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