(model_path, save_path, name, as_text=False, target_type='fp16', input_name=None, output_names=None):
    #生成新的图数据类型
    if target_type == 'fp16':
        dtype = types_pb2.DT_HALF
    elif target_type == 'fp64':
        dtype = types_pb2.DT_DOUBLE
    else:
        dtype = types_pb2.DT_FLOAT

    #加载需要转换的模型
    source_sess = load_graph(model_path)
    source_graph_def = source_sess.graph.as_graph_def()
    #创建新的模图对象
    target_graph_def = graph_pb2.GraphDef()
    target_graph_def.versions.CopyFrom(source_graph_def.versions)
    #对加载的模型遍历计算节点
    for node in source_graph_def.node:
        # 对FusedBatchNorm计算节点替换为FusedBatchNormV2
        if node.op == "FusedBatchNorm":
            rewrite_batch_norm_node_v2(node, target_graph_def, target_type=target_type)
            continue
        # 复制计算节点
        new_node = target_graph_def.node.add()
        new_node.op = node.op
        new_node.name = node.name
        new_node.input.extend(node.input)

        #对attrs属性进行复制，attrs属性主要关注
        attrs = list(node.attr.keys())
        # BatchNorm属性保持不变
        if ("BatchNorm" in node.name) or ('batch_normalization' in node.name):
            for attr in attrs:
                new_node.attr[attr].CopyFrom(node.attr[attr])
            continue
        # 除了BatchNorm以外其他计算节点的属性单独
        for attr in attrs:
            # 对指定的计算节点保持不变
            if node.name in keep_fp32_node_name:
                new_node.attr[attr].CopyFrom(node.attr[attr])
                continue
            #将Float类型修改为设置的目标类型
            if node.attr[attr].type == types_pb2.DT_FLOAT:
                # modify node dtype
                node.attr[attr].type = dtype
                
            #重点关注value，weights都是保存在value属性中
            if attr == "value":
                tensor = node.attr[attr].tensor
                if tensor.dtype == types_pb2.DT_FLOAT:
                    # if float_val exists
                    if tensor.float_val:
                        float_val = tf.make_ndarray(node.attr[attr].tensor)
                        new_node.attr[attr].tensor.CopyFrom(tf.make_tensor_proto(float_val, dtype=dtype))
                        continue
                    # if tensor content exists
                    if tensor.tensor_content:
                        tensor_shape = [x.size for x in tensor.tensor_shape.dim]
                        tensor_weights = tf.make_ndarray(tensor)
                        # reshape tensor
                        tensor_weights = np.reshape(tensor_weights, tensor_shape)
                        tensor_proto = tf.make_tensor_proto(tensor_weights, dtype=dtype)
                        new_node.attr[attr].tensor.CopyFrom(tensor_proto)
                        continue
            new_node.attr[attr].CopyFrom(node.attr[attr])
    # transform graph
    if output_names:
        if not input_name:
            input_name = []
        transforms = ["strip_unused_nodes"]
        target_graph_def = TransformGraph(target_graph_def, input_name, output_names, transforms)
    # write graph_def to model
    tf.io.write_graph(target_graph_def, logdir=save_path, name=name, as_text=as_text)
    print("Converting done ...")

save_path = "test"
name = "output_fp16.pb"
model_path="test.pb"
as_text = False
target_type = 'fp16'
convert_graph_to_fp16(model_path, save_path, name, as_text=as_text, target_type=target_type, input_name=input_name, output_names=output_names)
# 测试一下转换后的模型是否能够加载
sess = load_graph(save_path+"/"+name)