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# Design Doc: Session | ||
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## Abstract | ||
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The *session* object encapsulates the environment in which the | ||
computation graph is executed. | ||
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We will have the *local* session and *remote* session, they offer the | ||
same [interface](#interface). The local session encapsulates the local | ||
runtime environment and the remote session encapsulates the cluster | ||
runtime environment. | ||
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The local runtime environment contains: | ||
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1. computation devices (i.e., CPU, GPU) handles, and | ||
1. the [scope](../scope.md) which holds all variables. | ||
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The remote runtime environment contains: | ||
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1. computation devices (i.e., CPU and GPU on node 0, 1) in a cluster, | ||
and | ||
1. the distributed [scope](../scope.md) in a cluster which holds all | ||
variables. | ||
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The user can create a remote session on Paddle Cloud and evaluate the | ||
computation graph with it. In this way, the user can control the | ||
remote computation resource in a cluster from his local computer. | ||
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## Background | ||
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The current design has an implicit global session in which | ||
`paddle.eval()` is executed. The pain point is: | ||
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Since the user is not able to explicitly switch between runtime | ||
environments, the user cannot run a topology in two independent | ||
environments. | ||
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For example, in reinforcement learning, the user may want to have a | ||
stale model for inference and a fresh model for training, and only | ||
replace the stale model with the fresh model periodically. | ||
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Furthermore, we have no concept that encapsulates a remote environment | ||
that executes a computation graph. | ||
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We need the session object to address above issues. | ||
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## Session | ||
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A session is an object that owns the runtime environment. All | ||
computations are executed through `session.eval()`. | ||
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### Interface | ||
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```python | ||
eval( | ||
targets, | ||
feed_dict=None, | ||
) | ||
``` | ||
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Evaluates the target Operations or Variables in `targets`. | ||
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- *targets*: the evaluation targets. Can be a single Operation or | ||
Variable, or a list with the Operations or Variables as | ||
elements. The value returned by `eval()` has the same shape as the | ||
`target` argument. | ||
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The PaddlePaddle program is represented by | ||
the [ProgramDesc](../design/program.md), `eval()` will infer the | ||
ProgramDesc from the given targets and run the PaddlePaddle | ||
program. Please | ||
see | ||
[this graph](./distributed_architecture.md#local-training-architecture) for | ||
the detailed illustration for the local session | ||
and | ||
[this graph](./distributed_architecture.md#distributed-training-architecture) for | ||
the detailed illustration for the remote session. | ||
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- *feed_dict*: a dictionary that contains the tensors which override | ||
the edges of the computation graph. | ||
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feed_dict not only can provide the input data, it can override any | ||
OP's input as well: | ||
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```python | ||
a = pd.constant(2.0, name="a") | ||
b = pd.variable(name="b") | ||
c = pd.mul(a,b) | ||
sess.eval(targets=c, feed_dict={"b":3.0}) # returns 6.0 | ||
``` | ||
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```python | ||
close() | ||
``` | ||
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Closes the session and releases the scope that the session owns. | ||
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### Create a Local Session | ||
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```python | ||
session( | ||
devices=None | ||
) | ||
``` | ||
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Creates a new session. One session owns one global scope, so creating | ||
multiple sessions will create different scopes. | ||
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- *devices*: a single `string` or a list of `string` of device names, | ||
the corresponding devices will be the computation devices for | ||
`eval()`. If not specified, all available devices (e.g., all GPUs) | ||
will be used. The user doesn't need to specify the CPU device since | ||
it will be always used. Multiple sessions can use the same device. | ||
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#### Example | ||
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```Python | ||
a = paddle.constant(1.0) | ||
b = paddle.constant(2.0) | ||
c = a + b | ||
sess = paddle.session(devices=["gpu:0", "gpu:1", "fpga:0"]) | ||
sess.eval(c) | ||
sess.close() | ||
``` | ||
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### Create a Remote Session | ||
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```python | ||
create_cloud_job( | ||
name, | ||
num_trainer, | ||
mem_per_trainer, | ||
gpu_per_trainer, | ||
cpu_per_trainer, | ||
num_ps, | ||
mem_per_ps, | ||
cpu_per_ps, | ||
) | ||
``` | ||
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Creates a Paddle Cloud job. Fails if the job name exists. | ||
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```python | ||
get_cloud_job( | ||
name | ||
) | ||
``` | ||
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Gets a Paddle Cloud job. | ||
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```python | ||
remote_session( | ||
job | ||
) | ||
``` | ||
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- *job*: the Paddle Cloud job. | ||
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#### Example | ||
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```Python | ||
reader = paddle.reader.recordio("/pfs/home/peter/mnist-train-*") # data stored on Paddle Cloud | ||
image = reader.column(0) | ||
label = reader.column(1) | ||
fc1 = paddle.op.fc(image, size=256, act="sigmoid") | ||
fc2 = paddle.op.fc(fc1, size=10, act="softmax") | ||
cost = paddle.op.cross_entropy(fc2, label) | ||
opt = paddle.optimizer.sgd(cost) | ||
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job = paddle.create_cloud_job("test", 3, "1G", 1, 1, 2, "1G", 1) | ||
sess = paddle.remote_ession(job) | ||
for i in range(1000): | ||
sess.eval(opt) | ||
sess.close() | ||
``` |
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