在 Keras 中使用 multi_gpu_model 冻结层

Question

我正在尝试在 Keras 中微调修改后的 InceptionV3 模型。

我按照this page 上的示例“在一组新的类上微调 InceptionV3”。

所以我首先使用以下代码训练了添加到 InceptionV3 基础模型的顶层密集层：

model = Model(inputs=base_model.input, outputs=predictions)

for layer in base_model.layers:
    layer.trainable = False

parallel_model = multi_gpu_model(model, gpus=2)

parallel_model.compile(optimizer='rmsprop', loss='categorical_crossentropy')

history = parallel_model.fit_generator(generate_batches(path), steps_per_epoch = num_images/batch_size, epochs = num_epochs)

之后，我尝试微调 InceptionV3 中的前 2 个初始块。根据示例，我应该做的是：

for layer in model.layers[:249]:
   layer.trainable = False
for layer in model.layers[249:]:
   layer.trainable = True

model.compile(optimizer=SGD(lr=0.0001, momentum=0.9), loss='categorical_crossentropy')

model.fit_generator(...)

但我使用的是multi_gpu_model，所以我不知道如何冻结前 249 层。

我的意思是，如果我冻结 no-gpu 模型中的层（如示例），并使用 parallel_model = multi_gpu_model(model, gpus=2) 冻结 parallel_model 中的层，那么刚刚训练的顶部密集层中的权重并且包含在parallel_model 中的会被覆盖，对吧？

另一方面，我尝试直接使用for layer in parallel_model.layers[:249]: layer.trainable = False，但是当我检查parallel_model中的图层时，它显示：

for i, layer in enumerate(parallel_model.layers):
   print(i, layer.name)

(0, 'input_1')
(1, 'lambda_1')
(2, 'lambda_2')
(3, 'model_1')
(4, 'dense_3')

那么“lambda_1”、“lambda_2”和“model_1”层是什么？为什么它在parallel_model中只显示5层？

更重要的是，如何冻结parallel_model中的图层？

Answer 1

这个例子有点复杂，因为你要嵌套一个基础模型

base_model = InceptionV3(weights='imagenet', include_top=False)

进入一个添加你自己的密集层的模型，

model = Model(inputs=base_model.input, outputs=predictions)

然后调用 multi_gpu_model 当它使用 lambda 为每个 GPU 拆分模型一次时再次嵌套模型，然后将输出连接在一起以便将模型分布到多个 GPU 上。

parallel_model = multi_gpu_model(model, gpus=2)

在这种情况下，请记住两件事：更改 base_model 中层的可训练性并将非并行模型加载到 cpu 上以获得最佳性能。

这是完整的微调示例，只需更新 train_data_dir 以指向您自己的数据位置。

import tensorflow as tf
from keras import Model
from keras.applications.inception_v3 import InceptionV3, preprocess_input
from keras.layers import Dense, GlobalAveragePooling2D
from keras.optimizers import SGD
from keras.preprocessing.image import ImageDataGenerator
from keras.utils import multi_gpu_model

train_data_dir = '/home/ubuntu/work/data/train'
batch_size_per_gpu = 32
nb_classes = 3
my_gpus = 2
target_size = (224, 224)
num_epochs_to_fit_dense_layer = 2
num_epochs_to_fit_last_two_blocks = 3

batch_size = batch_size_per_gpu * my_gpus
train_datagen = ImageDataGenerator(preprocessing_function=preprocess_input)
train_iterator = train_datagen.flow_from_directory(
    train_data_dir,
    target_size=target_size,
    batch_size=batch_size,
    class_mode='categorical',
    shuffle=True)

# Check to make sure our model will match our data
assert nb_classes == train_iterator.num_classes

# Create base and template models on cpu
with tf.device('/cpu:0'):
    base_model = InceptionV3(weights='imagenet', include_top=False)
    for layer in base_model.layers:
        layer.trainable = False

    # Add prediction layer to base pre-trained model
    x = base_model.output
    x = GlobalAveragePooling2D()(x)
    x = Dense(1024, activation='relu')(x)
    predictions = Dense(nb_classes, activation='softmax')(x)

    template_model = Model(inputs=base_model.input, outputs=predictions)

    # If you need to load weights from previous training, do so here:
    # template_model.load_weights('template_model.h5', by_name=True)

# Create parallel model on GPUs
parallel_model = multi_gpu_model(template_model, gpus=2)
parallel_model.compile(optimizer='adam', loss='categorical_crossentropy')

# Train parallel model.
history = parallel_model.fit_generator(
    train_iterator,
    steps_per_epoch=train_iterator.n // batch_size,
    epochs=num_epochs_to_fit_dense_layer)

# Unfreeze some layers in our model
for layer in base_model.layers[:249]:
   layer.trainable = False
for layer in base_model.layers[249:]:
   layer.trainable = True

# Train parallel_model with more trainable layers
parallel_model.compile(optimizer=SGD(lr=0.0001, momentum=0.9), loss='categorical_crossentropy')
history2 = parallel_model.fit_generator(
    train_iterator,
    steps_per_epoch=train_iterator.n // batch_size,
    epochs=num_epochs_to_fit_last_two_blocks)

# Save model via the template model which shares the same weights as the parallel model.
template_model.save('template_model.h5')