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Training ML-Agents

Table of Contents

For a broad overview of reinforcement learning, imitation learning and all the training scenarios, methods and options within the ML-Agents Toolkit, see ML-Agents Toolkit Overview.

Once your learning environment has been created and is ready for training, the next step is to initiate a training run. Training in the ML-Agents Toolkit is powered by a dedicated Python package, mlagents. This package exposes a command mlagents-learn that is the single entry point for all training workflows (e.g. reinforcement leaning, imitation learning, curriculum learning). Its implementation can be found at ml-agents/mlagents/trainers/learn.py.

Training with mlagents-learn

Starting Training

mlagents-learn is the main training utility provided by the ML-Agents Toolkit. It accepts a number of CLI options in addition to a YAML configuration file that contains all the configurations and hyperparameters to be used during training. The set of configurations and hyperparameters to include in this file depend on the agents in your environment and the specific training method you wish to utilize. Keep in mind that the hyperparameter values can have a big impact on the training performance (i.e. your agent's ability to learn a policy that solves the task). In this page, we will review all the hyperparameters for all training methods and provide guidelines and advice on their values.

To view a description of all the CLI options accepted by mlagents-learn, use the --help:

mlagents-learn --help

The basic command for training is:

mlagents-learn <trainer-config-file> --env=<env_name> --run-id=<run-identifier>

where

  • <trainer-config-file> is the file path of the trainer configuration yaml. This contains all the hyperparameter values. We offer a detailed guide on the structure of this file and the meaning of the hyperparameters (and advice on how to set them) in the dedicated Training Configurations section below.
  • <env_name>(Optional) is the name (including path) of your Unity executable containing the agents to be trained. If <env_name> is not passed, the training will happen in the Editor. Press the Play button in Unity when the message "Start training by pressing the Play button in the Unity Editor" is displayed on the screen.
  • <run-identifier> is a unique name you can use to identify the results of your training runs.

See the Getting Started Guide for a sample execution of the mlagents-learn command.

Observing Training

Regardless of which training methods, configurations or hyperparameters you provide, the training process will always generate three artifacts:

  1. Summaries (under the summaries/ folder): these are training metrics that are updated throughout the training process. They are helpful to monitor your training performance and may help inform how to update your hyperparameter values. See Using TensorBoard for more details on how to visualize the training metrics.
  2. Models (under the models/ folder): these contain the model checkpoints that are updated throughout training and the final model file (.nn). This final model file is generated once either when training completes or is interrupted.
  3. Timers file (also under the summaries/ folder): this contains aggregated metrics on your training process, including time spent on specific code blocks. See Profiling in Python for more information on the timers generated.

These artifacts (except the .nn file) are updated throughout the training process and finalized when training completes or is interrupted.

Stopping and Resuming Training

To interrupt training and save the current progress, hit Ctrl+C once and wait for the model(s) to be saved out.

To resume a previously interrupted or completed training run, use the --resume flag and make sure to specify the previously used run ID.

If you would like to re-run a previously interrupted or completed training run and re-use the same run ID (in this case, overwriting the previously generated artifacts), then use the --force flag.

Loading an Existing Model

You can also use this mode to run inference of an already-trained model in Python by using both the --resume and --inference flags. Note that if you want to run inference in Unity, you should use the Unity Inference Engine.

Alternatively, you might want to start a new training run but initialize it using an already-trained model. You may want to do this, for instance, if your environment changed and you want a new model, but the old behavior is still better than random. You can do this by specifying --initialize-from=<run-identifier>, where <run-identifier> is the old run ID.

Training Configurations

The Unity ML-Agents Toolkit provides a wide range of training scenarios, methods and options. As such, specific training runs may require different training configurations and may generate different artifacts and TensorBoard statistics. This section offers a detailed guide into how to manage the different training set-ups withing the toolkit.

More specifically, this section offers a detailed guide on four command-line flags for mlagents-learn that control the training configurations:

  • <trainer-config-file>: defines the training hyperparameters for each Behavior in the scene
  • --curriculum: defines the set-up for Curriculum Learning
  • --sampler: defines the set-up for Environment Parameter Randomization
  • --num-envs: number of concurrent Unity instances to use during training

Reminder that a detailed description of all command-line options can be found by using the help utility:

mlagents-learn --help

It is important to highlight that successfully training a Behavior in the ML-Agents Toolkit involves tuning the training hyperparameters and configuration. This guide contains some best practices for tuning the training process when the default parameters don't seem to be giving the level of performance you would like. We provide sample configuration files for our example environments in the config/ directory. The config/trainer_config.yaml was used to train the 3D Balance Ball in the Getting Started guide. That configuration file uses the PPO trainer, but we also have configuration files for SAC and GAIL.

Additionally, the set of configurations you provide depend on the training functionalities you use (see ML-Agents Toolkit Overview for a description of all the training functionalities). Each functionality you add typically has its own training configurations or additional configuration files. For instance:

  • Use PPO or SAC?
  • Use Recurrent Neural Networks for adding memory to your agents?
  • Use the intrinsic curiosity module?
  • Ignore the environment reward signal?
  • Pre-train using behavioral cloning? (Assuming you have recorded demonstrations.)
  • Include the GAIL intrinsic reward signals? (Assuming you have recorded demonstrations.)
  • Use self-play? (Assuming your environment includes multiple agents.)

The answers to the above questions will dictate the configuration files and the parameters within them. The rest of this section breaks down the different configuration files and explains the possible settings for each.

Trainer Config File

We begin with the trainer config file, <trainer-config-file>, which includes a set of configurations for each Behavior in your scene. Some of the configurations are required while others are optional. To help us get started, below is a sample file that includes all the possible settings if we're using a PPO trainer with all the possible training functionalities enabled (memory, behavioral cloning, curiosity, GAIL and self-play). You will notice that curriculum and environment parameter randomization settings are not part of this file, but their settings live in different files that we'll cover in subsequent sections.

BehaviorPPO:
  trainer: ppo

  # Trainer configs common to PPO/SAC (excluding reward signals)
  batch_size: 1024
  buffer_size: 10240
  hidden_units: 128
  learning_rate: 3.0e-4
  learning_rate_schedule: linear
  max_steps: 5.0e5
  normalize: false
  num_layers: 2
  time_horizon: 64
  summary_freq: 10000
  vis_encoder_type: simple
  init_path: null

  # PPO-specific configs
  beta: 5.0e-3
  epsilon: 0.2
  lambd: 0.95
  num_epoch: 3
  threaded: true

  # memory
  use_recurrent: true
  sequence_length: 64
  memory_size: 256

  # behavior cloning
  behavioral_cloning:
    demo_path: Project/Assets/ML-Agents/Examples/Pyramids/Demos/ExpertPyramid.demo
    strength: 0.5
    steps: 150000
    batch_size: 512
    num_epoch: 3
    samples_per_update: 0

  reward_signals:
    # environment reward
    extrinsic:
      strength: 1.0
      gamma: 0.99

    # curiosity module
    curiosity:
      strength: 0.02
      gamma: 0.99
      encoding_size: 256
      learning_rate: 3.0e-4

    # GAIL
    gail:
      strength: 0.01
      gamma: 0.99
      encoding_size: 128
      demo_path: Project/Assets/ML-Agents/Examples/Pyramids/Demos/ExpertPyramid.demo
      learning_rate: 3.0e-4
      use_actions: false
      use_vail: false

  # self-play
  self_play:
    window: 10
    play_against_latest_model_ratio: 0.5
    save_steps: 50000
    swap_steps: 50000
    team_change: 100000

Here is an equivalent file if we use an SAC trainer instead. Notice that the configurations for the additional functionalities (memory, behavioral cloning, curiosity and self-play) remain unchanged.

BehaviorSAC:
  trainer: sac

  # Trainer configs common to PPO/SAC (excluding reward signals)
  # same as PPO config

  # SAC-specific configs (replaces the "PPO-specific configs" section above)
  buffer_init_steps: 0
  tau: 0.005
  steps_per_update: 1
  train_interval: 1
  init_entcoef: 1.0
  save_replay_buffer: false

  # memory
  # same as PPO config

  # pre-training using behavior cloning
  behavioral_cloning:
    # same as PPO config

  reward_signals:
    reward_signal_num_update: 1 # only applies to SAC

    # environment reward
    extrinsic:
      # same as PPO config

    # curiosity module
    curiosity:
      # same as PPO config

    # GAIL
    gail:
      # same as PPO config

  # self-play
  self_play:
    # same as PPO config

We now break apart the components of the configuration file and describe what each of these parameters mean and provide guidelines on how to set them. See Training Configuration File for a detailed description of all the configurations listed above.

Curriculum Learning

To enable curriculum learning, you need to provide the --curriculum CLI option and point to a YAML file that defines the curriculum. Here is one example file:

BehaviorY:
  measure: progress
  thresholds: [0.1, 0.3, 0.5]
  min_lesson_length: 100
  signal_smoothing: true
  parameters:
    wall_height: [1.5, 2.0, 2.5, 4.0]

Each group of Agents under the same Behavior Name in an environment can have a corresponding curriculum. These curricula are held in what we call a "metacurriculum". A metacurriculum allows different groups of Agents to follow different curricula within the same environment.

Specifying Curricula

In order to define the curricula, the first step is to decide which parameters of the environment will vary. In the case of the Wall Jump environment, the height of the wall is what varies. Rather than adjusting it by hand, we will create a YAML file which describes the structure of the curricula. Within it, we can specify which points in the training process our wall height will change, either based on the percentage of training steps which have taken place, or what the average reward the agent has received in the recent past is. Below is an example config for the curricula for the Wall Jump environment.

BigWallJump:
  measure: progress
  thresholds: [0.1, 0.3, 0.5]
  min_lesson_length: 100
  signal_smoothing: true
  parameters:
    big_wall_min_height: [0.0, 4.0, 6.0, 8.0]
    big_wall_max_height: [4.0, 7.0, 8.0, 8.0]
SmallWallJump:
  measure: progress
  thresholds: [0.1, 0.3, 0.5]
  min_lesson_length: 100
  signal_smoothing: true
  parameters:
    small_wall_height: [1.5, 2.0, 2.5, 4.0]

The curriculum for each Behavior has the following parameters:

Setting Description
measure What to measure learning progress, and advancement in lessons by.

reward uses a measure received reward, while progress uses the ratio of steps/max_steps.
thresholds Points in value of measure where lesson should be increased.
min_lesson_length The minimum number of episodes that should be completed before the lesson can change. If measure is set to reward, the average cumulative reward of the last min_lesson_length episodes will be used to determine if the lesson should change. Must be nonnegative.

Important: the average reward that is compared to the thresholds is different than the mean reward that is logged to the console. For example, if min_lesson_length is 100, the lesson will increment after the average cumulative reward of the last 100 episodes exceeds the current threshold. The mean reward logged to the console is dictated by the summary_freq parameter defined above.
signal_smoothing Whether to weight the current progress measure by previous values.
parameters Corresponds to environment parameters to control. Length of each array should be one greater than number of thresholds.

Training with a Curriculum

Once we have specified our metacurriculum and curricula, we can launch mlagents-learn using the –curriculum flag to point to the config file for our curricula and PPO will train using Curriculum Learning. For example, to train agents in the Wall Jump environment with curriculum learning, we can run:

mlagents-learn config/trainer_config.yaml --curriculum=config/curricula/wall_jump.yaml --run-id=wall-jump-curriculum

We can then keep track of the current lessons and progresses via TensorBoard.

Note: If you are resuming a training session that uses curriculum, please pass the number of the last-reached lesson using the --lesson flag when running mlagents-learn.

Environment Parameter Randomization

To enable parameter randomization, you need to provide the --sampler CLI option and point to a YAML file that defines the curriculum. Here is one example file:

resampling-interval: 5000

mass:
  sampler-type: "uniform"
  min_value: 0.5
  max_value: 10

gravity:
  sampler-type: "multirange_uniform"
  intervals: [[7, 10], [15, 20]]

scale:
  sampler-type: "uniform"
  min_value: 0.75
  max_value: 3

Note that mass, gravity and scale are the names of the environment parameters that will be sampled. If a parameter specified in the file doesn't exist in the environment, then this parameter will be ignored.

Setting Description
resampling-interval Number of steps for the agent to train under a particular environment configuration before resetting the environment with a new sample of Environment Parameters.
sampler-type Type of sampler use for this Environment Parameter. This is a string that should exist in the Sampler Factory (explained below).
sampler-type-sub-arguments Specify the sub-arguments depending on the sampler-type. In the example above, this would correspond to the intervals under the sampler-type multirange_uniform for the Environment Parameter called gravity. The key name should match the name of the corresponding argument in the sampler definition (explained) below)

Included Sampler Types

Below is a list of included sampler-type as part of the toolkit.

  • uniform - Uniform sampler
    • Uniformly samples a single float value between defined endpoints. The sub-arguments for this sampler to specify the interval endpoints are as below. The sampling is done in the range of [min_value, max_value).
    • sub-arguments - min_value, max_value
  • gaussian - Gaussian sampler
    • Samples a single float value from the distribution characterized by the mean and standard deviation. The sub-arguments to specify the Gaussian distribution to use are as below.
    • sub-arguments - mean, st_dev
  • multirange_uniform - Multirange uniform sampler
    • Uniformly samples a single float value between the specified intervals. Samples by first performing a weight pick of an interval from the list of intervals (weighted based on interval width) and samples uniformly from the selected interval (half-closed interval, same as the uniform sampler). This sampler can take an arbitrary number of intervals in a list in the following format: [[interval_1_min, interval_1_max], [interval_2_min, interval_2_max], ...]
    • sub-arguments - intervals

The implementation of the samplers can be found in the sampler_class.py file.

Defining a New Sampler Type

If you want to define your own sampler type, you must first inherit the Sampler base class (included in the sampler_class file) and preserve the interface. Once the class for the required method is specified, it must be registered in the Sampler Factory.

This can be done by subscribing to the register_sampler method of the SamplerFactory. The command is as follows:

SamplerFactory.register_sampler(*custom_sampler_string_key*, *custom_sampler_object*)

Once the Sampler Factory reflects the new register, the new sampler type can be used for sample any Environment Parameter. For example, lets say a new sampler type was implemented as below and we register the CustomSampler class with the string custom-sampler in the Sampler Factory.

class CustomSampler(Sampler):

    def __init__(self, argA, argB, argC):
        self.possible_vals = [argA, argB, argC]

    def sample_all(self):
        return np.random.choice(self.possible_vals)

Now we need to specify the new sampler type in the sampler YAML file. For example, we use this new sampler type for the Environment Parameter mass.

mass:
  sampler-type: "custom-sampler"
  argB: 1
  argA: 2
  argC: 3

Training with Environment Parameter Randomization

After the sampler YAML file is defined, we proceed by launching mlagents-learn and specify our configured sampler file with the --sampler flag. For example, if we wanted to train the 3D ball agent with parameter randomization using Environment Parameters with config/3dball_randomize.yaml sampling setup, we would run

mlagents-learn config/trainer_config.yaml --sampler=config/3dball_randomize.yaml
--run-id=3D-Ball-randomize

We can observe progress and metrics via Tensorboard.

Training Using Concurrent Unity Instances

In order to run concurrent Unity instances during training, set the number of environment instances using the command line option --num-envs=<n> when you invoke mlagents-learn. Optionally, you can also set the --base-port, which is the starting port used for the concurrent Unity instances.

Some considerations:

  • Buffer Size - If you are having trouble getting an agent to train, even with multiple concurrent Unity instances, you could increase buffer_size in the config/trainer_config.yaml file. A common practice is to multiply buffer_size by num-envs.
  • Resource Constraints - Invoking concurrent Unity instances is constrained by the resources on the machine. Please use discretion when setting --num-envs=<n>.
  • Result Variation Using Concurrent Unity Instances - If you keep all the hyperparameters the same, but change --num-envs=<n>, the results and model would likely change.