提交 e4eacd58 编写于 作者: Y Yi Wang

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# Design Doc: PaddlePaddle API
# PaddlePaddle API
## Ingredients
......@@ -27,8 +27,8 @@ indicates a *reference*, and `-` marks a "class method".
### Model
We used to think that parameters are part of the toplogy (or layers).
But that is not true, because multiple layers could share the same
We used to think that parameters are part of the topology (or layers).
But that is not true because multiple layers could share the same
parameter matrix. An example is a network that compares two text
segments in a semantic space:
......@@ -56,8 +56,7 @@ parameter sharing, please refer to [TODO: API].
Supposed that we have a trained ranking model, we should be able to
use it in our search engine. The search engine's Web server is a
concurrent program so to serve many HTTP requests simultaneously. It
doens't make sense for each of these threads to have its own copy of
model, because that would duplicate topologies and parameters.
doesn't make sense for each of these threads to have its own copy of the model because that would duplicate topologies and parameters.
However, each thread should be able to record layer outputs, i.e.,
activations, computed from an input, derived from the request. With
*Evaluator* that saves activations, we can write the over-simplified
......@@ -86,5 +85,193 @@ Hence the `GradientMachine`.
None of Model, Evaluator, nor GradientMachine implements the training
loop, hence Optimizer. We can define a concurrent optimizer that runs
multiple simultaneious threads to train a model -- just let each
multiple simultaneous threads to train a model -- just let each
thread has its own GradientMachine object.
Most models should be able to be trained using the
`paddle.optimizer.SGD` by calling its `train` method. Many
customizations to the SGD algorithm happens with the update equation,
e.g., momentum and the Adam SGD algorithm. We make `train` calls
`update` to do an update, so that we can derive a `paddle.optimizer.Adam`
from `paddle.optimizer.SGD` by overrides only the `update` method.
## Programming
A fictive example of PaddlePaddle program looks like the following:
```python
import paddle
def read(args):
f = open_file(args["filename"])
mb = read_a_minibatch(f)
end_pass = eof(f)
if end_pass:
f = open_file(args["filename"]) # rewind for reading again
yield mb, end_pass
input = paddle.layer.data(...)
intermediate = paddle.layers.fc(input)
output = paddle.layer.softmax(intermediate)
model = paddle.model.create(output)
paddle.train(model, data_provider=read)
```
This shows some important part of a program:
1. Define how to read (and augment) data by defining a function, in
this example, `read`, that `yields` a minibatch and a boolean flag
`eof_of_pass`.
1. Define the topology, `input`, `intermediate`, and `output` in this
example.
1. Create parameters from the topology thus forms the model by calling
`paddel.model.create`.
1. Train the model by calling `paddle.train`.
### Reader
Not all programming frameworks allow users to define I/O functions.
An example is Google MapReduce, which can only read from text,
SSTable, and RecordIO files. Hadoop MapReduce allows users to define
readers and writers by deriving from base classes `Reader` and
`Writer`. The former is less flexible but also less error-prone. We
decide to provide the flexibility to users to define their readers.
#### A Synthetic Data Reader
Sometimes we want to test a topology and/or a training algorithm using
synthetic data. We can do this by defining the reader a synthesizer:
```python
def read(args):
x = sample_from_uniform(0.0, 1.0)
y = sample_from_gauss(2 * x, sigma)
yield {x, y}, False # no end-of-file so no end-of-pass
```
#### A Reader for Online Learning
Readers can also read an infinite data stream, e.g., a log stream from
a search engine and collected by Kafka:
```python
def read(args):
log_stream = kafka.open_channel(args["kafka channel name"])
yeild log_stream.read(), False # no end-of-pass in online learning
```
### Topology
By default, layers don't have names. But if we want to refer to a
layer later some time, for example, when we do serving using the model
and wants activations/outputs of a layer, we should give it a name.
```python
input = paddle.layer.data(...)
intermediate = paddle.layer.fc(input, name="inter", ...)
output = paddle.layer.softmax(intermediate, name="output", ...)
m = paddle.model.create(output)
e = paddle.evaluator.create(model)
e.forward(read_an_input()) # compute activations of all layers.
print e.activations(layer="inter") # retrieve the activations of layer "inter"
print e.activations(layer="output") # retrieve the activations of layer "output"
```
#### Sharing Parameters
In [above section](#model) we shows a network whose two layers share
the same parameter matrix. To specify such cases, we give "parameter
names" to layers. If some layers have the same paraemter names,
`paddle.model.create` creates a single parameter matrix for these
layers:
```python
text1 = paddle.layer.data(...)
sematic1 = paddle.layer.fc(text1, ..., parameter_name="sematic_projection")
text2 = paddle.layer.data(...)
sematic2 = paddle.layer.fc(text2, ..., parameter_name="sematic_projection")
out = paddle.layer.cosine(semantic1, semantic2)
```
We can also share parameter matrices between layers in different
models. To do this, we need an additional parameter that refers to a
model:
```python
model1_input = paddle.layer.data(...)
model1_output = paddle.layer.softmax(model1_input, ...,
parameter_name="a_parameter_matrix")
model1 = paddle.model.create(model1_output)
# Another model
model2_semantic = paddle.layer.fc(text2, ...,
parameter_name="a_parameter_matrix",
parameter_model=model1)
```
### Training
The recommended way to training a model is to call `paddle.train`,
which simply calls `paddle.optimizer.Default`, a global variable of
type `paddle.optimizer.SGD`. Equivalently, we can do
```python
opt = paddle.optimizer.SGD(...)
opt.train(model, reader=read, ...)
```
#### Distributed Training
If users want to do distributed training on a cluster, s/he should
call `paddle.dist_train` and provides access tokens to the cluster as
a parameter.
For example, if the user has a TLS certificate that allows him to
access a Kubernetes cluster, s/he should be able to call
```python
paddle.dist_train(model,
reader=read,
optimizer=paddle.optimizer.SGDOptimizer(...),
k8s_user="yi",
k8s_token="kube_cluster_tls.pem",
k8s_job="hello",
num_parameter_servers=15)
```
The pseudo code if `paddle.dist_train` is as follows:
```python
def dist_train():
if os.getenv("KUBERNETES_SERVICE_HOST") == None:
image_name = k8s_user + '/' + k8s_job
docker_build(image_name)
docker_push()
kube_ctrl_start_job(image_name, k8s_user, k8s_token)
else:
rank = kube_list_containers_in_job_and_return_current_containers_rank()
if rank == 0:
master()
elif rank < 15:
parameter_server()
else:
optimizer.train(model, reader=read)
```
Please be aware that if a process is running on the Kubernetes
cluster, it will have some environment variables pre-defined.
If `dist_train` doesn't see these environment variables, it knowns
that it's running on users' personal computer, and it should work as a
*launcher*. Otherwise, it knows that it's running on the cluster and
need to figure out its role as either the master, or a trainer, or a
parameter server.
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