[英]Gradient Accumulation with Custom model.fit in TF.Keras?
请对您的想法添加最低限度的评论,以便我可以改进我的查询。 谢谢。 :)
我正在尝试使用梯度累积(GA)训练tf.keras
model。 但我不想在自定义训练循环( like )中使用它,而是通过覆盖train_step
来自定义.fit()
方法。这可能吗? 如何做到这一点? 原因是如果我们想从keras
内置功能(如fit
、 callbacks
)中受益,我们不想使用自定义训练循环,但同时如果train_step
某种原因(如 GA 或否则)我们可以自定义fit
方法,并且仍然可以使用这些内置函数。
而且,我知道使用GA的优点,但使用它的主要缺点是什么? 为什么它不是默认的,而是框架的可选功能?
# overriding train step
# my attempt
# it's not appropriately implemented
# and need to fix
class CustomTrainStep(tf.keras.Model):
def __init__(self, n_gradients, *args, **kwargs):
super().__init__(*args, **kwargs)
self.n_gradients = n_gradients
self.gradient_accumulation = [tf.zeros_like(this_var) for this_var in \
self.trainable_variables]
def train_step(self, data):
x, y = data
batch_size = tf.cast(tf.shape(x)[0], tf.float32)
# Gradient Tape
with tf.GradientTape() as tape:
y_pred = self(x, training=True)
loss = self.compiled_loss(y, y_pred, regularization_losses=self.losses)
# Calculate batch gradients
gradients = tape.gradient(loss, self.trainable_variables)
# Accumulate batch gradients
accum_gradient = [(acum_grad+grad) for acum_grad, grad in \
zip(self.gradient_accumulation, gradients)]
accum_gradient = [this_grad/batch_size for this_grad in accum_gradient]
# apply accumulated gradients
self.optimizer.apply_gradients(zip(accum_gradient, self.trainable_variables))
# TODO: reset self.gradient_accumulation
# update metrics
self.compiled_metrics.update_state(y, y_pred)
return {m.name: m.result() for m in self.metrics}
请运行并检查以下玩具设置。
# Model
size = 32
input = tf.keras.Input(shape=(size,size,3))
efnet = tf.keras.applications.DenseNet121(weights=None,
include_top = False,
input_tensor = input)
base_maps = tf.keras.layers.GlobalAveragePooling2D()(efnet.output)
base_maps = tf.keras.layers.Dense(units=10, activation='softmax',
name='primary')(base_maps)
custom_model = CustomTrainStep(n_gradients=10, inputs=[input], outputs=[base_maps])
# bind all
custom_model.compile(
loss = tf.keras.losses.CategoricalCrossentropy(),
metrics = ['accuracy'],
optimizer = tf.keras.optimizers.Adam() )
# data
(x_train, y_train), (_, _) = tf.keras.datasets.mnist.load_data()
x_train = tf.expand_dims(x_train, -1)
x_train = tf.repeat(x_train, 3, axis=-1)
x_train = tf.divide(x_train, 255)
x_train = tf.image.resize(x_train, [size,size]) # if we want to resize
y_train = tf.one_hot(y_train , depth=10)
# customized fit
custom_model.fit(x_train, y_train, batch_size=64, epochs=3, verbose = 1)
我发现其他一些人也试图实现这一目标并最终遇到了同样的问题。 一个有一些解决方法, here ,但它太乱了,我认为应该有一些更好的方法。
是的,可以通过在没有自定义训练循环的情况下覆盖train_step
来自定义.fit()
方法,下面的简单示例将向您展示如何训练具有梯度累积的简单 mnist 分类器:
import tensorflow as tf
# overriding train step
# my attempt
# it's not appropriately implemented
# and need to fix
class CustomTrainStep(tf.keras.Model):
def __init__(self, n_gradients, *args, **kwargs):
super().__init__(*args, **kwargs)
self.n_gradients = tf.constant(n_gradients, dtype=tf.int32)
self.n_acum_step = tf.Variable(0, dtype=tf.int32, trainable=False)
self.gradient_accumulation = [tf.Variable(tf.zeros_like(v, dtype=tf.float32), trainable=False) for v in self.trainable_variables]
def train_step(self, data):
self.n_acum_step.assign_add(1)
x, y = data
# Gradient Tape
with tf.GradientTape() as tape:
y_pred = self(x, training=True)
loss = self.compiled_loss(y, y_pred, regularization_losses=self.losses)
# Calculate batch gradients
gradients = tape.gradient(loss, self.trainable_variables)
# Accumulate batch gradients
for i in range(len(self.gradient_accumulation)):
self.gradient_accumulation[i].assign_add(gradients[i])
# If n_acum_step reach the n_gradients then we apply accumulated gradients to update the variables otherwise do nothing
tf.cond(tf.equal(self.n_acum_step, self.n_gradients), self.apply_accu_gradients, lambda: None)
# update metrics
self.compiled_metrics.update_state(y, y_pred)
return {m.name: m.result() for m in self.metrics}
def apply_accu_gradients(self):
# apply accumulated gradients
self.optimizer.apply_gradients(zip(self.gradient_accumulation, self.trainable_variables))
# reset
self.n_acum_step.assign(0)
for i in range(len(self.gradient_accumulation)):
self.gradient_accumulation[i].assign(tf.zeros_like(self.trainable_variables[i], dtype=tf.float32))
# Model
input = tf.keras.Input(shape=(28, 28))
base_maps = tf.keras.layers.Flatten(input_shape=(28, 28))(input)
base_maps = tf.keras.layers.Dense(128, activation='relu')(base_maps)
base_maps = tf.keras.layers.Dense(units=10, activation='softmax', name='primary')(base_maps)
custom_model = CustomTrainStep(n_gradients=10, inputs=[input], outputs=[base_maps])
# bind all
custom_model.compile(
loss = tf.keras.losses.CategoricalCrossentropy(),
metrics = ['accuracy'],
optimizer = tf.keras.optimizers.Adam(learning_rate=1e-3) )
# data
(x_train, y_train), (_, _) = tf.keras.datasets.mnist.load_data()
x_train = tf.divide(x_train, 255)
y_train = tf.one_hot(y_train , depth=10)
# customized fit
custom_model.fit(x_train, y_train, batch_size=6, epochs=3, verbose = 1)
输出:
Epoch 1/3
10000/10000 [==============================] - 13s 1ms/step - loss: 0.5053 - accuracy: 0.8584
Epoch 2/3
10000/10000 [==============================] - 13s 1ms/step - loss: 0.1389 - accuracy: 0.9600
Epoch 3/3
10000/10000 [==============================] - 13s 1ms/step - loss: 0.0898 - accuracy: 0.9748
梯度累积是一种将用于训练神经网络的样本批量拆分为几个小批量样本的机制,这些样本将按顺序运行
Because GA calculates the loss and gradients after each mini-batch, but instead of updating the model parameters, it waits and accumulates the gradients over consecutive batches, so it can overcoming memory constraints, ie using less memory to training the model like it using large批量大小。
示例:如果您以 5 步和 4 幅图像的批量大小运行梯度累积,它的目的几乎与以 20 幅图像的批量大小运行相同。
我们还可以在使用 GA 时并行训练,即聚合来自多台机器的梯度。
这种技术效果很好,因此被广泛使用,在使用它之前需要考虑的事情很少,我认为它不应该被称为缺点,毕竟 GA 所做的只是将4 + 4
变为2 + 2 + 2 + 2
.
如果你的机器有足够的memory来满足已经足够大的batch size那么就没有必要使用它,因为众所周知batch size太大会导致泛化性差,如果你使用GA肯定会运行得更慢达到您机器的 memory 已经可以处理的相同批量大小。
参考:
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