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Submit a Distributed Training Job

The user can submit a distributed training job with Python code, rather than with a command-line interface.

Runtime Environment On Kubernetes

For a distributed training job, there is two Docker image called runtime Docker image and base Docker image. The runtime Docker image is the Docker image that gets scheduled by Kubernetes to run during training. The base Docker image is for building the runtime Docker image.

Base Docker Image

Usually, the base Docker image is PaddlePaddle product Docker image including paddle binary files and python package. And of course, users can specify any image name hosted on any docker registry which users have the access right.

Runtime Docker Image

The trainer package which user upload and some Python dependencies are packaged into a runtime Docker image based on base Docker image.

Handle Python Dependencies

You need to provide requirements.txt file in your trainer-package folder. Example:
```
pillow
protobuf==3.1.0
```
More details about requirements, an example project looks like:
```
  paddle_example
    |-quick_start
      |-trainer.py
      |-dataset.py
      |-requirements.txt
```

Submit Distributed Training Job With Python Code

paddle.job.dist_train() will call the Job Server API /v1/packages to upload the trainer package and save them on CephFS, and then call /v1/trainer/job to submit the PaddlePaddle distributed job.
/v1/trainer/job will start a building job for preparing the runtime Docker image. When the building job is finished, Job Server will submit the PaddlePaddle distributed job to Kubernetes.
NOTE: For the first version, we will not prepare the runtime Docker image, instead, the package is uploaded to Paddle Cloud, and Paddle Cloud will mount the package in a temporary folder into the base Docker image. We will not support custom Python dependencies in the first version as well.

You can call paddle.job.dist_train and provide distributed training configuration as the parameters:

paddle.job.dist_train(
  trainer=dist_trainer(),
  paddle_job=PaddleJob(
    job_name = "paddle-cloud",
    entry_point = "python %s"%__file__,
    trainer_package = "/example/word2vec",
    image = "yancey1989/paddle-job",
    trainers = 10,
    pservers = 3,
    trainer_cpu = 1,
    trainer_gpu = 1,
    trainer_mem = "10G",
    pserver_cpu = 1,
    pserver_mem = "2G"
  ))

The parameter trainer of paddle.job.dist_train is a function and you can implement it as follows:

def dist_trainer():
  def trainer_creator():
    trainer = paddle.v2.trainer.SGD(...)
    trainer.train(...)
  return trainer_creator

The pseudo code of paddle.job.dist_train is as follows:

def dist_train(trainer, paddle_job):
  # if the code is running on cloud, set PADDLE_ON_CLOUD=YES
  if os.getenv("RUNNING_ON_CLOUD", "NO") == "NO":
    #submit the paddle job
    paddle_job.submit()
  else:
    #start the training
    trainer()

PaddleJob Parameters

parameter	type	explanation
job_name	str	the unique name for the training job
entry_point	str	entry point for startup trainer process
trainer_package	str	trainer package file path which user have the access right
image	str	the base image for building the runtime image
pservers	int	Parameter Server process count
trainers	int	Trainer process count
pserver_cpu	int	CPU count for each Parameter Server process
pserver_mem	str	memory allocated for each Parameter Server process, a plain integer using one of these suffixes: E, P, T, G, M, K
trainer_cpu	int	CPU count for each Trainer process
trainer_mem	str	memory allocated for each Trainer process, a plain integer using one of these suffixes: E, P, T, G, M, K
trainer_gpu	int	GPU count for each Trainer process, if you only want CPU, do not set this parameter

Deploy Parameter Server, Trainer and Master Process

Deploy PaddlePaddle Parameter Server processes, it's a Kubernetes ReplicaSet.
Deploy PaddlePaddle Trainer processes, it's a Kubernetes Job.
Deploy PaddlePaddle Master processes, it's a Kubernetes ReplicaSet.

Job Server

RESTful API

Job server provides RESTful HTTP API for receiving the trainer package and displaying PaddlePaddle job related informations.
- POST /v1/package receive the trainer package and save them on CephFS
- POST /v1/trainer/job submit a trainer job
- GET /v1/jobs/ list all jobs
- GET /v1/jobs/<job-name> the status of a job
- DELETE /v1/jobs/<job-name> delete a job
- GET /v1/version job server version
Build Runtime Docker Image on Kubernetes

paddle.job.dist_train will upload the trainer package to Job Server, save them on the distributed filesystem, and then start up a job for building the runtime Docker image that gets scheduled by Kubernetes to run during training.

There are some benefits for building runtime Docker image on JobServer:
- On Paddle Cloud, users will run the trainer code in a Jupyter Notebook which is a Kubernetes Pod, if we want to execute docker build in the Pod, we should mount the host's docker.sock to the Pod, user's code will connect the host's Docker Engine directly, it's not safe.
- Users only need to upload the training package files, does not need to install docker engine, docker registry as dependencies.
- If we want to change another image type, such as RKT, users do not need to care about it.
Deploy Parameter Server, Trainer and Master Processes

POST /v1/trainer/job receives the distributed training parameters, and deploy the job as follows:
- Deploy PaddlePaddle Parameter Server processes, it's a Kubernetes ReplicaSet.
- Deploy PaddlePaddle Trainer processes, it's a Kubernetes Job.
- Deploy PaddlePaddle Master processes, it's a Kubernetes ReplicaSet.

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