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regularizer.rst
transpiler.rst
unique_name.rst
recompute.rst
Forward Recomputation Backpropagation
=============
Context
---------
As the amount of training data increases, training deeper neural network models becomes more and more popular. Current deep-learning training usually keeps the hidden layer outputs in memory during the forward propagation,
and the number of outputs increases linearly with
the increase of the number of model layers,
which becomes a challenge of the memory size
for common devices.
Theory
---------
As we know, a training process of a deep-learning network contains 3 steps:
- **Forward Propagation**:Running forward operators and generate temporary variables as output
- **Backward Propagation**:Running backward operators to compute gradients of parameters
- **Optimization**:Applying optimization algorithm to update parameters
When the model becomes deeper, the number of temporary variables
generated in the forward propagation process can reach tens
of thousands, occupying a large amount of memory.
The `Garbage Collection mechanism <https://paddlepaddle.org.cn/documentation/docs/zh/advanced_usage/best_practice/memory_optimize.html>`_
in Paddle can delete useless variables for the sake of saving memory.
However, some variables serve as inputs of backward operators,
they must be kept in memory until particular operator finish.
Take a simple example, define a network contains two `mul` operators,
the forward propagation works as follows:
.. math::
y = W_1 * x
z = W_2 * y
where :math:`x, y, z` are vectors, :math:`W_1, W_2` are matrix。It is easy to conduct that the gradient of :math:`W_2` is:
.. math::
W_{2}^{'} = z^{'} / y
We can see that :math:`y` is used in the backward propagation process,
thus it must be kept in the memory during the whole forward propagation.
When network grows deeper, more 'y's need to be stored,
adding more requirements to the memory.
Forward Recomputation Backpropagation(FRB) splits a deep network to k segments.
For each segment, in forward propagation,
most of the temporary variables are erased in time,
except for some special variables (we will talk about that later);
in backward propagation, the forward operators will be recomputed
to get these temporary variables before running backward operators.
In short, FBR runs forward operators twice.
But how to split the network? A deep learning network usually consists
of connecting modules in series:
ResNet-50 contains 16 blocks and Bert-Large contains 24 transformers.
It is a good choice to treat such modules as segments.
The variables among segments are
called as checkpoints.
The following picture is a network with 4 fc layers, 3 relu layers,
1 sigmoid layer and 1 log-loss layer in series.
The left column is the forward propagation,
the middle column is the normal backward propagation,
and the right column is the FRB.
Rectangular boxes represent the operators, red dots represent
the intermediate variables in forward computation, blue dots
represent checkpoints and arrows represent the dependencies between operators.
.. image:: image/recompute.png
Note: the complete source code of this example: `source <https://github.com/PaddlePaddle/examples/blob/master/community_examples/recompute/demo.py>`_
After applying FBR, the forward computation only needs to store
2 variables (the blue dots) instead of 4 variables (the red
dots), saving the corresponding memories. It is notable that
recomputing operators generate new intermediate variables at the same time,
a trade-off needs to be considered in this situation.
While according to our experiments,
FBR usually saves rather than increase the memory load.
Usage
---------
We have implemented the FRB algorithm named "RecomputeOptimizer"
based on Paddle. More information about this algorithm can
be learned by the `source code <https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/optimizer.py>`_
and the
`document <https://www.paddlepaddle.org.cn/documentation/docs/zh/api_cn/optimizer_cn/RecomputeOptimizer_cn.html>`_
of RecomputeOptimizer.
There are 2 methods to apply RecomputeOptimizer in your Paddle
program: call RecomputeOptimizer directly or use it with Fleet
API. For single-GPU card training or CPU training, we recommend
directly calling; For multi-GPU training, we
recommend using with Fleet API.
**1. Directly calling**
Calling RecomputeOptimizer is very easy: first, define a classic
optimizer, such as Adam; second, wrap it with RecomputeOptimizer;
third, set the checkpoints.
.. code-block:: python
import paddle.fluid as fluid
# Define the network
def mlp(input_x, input_y, hid_dim=128, label_dim=2):
print(input_x)
fc_1 = fluid.layers.fc(input=input_x, size=hid_dim)
prediction = fluid.layers.fc(input=[fc_1], size=label_dim, act='softmax')
cost = fluid.layers.cross_entropy(input=prediction, label=input_y)
sum_cost = fluid.layers.reduce_mean(cost)
return sum_cost, fc_1, prediction
input_x = fluid.layers.data(name="x", shape=[32], dtype='float32')
input_y = fluid.layers.data(name="y", shape=[1], dtype='int64')
cost, fc_1, pred = mlp(input_x, input_y)
# define RecomputeOptimizer
sgd = fluid.optimizer.Adam(learning_rate=0.01)
sgd = fluid.optimizer.RecomputeOptimizer(sgd)
# set checkpoints
sgd._set_checkpoints([fc_1, pred])
# apply optimization
sgd.minimize(cost)
In principle, recompute is for all kinds of optimizers in Paddle.
**2. Using Recompute in Fleet API**
`Fleet API <https://github.com/PaddlePaddle/Fleet>`_
is a high-level API for distributed training in Fluid. Adding
RecomputeOptimizer to Fluid takes two steps:
- set dist_strategy.forward_recompute to True
- set dist_strategy.recompute_checkpoints
.. code-block:: python
from paddle.fluid.incubate.fleet.collective import fleet, DistributedStrategy
dist_strategy = DistributedStrategy()
dist_strategy.forward_recompute = True
dist_strategy.recompute_checkpoints=checkpoints
optimizer = fleet.distributed_optimizer(optimizer, strategy=dist_strategy)
optimizer.minimize(loss)
We supply some examples of using recompute in Fleet API for users.
We also post corresponding training speed,
test results and memory usages of these examples for reference.
- Fine-tuning Bert Large model with recomputing: `source <https://github.com/PaddlePaddle/Fleet/tree/develop/examples/recompute/bert>`_
- Training object detection models with recomputing:developing.
Q&A
-------
- **Does RecomputeOptimizer support operators with random outputs?**
We currently found that the dropout operator has random results
and RecomputeOptimizer is able to keep the outputs of
first-computation and recomputation consistent.
- **Are there more official examples of Recompute?**
More examples will be updated at `examples <https://github.com/PaddlePaddle/examples/tree/master/community_examples/recompute>`_
and `Fleet <https://github.com/PaddlePaddle/Fleet>`_ . Feel free to
raise issues if you get any problem with these examples.
- **How should I set checkpoints?**
The position of checkpoints is important:
we suggest setting the variable between the sub-model as checkpoints,
that is, set a variable as a checkpoint if it
can separate the network into two parts without short-cut connections.
The number of checkpoints is also important:
too few checkpoints will reduce the memory saved by recomputing while
too many checkpoints will occupy a lot of memory themselves.
We will add a tool to estimate the memory usage with specific checkpoints,
helping users to choose checkpointing variables.
[1] Tianqi Chen, Bing Xu, Chiyuan Zhang, and Carlos Guestrin . Training deep nets with sublinear memory cost.
arXiv preprint, arXiv:1604.06174, 2016.
[2] Audrunas Gruslys , Rémi Munos , Ivo Danihelka , Marc Lanctot , and Alex Graves. Memory efficient
backpropagation through time. In Advances in Neural Information Processing Systems (NIPS), pages 4125 4133,
2016.
[3] Kusumoto, Mitsuru, et al. "A Graph Theoretic Framework of Recomputation Algorithms for Memory-Efficient Backpropagation." arXiv preprint arXiv:1905.11722 (2019).
......@@ -27,4 +27,3 @@ API Reference
regularizer_cn.rst
transpiler_cn.rst
unique_name_cn.rst
recompute.rst
Forward Recomputation Backpropagation
=============
背景
---------
随着训练数据规模的逐渐增加,训练更大、更深的深度学习模型成为一个主流趋势。目前的深度学习模型训练,通常要求保留前向计算的隐层结果,并且需要保存结果的数量会随着模型层数的增加线性增加,这对于目前能够使用的AI芯片的内存大小是个挑战。Forward Recomputation Backpropagation(FRB)可以在额外增加少量计算的情况下,显著增加模型的层数和宽度,同时也可以显著提升模型训练的batch大小。
原理
---------
我们知道,深度学习网络的一次训练迭代包含三个步骤:
- **前向计算**:运行前向算子(Operator) 来计算中间隐层(Variable)的值
- **反向计算**:运行反向算子来计算参数(Parameter)的梯度
- **优化**:应用优化算法以更新参数值
在前向计算过程中,前向算子会输出大量的中间计算结果,在Paddle中,使用
Variable来存储这些隐层的中间结果。当模型层数加深时,其数量可达成千上万个,
占据大量的内存。Paddle的 `显存回收机制 <https://paddlepaddle.org.cn/documentation/docs/zh/advanced_usage/best_practice/memory_optimize.html>`_
会及时清除无用的中间结果,以节省存储。
然而,有些中间结果是反向算子的输入,这些Variable必须存储在内存中,直到相应的反向算子计算完毕。
举个简单的例子, 我们定义一个由mul算子构成的网络,其前向计算为:
.. math::
y = W_1 * x
z = W_2 * y
其中 :math:`x, y, z` 为向量, :math:`W_1, W_2` 为矩阵。容易知道,求 :math:`W_2` 梯度的反向计算为:
.. math::
W_{2}^{'} = z^{'} / y
可以看到反向计算中用到了前向计算生成的变量 :math:`y` ,因此变量 :math:`y` 必须存储在内存中,直到这个反向算子计算完毕。当模型加深时,我们会有大量的“ :math:`y` ”,占据了大量的内存。
Forward Recomputation Backpropagation(FRB)的思想是将深度学习网络切分为k个部分(segments)。对每个segment而言:前向计算时,除了小部分必须存储在内存中的Variable外(我们后续会讨论这些特殊Variable),其他中间结果都将被删除;在反向计算中,首先重新计算一遍前向算子,以获得中间结果,再运行反向算子。简而言之,FRB和普通的网络迭代相比,多计算了一遍前向算子。
我们把切分网络的变量叫做checkpoints。
那么问题来了,如何选择checkpoints呢?自从FRB方法提出以来 \ :sup:`[1], [2]`,大量学者在研究这一关键问题。
我们知道深度学习网络通常是由一个个模块串联得到的,比如ResNet-50由16个block串联而成,
Bert-Large由24个transformer串联而成,以两个子模块中间的变量作为切分点就是一个很好的选择。
对于非串联的网络(比如含有大量shortcut结构的网络),FRB也支持对其做切分,
只是可能多耗费一点内存(用于存储shortcut的Variable)。
Mitsuru Kusumoto \ :sup:`[3]` 等提出了一种基于动态规划的算法,
可以根据指定的内存自动搜索合适的checkpoints,支持各种各样的网络结构。
下图是由4个fc Layer、3个relu Layer、1个sigmoid Layer和1个log-loss Layer串联而成的一个网络:最左侧为其前向计算流程、中间是普通的前向计算和反向计算流程、最右侧为添加FRB后的前向计算和反向计算流程。其中方框代表算子(Operator),红点代表前向计算的中间结果、蓝点代表checkpoints。
.. image:: image/recompute.png
注:该例子完整代码位于 `source <https://github.com/PaddlePaddle/examples/blob/master/community_examples/recompute/demo.py>`_
添加FRB后,前向计算中需要存储的中间Variable从4个(红点)变为2个(蓝点),
从而节省了这部分内存。当然了,重计算的部分也产生了新的中间变量,
这就需要根据实际情况来做权衡了。这个例子里的网络比较浅,通常来讲,
对层数较深的网络,FRB节省的内存要远多于新增加的内存。
使用方法
---------
我们实现了基于Paddle的FRB算法,叫做RecomputeOptimizer,
您可以根据其 `源码 <https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/fluid/optimizer.py>`_
`文档 <https://www.paddlepaddle.org.cn/documentation/docs/zh/api_cn/optimizer_cn/RecomputeOptimizer_cn.html>`_
更深入地了解这一算法。我们为用户提供了两个使用RecomputeOptimizer的方法:
直接调用与Fleet API中使用。在单机单卡或者CPU训练中建议您直接调用RecomputeOptimizer,
在多卡训练或者多机训练任务上建议您在Fleet API中使用Recompute。
**1. 直接调用**
直接调用RecomputeOptimizer非常简单,首先要定义一个经典的Optimizer,比如Adam;
然后在外面包一层RecomputeOptimizer;最后设置checkpoints即可。
.. code-block:: python
import paddle.fluid as fluid
# 定义网络
def mlp(input_x, input_y, hid_dim=128, label_dim=2):
print(input_x)
fc_1 = fluid.layers.fc(input=input_x, size=hid_dim)
prediction = fluid.layers.fc(input=[fc_1], size=label_dim, act='softmax')
cost = fluid.layers.cross_entropy(input=prediction, label=input_y)
sum_cost = fluid.layers.reduce_mean(cost)
return sum_cost, fc_1, prediction
input_x = fluid.layers.data(name="x", shape=[32], dtype='float32')
input_y = fluid.layers.data(name="y", shape=[1], dtype='int64')
cost, fc_1, pred = mlp(input_x, input_y)
# 定义RecomputeOptimizer
sgd = fluid.optimizer.Adam(learning_rate=0.01)
sgd = fluid.optimizer.RecomputeOptimizer(sgd)
# 设置checkpoints
sgd._set_checkpoints([fc_1, pred])
# 运行优化算法
sgd.minimize(cost)
Recompute原则上适用于所有Optimizer。
**2. 在Fleet API中使用Recompute**
`Fleet API <https://github.com/PaddlePaddle/Fleet>`_
是基于Fluid的分布式计算高层API。在Fleet API中添加RecomputeOptimizer
仅需要2步:
- 设置dist_strategy.forward_recompute为True;
- 设置dist_strategy.recompute_checkpoints。
.. code-block:: python
from paddle.fluid.incubate.fleet.collective import fleet, DistributedStrategy
dist_strategy = DistributedStrategy()
dist_strategy.forward_recompute = True
dist_strategy.recompute_checkpoints=checkpoints
optimizer = fleet.distributed_optimizer(optimizer, strategy=dist_strategy)
optimizer.minimize(loss)
为了帮助您快速地用Fleet API使用Recompute任务,我们提供了一些例子,
并且给出了这些例子的计算速度、效果和显存节省情况:
- 用Recompute做Bert Fine-tuning: `source <https://github.com/PaddlePaddle/Fleet/tree/develop/examples/recompute/bert>`_
- 用Recompute做目标检测:开发中.
Q&A
-------
- **是否支持带有随机性的Op?**
目前Paddle中带随机性的Op有:dropout,Recompute支持
dropout Operator,可以保证重计算与初次计算结果保持一致。
- **有没有更多Recompute的官方例子?**
更多Recompute的例子将更新在 `examples <https://github.com/PaddlePaddle/examples/tree/master/community_examples/recompute>`_
和 `Fleet <https://github.com/PaddlePaddle/Fleet>`_ 库下,欢迎关注。
- **有没有添加checkpoints的建议?**
我们建议将子网络连接部分的变量添加为checkpoints,即:
如果一个变量能将网络完全分为前后两部分,那么建议将其加入checkpoints。
checkpoints的数目会影响内存的消耗:如果checkpoints很少,
那么Recompute起的作用有限;如果checkpoints数量过多,
那么checkpoints本身占用的内存量就较大,内存消耗可能不降反升。
我们后续会添加一个估算内存用量的工具,
可以对每个Operator运算前后的显存用量做可视化,
帮助用户定位问题。
[1] Tianqi Chen, Bing Xu, Chiyuan Zhang, and Carlos Guestrin . Training deep nets with sublinear memory cost.
arXiv preprint, arXiv:1604.06174, 2016.
[2] Audrunas Gruslys , Rémi Munos , Ivo Danihelka , Marc Lanctot , and Alex Graves. Memory efficient
backpropagation through time. In Advances in Neural Information Processing Systems (NIPS), pages 4125 4133,
2016.
[3] Kusumoto, Mitsuru, et al. "A Graph Theoretic Framework of Recomputation Algorithms for Memory-Efficient Backpropagation." arXiv preprint arXiv:1905.11722 (2019).
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